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Synthesis, characterization and applications of ethylenevinylalcohol copolymersCitation for published version (APA):Ketels, H. H. T. M. (1989). Synthesis, characterization and applications of ethylene vinylalcohol copolymers.Eindhoven: Technische Universiteit Eindhoven. https://doi.org/10.6100/IR316591
DOI:10.6100/IR316591
Document status and date:Published: 01/01/1989
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SYNTHESIS, CHARACTERIZATION
and APPLICATIONS of
ETHYLENE VINYLALCOHOL COPOLYMERS
H.H.T.M. Ketels
SYNTHESIS, CHARACTERIZATION
and APPLICATIONS of
ETHYLENE VINYLALCOHOL COPOLYMERS
SYNTHESIS, CHARACTERIZATION
and APPLICATIONS of
ETHYLENE VINYLALCOHOL COPOLYMERS
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de
Technische Universiteit Eindhoven, op gezag van
de Rector Magnificus, prof. ir. M. Tels, voor
een commissie aangewezen door het College van
Dekanen in het openbaar te verdedigen op
dinsdag 12 september 1989 te 16.00 uur
door
Hendrikus Hubertus Theodoor Maria Ketels
geboren te Tegelen
druk. wrbr'O dlss~rt:atn::idrukkerq. helmand
Dit proefschrift is goedgekeurd door
de promotoren: Prof. Dr. P.J. Lemstra
Prof. Dr. Ir. H.E.H. Meijer
en de copromotor: Dr. G.P.M. van der Velden
CONTENTS
CHAPTER 1
INTRODUCTION
1.1 EVOH copolymers
1.2 Permeability of polymers
1.3 Factors influencing permeability
1.4 Background of the present investigation
1.4.1 EVOH copolymers used as harrier resins
1.4.2 EVOH copolymers for fiber production
1.5 Aim of the present investigation
1.6 Survey of thesis
1.7 References
CHAPTER 2
SYNTHESIS OF EVOH CO POL YMERS
2.1 Introduetion
2.2 Copolymerization of ethylene and vinylacetate
2.3 Experimental
2.3.1 Principle of operation
2.3.2 Apparatus
2.3.3 Copolymerization process
2.3.4 Characterization techniques
2.4 Results and discussion
2.4.1 Ethylene content
2.4.2 Molecular weight
2
5
7
7
10
11
11
14
15
16
21
21
21
24
25
26
26
27
2.5 Conclusions
2.6 References
CHAPTER3
29
31
MICROSTRUCIURE OF EVOH CO POL YMERS AS STUDlED BY 1H AND 13C NMR
3.1 Introduetion
3.2 Experimental
3.2.1 Materials
3.2.2 NMR measurements
3.3 Results and discussion
3.3.1 1H NMR
3.3.2 13C NMR
3.4 Conclusions
3.5 References
CHAPTER4
EVOH/NYWN-6 BLENDS
4.1 Introduetion
4.2 Experimental
4.2.1 Materials
4.2.2 Films
4.2.3 Characterization techniques
4.3 Results and discussion
4.3.1 Extrusion blending of EVOH and Nylon-6
4.3.2 Barrier properties of EVOH/Nylon-6 films
ii
33
34
34
34
35
35
43
51
53
55
56
56
56
56
58
58
59
4.3.3 Miscibility of EVOH and Nylon-6 61
4.3.4 Mechanical properties of EVOH/Nylon-6
films 64
4.3.5 Transmission Electron Microscopy 65
4.3.6 Solid State 13C NMR 66
4.3.6.1 Chemica! shifts 66
4.3.6.2 Spin lattice relaxation times 69
4.4 Conclusions 72
4.5 References 73
CHAPTER 5
EVOH/PET AND EVOH/PE BLENDS
5.1 Introduetion 75
5.2 Experimental 80
5.2.1 Materials 80
5.2.2 Blends and films 81
5.2.3 Irradiation 83
5.2.4 Morphology 83
5.3 Results and discussion 83
5.3.1 Stratified pellets 83
5.3.2 Compatibilizers 84
5.3.3 Irradiation 86
5.3.4 Films 87
5.4 Conclusions 92
5.5 References 94
111
CHAPTER6
PREPARATION AND CHARACfERIZATION OF SOLUTION
(GEL-SPUN) EVOH FIBERS
6.1 Introduetion 95
6.2 Experimental 96
6.2.1 Materials 96
6.2.2 Fiber-spinning/drawing 96
6.2.3 Characterization techniques 97
6.3 Results and discussion 97
6.3.1 Synthesis of EVOH copolymers 97
6.3.2 Properties of EVOH fibers 98
6.3.3 Development of Young's modulus
as a function of draw ratio .À 99
6.4 Conclusions 102
6.5 References 103
APPENDIX
Al Introduetion 105
A2 Experimental 105
A.2.1 Materials 105
A.2.2 NMR measurements 106
A3 Results and discussion 106
A4 Conclusions 114
A5 References 115
iv
SUMMARY 117
SAMENVATTING 121
CURRICULUM VITAE 125
V
1.1 EVOH copolymers
CHAPTER 1
INTRODUeTION
Copolymerization represents one of the most powerfut tools in tailoring polymer
systems to meet specific engineering needs and sametimes extends the range of
utility of monomers that would otherwise be of Iimited value1•3
•
Ethylene vinylalcohol copolymers are paradigmatic in this respect. The pure
homopolymer polyvinylalcohol (PVOH) possesses intrinsic limitations
concerning processability and moisture sensitivity. The abundance of -OH side
groups promotes the absorption of water, resulting in a deterioration of physical
and mechanica! properties, and moreover degradation occurs during processing
via the melt. At relative low humidities however, the pure homopolymer PVOH
possesses interesting properties. The -OH side group is relatively smalt and
consequently PVOH is crystallizable, despite its atactic character, into a
distorted monoclinic version of the polyethylene structure. Due to its crystal
linity, but notably due to the high glass transition temperature Tg (approx. 80
oe at 0 % relative humidity (R.H.)), PVOH possesses excellent harrier
properties (low permeability) against gases like oxygen (02) and carbon dioxide
(C02), at low relative humidities.
The similarity of PVOH and polyethylene with respect to the crystal structure
stimulated research efforts to produce high-strength/high-modulus fibers
similar to developments based on polyethylene Iike melt-spinning and/or
solution-spinning/ultra-drawing4•
Developments in using PVOH as harrier film and high-performance fibers have
only been partly succesfull, mainly as a consequence of the moisture sensitivity
and lack of processability.
A salution to this problem could be the incorporation of ethylene in the PVOH
chain which results in the EVOH copolymer. On a technological scale, PVOH
is produced via complete hydralysis (methanolysis) of polyvinylacetate (PVA).
Direct synthesis from the corresponding monoroer "vinylalcohol" is impossible
since this species exists in its more stable, tautomerie, keto-form: acetaldehyde.
It is well known that ethylene and vinylacelate copolymerize readily into a
copolymer, ethylene vinylacelate (EVA) copolymer, in which the ethylene and
vinylacetate units are arranged at random along the molecule. Hydralysis of
these EVA copolymers results in ethylene vinylalcohol (EVOH). The
hydrapbobic ethylene segments willlower the moisture sensitivity.
EVOH copolymers are quite unique because they are co-crystallizable over the
whole composition range5• This results in a, relatively, smalt decrease of melting
temperature and crystallinity of the EVOH copolymers with changing
composition. Moreover, the T 1 of EVOH copolymers decreases, in comparison
with PVOH, only slightly with increasing ethylene content (e.g. Tg EVOH (44
mole% ethylene) = 65 oe at 0% R.H.). The decomposition temperature of the
EVOH copolymers is, in contrast to PVOH, above the melting point and
consequently melt processing is possible.
A disadvantage of EVOH copolymers is their brittleness due to their relative
low molar mass as will be discussed in chapter 2.
In this thesis the possibilities of EVOH copolymers for the production of fibers
and fortheuse as a harrier resin will be investigated.
For a better understanding of the harrier properties of polymers in general,
some introductory remarks about the permeability of polymers are given in the
next section.
1.2 Penneability of p«)}ymers
The process of permeation through non-porous polymers is generally explained
in terros of the solution-diffusion model6• This model postulates that the
permeation of a gas through a polymer film occurs in three stages: 1 sorption
of the penetrant in the surface of the polymer, ~ diffusion through the polymer
2
and l desorption from the other face. The permeability P is a combination of
the diffusivity D of the gas dissolved in the polymer and its concentration
gradient, which in turn is proportional to the gas solubility Sin the polymer.
Above the glass transition temperature T1 of the polymerjpenetrant system the
solubility of gases usually obeys Henry's law and the sorption isotherm shows
a linear dependenee of concentration C versus pressure p:
S = C/p (I)
Since the solubility of gases in rubbery polymers is sufficiently small ( < 0.2 o/o)
the concentration dependenee of D is negligible. The diffusion coefficient is
aften well approximated by a constant D.
From Fick's law. the flux J of a gas diffusing through unit area of film with
time (steady-state) is given by:
(2)
where C1 and C2 are the concentrations at the surface and x is the thickness
of the film.
In practical systems, the surface concentration is not always known and it is
convenient to express the flux per unit area (J), in terms of the concentrations
of the external phases Cex~ or for gases in external pressures.
Combining eqs. (l) and {2) results in:
(3)
where P is the permeability coefficient.
3
In the case of gas transport through rubbery films:
P = D·S (4)
At or below T1, penetrant diffusion and sorption processes are considerably
more complicated. Both simpte gases such as e.g. C02, but also hydrocarbon
vapours in glassy polymers exhibit a pressure dependent sorption isotherm
which varies as:
C C8 '·b S(p) = - = s +
p 1 + b·p (5)
where C8 ' is a hole or site saturation constant and b is a hole or site affinity
constant.·
Originally it was supposed that one popuiadon of the sorbed molecules is
molecularly dissolved and obeys Henry's law (the first term) and the second
polpulation is sorbed into "microvoids" (holes) and follows a Langmuir isotherm
(second term).
lt was assumed that the dissolved molecules are free to diffuse down a
concentration gradient whilst the second popuiadon is bound to a fixed number
of absorption sites and holes within the polymer. Bound and mobile molecules
however are in equilibrium.
The mean permeability P is given by the product of the Henry's law's constant
S and the diffusivity of the mobile fraction of the sorbed molecules:
(6)
Later, this model was modified to allow some limited mobility of the "immobi
lized" penetrant molecules:
4
K·R p = S·D<~ (1 + 1 + b·p ) (7)
where K = CH'·b/S and Ris a measure of the degree of immobilization.
If R = 0, total immobilization, eq. (7) reduces to eq. (6) and for R = 1 the
permeability becomes pressure dependent
The model presenled above for sorption and diffusion of gas molecules through
glassy polymers is only one out of many models and theories7 which have been
prepared for transport phenomena in polymer systems.
Glassy polymers are of course structurally more complex than rubbery systems.
However, two-phase systems such as polymer blends and semi-crystalline
polymers are even more complex.
Detailed rnadelling of transport phenomena in polymer systems is beyoud the
scope of the present thesis. The reader who is interested in more detailed
information is referred to more recent reviews6.7•
In the next section, the permeability coefficients of polymer systems are
presented and the effects of various parameters are discussed.
1.3 Factors influencing penneability
The permeability P of a polymer system is expressed as:
(amount of gas)(film thickness) p = (8)
(film area)(time)(pressure)
Huglin and Zakaria8 noted 29 different units for P which appeared in the
literature.
5
Based on experimental evidence the main factors influencing permeability are:
oenetrant size
An increase in size in a series of chemically similar penetrants generally leads
to an increase in the solubility coefficients due to their increased boiling
points, but will also lead to a decrease in their diffusion coefficients due to the
increased activation energy needed for diffusion. The overall effect of these
opposing trends is that the permeability generally decreases with increasing
penetrant size, since for many polymerjpenetrant pairs the sorption coefficient
will only increase by perhaps a factor of 10 whilst the diffusion coefficient can
vary by I 0 orders of magnitude.
polymer molecular weight
When the polymer molecular weight increases, the number of chain ends
decreases. The chain ends represent a discontinuity and may form absorption
sites for penetrant molecules in glassy polymers resulting in an increase of
the permeability.
functional groups
The permeability of penetrants which interact with functional groups present
in the polymer can be expected to decrease as the cohesive energy of the
polymer increases. For example by increasing the polarity of the substituent
group on a vinyl polymer backbone, oxygen permeability is reduced by almost
50,000 times. Functional groups which have specific interactions with a
penetrant act to increase its solubility in the polymer. This leads to plasticiza
tion and hence enhanced permeability.
density and polymer structure
Generally, a reduction in density in a series of polymers results in an increase
in permeability.
crosslinking
In non-crystalline polymers, diffusion coefficients decrease approximately
6
linearly with crosslink density at low to moderate levels of crosslinking.
Generally, the solubility coefficient is relatively unaffected, except at high
degrees of crosslinking or when the penetrant swells the polymer significantly.
However, crosslinking reduces the mobility of the polymer segments and tencts
to make the diffusivity more dependent on the size and shape of the penetrant
molecules and the penetrant concentration.
crystallinity
In crystalline polymers, the crystalline areas act as impermeable harriers to
penetrating molecules and have the same effect as inert filters, i.e. they force
the penetrant molecules to diffuse along longer path lengths.
filled polymers
The actdition of inert filters may either increase or decrease permeahility
depending upon their degree of acthesion and compatihility with the polymer.
An inert filler which is compatible with the polymer matrix wil he
impermeable to the penetrant molecules and so permeation will he slower due
to the reduced area for transport. When the filler is incompatihle with the
polymer, voids tend to occur at the interface which leads to an increase in free
volume of the system and, consequently, an increase in permeahility.
In general, po lar polymers which usually possess a high T g• show the lowest
permeability, see also next section.
1.4 Background of the present investigation
1.4.1 EVOH topolymers used as barrier resins
Table 1.1 clearly shows the excellent harrier properties of PVOH for 0 2 at low
relative humidity, in fact the lowest permeability coefficient of all synthetic
polymers (generally, the perrneability of co2 is 3-5 tirnes larger in cornparison
with OJ. However, PVOH ahsorhs water resulting in a decrease of the harrier
properties because water molecules plasticize the arnorphous parts (lowering of
7
T1) and water itself can act as a gas carrier.
Table 1.1
Oxygen permeability coefficients of various polymers at 20 "C, at 0 % R.H.
(relative values basedon average literature data with reference to polyethylenet
Polyethylene (low density) 100
(high density) 46
Polypropylene 50
(oriented) 29
Polystyrene 98
Pol ycarbonate 50
Poly( vinylchloride) (rigid) 1.7
Poly( ethyleneterephthalate) 0.9
Polyamide 1.0
(oriented) 0.6
Poly(acrylonitrile) 0.17
Poly(vinylidenechloride) 0.17
PVOH 0.83 x 10-3
EVOH (29 mole% ethylene) 4.2 x to·3
EVOH (38 mole % ethylene) 17 x 10-3
The use of EVOH, which contains the hydrophobic ethylene group, could be
a solution to this problem: the sensitivity towards moisture deercases with
increasing ethylene content.
Ho wever, with an increase of the ethylene content both intermo1ecular and
intramolecular hydrogen bond strengtbs decrease. Thus, in EVOH with a higher
ethylene content, segment movement becomes easier and the permeability
coefficients become higher as can be observed in Table l.I.
8
In Figure 1.1 the oxygen permeability of various EVOH films at different
R.H.-values is shown9'10
• At R.H.-values < 70% the EVOH containing the lowest
amount of ethylene possesses the best harrier properties but above 70-80 o/o R.H.
the influence of ethylene becomes noticable: EVOH copolymer with the highest
amount of ethylene possesses the best harrier properties.
10
0 20 40 80
RH(%)
I I I
PVOH/
f EPF
I I i I
) I EPH
/lPE
Figure 1.1 Oxygen permeability of various EVOH films
at different relative humidities
9
Also important in this context is the fact that although ethylene and vinylalco
hol are co-crystallizable over the whole range of composition, a decrease of the
crystallinity in comparison with the homopoJymers can be observed4• This will
also result in an increase of the permeability (see section 1.2).
Consequently, for practical applications it is necessary to select an EVOH
copolymer which is a compromise between harrier properties and moisture
sensitivity.
A possibility of a further reduction of the moisture sensitivity of EVOH is
coextrusion into multilayer films, botties etc10•11
•
Typically, EVOH is coextruded between layers of high moisture harrier
materials such as PE and polypropylene (PP). With this construction, the high
gas-harrier qualities of EVOH can be effectively maintained. Over 80% of the
EVOH resins are used nowadays in combination with PE or PP. Since the highly
polar EVOH resins do not adhere well to non-polar polyolefins, an adhesive
layer must be used between these two polymers. Thus a typical structure
involving EVOH resins might be PP/adhesive/EVOH/adhesive/PP.
A drawback of EVOH still remains: its poor mechanical properties. The
mechanical properties of EVOH decrease with increasing ethylene content9•
Generally, there are two possibilities to solve this problem: 1 the already
mentioned coextrusion process; in the multilayer structures the outer layers
provide for the (enhanced) mechanical properties10• and .2. blending; EVOH is
blended with a polymer which possesses good mechanica! properties.
The coextrusion process bas several disadvantages, notably the impossibility to
re-process the films/containers. A much more flexible system could be a blend
of EVOH with other polymers.
1.4.2 EVOH copolymers for fiber production
Using PVOH in the so-called gel-spinning process, originally developed to
produce fibers of ultra-high molecular weight PE with excellent mechanical
properties, interesting results have been obtained5• PVOH fibers have some
advantages in comparison with PE fibers (higher melting point and lower creep)
but their major drawback is, as was pointed out before, their sensitivity towards
10
moisture: mechanical properties decrease at higher humidities. Again, the
incorporation of ethylene in PVOH could be an advantage in this respect.
1.5 Aim of the present investigation
The aim of the present investigation is:
- The synthesis of EVOH copolymers with a narrow composition and molecular
weight distribution to prevent a spreading of the properties.
- A precise determination of the microstructure of EVOH copolymers to obtain
more knowledge about the relationship between structure and properties.
- The preparation of blends basedon EVOH copolymers and the determination
of harrier and mechanical properties of their films. Also the miscibility /mor
phology of the blends have to be studied for a better understanding of the
properties.
- The preparation and characterization of gel-spun EVOH fibers, and a study
of the effect of the incorporated ethylene on the moisture sensitivity and
mechanical properties of the fibers.
1.6 Survey of thesis
Chapter 2 deals with the salution copolymerization of ethylene and vinylacetate
and the subsequent hydralysis to obtain the EVOH copolymer. The used
apparatus and metbod are described. The results of the different characteriza
tion methods are compared and discussed.
In chapter 3 the microstructure of EVOH copolymers is studied via salution 1H
and 13C NMR. An assignment of the resonances in the spectra is given.
Attention bas been paid to the sequence distribution, alkyl chain branching,
anomalous linkages and tacticity of the copolymers. This chapter has been
11
The blends of EVOH with Nylon-6 are described in chapter 4. The miscibility
is studied using Differential Scanning Calorimetry (DSC), Pulse Induced Critica!
Scattering (PICS) and optica! microscopy. From these results it can be concluded
that EVOH and Nylon-6 are miscible in the melt (at least at temperatures just
above the melting point of Nylon-6 and for EVOH copolymers with lower
ethylene contents) and phase separate upon cooling. By consecutive crystalliza
tion a so-called structured blend is obtained: a blend exhibiting a specific
morphology. The harrier properties of films of these blends are explained taking
into account this specific morphology. The mechanica! properties of the films
are studied. The morphology of blown films is investigated using Transmission
Electron Microscopy (TEM} and Solid State 13C NMR. Parts of this chapter have
been published14 or are submitted for publication15•
In contrast to the EVOH/Nylon-6 blends, the blends of EVOH with poly
(ethyleneterephthalate) and polyolefines, which are discussed in chapter 5, are
immiscible. Using the so-called Multiflux static mixer, layer structures in pellets
of these blends can be obtained. Because of this specific morphology these
blends also belong to the class of structured blends. Attempts have been made
to preserve this specific morphology upon further processing (film blowing)
because stratified films are expected to possess excellent harrier properties.
In chapter 6 the preparation and characterization of gel-spun EVOH fibers are
descri bed. The moisture sensitivity decreases, in comparison with PVOH fibers,
with increasing ethylene content but also the efficiency of draw and maximum
obtainable draw ratio (>.max) decrease. The decrease in Àmax is due to a reduction
in overall chain length with increasing ethylene content. Consequently, the
mechanical properties of EVOH copolymers, obtained via the process described
in this thesis, are intrinsically limited.
This chapter is submîtted for publication16•
12
In the aooendîx the results of a Solîd State 13C NMR study of EVOH copolymers
are presented. The observed splitting in the methine carbon region can be
explained taking into account both sequence and tacticity effects. These results
have been published17 or are submitted for publication18•
13
1. 7 Keferences
1. Alfrey Jr., T., Bohrer, J.J., Mark, H., Copolymerization, Interscience,
New York, 1952.
2. Ham, G.E., ed., Copolymerization, lnterscience, New York, 1964.
3. Ham, G.E., in "Kinetics and Mechanisms of Polymerizations", vol.l, Vinyl
Polymerization, Part 1, G.E. Ham ed., Dekker, New York, 1967.
4. Tanaka, H., Suzuki, M., Ueda, F., Toray Industries, Eur. Patent 146,084
(1985).
5. Matsumoto, T., Nakamae, K., Ogoshi, N., Kawazoe, M., Oka, H.,
Kobunshikagaku (Polymer Chemistry), 28,, 610, 1971.
6. de V. Naylor, T., in "Comprehensive Polymer Science, vol. 2, C. Boothand
C. Price eds., Pergamon Press, 1989.
7. Frisch, H.L., Polym. Eng. Sci., 2.0.(1), 2, 1980.
8. Huglin, M.B., Zakaria, M.B., Angew. Makromol. Chemie, 117, 1983.
9. Iwanami, T., Hirai, Y., Tappi Joumal, 22,(10), 85, 1983.
10. Blackwell, A.L., J. Plastic Film and Sheeting, 1. 205, 1985.
11. Culter, J.D., J.Plastic Film and Sheeting,l, 215, 1985.
12. Ketels, H., v.d. Velden, G., in "The Integration of Fundamental Polymer
Science and Technology 3", L. Kieintjens and P.J. Lemstra eds., Elsevier,
New York and London, 1988.
13. Ketels, H., Beulen, J., v.d. Velden, G., Macromolecules, 2.1. 2032, 1988.
14. Ketels, H., v.d. Ven, L., Aerdts, A., v.d. Velden, G., Polymer Comm., 30,
80, 1989.
15. Ketels, H., Nies, E., Lemstra, P.J., submitted for publication in Polymer.
16. Schellekens, R., Ketels, H., submitted for publication in Polymer Comm ..
17. Ketels, H., de Haan, J., Aerdts, A., v.d. Velden, G., in "The Inlegration of
Fundamental Polymer Science and Technology 4", L. Kieintjens and P.J.
Lemstra eds., Elsevier, New York and London, 1989.
18. Ketels, H., de Haan, J., Aerdts, A., v.d. Velden, G., submitted for pubti
cation in Polymer.
14
CHAPTER2
SYNTHESIS OF EVOH CO POL YMERS
2.1 Introduetion
As was mentioned in chapter I, the EVOH copolymers are hydrolyzed EVA
copolymers (Figure 2.1)\ (in this thesis ethylene vinylacetate and ethylene
vinylalcohol copolymers will be indicated as EVA and EVOH respectively).
n ~
CH2 + m CH2 y 0 I
c""o I CH3
1
EVOH
Figure 2.1 Synthesis of EVA and EVOH copolymer
EVA can be produced by various polymerization methods like salution-,
suspension-, bulk- and emulsion polymerization2•5
• For the production of EVOH
copolymers, the precursor EVA is generally obtained via salution polymeriza
tion in view of a better control of copolymer composition, randomness of the
copolymer, branching, degree of polymerization and distribution.
15
2.2 Copolymerization of ethylene and vinylacetate
The copolymerization of ethylene and vinylacetate under high pressure was
publisbed in the patent literature as early as 19386• However, no attempts were
made to describe the copolymerization behaviour and the products were
prepared in a merely empirical way. In spite of the fact that in 19447.s models
were reported for the description of copolymerization reactions in general, it
lasted until the period 1962-1977 befare more attention was paid to the
copolymerization reaction of ethylene and vinylacetate9-14•
The derivation of the copolymerization equation considers the four propagation
reactions:
-a· +a --> -a· -a· + b --> -b·
-b· + a --> -a· -b· + b --> -b·
with rate constant k ..
with rate constant kv, with rate constant kba
with rate constant ~
Where -a· + -b· are polymer ebains ending with radicals formed of the
monomers a and b. The reactions with rate constants k .. and kbb are known as
self -propagation and those with constants k.., and ~are called cross-propaga
tion.
If it is assumed that the reactivity of the growing chain is independent of the
chain length and dependent only upon the nature of the terminal group, then
the rate of consumption of rnanamers a and b is given by :
dn. --- = k .. n.I:n8 + kban.I:llt,· (1)
dt
dnb --- = Vbi:n1,- + k..,nbi:n_. (2)
dt
16
where n., nb are the number of moles a and b respectively and En.·, Enb· are
the concentrations of all the active centers terminating in a and b units.
At any instant during the reaction, the ratio of the amounts of each type of
monoroer being incorporated into the copolymer is found by dividing eqs. (I)
and (2):
(3)
The ratio En&/Enb· can be evaluated by a steady-state approximation, the
concentration of each of the two types of active center remains constant:
dEn& = 0 and = 0
dt dt
The most important reactions in which the active centers are created and
destroyed are the cross-propagation reactions. If now only the a· ebains are
considered, then the rate of creation and destruction is respectively, given by:
dEn.· dEn.·
dt dt
At steady-state the rate of creation equals the rate of destruction and so:
(4)
17
Substituting eq. (4) in eq. (3) and simplifying leads to7.t!:
= (5) d~ r~Jn. + 1
in which n. = number of moles a in the reactor
nb = number of moles b in the reactor
r. • k .. I kab
rb ==~I kba r. and rb are the monomer reactivity ratios.
The reactivity ratios indicate the preferenee exhibited by a certain radical at the
end of the growing chain fora monomerunit of the samekindtoa monomer
unit of the other kind.
The copolymerization eq. (5) can be integrated yielding an exact relationship
between the monomer feed composition and the degree of conversion7·ts-
17:
in which Q; = (njnb)i
qi = (njnb)i
rb- 1 R=
- r. (fb)i
Fb= 100 {1- (fJi}
100- Fb
100 =0
(6)
fb = 100 (I - nJ(nb)0)% and (nb)o is the initial quantity of moles in
the reactor.
18
German13 proved, using eq. (6), the validity of the Alfrey model for the
ethylene/vinylacetate solution copolymerization at low pressure (35 kgfjcm2)
and 62 oe in tertiary-butyl alcohol (TBA).
Consictering the r-values, three types of copolymerization processes can be
distinguished:
1: r. = 1/rb or r. x rb = 1 (ideal):
the two types of unit are arranged at random along the polymer chain in
relative amounts determined by the composition of the feed and the
relative activities of the two monomers.
2: r. = rb = o (alternating):
the monomers alternate regularly along the chain.
3: r. > 1 and rb > 1 :
in the extreme case, this would result in simultaneous homopolymeriza
tion.
In most cases copolymerization behaviour of monosubstituted vinyl monomers
lies in between the ideal and the alternating systems, with o < ra x rb < 118•
In the period 1962 - 1977 several experiments were performed todetermine the
re- and rv- values of ethylene and vinylacetate in the copolymerization.
The results and reaction conditions used are shown in Table 2.1.
From Table 2.1 it can be concluded that both parameters are near unity under
high pressure and/or high temperature conditions. The values obtained by
Erussalimsky et al12 seem to be quite exceptional. Ho wever, the results o btained
by German13 show that the difference between re and rv increases at lower
pressures, while the constant value of re x rv indicates that the copolymer
remains almast ideal.
19
reference
2(1963) 10(1963) 10(1963) 6(1964) 12(1967) 12(1967) 13(1971) 14(1971) 14(1977)
Table 2.1
Literature values of re and rv concerning the copo1y
merization of ethylene and vinylacetate
reaction conditions r.., rv re·rv temp pressure solvent
(OC) (kgf/cm2)
1.07 1.08 1.16 90 1000 toluene 0.77 1.02 0.79 70 400 benzene 0.97 1.02 0.99 130 400 benzene 1.01 1.00 1.01 150 840 0.16 1.14 0.18 60 100 0.70 3.70 2.59 60 1200 0.74 1.50 1.11 62 35 TBA 0.78 1.42 1.11 62 600 TBA 0.80 1.37 1.10 62 1200 TBA
In free-radical actdition it is often found that growing ebains are terminated in
chain transfer reactions. In these reactions a hydrogen atom is normally
abstracted from the transfer agent although other types of atoms can be
abstracted in certain cases. The radica1 formed may initiatea monomermolecule
and the active center is thus maintained. Since the growing ebains are
terminated premature1y, the degree of polymerization is reduced.
A wide variety of substances can act as a chain transfer agent, e.g. polymers,
monomers, solvents and contaminations. Consequently, in a polymerization
reaction where a high degree of polymerization bas to be obtained, a solvent
with a low chain transfer constant is required.
20
2.3 Expertmental
2.3.1 Principle of operation
The reactor, of the type as originally used by German13, is a vertically placed
cylindrical vessel provided with a piston. The upper compartment serves as
reaction chamber, the lower compartment to control the pressure.
The liquid monomer (vinylacetate (V Ac)) and the solvent (TBA, low chain
transfer constane9), containing the radical initiator (a,a'-azodiisobutyronitrile
(AIBN)) are introduced into the reaction chamber. The upper campartment is
supplied with the gaseous monomer (ethylene) and the pressure is maintained
constant during the reaction.
During the copolymerization the composition of the reaction mixture is studied
and controlled by means of a gas chromatograph. Using a specific sample
system13 the reaction mixture can directly be introduced into the gas chromato
graph. Copolymer present in the sample is retained by a precolumn. The peak
areas of the three remaining components (ethylene, VAc and TBA) are
determined by electrooie integration of the detector signal and printed out by
a digital printer. Depending on the composition of the reaction mixture, new
V Ac is introduced into the reactor to keep the ratio of the concentrations of the
monomers constant during the copolymerization. This is of utmost importance
in view of maintaining a narrow distribution with respect to composition and
molecular weight of the EVA copolymers.
Aftera eertaio reaction time the copolymerization is stopped using hydrochi
non. Subsequently, the EVA's are hydrolyzed to obtain the EVOH copolymers1.S.20.21.
2.3.2 Apparatus
A block diagram of the system and its components is given in Figure 2.2.
The gas chromatograph used is a Hewlett-Packard model 5710A. The column
is a3-meter stainless steel coiled tubing t "o.d. with cbramosorb G 60-80 mesh
HP as solid support and as stationairy phase: I 0 % by weight of a mixture of
21
diglycerol and carbowax 400 (60/40 by weight). The carrier gas is helium dried
over Linde molecular sieves. The column temperature is 75 °C.
A drawing of the reactor is given in Figure 2.3.
The reaction takes place in the campartment above the piston. Below the piston
helium can be introduced in order to pressurize the vessel.
Outlet H is used to collect the reaction mixture after the copolymerization
process.
r
'
'----------'0 --{_E J A: reactor
B: pressure control
C: sampling device
D: gas chromatograph
E: integrator
F: recorder
G: printer
H: pressure control
J: vinylacetate
Figure 2.2 Scheme of the inlegral equipment
22
' '
[J=J[§]
~~l}--~~-c
r-~T).!;'I--~-~--- 0 :;:>..----E
L--1*--~-~C
A: inlet
B: vent
C: stirrer
D: heating coil
E: thermo well
F: piston
L--1+-~-~-----G G: helium supply
L----~--- H H: drain
J: sampling tube
Figure 2.3 Reactor
23
2.3.3 Copolymerizatioo process
Materials
Ethylene, supplied by DSM (Geleen, The Netherlands) and of polymerization
quality was used without further purification. V Ac (AKZO, Arnhem, The
Netherlands) was purified by distillation. TBA, n-hexane, AIBN and hydrochi
non (all Merck) were used without further purification.
The chemieals used in the acetoxy-hydroxide transformation, methanol and
acetone (both Merck) and NaOH (Aldrich), were also used without further
purification.
EVA cooolymer
The initiator AIBN was dissolved in a mixture of V Ac and TBA, and intro
doeed into the reactor.
After the liquid components were introduced, the reactor was flusbed with
ethylene, closed and heated by means of a thermostat.
After the reaction temperature was reached, ethylene was introduced into the
reactor vessel, up to the reaction pressure, under stirring of the solution. The
ethylene pressure was kept constant during the polymerization process.
During the reaction, samples of the reaction mixture were directly introduced
in the gas chromatograph.
Using these results the ratio of the concentrations of ethylene and V Ac in the
reaction mixture was kept constant by adding V Ac during the polymerization
process using a pump.
After a certain reaction time the polymerization was stopped by adding
hydrochinon. The reaction mixture was collected and poured out in a non
solvent (n-hexane).
The solid EVA copolymer was collected and dried under vacuum at 50 oe for
24 hours.
24
EVOH copolymer
The EVA copolymer was dissolved in methanol (50 g EV A/1) at 50 oe. A NaOH
solution in methanol (30 mg NAOH/g EVA) was added to the solution under
stirring. The EVOH copolymer precipitated and was collected. To obtain a high
degree of hydralysis the copolymer was dissolved again in a methanoljH20
mixture (l/1 mixture, 50 g EVA/1). Subsequently, a NaOH solution was added
(30 mg NAOH/g EVA) and the solution was stirred and refluxed for approxi
mately 4 hours. After cooling, acetone was added to the reaction mixture to
precipitate the EVOH copolymer. The EVOH copolymer was collected and
dried under vacuum at 60 oe for 24 hours.
From NMR measurements (absence of acetate resonance, see chapter 3) the
degree of hydralysis appeared to be higher than 99% for all EVOH copolymers.
2.3.4 Characterization techniques
Ethylene content determination
The ethylene content of the EVOH copolymers was determined by 1H and 13e
NMR (for equations and detailed description, see chapter 3).
200 MHz 1H NMR spectra have been recorded with a Varian XL-200
spectrometer at 50 oe. Sample concentration was approximately 3% (w /v) using
perdeuteriated DMSO (Merck) as solvent and internat locking agent.
50 MHz 13e NMR spectra of EVOH copolymers were obtained with a Varian
XL-200 spectrometer at 40 oe. Sample concentration was 20% (w /v) in op or
in a 80/20 (wjw) phenol/DP (10-15% (wjv)) mixture.
The ethylene content was also determined by dynamic thermogravimetrie
analysis (TGA)22-24
, using a Perkin-Elmer TGS - apparatus. The EVA copoly
mers ( 4-6 mg) were heated ( 40 oe;min) to 320 oe under nitrogen (gasflow =
2 1/h), and kept there for 30 minutes. The loss of weight (acetic acid) was
determined.
25
Molecular weight determination
The number average molecular weight (Mn) of the EVA copolymers and the
reacetylated25 EVOH copolymers were determined using a Hewlett- Packard
High Speed Membrane Osmometer, model 502 and toluene as solvent.
The ratio Mw (weight average molecular weight)/Mn of the reacetylated EVOH
copolymers was determined by gel permeation chromatography (GPC) using a
Waters chromatograph connected to a differential refractometer and a
ultraviolet (UV) detector (254 nm). Waters ~-Styragel GPC-columns (lOS, 10\
10\ 102 Á) were used with tetrahydrofurane (THF) as the mobile pha8e (0.9
mi/min). Calibration was performed using polystyrene samples with a narrow
molecular weight distribution.
The EVOH copolymers had to be reacetylated before determining Mn and
Mw/Mn because they do not dissolve in toluene (osmometry) and THF (GPC).
2.4 Results and dlscussion
2.4.1 Ethylene content
In Table 2.2 the ethylene content of the EVOH copolymers synthesized at
different ethylene pressures is shown. The ethylene content is determined by
NMR ctH and 13C, more details in chapter 3) and by TGA.
In comparison with the values obtained via 1H NMR the values from 13C NMR.
are lower (ca. 0.03), possibly due to increased error propagation for small values
of the ethylene content or to the neglect of anomalous linkages (chapter 3).
Therefore, the 1H NMR results are more reliable than the 13C NMR results. A
linear relationship can be observed between the ethylene pressure in the
polymerization process arid the ethylene content of the EVOH copolymers.
The ethylene contents of the EVOH copolymers determined with TGA, via the
conesponding reacetylated EVOH copolymers, appear to be higher for all
samples than those obtained with NMR.
1t is not clear yet how this discrepancy arises. Possibly not all present acetate
groups in the EVA copolymer are abstracted from the polymer chain or the
26
reacetylation of the EVOH copolymers is not complete resu1ting in a 1ower
value for the acetate contentand consequently a higher value for the ethylene
content of the copolymers.
Table 2.2
Molar ethylene content of EVOH copolymers *
FE
EVOH pethyleru: 1HNMR 13C NMR TGA
(kgf/cm2)
A 2 0.08 0.05 0.13 B 5 0.13 0.10 0.17 c 10 0.22 0.18 0.26 D 15 0.30 0.29 0.36 E .. 0.34 0.31 0.36
* prepared under same conditions: TBA: 480 ml, V Ac: 160 ml, AIBN: 400 mg,
T IX = 55 oe, tiX = 7 h
**sample E = EVOH obtained from Kuraray (= EVAL-EPF).
2.4.2 Molecular weight
In Table 2.3 the Mn and Mw values of the EVA, EVOH and reacetylated EVOH
copolymers (EVOH(R)) are given.
U pon comparing the Mn values of EVA copolymers and reacety1ated EVOH
copo1ymers as obtained by osmometry, it can be conc1uded that with copo1ymers
preparedat low ethylene pressures (2 kgfjcm2) a large decrease of the Mn va1ue
can be observed. This is caused by the fact that the amount of hydrolyzab1e
chain branches increases with increasing conversion which is obvious because
at higher conversion the polymer concentration increases and consequently the
amount of chain transfer to the polymer increases.
27
Table 2.3
Mn and Mw values of EVA, EVOH and EVOH(R) copolymers
in kg/mole
Osmometry GPC conv.(%)
Mn Mw/Mn Mwca~c DPw Mwc:ak:
EVA EVOH(R) EVA EVOH(R) EVOH(R) EVOH
A 316 129 1.9 2.0 258 3170 135 6.1 B 119 109 1.8 1.9 207 2640 111 54 c 80 74 1.8 1.9 140 1910 77 45 D 56 48 1.8 2.0 96 1400 55 39
Using GPC, the Mw /Mn distri bution of the reacetylated EVOH copolymers is
determined. For all copolymers the value is about 2. Using the Mn values
obtained by osmometry, and Mw/Mn values, obtained by GPC, the Mw of the
reacetylated EVOH copolymers is calculated (Mw~EVOH(R)) Table 2.3).
With these values the weight average degree of polymerization can be
determined using eq. (7):
Mw~EVOH(R)) DPw = (7)
28 Fa+ 86 (1 - FJ
where Fa is the molar ethylene content as obtained by 1H NMR
From the DPw results also :the Mw's of the EVOH copolymers can be calculated:
Mw~EVOH) = 28 DPw F8 + 44 DPw (1- FJ (8)
These values are also given in Table 2.3. It is obvious that the Mw values of the
EVOH copolymers decrease with increasing ethylene content, i.e. higher
28
ethylene pressure during the copolymerization process. This decrease in
molecular weight is caused by the higher propensity of the ultimate polymerie
ethylene radical to chain transfer reactions to monomeric species.
From Table 2.1 (section 2.2, ref. 13 and 14) it appeared that re< 1 and rv > 1.
Furthermore, we may assume that k. << kw and that all terminalion reactions
are determined by diffusion. Consequent1y, this implies that at both radical
chain ends (-e· or -v·) ethylene addition takes place slower than V Ac
addition26• At higher ethylene pressures the ethylene concentration in the
reaction mixture increases which results in a slower copolymerization reaction
and consequently a decrease of the conversion rate as can be observed (see
Table 2.3).
2.5 Conclusions
1H NMR appears to be a more reliab1e metbod to determine the ethylene
content of EVOH copolymers than 13C NMR because e.g. in the 13C NMR
metbod some specific structures are neglected in the calculations (chapter 3).
From the Mw/Mn ratio(= 1.8) of the EVA copolymers it can be concluded that
the molecular weight distribution is narrow.
The Mw/Mn ratio(= 2.0) of the reacetylated EVOH copolymers is a little larger
than that of the EVA copolymers. This implies that the hydrolysis of the EVA's
does not result in a significant increase of the molecular weight distribution: the
molecular weight distribution of the hydrolyzable chain branches is, comparable
with that of the backbone chains, also narrow.
At lower ethylene pressures the conversion rate increases because the concentra
tion of the "slow reacting" ethylene decreases. However, at higher conversion
rates the concentration of the po1ymer in the reaction mixture increases which
results in a higher chain transfer rate to the polymer: a high amount of
(hydrolyzable) chain branches is obtained.
An observation of utmost importanceis that EVOH copolymers can be obtained
with Mw > 105 gjmole with an ethylene content< 15 %. Increasing the ethylene
content results in a decrease of the Mw value caused by an increased transfer
29
of the ethylenic chain radical to the monomers at higher ethylene concentrations
(i.e. pressures). This change in molecular weight is very important in respect to
the physical properties of e.g. EVOH fibers (chapter 6).
To obtain more knowledge about the microstructure of the EVOH copolymers
a detailed NMR study was performed on these polymers (chapter 3).
30
2.6 Keferences
1. Koopmans, R.J., v.d. Linden, R., Vansant, E.F., Polym. Eng. Sci., 22(10),
645, 1982.
2. Burkhart, R.D., Zutty, N.L., J. Polym. Sci. A, 1. 1137, 1963.
3. Tsuchihara, T., Kobunshi Ronbunshu, ~ (7), 473, 1979.
4. Lohr, G., Plast. Rubber: Mater. Appl., !1(4), 141, 1979.
5. Koopmans, R.J., v.d. Linden, R., Vansant, E.F., Polym. Eng. Sci, 23(6), 306,
1983.
6. Perrin, M.W., et al., B.P. 497,643 (1938) and U.S.P. 2,200,429 (1940).
7. Mayo, F.R., Lewis, F.M., J. Am. Chem. Soc., 66, 1594, 1944.
8. Alfrey Jr., T., Go1dfinger, G., J. Chem. Phys., 12.. 205, 1944.
9. Zutty, N.L., Burkhart, R.D., Advan. Chem. Ser., 34, 52, 1962.
10. Terteryan, R.A., Dintses, A.I., Rysakow, M.V., Neftekhimiya, ,J., 719, 1963.
11. Brown, F.E., Ham, G.E., J. Polym. Sci. A, 2_, 3623, 1964.
12. Erussalimsky, B., Tumarkin, N., Duntoff, F., Lyubetzky, S., Goldenberg,
A., Makromol. Chem., 104, 288, 1967.
13. German, A.L., Heikens, D., J. Polym. Sci. Al, .2. 2225, 1971.
14. van der Meer, R., German, A.L., Heikens, D., J. Polym. Sci., Po1ym. Chem.
Ed., ll. 1765, 1977.
15. Behnken, D.W., J. Po1ym. Sci. A, 2_, 645, 1964.
16. Meyer, V.E., Lowry, G.G., J. Polym. Sci. A, 1. 2843, 1965.
17. Ring, W., Makromol. Chem., 101, 145, 1967.
18. Young, L.J., J. Polym. Sci., _H, 411, 1961.
19. Bautl, H., U.S.P., 2,947,735 (1957).
20. Koopmans, R.J., v.d. Linden, R., Vansant, E.F., J. Adhesion, 11. 191, 1980.
21. Iwasaki, H., Yonezu, K., Kobunshi Ronbunshu, 32_(1), 33, 1978.
22. Terteryan, R.A., Shapkina, L.N., Polym. Sci. USSR, U. 1813, 1971.
23. v.d. Meer, R., German, A.L., Angew. Makromol. Chemie., 56, 27, 1976.
24. Park, W.R.R., 7thTherm. Anal. Proc. Int. Conf., 2_, Miller Bernard ed., Wiley
Chichester UK, 1057, 1982.
25. Tubbs, R.K., J. Polym. Sci, Polym. Chem. Ed., i. 623, 1966.
31
26. Odian, G., in "Principles of Polymerization", 2nd ed., Wiley-Interscience,
New York, 1981.
32
CHAPTER 3*
MICROSTRUCTURE OF EVOH COPOLYMERS AS STUDlED BY 1H AND 13C NMR
3.1 Introduetion
Structure-property relationships have been studied extensively exploiting arnong
other techniques detailed rnicrostructural analysis via NMR rnethods. In the
past, several papers have demonstraled the usefulness of high-resalution 1H
and/or 13C NMR methods in determining composition, sequence distribution
and cotacticity of these copolymers. Detailed microstructural analysis has been
carried out for the following copolymers: EVA1.2, EVOH3.4, EVCI5
•6
, VA- VOH7•8
and VA-VCI9•10
• Moreover, terpolymers like E-VA-VOH3 and E-VA-VCln have
been studied, however, in these cases only quantitative termonoroer composi
tions could be obtained.
Concentrating on the information available for EVOH copolymers, composi
tional sequence distribution has been obtained from either salution 1H or 13C
NMR data of the compositional VOH centered methine triacts eH NMR)3 or
alternatively via studies of the compositional dyads e3C NMR)4• Configuration
ally (i.e. tacticity) induced splittings have been analysed only partially via
salution 1H NMR of the VOH centered hydroxyl triads.
Similar to PVOH, also for EVOH copolymers anomalous linkages are possible
bes i des the normal head-to-taillinkage (I ,3-diol), i.e., head-to-he ad ( 1 ,2-diol)
and tail-to-tail ( 1,4-diol).
In previous salution NMR studies it was possible to detect specific resonances
of 1,2-diol and I ,4-diol structures in poly(vinylalcohol-co-crotonic acid) and
PVOH using ultra high field NMR equipment, respectively 125 and 100 MHz
*) Reproduced from H. Ketels, J. Beulen and G. v.d. Velden, Macrornolecules,
ll, 2032, 1988, by permission of the American Chemical Society.
33
13C NMR12.13. 1,4-Diol structures have also been detected in EVOH copoly
mers4·14, whereas the corresponding 1,4-diacetates could not be observed in EVA
copolymers4. Up to now, in spite of several attempts, resonances due to I ,2-diol
structures or adjointed 1,2-l ,4-diol structures havenotbeen detected in EVOH
copolymers4'14, possibly due to two causes: low molar ethylene ratio~ 30 mole
%in the copolymers) and detection limits (22.5 MHz t~c NMRt·14.
It is the aim of this chapter to show in a combined 1H and 13C salution NMR
study the relative merits of both techniques, concentrating on tacticity,
compositional sequence distribution, branching and anomalous linkages, because
a better onderstanding of the microstructure might be useful for explaining
macrostructural properties.
3.2 Experimental
3.2.1 Materials
The four samples of EVOH copolymers (A-D) as described in chapter 2 were
used in this study.
Two commercially available EVOH copolymers have been obtained from
Kuraray coded EPF (sample E) and ECF (sample F). PVOH has been obtained
from Hoechst (Mowiol 66-100) and is characterized by a viscosity of 66±4
centipoise ( cP) ( 4 % water salution at 20 °C).
3.2.2 NMR measurements
200 MHz and on one occasion 300 MHz 1H NMR spectra have been recorded
witheither a Varian XL-200 or a Varian SC-300 spectrometer at 50 oe. Sample
concentration was approximately 3% (w/v) using perdeuteriated dimethylsul
foxide (Me~O-d6, Merck) as solventand internallocking agent. Five millimeter
tubes were used. The speetral width of the 200 MHz NMR spectrometer
operated in the FT mode amounted to ± 2600 Hz, the acquisition time to 1.5 s,
and a putse delay of 10 s and a putse width of 5 ps (60° flip angle) were chosen.
The 50 MHz 13C NMR spectra were obtained with a Varian XL-200 spectrome-
34
ter. The sample concentration was 20% (w/v) in DP or in a 80/20 (w/w)
phenol/DzÜ (10-15% (w/v)) mixture. Spectra were generally obtained at 40 oe, using braadband decoupling, a pu1se delay of 5 s, accumulating 15000-45000
scans and a digital resolution of 0.69 Hz point, corresponding to a speetral
width of 11000 Hz and a data length of 16 K. Monomer sequence placements
were determined by camparing the relative peak areasof the proton or carbon
atoms involved. In performing quantitative NMR measurements via composi
tional or configurational sequence placements, one should take into account
differences in nuclear Overhauser effects (NOE) and spin-lattice relaxation
times (T1). No NOE's or T1's have been determined but one additional 13e NMR
experiment has been performed on sample D using a much long er delay (15 s)
and gating off the decoupler to remave the NOE. The results were identical
with those obtained via 13e NMR using the standard methods.
Implicitly we have assumed that no differential spin-lattice relaxation times are
present for different stereoisomerie (mm, mr and rr) triads ar compasitianal
triad (VOH, VOH, VOH), (VOH, VOH, E), (E, VOH, E) or analagaus dyad
sequences in the methine or hydroxyl resonances in the 1H NMR (bath) or 13e NMR (only methine) spectra. No differential 1H NOE's have been considered
to occur. Withinthese limits relative peak areas are proportional to the numbers
of proton and carbon atoms involved.
3.3 Results and discussion
3.3.1 1H NMR
Assignments
Figure 3.1 depiets as a typical example, the 300 MHz 1H NMR spectrum of an
EVOH copolymer (sample F) recorded in DSMO-d6 at 50 oe. Approximately
the same resolution as shown in this Figure could be obtained when 200 MHz
NMR equipment was used.
35
5
H20
methylene
(E, VOH)
(E, E)
(VOH, VOH)
hydroxyl methine ~--
DMSO-d5h t (VA)
methyl methyl
! 4 3 2
ppm....,....__
Figure 3.1 300 MHz 1H NMR spectrum of EVOH (F)
recorded at 50 oe
Using the results from temperature dependent studies, double resonance
experiments (cf Figure 3.2), shift additivity rules and the earlier preliminary
assignments of Wu3, we could assign all resonances. The complete assignment
is given in Table 3.1 and will be discussed insome detail now.
In the following sections, the mole fractions will be denoted by FE and FvoH·
The kinds of monads will be denoted by E and VOH while the three kind of
dyad sequences will be given by (E, E), (E, VOH) and (VOH, VOH). A similar
notation is used for the three different kind of triads, i.e. (VOH, VOH, VOH),
(VOH, VOH, E) and (E, VOH, E). Configurational sequence placements
(tacticity induced splittings) are denoted by m or r (dyads) or mm, mr, rr
(triads).
36
Table 3.1
Speetral assignments for EVOH copo1ymers,
measured with 300 MHz 1H NMR
chem. shifts, ppm
1.25 1.34 1.41, 1.44 3.38, 3.41 3.62 3.65, 3.67 3.85 3.88 3.92 3.92, 3.95 4.01 4.07, 4.08 4.26 4.30, 4.31 4.50
protons
methylene methylene methylene methine methine methine methine methine methine hydroxyl hydroxyl hydroxyl hydroxyl hydroxyl hydroxyl
dyads or triads
EE EO 00 EOE EOO,m EOO, r 000, mm OOO,mr 000, rr EOE EOO, r 000, rr EOO,m 000, mr OOO,mm
The methylene proton resonances, centeredat 1.25, 1.34, 1.41 and 1.44 ppm are
rather broad due to a combination of spin-spin coupling and configurational
splittings but have been assigned tentatively to three compositional dyads (see
Fig. 3.1). Quantitative information about the methylene dyads is as a conse
quence hard to extract and is much more easily obtained from an analysis of the 13C NMR methylene dyad and tetrad data (see section 3.3.2). Not counting
resonances that are due to Me;$0-d6 and H20, the number of the remaining low
field resonance patterns visible is six, in two separate groups of three lines. In
an attempt to clarify the assignments in this particular region (3-5 ppm) in
considerable more detail than before3, double resonance and temperature
dependent studies have been performed on sample F. In Fig. 3.2 expanded 300
MHz 1H NMR spectra of this particular EVOH copolymer are depicted showing
only the expanded methine and hydroxyl resonance region.
37
rr (000) methine + (EOE) (Hydroxyl)
r (OOE) jmm (000) + mr (000)
rr (000)1 j (EOO) m(OOE) 1 t
mr(ooof! -
70 oe
mm (000)
d-----.J
70 oe
c-----
50 oe
b---_.." Hydroxyl methlne -------
mr (000) + m (OOE)
50 oe f rr (000) + r (OOE) + (EOE)
t
a ___ _,
5 4.5 4 3 3.5 ppm-1111111...----
Figure 3.2 Expanded 300 MHz 1H NMR spectra of EVOH (F),
showing only methine and hydroxyl resonances.
VOH units have been truncated to 0
38
As is evident from Fig. 3.2 all these resonances appear to be appreciably
broadened as a consequence of spin-spin coupling effects, tacticity induced
splittings and compositionally induced chemical sequence placements.
Neglecting for the moment spin-spin splitting effects, already on the basis of
stereoregularity (tacticity) and chemical sequence placements alone there are as
many as six different hydroxyl triads and six methine triacts to be expected (12
different peaks).
As already apparent from a comparison of the spectra recorded at 50 and 70 oe
(Fig. 3.2a and 3.2c) the hydroxyl resonances (the chemica! shifts being
dependent on temperature) can be easily addressed: all resonances which
resonate downfield from 3.9 ppm onwards (at 50oe). Hidden splittings can be
resolved (both at 50 and 70 oq provided that the methylene dyads are
irradiated at 1.34 ppm.
No resolution enhancement occurs for the hydroxyl triacts (cf Fig. 3.2a and
3.2b or 3.2c and 3.2d respectively) due to the absence of any coupling between
the methylene and hydroxyl protons. Therefore, the remaining splittings have
to be assigned to a combination of tacticity induced splittings (± 15 Hz? and
spin-spin splittings between hydroxyl and methine protons (± 5MHz)3•
On the other hand resolution enhancement is observed for the methine centered
triacts (cf Fig. 3.2a and 3.2b) on decoupling. The remaining splitting has to be
assigned, in an analogous manner as for the hydroxyl protons, to the combined
effects of tacticity induced splittings (± 15Hz) and methine-hydroxyl spin-spin
splittings (± 5 Hz)3• Using an observation obtained by Moritani and coworkers
for PVOH15, i.e. the observed methine-hydroxyl spin-spin coupling constants
increase from mm < mr < rr and m < r, one can straightforwardly assign all
observed resonances. By decoupling at 50 oe (see Fig. 3.2b) the methine (VOH,
VOH, VOH) triad can be seen to be nicely split in three subpeaks, which can
be assigned, using the above mentioned rule of thumb15 to the three tacticity
induced mm, mr and rr (VOH, VOH, VOH) methine triacts (see Table 3.1 ).
As is evident from Fig. 3.2a-d overlap of methine and hydroxyl resonances
could occur (see Fig. 3.2c, at 70 oq teading to incorrect composition sequence
data ( concentrating on methine triads), however at the lower temperature of 50
oe the tacticity induced splittings of the hydroxyl triads, which are clearly
39
visible at 70 oç, (Fig. 3.2c) are hardly distinguishable. Therefore, the subtie
balance between sample concentration, measuring temperature and the acidity
of the NMR salution will render the recording of 'H NMR spectra of EVOH
copolymers in this region to be a non-trivial task.
Copolymer composition
As indicated by Wu3 the copolymer composition can be calculated from the total
spectrum (see Fig. 3.1) using the following equation:
FvoH = (I)
where A1 and Ah represent respectively the total peak areas at low ~ 3.3 ppm)
and high field (?:_ 1.6 ppm).
The copolymer composition for samples A-F has been given in Table 3.2,
expressed as the ethylene molar fraction F~~a.
Table 3.2
Molar composition and tacticity in EVOH copolymers *
EVOH F~~a FBb F& m
A 0.08 0.06 0.05 0.49 B 0.13 0.08 0.10 0.51 ç -0.22 0.15 0.18 0.47 D 0.30 0.26 0.29 0.50 E 0.34 0.29 0.31 0.47 F 0.34 0.26 0.31 0.54
• F~~a, FBb and F& determined via eq. (1), (4) and (5) respectively (see text).
40
----···-·------------
Seguence analysis
In fortunate cases as for copolymer F (see Fig. 3.2) or in less fortunate cases by
adding some dropiets of DCI to the NMR tube3 teading to a rapid exchange of
all hydroxyl protons, a sequence analysis can be performed most easily on the
VOH centered methine triad resonances. For all copolymers the composi
tion-averaged propagation statistics could be described as Bernoullian for
moderately conversion (± 25 %) copolymers (A-D) or presumably higher
conversion copolymers (E, F) as atready has been demonstrated by Wu on
similar EVA 2 and EVOH3 copolymers.
Tacticity analysis
In normal, i.e. non-decoupled 1H NMR spectra, the tacticity of these copoly
mers can be calculated (absence of anomalous structures etc.) provided that the
resonance peak at 4.50 ppm (70 oe, see Table 3.1) is clearly resolved:
t -1 m = (2 Aj A1) (FvoH) (2)
where m = the probability that a growing polymer chain will forma meso
sequence,
A. = the peak area of the mm (VOH, VOH, VOH) triad,
A1 = the total peak area at low field (.:5_ 3.3 ppm).
Although other alternatives exist, e.g. consirlering only the hydroxyl resonances
(see Fig. 3.2a), in our opinion only eq. (2) can be used in an unambiguous
manner, because depending on a subtie balance of temperature, concentration
etc., complex partially overlapping resonance areas can occur (Fig. 3.2a and
3.2c).
The values calculated for m have been depicted in Table 3.2 for all polymers.
From this Table it can be concluded that m is approximately independent of the
copolymer composition, and approximately ideally atactic.
41
Anomalous structures
Methine or hydroxyl resonances which could be assigned to anomalous linkages,
i.e. signals other than due to 1 ,3-diols have not been observed not even in
decoupled 1H NMR spectra (cf Fig. 3.2b, 3.2d). This means implicitly that in
the above mentioned quantitative analysis intrinsically the amount of 1 ,2-diol,
1 ,4-diol or adjoint 1,2-1 ,4-diol structures bas been included.
Branching
No evidence bas been found for short-chain non-hydrolyzable VOH branching.
The only evidence for non-hydrolyzable short and/or long chain alkyl
branching is the presence of a weak CH3 resonance at 0.83 ppm in the 1H NMR
spectrum (Fig. 3.1 ).
In EVA copolymers with much higher molar ethylene ratio, the amount of alkyl
chain branching was determined16,
17 and the type of branches cou1d be
identified. The amount of chain branching can now be calculated:
2ACH3 = (3)
1000
The total amount of branching bas been plotted in Figure 3.3 and increases
steadily for samples A-D with increasing ethylene contents. Since the two
commercial polymers (E, F) may have been synthesized under quite different
conditions from those used for the preparation of samples A to D, it is not
surprising that the amountof branching in these does not follow the same trend.
Possibly, terminal CH3 groups of the main chain interfere with the analysis
mentioned above. At a fieldstrengthof 4.6 T there may very well be overlap
between the 1H NMR signals of terminal methyl groups of the main chain and
those of some alkyl side chains.
42
No 13C NMR measurements have been performed in order to be able to assign
the various alkyl groups17•
6
5
4
3
2
CH3 /1000 C (0)
! • I (C) (F)
• x
(B)
• (E)
(A) x
•
~ mole% '---.-------,.--------,----.----,----.----.---- ethylene
5 10 15 20 25 30 35
Figure 3.3 Amount of non-hydrolyzable chain branching determined
via 1H NMR versus the molar ethylene ratio
3.3.2 uc NMR
Assignments
Figure 3.4 shows the 13C NMR spectra of a series of EVOH copolymers.
The spectra can be subdivided into a low field region (65-80 ppm) which is
assigned to all methine carbon resonances and a high field region (20-53 ppm)
which is assigned to all methylene carbon resonances3•
In Fig. 3.4 the nomenclature proposed for ethylene-propylene copolymers18 is
used again. The assignments X, Y, Z are discussed later (tacticity).
43
B.
70
asy
y{xsay ~ sa/)
(3asj)p+
y(3say- -l ! [3s/) mole o/o
yasaf3- T ethylene
- - 8 t
13
60 50 40 30 ppm _,... ___ _
Figure 3.4 50 MHz 13C NMR spectra of EVOH copolymers
44
Copolymer composition
Using eq. (4) the molar VOH ratio can be calculated:
A65-80 FvoH =
A65-80 + (A2D-S2 - A65-80)/2
(4)
2A65-80 =
A65-80 + A:ID-sz
where A 65 etc. denote the peak areas of the resonances indicated by their
respective chemical shifts.
The molar ethylene content can also be evaluated from the relative areas of the
different methylene carbon resonances, providing minor amounts of anomalous
structures are neglected. Adopting for EVOH copolymers a nomenclature
originally proposed for ethylene-propylene copolymers18, the following equation
is arrived at
(5)
In Table 3.2 the ethylene molar ratio using eq. (I) (FEa) (H NMR), (4)(FEb) and
(5)(FEc) (both 13C NMR) has been tabulated. A reasonably good agreement can
be found between the three independent methods, i.e. 1H NMR ( eq. (I)), 13C
NMR (eq. (4), utilizing only total methine and methylene carbon resonances
including anomalous structures), or alternatively solely the methylene carbon
resonances (eq. (5)). However in comparison to the results obtained via 1H NMR
a systematic deviation occurs, the results of both 13C NMR methods being
0.02-0.05 units (in absolute values) too low, possibly due to increased error
45
propagation (for small F8-values) (eq. (4)) or to the neglect of the anomalous
structures (eq. (5) vide infra). Therefore, the 1H NMR results are used further
throughout this work.
Seguence analysis
In the high field region of the spectra at least six well resolved resonance
patterns are observed for the methylene carbons (see Fig. 3.4).
Configurational splittings as shown by Moritani4 apparently only contribute to
the line widths and could possibly be assigned to peaks occurring within one
resonance pattern.
The methylene resonances have been assigned in a similar manner to that
described by Moritani4, except that we have adopted the nomenclature18 used
in eq. (5). For the different EVOH copolymers, it is possible to calculate from
the area intensities of the methylene lines, expressed as percentages of the total
area, the number average methylene sequence length distribution i.e. n1, n2, n3
and n4/8
•
%-CH2
100 t A B c D E F
1 2 3 ~4 1 2 3 ;;:;4 1 2 3 ~4 1 2 3 >4 1 2 3 2:4 1 2 3 ;;:;4
Figure 3.5 Number-average methylene sequence distribution
of the EVOH copolymers
In Figure 3.5 the methylene sequence distribution has been plotted for all
copolymers as a function of the methylene sequence length. Obvious conclusions
are: approximately constant level of n2, and the increase of long methylene
46
sequences with increasing ethylene content. The methylene sequence data
confirm again that the distribution is Bernoullian, as has been done by others
in earlier work on the parent EVA copolymers2•
Tacticity analysis
In Fig. 3.4 the low field region of the spectra shows five to six methine carbon
resonances which have been assigned tentatively to the six VOH triad sequences:
(E, VOH, E), m (E, VOH, VOH), mm (VOH, VOH, VOH), r (E, VOH, VOH),
mr and rr (VOH, VOH, VOHt.
This mixed configurational-compositonal VOH methine centered triad has
however not been used for a tacticity analysis, although 13C NMR offers in
comparison with 1H NMR the advantage that the methine resonance area is free
from anomalous resonances. Under our measuring conditions, the six possible
VOH centered triads betonging to normal head-to-tail sequences, lead to the
following set of equations (see also Fig. 3.4):
X = (E, VOH, VOH) + m (E, VOH, VOH) + mm (VOH, VOH, VOH)
Y = r (E, VOH, VOH) + 2mr (VOH, VOH, VOH)
Z = rr (VOH, VOH, VOH) (6)
where X, Y, Z represent the measured areasof the VOH methine centered
resonances at respectively low, central and high field.
This set of equations is exactly identical with the earlier proposed assignment
in vinylchloride methine centered resonances in V A-VCI copolymers10 and the
carbonyl triple region in the 13C NMR spectra of V A-VOH copolymers8•
Eq. (6) can be put in a handier form, having already proven (via 1H NMR)2
Bernoullian statistics to hold for the compositional sequence distribution:
Y 2 2m = (Fe/(1-Fe))+ (7)
Z r r
47
Eq. (6) can be solved numerically for m using the experimental values of the
VOH methine centered resonances (200-300 MHz 1H NMR spectra} for each
copolymer (see Fig. 3.2a) and the measured 13C NMR peak areas X, Y and Z
(see Fig. 3.4).
Without explicit knowledge of the VOH centered compositional triads, a
graphical analysis can be used (eq. (7)), assuming the value m to be constant
over the whole series of copolymers. Because it bas been proven, that m is
approximately constant for this series of copolymers (see Table 3.2) a plot of
Y/Z vs (F.J/(1-(Fs)) is represented in Fig. 3.6.
3
2 B ..,._.llllflllll- ...
A --·-_.-
E o---'1
c --- -· • e --- F -- --
0.05 0.10 0.20 0.30 0.40 0.45
Figure 3.6 Plot of Y /Z versus (Fe)!( I -(FE)) ratio
From the slope an average value for m can be calculated, being m = 0.50.
Theoretically the intercept should be equal to 2 for this particular case, the
experimentally determined intercept however being 1.64. An error analysis
shows that slight deviations of the average m value (e.g. m being 0.45) already
lead to an intercept of 1.63, therefore the m parameter determined from the
slope is a more reliable value.
Anomalous structures
The anomalous structures, i.e. 1,2-diol and 1.4-diol structures in PVOH which
could not be observed via 1H NMR, can easily be detected with UC NMR, as is
48
evident in Figure 3.7, where a 50 MHz 13C NMR spectrum of PVOH is
represented.
/kt s ~y +
yasaJ3
i
70 60 50 40 30
ppm -olllllll..,.._ __
Figure 3.7 50 MHz 13C NMR spectrum of PVOH recorded
in D:/) showing anomalous sequences
The theoretically calculated (using additivity increments12•13
'19
) chemical shifts
of the methylene carbons in the diol structures are listed in Table 3.3. The
resonances 1aSaB, BaSB1+ and 1BSa1 which can be attributed toa fragment of
coupled 1,2- and 1,4-diol structures (a, b, c of fragment I in Table 3.3) are
clearly observed.
49
Table 3.3
Calculated 13C NMR chemica! shifts of the methylene carbons
in the diol structures of EVOH copolymers
fragment
• • • 1 -c-c-c-c-c-c-c-c-c I I I I I OH OHOH OH OH . . .
u-c-c-c-c-c-c-c-c-c I I I I I
OH OH OH OH OH
b e I lt liJ -c-c-c-c-c-c-c-c-c
I I I I OH ÓH OH OH
type
1,2 + 1,4
1,2
1,4
ll,ppm
42.2 30.5 34.2
42.2 47.0
34.2 47.0
a,"YaSatJ b,"Y+fjaS(J-y+ C,"YfJSa-y
a, {JaSa-y b,-yaSa-y
In the 13C NMR spectra of all EVOH copolymers (see Fig. 3.4), evidence for the
presence of the 1,4-diol structure can easily be derived from the presence of a
resonance at 36.2 ppm. The contents of the 1,4-diol structure can be calculated
using the formula for copolymers containing only 1,4-diol:
Isaa mole % 1,4-diol = (8)
where Isaa is the area intensity of the 1BSa1 resonance at 36.2 ppm and 11 is
the total area intensity of the methine and methylene lines.
The factor 2 bas to be left out for copolymers containing 1,2- and 1 ,4-diol
structures. The assumption bas been made that in an EVOH copolymer either
coupled 1,2- and 1 ,4-diol structures are present or only 1 ,4-diol but not both.
The calculated contentsof 1,4- and 1,2-diol structures are listed in Table 3.4.
As can be concluded from Table 3.4 the contents of the 1,4-diol structure in
PVOH and EVOH copolymers are in the same order. Coupled 1,2- and
1,4-units (fragment I, Table 3.3) have been observed in the 13C NMR spectrum
of polymers A en B. Isolated 1,4-units are observed for the polymers C, D, E
and F. No evidence for the presence of isolated 1 ,2-diol structures in these
50
copolymers (C-F) has been found (absence of a resonance at 42.2 ppm).
1,4-diol 1,2-diol
Table 3.4
Calculated contents of I ,2- and 1 ,4-diol structures
of the EVOH copolymers
PVOH A
1.2 1.2
1.0 1.0
B
1.9 0.5
c D E
1.1 0.9 0.8
F
0.4
However, in the polymers A and B where coupled 1,2- and 1,4-units are
present, the presence of an additional isolated 1 ,2-unit can not be excluded
(overlapping 13C NMR resonances). The presence of isolated 1,4-diol structures
has to be attributed to the insertion of ethylene fragments (at least for the
polymers C, D, E and F). For the polymers where adjointed 1,2- and 1 ,4-diol
structures are observed (A, B) both possibilities have to be considered
(experiments with 13C enriched ethylene would in principal unravel this
phenomenon). Thus it is possible to detect the I ,2-diol structure in EVOH
copolymers with an ethylene content to approximately 10 mole %.
Branching
No evidence for non-hydrolyzable VOH or alkyl branching has been found in
the salution 13C NMR spectra.
3.4 Conclusions
Using 1H and 13C NMR an assignment could be made for the methine and
methylene carbon resonances.
In the 1H NMR spectra the different hydroxyl resonances could be observed.
In determining the molar ethylene content in the EVOH copolymers, a
reasonably good agreement was found between the 1H and 13C NMR methods,
51
as was also concluded in chapter 2. Ho wever, a systematic deviation occurs
between both methods, possibly due to increased error propagation or to the
neglect of anomalous structures. 1H and 1~ NMR appeared to be useful for studying the tacticity of the
copolymers. With both methods it was proved that the copolymers are nearly
ideally atactic.
To obtain more knowledge about the sequence distribution and the anomalous
structures in the EVOH copolymers, 13C NMR is more useful than 1H NMR:
both 1,2-diol and 1,4-diol structures could be observed in the EVOH copoly
mers, using ~ NMR.
The only evidence for non-hydrolyzable short and/or long chain alkyl
branching was found in the 1H NMR spectra.
It can be concluded that a combination of 1H and 13C NMR methods is
necessary to obtain profound knowledge about the microstructure of EVOH
copolymers. This knowledge is important for a better understanding of the
properties of e.g. EVOH fibers (chapter 6).
S2
3.5 References
I. Wu, T.K., J. Polym. Sci. A2, .8., 1167, 1970.
2. Wu, T.K., Ovenall, D.W., Reddy, G.S., J. Polym. Sci., Polym. Phys. Ed.,
Jl., 901, 1974.
3. Wu, T.K., J. Polym. Sci., Polym. Phys. Ed., 14, 343, 1976.
4. Moritani, T., Iwasaki, H., Macromolecules, il. 1251, 1978.
5. Keiler, F., Plaste Kautsch., ll(IO), 730, 1978.
6. Zambelli, A., Gatti, G., Macromolecules, ll, 485, 1978.
7. Moritani, T., Fujiwara, Y., Macromolecules, 10, 532, 1977.
8. Van der Velden, G ., Beulen, J ., Macromolecu1es, li. 1071, 1982.
9. Okada, T., Hashimoto, K., Ikushige, T., J. Polym. Sci., Polym. Chem. Ed.,
12.. 184, 1981.
10. Van der Velden, G., Macromolecules, lQ., 1336, 1983.
11. Van der Velden, G., unpublished results, 1985.
12. Amiya, S., Uetsuki, M., Macromolecu1es, li. 166, 1982.
13. Ovenall, D.W., Macromolecules, .!I, 1458, 1984.
14. Amiya, S., Iwasaki, H., Fujiwara, Y., Nippon Kagaku Kaishi (J. Chem.
Soc. Japan), il, 1698, 1977.
15. Moritani, T., Kuruma, I., Shibatani, K., Fujiwara, Y., Macromolecules,
577, 1972.
16. Wagner, T., Schlothauer, K., Schneider, H., Plaste Kautsch., 29(11), 637,
1982.
17. Grenier-Loustalot, M.F., Eur. Polymer J., 2.1. 361, 1985.
18. Carman, C.J., Wilkes, C.E., Rubber Chem. Techn., 44, 781, 1971.
19. Vercauteren, F., Donners, W.A.B., Polymer, 27, 993, 1986.
53
54
4.1 Introduetion
CHAPTER 4
EVOH/NYWN-6 BLENDS
In previous chapters, I and 2, it was described that EVOH copolymers are
brittie partly due to their relàtively low molar mass which is related to chain
transfer reactions during copolymerization of ethylene and vinylacetate.
In chapters 4 and 5, the possibility of bleuding EVOH withother polymers will
be investigated, aiming to achieve polymer blend systems combining barrier
properties (EVOH)1.z and acceptable mechanical properties.
In this chapter the combination of EVOH and the polyamide Nylon-6 was
chosen. Polyamides are well known fortheir toughness and ease of processing.
Moreover, Nylon-6 is used for barrier film applications. However, as shown in
Table 1.1 in chapter I, the barrier properties of Nylon-6 are not impressive.
Combination of polymers can result in miscible, partially miscible or immiscible
systems3.4. A miscible system is the result of molecular mixing on a segmental
level, in contrast to immiscible blends which are multipbase systems in which
the major component forms the continuous phase or matrix and the other
component makes up the dispersed phase. The partially miscible blends
constitute of two or more phases each showing a certain level of molecular
mixing. The physical properties of polymer blends (e.g. mechanica} properties,
barrier properties) depend on its state of mixing. This state is determined by the
thermodynamics of interactions between the blend components and this
interaction is a function of their physical and chemica! structures.
Miscibility inthemelt was investigated with DSC, PICS and optical microscopy.
The miscibility /morphology in the solid state (films) was discussed using results
obtained from TEM and Solid State 13C NMR. DSC methods could not be used
because the EVOH copolymers, as used in this study, and Nylon-6 possess
similar Tg's.
55
4.2 Experimental
4.2.1 Materials
Nylon-6, Akulon 135 C (Mn = 30 kg mole'1) was provided by AKZO. EVOH
copolymers coded EPL (Mn = 32 kg mote·1, ethylene content= 27 mole %), EPF
(Mn = 21 kg mole"\ ethylene content= 32 mole %), EPG (Mn = 20 kg mole- 1,
ethylene content= 47 mole %) were obtained from Kuraray Co.
4.2.2 Films
Blends (films) of Nylon-6 and EVOH copolymers were made using a single
screw extruder with a tubular film die head. The temperature of the die was
250 oe, well above the melting point of both polymers. The residence time was
about 1 minute. The winding-speed of the films was 2 m/min. The thickness
of the films was about 20 pm.
Permeability measurements at 0 % R.H. were performed at TNO (Delft, the
Netherlands)5• Measurements at 60, 70 and 80% R.H. were performed at DSM.
Carbon dioxide (COJ was used as testing gas.
Impact performance was measured by an instrumented dart-test. A hemispheri
cal dart was used. Impact energies were obtained by recording the load-time
curve during penetration. From this curve the total energy could be determined.
4.2.3 Charaderization techniques
Differential Scanning Calorimetry (DSC)
DSC measurements were performed using a Perkin-Elmer DSC-7 calorimeter.
Indium was used for the calibration (Tm= 156.6 oe). A standard heating and
cooling rate of 5 °C/min was adopted.
56
Putse Induced Critical Scattering (PICS)
PICS measurements were performed using a Mark 11 instrument6• The blends
of EVOH-EPF and Nylon-6 (25/75 and 50/50 (wjw)), homogenized in aso
called "centrifugal homogenizer", were heated from 150 to 240 oe with a
heating rate of 2 oC/min. The capillary cell containing the polymer blend was
illuminated by a low-power ~e-Ne laser beam. The PICS apparatus was used
in the cloud point mode.
Optical microscopy
A Zeiss optica! microscope equiped with a Mettier FP hot stage was used.
Thin films of the blends (EVOH-EPF/Nylon-6 = 25/15 and 50/50 (w/w)) were
heated from 150 to 240 oe with a heating rate of 2 °C/min.
Transmission Electron Microscopy (TEM)
Films of Nylon-6 and EVOH-EPF/Nylon-6 blends were imbedded in an epoxy
matrix. Before staining, the samples were trimmed in an appropriate manner for
sectioning. The samples were treated with phosphotungstic acid7 (PT A
(Aldrich), 10% aq. solution, 24 h at room temperature) or ruthenium tetra
oxide8.9 (Ru04) (0.2 g RuCI3·3H . .P (Aldrich) in 10 milS% aq. sodium hypochlo
rite). Microtomy was performed at room temperature using an ultramicrotome
Reichert Jung 4F and diamond knife. Sections of 50-70 nm thick were
obtained. A JEOL 2000 FX TEM operating at 80 KV was used for the
investigation.
Solid State 13C NMR
NMR data were acquired at room temperature on a Bruker CXP 200 spectrome
ter operating at 50.3 KHz. Samples were loaded into an air driven two
component Beams-Andrew BN-POM rotor with a polyoxymethylene (POM)
cap and spun at 3.0-3.5 kHz. The chemica! shifts were referenced to the
57
external crystalline and amorphous POM-resonances (both at 88.8 ppm).
Typical cross-polarization (CP) putse sequences implied 4 p.s 90° pulse, 1 ms
contact time and a 4 s recycle time, collecting 2048 points in the time domaio
for a 20 kHz speetral width. Depending on the sample, 1000-15000 FID's we re
collected for the Dipolar-Decoupling/CP/Magic Angle Spinning (DD/CP/
MAS)-experiments1o.t1. Spin-lattice relaxation times (of the crystalline phase
(T1J and the amorphous phase (T1J) were determined by an inversion recovery
sequence using CP and MAS. Depending on the sample, 500-750 FID's were
colleeled for 10 recovery delays which were 0.5 p.s to 100 s in length. The
amplitudes at the peak positions have been fitted to an equation of the form:
l(t) = I(oo) + A{a·exp(-r/T1J + (l-a)·exp(-r/T1J}
where ris the delay between the two pulses and I{oo) is the equilibrium
magnetization. A non-linear least square regression metbod has been used.
Initial estimates of I( oo ), A, a, TJa and T te are needed.
4.3 Results and discussion
4.3.1 Extrusion blending of EVOH and Nylon-6
Melt blending of Nylon-6 and EVOH is rather complicated due to the
formation of gellike particles during extrusion. From IR measurements it was
concluded that these gels were not due to a degradation of the EVOH.
Consequently, areaction ofEVOH with Nylon-6, or possibly present oligomers,
could be the only cause forthese gels. After extraction of Nylon-6 with water
{90 oe, 24 h) the concentration of oligomers (lactams) in Nylon-6 was
decreased. Using the extracted Nylon-6 in the film blowing process, the
formation of gels could be prevented.
58
4.3.2 Harrier properties of EVOH/Nyloo-6 films
In Figure 4.1 the C02-permeability versus blend composition is shown. The
observed dependency is ooo-lioear: addition of a small amount of EVOH to
Nylon-6 results in a large decrease of the permeability.
1
• • EPG(47moln ethJ
". EPF (32mole% eth.)
o ·EPL(28mole% eth.)
%(Wfw)EV0H ---
Figure 4.1 C02-permeability versus blend composition of
different EVOH/Nylon-6 films
59
From Fig. 4.1 it can be concluded that the harrier properties of the films
increase with decreasing ethylene content of the EVOH in the blend: EVOH (27
% eth.) > EVOH (32 % eth.) > EVOH (47 % eth.).
Fig. 4.1 clearly demonstrates that the harrier properties of EVOH/Nylon-6
films are mainly determined by the EVOH copolymer (concentration, ethylene
content) used in the blend.
The non-linear relationship between harrier properties and EVOH content is
also confirmed by Figure 4.2, showing the C02-permeability of EVOH
EPL/Nylon-6 films at different R.H.-values (Please note that the vertical scale
is a logarithmic scale now).
!] ~
103 ..;
~
i s. 104
;C\1 g
105 0..
40 60 80 100 RH(%)
Figure 4.2 C02-permeability versus R.H. of EVOH-EPL/Ny/on-6 films
The addition of 25 % EVOH to Nylon-6 results in a large increase of the harrier
properties, whereas the difference between the films containing 25 and 50 %
EVOH is rather smalt. In facta ten-fold decrease in permeability is obtained
in the case of EVOH-EPL (25 %)/Nylon-6 blend in comparison with pure
60
Nylon-6. Consequently, this system falls in the same category as high barrier
resins such as poly(acrylonitrile) and poly(vinylidenechloride) (see Table 1.1,
chapter 1).
At higher R.H.-values the barrier properties decrease. The penetrating water
molecules destroy the hydrogen boncts and act as plasticizer, which will result
in an increase of the polymer chain mobility and consequently the permeability.
4.3.3 Miscibility of EVOH and Nylon-6
Both EVOH and Nylon-6 are capable of crystallization, although in different
crystalline structures12"14. Based on thermodynamica! equilibrium considera
tîons15, one can expect an eutectic phase-diagram, provided that the components
are miscible in the melt. For polymer mixtures, thermodynamic equilibrium is
difficult to reach. Therefore, it is common practice to discuss experimental
melting data obtained for similar experimental conditions. In this way possible
differences in morphology in the mixtures and the pure components are kept
toa minimum.
coolingline
EVOH
125 150
T["CJ
Figure 4.3 DSC cooling-line (5 OC/min) of
a EVOH-EPF /Nylon-6 (50/50 (wjw)) blend
61
In Figure 4.3 an example is shown of a DSC cooling-line (5 oe; min) of a 50/50
EVOH-EPF /Nylon-6 blend. It is obvious that consecutive crystallization occurs
upon cooling the blend. The first crystallization peak (A) belongs to Nylon-6,
the second (B) is the crystallization peak of the EVOH copolymer.
In Table 4.1 the T.,- and Tm-values of EVOH-EPF and Ny1on-6 obtained from
DSC measurements are shown.
Ny-6 80/20 60/40 40/60 20/80 EPF
Table 4.1
T"-and Tm-va1ues of EVOH-EPF and Nylon-6
for different blend compositions
Ny-6 Te Tm
192.2 225.0 192.0 225.0 190.1 221.4 184.5 219.3 165.5 183.8 177.5 216.4 165.0 185.3
165.0 185.8
Similar differences in crystallization behaviour, as can be noticed in Tab1e 4.1,
have been observed for polymer solutions and blends and are due to kinetic
effects15• The mutua1 differences between the crystallization and melting
temperatures are approximately constant indicating that the experiments were
performed under similar conditions. For comparative purposes it is convenient
to use the experimental crystallization and me1ting temperature instead of the
equilibrium temperature.
According to F1ory16, the Tm-depression is given by:
1
Tmt
1 -RV1
Tmt(O) = .ö.H1V1 {
+ (
62
where Tm is the melting temperature in the blend, T m(O) is the equilibrium
melting temperature, R is the gas constant, 8H is the heat of fusion per mole
repeating unit, V is the molar volume of repeating unit, <P is the volume
fraction in the blend, m is the relative chain length on the lattice, X12 is the
Flory-Huggins interaction parameter and V, is the molar volume of the
lattice site. Subscripts I and 2 indicate component 1 and 2 in the blend.
Due to the uncertainty in the experimental data and the inherent non
equilibrium character of the experiments the exact value of X12 giving the best
fit to the experimental data according to above equation is not relevant. One
could conclude that in order to obtain the melting point depression of Nylon-
6 as noticeable in Table 4.1, the interaction parameter X12 must have a negative
value. This is an indication of favourable or specific interactions between both
blend components. This also means that the components are miscible in the melt
at least for temperatures just above the melting temperature of Nylon-6. For
example a liquid-liquid lower critical salution temperature (LCST) miscibility
curve could be present at more elevated temperatures.
The conclusion that EVOH-EPF and Nylon-6 are miscible in the melt is also
confirmed by the results of PICS and optical microscopy experiments. In the
PICS measurements no changes in scattered intensity could be observed in the
temperature range of 220-240 oe. Furthermore, optical microscopy experiments
using the same blends, showed no phase separation in the same temperature
range. Because the calculated difference in refractive index11 (n0 ) of both
components is about 0.02, phase separation should have been detected by these
optical techniques. These results support the conclusion that EVOH-EPF and
Nylon-6 are miscible in the melt.
It is obvious that the miscibility is accomplished by an interaction of Nylon-6
with the hydroxyl group of EVOH. Consequently, the miscibility of these
polymers will diminish with increasing ethylene content of the EVOH
copolymer.
As can be noticed in Fig. 4.3, upon cooling Nylon-6 and EVOH do not
crystallize simultaneously: Nylon-6 crystallizes at first and subsequently EVOH
63
follows. During the actual film blowing process the cooling rates are rather
high. Assuming that small Nylon-6 spherulites are formed, the EVOH phase is
forced to accumulale between crystalline areasof the Nylon-6 matrix, resulting
in a specific morphology. Therefore, this blend also belongs to the class of
structured blends1s.19•
The transport of molecules like 02 and co2 takes place through the amorphous
and intercrystalline regions of Nylon-6, but in this particular blend, the
amorphous matrix is enriched by EVOH. Because EVOH itself is an excellent
harrier resin, also excellent harrier blends are obtained.
4.3.4 Meehan1eal properties of EVOH/Nylon-6 films
In Table 4.2 the results of the dart-test obtained for EVOH-EPL/Nylon-6 films
are shown. By adding EVOH to Nylon-6 the impact energy decreases as was
expected. Blends containing more than 40% (w/w) EVOH copolymer possess
approximately the same impact strength as pure EVOH-EPL. But as was
concluded in the previous parts only minor amounts of EVOH (appr. 25%
(w/w)) are necessary in the blend to obtain blends with excellent harrier
properties. From Table 4.2 it can be concluded that EVOH/Nylon-6 films
containing minor amounts of the EVOH still possess good mechanica)
properties, provided by Nylon-6 which forms the matrix.
Table 4.2
Impact energies (KJ/m) of different EVOH-EPL/Nylon-6 films
(accuracy appr. 10 %)
Ny-6 75/25 60/40 50/50 40/60 EVOH-EPF
74.1 36.0 11.7 10.9 10.8 11.5
64
4.3.5 Transmission Electron Microscopy
Transmission Electron Microscopy is a well known technique for the charac
terization of the structure of heterogeneaus polymer systems. However, often
it is necessary to enhance image contrast for polymers using staining agents.
For this purpose, Nylon-6 film and EVOH-EPF/Nylon-6 film were stained
using PT A and Ru04• Figure 4.4 shows micrographs of ultra-thin sections of
Nylon-6 film and a film of EVOH-EPF/Nylon-6 (25/75 (wjw)) at high
magnifications, stained with Ru04• Using PT A camparabie results were
obtained.
A ,20nm,
B
Figure 4.4 Electron micrographof Nylon-6 (A)
and EVOH/Nylon-6 (B) film
No significant structural differences can be observed between the mkrographs
of pure Nylon-6 and the blend. As is known, selective staining causes the non
crystalline regions to show a dark contrast, whereas the interiors of the
crystalline sections appear as bright areas. Consequently, it can be assumed that
the bright regions in Fig. 4.4 arise from crystalline Nylon-6. From Fig. 4.4 it
can be concluded that the crystalline partsof Nylon-6 are very small caused by
the fast cooling of the melt during the film blowing process. It was impossible
to detect EVOH crystalline domains.
65
4.3.6 Solld State uc NMR
Since the pioneering workof Schaefer and Stejskal10'11 on the characterization
of solid polymers via Solid State 13C NMR several publications have appeared
dealing with the application of this metbod to polymer blends:zo..23• With complete
(or partial) miscibility the average environment of a given constituent molecule
will certainly be different from that in the case of immiscibility. In the latter
case, the majority of the molecules will be surrounded by neighbours of the
same type. The environment of a molecule will affect the electron density
around the atoms and/or the mobility of that molecule, which is reflected in the
chemical shifts and the relaxation times, respectively. Therefore, differences in
chemical shifts and NMR relaxation phenomena between pure polymers and the
same polymers in a blend will be an indication of a possible miscibility of the
constituents.
4.3.6.1 Chemical shifts
Figure 4.5 shows the 50 MHz 13C CP/MAS spectrum of the EVOH- EPF/Nyl
on-6 (50/50) blend (spectrum A).
The methine carbon resonances of the EVOH copolymer cao be observed (60-
80 ppm) but in the region 20-50 ppm the methylene carbon resonances of
Nylon-6 and the EVOH copolymer severely overlap.
By subtracting spectrum B (EVOH-EPF) from A by such a factor that the
methine carbon resonances of the EVOH copolymer disappear, the difference
spectrum C (Nylon-6 in the blend) cao be obtained and the chemica! shifts of
Nylon-6 in the blend cao be determined.
66
POM
0 11
- N-C1-C2-C3-C4-C5-C6 I
100 50
----B
10
-I ppm
Figure 4.5 50 MHz 13C CP /MAS spectra; A= EVOH-EPF /Nylon-6 (50/50),
B = EVOH-EPF; C =A-B (Nylon-6 in blend);
In Table 4.3 the 13C chemical shifts of two EVOH-EPF/Nylon-6 blends, pure
EVOH-EPF and Nylon-6 are listed. For semi-crystalline Nylon-6 two different
crystalline phases can he observed24: the thermodynamically more stabie a-phase
and the 1-phase. In Table 4.3 the 13C chemical shifts of the methylene carbons
are not given.
67
0\ 00
Table 4.3
Speetral assignments (in ppm) in two EVOH-EPF/Nylon-6 blends (in respect to
the POM-resonances (88.8 ppm))
c6 1 2 3 C17 c2,3 c4 Csa
EVOH-EPF/ Nylon-6
(100/0) 74.6 70.4 65.5
(50/50) 173.9 74.2 69.7 64.4 40.8 30.3 26.4 36.8
(25/75) 173.8 74.9 70.1 65.1 41.0 30.3 26.6 37.0
(0/100) 173.8 40.6 30.5 27.2 37.3
Cs-y
34.5
34.3
33.5
The spectra are a superposition of the spectra of crystalline and amorphous
materiaL However, crystalline material is generally better defined than its
amorphous counterpartand the "narrower" lines assigned bere are therefore
essentially due to the crystalline fraction.
No significant differences can be observed between the chemical shift of pure
Nylon-6 and EVOH-EPF and the analogous polymers being present in the
blend.
It is known that in Nylon-6 hydrogen boncts exist between amide protons and
carbonyl groups25, both within one chain and between different chains. These
boncts will influence 13C NMR chemical shifts26• A comparable situation exists
in the EVOH copolymer. It is very unlikely that in blends with compositions as
studied bere, no changes in hydrogen bonds, and thus in 13C NMR chemical
shifts, would occur upon intimate mixing of the two constituents.
4.3.6.2 Spin-lattice relaxation times
In the case of a miscible blend, the spin-lattice relaxation times (T1) of carbon
atoms in the polymer ebains which are mechanically coupled (m.c.) are similar.
In blends, this m.c. usually signifies close intermolecular distances. When the
carbon atoms are not coupled, each would have its own characteristic T 1•
Intermediate stages arealso possible: when polymers are not completely coupled,
butsome interaction between groups of the polymers is present, the T 1's of the
carbon atoms will change. The difference between the T1's of the carbons which
participate in those interactions will decrease upon increasing coupling
(interaction).
The T1a and T1c of the methylene carbons and the carbonyl carbon of Nylon-6
film and Nylon-6 in the blend films were determined using the equation as
described in the experimentalpart with a= 0.75. The values are listed in Table
4.4.
The results have to be compared with those of Nylon-6 film, because the blends
underwent the same manufacturing process.
69
Table 4.4
T1 values (ins) of the methylene carbon atoms and the carbonyl carbon atom of Nylon-6 in the amorphous
and crystalline phase in different films (accuracy = T1• (10%); T,.,J~Q%); T1 (C6) (30%))
ppm 173 43 37 30 27 41 34
Nylon-6 c6 Cla Csa c2,3 c4 c17 CS7
EVOH-EPF/ 'Nylon-6
(0/100) Tla 3.0 0.5 0.3 0.2 0.3 0.3
....:1 Tlc 5.0 10.0 5.8 6.2 4.0 5.1 0
(25/75) T,. 3.0 0.5 0.4 0.3 0.5
Tic 25.0 12.0 8.4 6.5 49.0
(40/60) T,. 8.0 0.6 0.5 0.3 0.6
Tlc 100.0 27.0 22.0 26.0 10.0
(50/50) T,. 12.0 0.6 0.5 0.8 0.9
Tlc 300.0 24.0 27.0 57.0 49.0
(60/40) T,. 0.6 0.4 0.2 0.6 0.2
Tlc 36.0 23.0 19.0 14.0 5.0
It can be noticed that the T1c values of the methylene carbons become larger
with increasing EVOH copolymer content in the blend. These differences
between Nylon-6 film and blend films cannot be caused by the presence of
radicals as is proved by the absence of EPR signals.
In the same way as for Nylon-6, the T1's of the methine carbon atoms in EVOH
copolymer film and EVOH copolymer in the blends were compared (Table 4.5).
Table 4.5
T1 values (ins) of the methine carbon atoms of EVOH copolymer
in the amorphous and crystalline phase in different films
ppm
EVOH
EVOH-EPF/Nylon-6
(25/75)
(40/60)
(50/50)
(60/40)
(100/0)
Tla Tic Tla Tlc
Tla Tic Tl• Tlc Tl• Tic
75
COH
2.5 21 1 24 2.6 17 1.1 16 2.5 15
70
COH
1.0 20 0.8 15 1.5 20 1.3 20 2.1 27
65
COH
2.0 24
1.4 16 0.6 16 3.0 16
Because of considerable overlap, the T1 values of separate methylene carbon
atoms cannot be determined accurately. The a values (0.4) in the bi-exponential
fit were constant for the different EVOH copolymer samples. As can be
concluded from Table 4.5, no differences can be observed between the T1's of
the methine carbon atoms in the EVOH samples.
Upon mechanical coupling in the blends with compositions as studied here,
changes in hydragen bonds would occur and consequently in relaxation times.
From the result that only T1c of Nylon-6 changes (not T1• of Nylon-6 and T1's
of EVOH), the conclusion can be made that there is no evidence for intimate
interactions.
71
The fact however, that the T1.,'s of the methylene carbon atoms of Nylon-6
change slightly, points to the existence of very small crystalline domains of
Nylon-6 in the blends. Because only in the case of small Nylon-6 crystallites,
an efficient contact between Nylon-6 and the surrounding amorphous phase
(Nylon-6 and EVOH), which probably affects the T1c of Nylon-6, is possible.
4.4 Conclusions
Based on experimental evidence, obtained from thermal analysis (DSC), light
scattering (PICS) and optical microscopy, one is tempted to conetude that EVOH
copolymers are miscible, in the molten state, with Nylon-6. The miscibility,
accomplished by favourable interactions of the polar groups in Nylon-6 and
EVOH, will depend on the ethylene content of the EVOH copolymer and
probably decreases with increasing ethylene content.
Upon cooling phase separation occurs caused by consecutive crystallization: at
first Nylon-6 crystallizes foliowed by the EVOH copolymer, resulting in a
specific morphology. Therefore, the EVOH/Nylon-6 blends also belong to the
class of structured blends1s.19•
Because the amorphous phase of Nylon-6, where transport of e.g. C02 takes
place, is enriched hy EVOH, films of these hlends exhihit excellent harrier
properties. Because the harrier properties increase highly non-linearly upon
adding EVOH, only minor amounts of EVOH are sufficient to ohtain films
possessing excellent harrier properties. Consequently, the mechanical properties
of the blend films remain good.
In the solidified EVOH/Nylon-6 hlends no information could he ohtained
concerning structural and morphological details using TEM. Ho wever, hased on
Solid State 13C NMR measurements one can conclude that complete phase
separation has occurred.
72
4.5 Keferences
1. Blackwell, A.L., J. Plastic Film and Sheeting, 1. 205, 1985.
2. Cutter, J.D., J. Plastic Film and Sheeting, 1. 215, 1985.
3. MacKnight, W.J., Karasz, F.E., Fried, J.R., in "Polymer Blends", Academie
Press, New York, 1978.
4. Olabisi, 0., Robeson, L.M., Shaw, M.T., in "Polymer-Polymer miscibility",
Academie Press, New York, 1979.
5. TNO, Delft, the Netherlands, internat report.
6. Galina, H., Gordon, M., Irvine, P., Kleintjens, L.A., Pure Appl. Chem.,
ll(2), 365, 1982.
7. Schaper, A., Hirte, R., Ruscher, C., Hillebrand, R., Walenta, E., Coll.
Polym. Sci., 22_4, 649, 1986.
8. Montezinos, D., Wells, B.G., Burns, J.L., J. Polym. Sci., Polym. Lett. Ed.,
2..3.. 421, 1985.
9. Trent, J.S., Scheinbeim, J.I., Couchman, P.R., Macromolecules, .!Q., 589,
1983.
10. Schaefer, J., Stejskal, E.O., J. Am. Chem. Soc., 98, 1031, 1976.
11. Schaefer, J., Stejskal, E.O., Buchdahl, R., Macromolecules, 10, 384, 1977.
12. Matsumoto, T., Nakamae, K., Ochiumi, T., Kawarai, S., Shioyama, T., J.
Soc. Fiber Sci. Japan, .3..3.(2), 49, 1977.
13. Nakamae, K., Kameyama, M., Matsumoto, T., Polym. Eng. Sci., .!2(8), 572,
1979.
14. Kinoshita, Y., Makromol. Chem., .3..3., 1,1959.
15. Smith, P., Ph.D. Thesis, University of Groningen, The Netherlands, 1976.
16. Flory, P.J., in "Principles of Polymer Chemistry", Cornell University Press,
Ithaca, NY, 1953.
17. Van Krevelen, O.W., in "Properties of Polymers, Correlation with Chemical
Structure", Elsevier Publishing Company, Amsterdam, 1981.
18. Meijer, H.E.H., Lemstra, P.J., Elemans, P.H.M., Makromol. Chem.,
Makromol. Symp., .!Q., 113, 1988.
19. Van Gisbergen, J.G.M., Meijer, H.E.H., Lemstra, P.J., accepted for
pubHeation in Polymer.
73
20. Stejskal, E.O., Schaefer, J., Sefcik, M.D., McKay, R.A., Macromolecules,
H. 275, 1981.
21. Crowther, M.W., Cabasso, I., Levy, G.C., Macromolecules, 21, 2924, 1981.
22. Parmer, J.F., Dickinson, L.C., Chien. J.C.W., Porter, R.S., Macromolecules,
~. 2308, 1987.
23. Dickinson, L.C., Yang, H., Chu, C.-W., Stein, R.S., Chien, J.C.W., Macro
molecules,~. 1757, 1987.
24. Weeding, T.L., Veeman, W.S., Angad Gaur, H., Huysmans, W.G.B.,
Macromolecules, .21.. 2028, 1988.
25. Arimoto, H., J. Polym. Sci. A,~. 2283, 1964.
26. Stothers, J.B., in "Carbon-13 NMR Spectroscopy", Academie Press, New
York, chapter 11, 1972.
74
CHAPTERS
EVOH/PET AND EVOH/PE BLENDS
5.1 Introduetion
In chapter 4, the miscible system EVOH/Nylon-6 was discussed. In this chapter
blends of poly(ethyleneterephthalate) (PET) and polyethylene (PE) with EVOH
copolymers will be studied. PET is used as a harrier resin, for example in
bottles, because of its excellent clearness and toughness. However, the harrier
properties are not impressive (see Table 1.1, chapter 1 ).
EVOH/PET and EVOH/PE blends are, in contrast to the EVOH/Nylon-6
blends, immiscible. Generally the group of immiscible blends can be divided in
compatible · and incompatible blends. The term compatibility is used to
characterize polymer mixtures possessing desired properties and practical
usefulness, like good acthesion between the constituents, improved mechanical
properties or ease of blending.
During the bleuding process a two-phase system is formed. Starting with a
coarse dispersion the spherical domains will become extended into thread1ike
structures which will finally break up into fine droplets. These droptets can be
extended as long as there is no equilibrium between shearing and interfacial
forces. At higher volume fractions of the dispersed phase the agglomeration
process becomes dominant. A1though this process is not very well understood,
the agglomeration rate depends strongly not only on the volume fractions, but
also on the viscosities of both phases and the interfacial properties. Consequent
ly, (in- )compatible polymer blends can have different, intrinsically unstable,
morpbologies like spheres, cylinders and lamellae depending on rheological
(viscosity, fluid elasticities) properties, interfacial (diffusion rate, solubility,
interfacial mobility and -energy) behaviour and the time effects in different
processing routes.
Properties, like harrier and mechanical properties, are highly dependent on the
morphology of the blend. Because the objective in this study is, similar to that
75
presented in chapter 4, to obtain blends with excellent harrier properties and
good mechanical properties, it is important to understand the correlation
between morphology and harrier properties.
A theory to describe transport in polymer blends is the Effective Medium
Theory (EMT)2• This theory provides a simpte quantitative prediction of the
overall conductivity of a multipbase material, given a minimal morphological
description of the composite. Using extensions of this theory proposed by
Kirkpatrick3, Davis4 and Barrer ilrul using the topological parameters of the
percoladon theory6.7, the effective or macroscopie diffusion coefficient Deu• of
ordered morpbologies (spheres, cylinders, lamellae) cao be determined8• It bas
to be taken into account that normally a blend exhibits a disordered macrostruc
ture but is built up from ordered microdomains as shown in Figure 5.1.
Lamellae Spheres Cylinders Lamellae
parallel series
Figure 5.1 Microstructured domains, subpartsof a
disordered macrostructure
In Figure 5.2 the calculated Detr-values of a EVOH/PP blend consisting of these
ordered structures are graphically shown. The results are representative for
PET /EVOH and PE/EVOH blends.
At a given volume fraction of EVOH in the blend, say 30% (w /w), the absolute
upper and lower bounds of Detr in Fig. 5.2 are given by the parallel and series
laws, respectively9.These bounds repcesent blends with an ordered macrostruc
ture (parallellayers and layers in series).
76
-(I) ......... C\1 E u -CD > --u CD --CD
0
1Ö6 ~-------------------------------------------,
10-7
10-8
10-9
parallel
-~ EVOH spheres
lamelle;---:-~
...... --- ~-- EVOH cylinders ---::::::::--
pp cylinders -- --:::::::-:----..-:---.-... --- ~
PP spheres
1Ö104---------~--------r---------r--------.---------i 0 0.2 0.4 0.6 0.8
vol. fractlon EVOH
Figure 5.2 Effective dijfusion coefficients in cm2/s
of EVOH/PP blends
From Fig. 5.2 it is obvious that layers in series of EVOH have to be present in
the matrix to obtain good harrier properties. A minor amount of EVOH in the
blend then can result in a significant increase of the harrier properties. This
highly non-linear relationship between harrier properties and EVOH content
in the blend was also observed in the study of the EVOH/Nylon-6 blends
(chapter 4). In contrast to the EVOH/Nylon-6 blends where the specific
crystallization process results in a structured morphology, specific production
processes are necessary to obtain EVOH layers in a matrix (i.e. PET), for
example:
77
1. By coextrusion multilayer constructionscan be obtained. Coextrusion is used
in various processing procedures: film blowing, blow moulding, cast films
and recently also injection moulding.
2. Films of a blend with spheres of EVOH in e.g. a PP matrix can be biaxially
drawn in the solid state: the EVOH spheres will be "stretched" and layer-Iike
structures of EVOH in the matrix are obtained. During this process not
only the barrier properties are achieved (compare Fig. 5.2) but also the
mechanica} and optica} properties of the matrix are considerably improved10•
3. Disadvantages of above mentioned possibilities are the high production costs
(I) and critical process conditions (2). Possibly this can be surpassed by the
use of stratified pellets, structured blends consequently, in different
subsequent production processes. When the morphology can be fixated it
could be possible to obtain stratified films which possess excellent harrier
properties (Fig. 5.2). Stratified blends can be obtained using mixers like
Kenics, Sulzer, Ross and the MultiClux static mixer11•
In the studies described in this chapter, the Multiflux static mixer is used to
prepare the stratified pellets. A proper choice of the polymerie materials has to
be made because parameters like viscosity and elasticity ratios of the used
polymers and the thickness ratio of the layers are important with respect to the
interfacial stability of the layers12•
As was pointed out, the specific morphology in the pellets will be unstable and
will change upon further processing. Therefore, it is important to preserve the
stratified morphology from distortion.
Generally, there are three possibilities to fixate/stabilize a eertaio morphology:
1 functionalizing the polymers in the blend with reactive groups, 2 adding
(block)copolymers to the blend12 and l crosslinking of the dispersed phase, e.g.
by the use of electron beam (EB) irradiation13'14
• The latter two methods were
tested in these studies.
The added (block)copolymer should contain different segments which are
78
chemically identical or compatible to those in the layers. These (block)copoly
mers alter the interface in the blend and are often referred to as "compatibili
zers" because they enhance the compatibility of the components in the blend15•
In stratified systems intermediate layers of compatibilizer conneet the layers.
When a decrease of interfacial tension can be established the start of the
deformation and breaking up process can be postporred which could result in
a prevention of the distortion of the layers during subsequent processing.
Using EB irradiation crosslinks can be induced in one (or both) polymer(s)
present in the stratified pellets. The presency of crosslinks could also result in
a delay of the start of the breaking up process because the mobility of the
polymer chains is decreased. A lower mobility implies a decreased flowablility
of the chains which results in a delay of the relaxation process.
In the case of (partially) broken layers the presence of compatibilizers and/or
crosslinks could also influence the agglomeration process. Agglomeration can
be delayed, because the interfaces are made more immobile16, or surpressed by
the existence of the intermediate layer. This implies that it could be possible to
postpone the start of the breaking up-, agglomeration- and finally coalescence
process. However, a complete prevention may he impossible. Not only the shear
rate and the total deformation but also the time in (further) processing is of
great importance with respect to a possible preservation of the stratified
structures.
Stratified PET/EVOH and PE/EVOH pellets were made using the Multiflux
static mixer. Compatibilizers and EB irradiation were used to fixate/stabilize
the specific morphology. Films were made of stratified PET/EVOH and
PET /EVOH pellets. The morphology (scanning electron microscopy (SEM)) and
the barrier properties of these films were investigated and compared with
properties of films obtained from turnbie mixtures.
79
5.2. Experimental
5.2.1 ~aterlals
PET (containing 10 mole% of the isophthalic group) was provided by AKZO.
EVOH copolymers coded EPF (Mn = 21 kg mote·1, 32 mole% ethylene) and
EPG (Mn = 20 kg mole·1, 47 mole% ethylene) were obtained from Kuraray Co.
Lineair Low Density PE-1026 (throughout this chapter defined as PE) was
supplied by DSM.
Tm C'e) fJ(Pas, i = 100 s·1)
PET 223 700 (250 oe)
EVOH-EPF 181 600 (250 oe)
EVOH-EPG 156 350 (200 °C)
PE 127 700 (200 oe)
eompatibilizers containing carboxylic groups after reaction bleuding with
maleic anhydride (MA) were obtained from DSM:
C-1 (based on EPDM)
C-2 (based on HOPE)
e-3 (based on PP)
e-4 (based on VLDPE)
C-5 (based on LDPE)
0.9 mole% MA
1.5 mole% MA
3.5 mole% MA
4.5 mole% MA
5.2 mole % MA.
Orevac 9307 (86 mole % ethy1ene, Tm= 94 oe), a modified EVA, was obtained
from A TO CHIMIE, France.
80
5.2.2 Bleods and films
Blends of PET /EVOH and PE/EVOH were made on a Berstorff ZE 25
Iabaratory corotating twin screw extruder.
Stratified pellets were made using a Multiflux static mixer11 • Polymer roelts
from different extruders flow tagether in the multiflux where the stream is
divided, deformed and rearranged (Figures 5.3 and 5.4).
Figure 5.3 Principle of a static mixer
Depending on the number of static elements in the Multiflux, layers can be
introduced in the melt following:
(I)
where N = number of elements
Z0 = number of layers present in the melt before reaching the
Multiflux.
Throughout the experiments five static elements were used in the Multiflux.
Consequently, stratified pellets (without compatibilizer) contained 64 layers.
Films of the blends were made on a Collin single screw Iabaratory extruder
fitted with a film die head.
Permeability measurements were performed at DSM.
81
direction of extrusion
Figure 5.4 Multi/lux static mixer
82
5.2.3 Irradiation
lrradiation of the blends was performed at IRI (Interfacultary Reactor Institute
in Delft, the Netherlands) using a 3 MeV Van de Graaff Accelerator in air at
room temperature. Maximum thickness of the samples was 1.0 cm and in order
to achieve a homogeneaus dose distribution, two-sided irradiation was
performed. Doses up to 20 MRad were used.
5.2.4 Morphology
SEM studies were carried out with a Cambridge Stereoscan Model S200. The
samples were plasma-etched with oxygen and subsequently sputter-coated with
gold.
5.3 Results and discussion
5.3.1 Stratified pellets
Using the Multiflux static mixer, stratified pellets we re made of PET /EVOH
EPF and PE/EVOH-EPG blends. In Figure 5.5, electron micrographs of the
blends obtained are shown.
A ,30.um,
I
B
Figure 5.5 Electron micrographs of stratified pellets of PET j EVOH (A)
and PE/EVOH ( B) blends, respectively
83
From Fig. 5.5 it is obvious that it is possible to obtain stratified pellets for both
types of blends. To obtain a first impression of the stability of the structure,
these pellets were used, without additional fixation, in the film blowing process.
The morphologies of the blown films were investigated via SEM (Figure 5.6).
5.um j
A B
Figure 5.6 Electron micrographs of blown films of PET / EVOH (A)
and PE/ EVOH blends ( B ), respectively, using stratified
pellets
The layer structure as present in the pellets appears not to be stabie enough to
remain during the film blowing process: no layers can be observed in the blown
films. During the manufacturing process the layers break up under influence
of the interfacial tension, agglomerate and subsequently coalesce resulting in a
morphology as shown in Fig. 5.6. Consequently, as described in the Introduc
tion, it is necessary to fixate the morphology before further processing.
5.3.2 Compatibilizers
For adding (block)copolymers to the blends, a third extruder was connected to
the Multiflux apparatus and a special die head was used by which a layer of the
(block)copolymer could be introduced between the blend layers (Figure 5.7).
84
r-1--
polymer B polymer A '----I-
polymer c
Figure 5.7 Scheme of apparatus used to introduce ( block)copolymer ( B)
between the blend layers
Two kinds of compatibilizer were tested in the PET/EVOH-EPF blends:
polyolefines with incorporated MA-groups which probably can react with the
hydroxyl groups in EVOH and the end-groups in PET, and a EVA copolymer
(86 mole% ethylene).
lt appeared that the MA-containing polyolefines could nat be used as
compatibilizers: at the processing temperature (250 °C) they decompose,
probably caused by an elimination reaction with C02 as teaving group.
Also the addition of the EVA copolymer did not result in any stabilization or
fixation of the layers: it was even difficult to obtain stratified pellets with EVA
as an intermediate layer. Decomposition and differences in viscosity between
the different layers at processing temperatures could be the reason for this
phenomenon.
However, in the PE/EVOH-EPG blends the MA-containing polyolefines could
be used because of the lower processing temperatures. In the obtained stratified
pellets some adhesion could be observed between the different layers, especially
when compatibilizers with higher MA-content were used.
Pellets, containing compatibilizer C-5 were used in the film blowing process.
The obtained results (morphology and harrier properties) will be discussed later
in this chapter, tagether with those of films made from other (un- )stratified
pellets.
85
5.3.3 Irradiation
Depending on their chemical structure polymers either crosslink or degrade
upon irradiation17•
The overall effect of crosslinking is that the molecular weight of the polymer
increases and a network is formed . Crosslinked polymers do not dissolve
completely in an usual solvent, and by determining the swollen gel fraction of
an irradiated polymer, the degree of crosslinking can be calculated.
In contrast, radiation degradation is a process in which the polymer chain
suffers random chain scission and consequently the molecular weight decreases
with radiation dose.
Polymers can be classified into two groups depending of the fact whether they
mainly crosslink or degrade upon irradiation18•
The polymers used in this study, EVOH copolymers, PET and PE, belong to the
group of polymers which crosslink upon irradiation, be it with different
rates17.18. By irradiating the stratified pellets of blends of the polymers some
crosslinking in the layers could possibly be established resulting in a decrease
of the chain mobility which postpones the start of the relaxation process by
which the layer structure is distorted. Of course toostrong an increase in overall
viscosity is not allowed because it will make subsequent processing impossible.
Different blends were irradiated: PET/EVOH-EPF (10, 20 MRad); PET/PE/
EVOH-EPF (5,10 MRad); PE/EVOH-EPG (2,5 MRad); PE/C-5/EVOH-EPG
(2,5 MRad). Todetermine the degree of crosslinking of the polymers, also PET,
EVOH-EPF and EPG, PE and C-5 were irradiated. Via the determination of
the gel fraction, crosslinks could only be indicated in the systems containing
PE and C-5.
No sta hilization of the stratified structure was obtained in the PET /EVOH
EPF blends; the layer structure is lost after compression moulding (5 min, 250 0 C}. A morphology of EVOH spheres in a PET matrix is obtained. This
indicates that under influence of the interfacial tension the EVOH layers break
up (see also 5.3.4 Films).
In contrast to the PET blends, the blends based on PE preserve the stratified
morphology during compression moulding as shown in Figure 5.8.
86
A B
c D
Figure 5.8 Electron micrographs of stratified
PE/EVOH-EPG pellets. A: 0 MRad, before compression moulding
( c.m.), B: 0 MRad, after c.m., C: 2 MRad, before c.m., D: 2 MRad,
a/ter c.m.
5.3.4 Films
In Figure 5.9 and 5.10 electron micrographsire shown offilms of PET/EVOH
and PE/EVOH blends, respectively made from tumble-mixtures, multiflux
blends withor without compatibilizer and irradiated multiflux-blends withor
without intermediate layer.
87
PET/EVOH-EPF
(70/30)
A
B
c
D
S,um
PET /PE/EVOH-EPF
(60/10/30)
E
F
G
H
turnbie-mixture
multiflux biend
irradiated
PET /EPF: I 0 MRad
PET /PE/EPF: 5 MRad
irradiated
PET /EPF: 20 MRad
PET /PE/EPF: I 0 MRad
Figure 5.9 Electron micrographs of PET /EVOH-EPF films
88
PE/EVOH-EPG
(70/30)
A
B
c
D
S.um
PE/C-5/EVOH-EPG
(60/10/ 30)
E
F
G
H
turnbie-mixture
multiflux blend
2 MRad
5 MRad
Figure 5.10 Electron micrographs of PE/EVOH- EPG films
89
Figure 5.9 clarifies again that no stabilization or fixation could be established
in the PET /EVOH blends by irradiation. The micrographs of the irradiated
samples show EVOH spheres in the PET matrix: the EVOH layers are broken
up.
Moreover, the films of PET/EVOH contained gellike particles: parts which do
nat melt at the regular melting point. Apart from the poor optica! and harrier
properties of these films (Table 5.1 ), the presence of these crosslinked particals
apparently maximize the radiation dose which can be used.
In contrast however, in the PE/EVOH-EPG samples an influence of irradiation
can be observed. In the films irradiated with 2 MRad layers are present. At
higher doses (5 MRad) however, the layers are distorted. The problem in the
blend systems used, is that bath phases might crosslink upon irradiation. A high
degree of crosslinking in PE results in an enhancement of the difference in
viscosity of both components in the blend, which can result in a faster
distartion of the layers12•
In the blends with an interlayer of compatibilizer the layers cannot be observed
clearly in the film. So the influence of the compatibilizer with respect to a
possible stabilization is negligible small.
The films as shown in Figures 5.9 and 5.10 were used in permeability
experiments (0% R.H.). The results are shown in Tab1e 5.1.
The permeability coefficients of the PE/EVOH-EPG films are rather similar
to those of PET /EVOH-EPF. Bearing in mind that PET is a much better harrier
resin than PE and that EVOH-EPF possesses better harrier properties than EPG
(chapter 4), the conclusion can be drawn that the influence of EVOH in the
PE/EVOH b1ends is more effective. This is in agreement with the results
presented in Figures 5.9 and 5.10 because in PE/EVOH-EPG films a layer
structure can still be noticed. At higher irradiation doses films are obtained in
which the layer structure is more difficult to abserve and consequently a
decrease of the harrier properties is noticed.
90
Table 5.1
Permeability of different blend films (cm3·cm/cm2·24 h·bar)
(Peru. lOs)
PET films Pco2 PE films Pc02
(Figure 5.9) (Figure 5.1 0)
A 1.2 A 1.2
B 9.7 B 1.5
c bad c 0.5
D bad D bad
E 2.7 E 1.7
F 9.6 F 3.6
G 8.0 G 4.6
H bad H bad
Upon comparing the harrier properties of films made from stratified pellets
with a reference experiment with films obtained from turnbie mixtures, it can
be concluded that the films from turnbie mixtures possess the best harrier
properties (or in one case (PE/EVOH-EPG), similar properties). From Figures
5.9 and 5.10 it is obvious that in these films, a "layered" structure is present. - --- · --
These layers (or extended) structures are induced by the biaxially strain
imposed on the blend during the film blowing procéss. This process is more
efficient when the volume fraction of the EVOH increases, because then a
course cocontinuous morphology is formed16, prior to the biaxially stretching
process, yielding layer structures as in the PP /EVOH example10 in the beginning
of this chapter. Also Dupont uses this principle in its Selar process to produce
biaxially drawn film of PElPolyamide blends. However, the morphology
induced with processes like these will strongly depend on the geometry and the
91
processing conditions (blowup ratio etc.) and are not as universal as the use of
pellets which al ready contain the des i red morphology.
5.4 Conclusions
To obtain films with excellent harrier properties, layer structures, perpendicular
to the direction of diffusion of the penetrant, have to be present in the films.
Using the Multiflux static mixer stratified pellets were obtained of PET j
EVOH-EPF and PE/EVOH-EPG blends. Using compatibilizers (e.g. MA
containing polyolefines) and/or EB irradiation attempts were made to stabilize
their specific structure during further processing.
Because of the high processing temperature (250 oq of PET, the used
compatibilizers decompose and consequently no stratified blends could be
obtained.
However, in the case of PE/EVOH-EPG blends the MA-containing polyole
fines could be used as compatibilizer. The polyolefines possessing the highest
content of MA (5.2 mole %) induced some adhesion between the layers in the
blend.
Upon irradiation no stabilization/fixation of the morphology could be
established in the pellets of the PET /EVOH-EPF blends. Already u pon
compression moutding the layers break up under influence of the interfacial
tension.
As could be concluded from compression moutding experiments with (un- )
stratified and (un-)irradiated pellets some degree of stabilization is introduced
in PE/EVOH-EPG blends irradiated with a low dose (2 MRad).
The different pellets (stratified, (un- )irradiated, turn bie-mixtures) were used
in the film blowing process. The results showed again that for PET /EVOH
EPF blends no stabilization of the stratified morphology occurs upon irradiating
and/ or adding compatibilizers. Ho wever, in films made of stratified PE/EVOH
EPG pellets irradiated with 2 MRad, layers are still noticeable indicating some
degree of stabilization.
From permeability experiments it revealed that films with layer (or extended)
structures of EVOH, obtained from turnbie mixtures or stratified pellets, possess
92
the best harrier properties indicating the validity of the calculations as
presented in Figure 5.2.
Summarizing, the aim of our investigation was to induce the desired morpho
logy already in the pellets. When the morphology can be completely fixated, a
variety of subsequent processes (like injection moulding, blow moulding,
extrusion, film blowing and casting) are possible, because the morphology and
consequently the harrier properties will not depend anymore on the process and
processing condîtions. However, our experiments show that up to now the
success of this approach was limited. At least for the blend systems chosen.
It wilt be very difficult to fixate the stratified morphology in PET/EVOH-EPF
blends because of the high processing temperature (decomposition of compati
bilizer) and the fact that higher irradiation doses will result in an increase of the
amount of gel like particles and consequently a loss of optical and harrier
properties.
In the case of PE/EVOH-EPG blends some stabilization could be established
upon irradiation with low doses, as could be concluded from compression
moutding experiments. However, during the subsequent film blowing process,
the stratified morphology is distorted. Higher doses did not improve the results
and moreover will make the matrix viscosity too high to allow for further
processing. Consequently, more research will be required to unravel the
possibilities of a stabilization/fixation of stratified structures.
93
5.5 References
1. Perkin, W., Polymer Bull., 12. 397, 1988.
2. Landauer, R., J. Appl. Phys., 2l, 779, 1952.
3. Kirkpatrick, S., Phys. Rev. Lett., 21., 1722, 1971.
4. Davis, H.T., J. Am. Ceramic Soc., 2Q., 499, 1977.
S. Barrer, R.H., in "Diffusion in Polymers", J. Crank and G.S. Park eds.,
Academie Press, London, 1968.
6. Shante, V.K.S., Kirkpatrick, S., Adv. Phys., ~. 325, 1971.
7. Fisher, M.E., Essam, J.W., J. Math. Phys., z., 609, 1961.
8. Sax, J., Ottino, J.M., Polym. Eng. Sci., 2l(3), 165, 1983.
9. Hashin, Z., Shtrikman, S., J. Appl. Phys., Jl, 3125, 1962.
10. Lemstra, P.J., Kirschbaum, R., Polymer, 2.6,, 1372, 1985.
11. Sluyters, R., De Ingenieur, J., 33, 1965.
12. Kahn, A.A., Han, C.D., Trans. Soc. Rheol., ll, 101, 1977.
13. Meijer, H.E.H., Lemstra, P.J., Elemans, P.H.M., Makromol. Chem.,
Makromol. Symp., 12. 113, 1988.
14. van Gisbergen, J.G.M., Meijer, H.E.H., Lemstra, P.J., accepted for
publication in Polymer.
15. Paul, D.R., in "Polymer Blends", Academie Press, New Vork, 1978.
16. Elemans, P.H.M., Ph.D. Thesis, University of Eindhoven, the Nethetlands,
1989.
17. Charlesby, A., Nucleonics, ll( 6), 18, 1954.
18. Chapiro, A., in "Radiation Chemistry of Polymerie Systems", Interscience
publishers, New Vork and London, 1962.
94
CHAPTER6
PREPARATION AND CHARACfERIZATION OF
SOLUTION (GEL-SPUN) EVOH FIBERS
6.1 Introduetion
Solution-spinning of ultra-high molecular weight PE (UHMW-PE), the so
called gel-spinning process, made it possible to produce fibers with excellent
mechanica! properties. Tenacities in the range of 3-4 GPa and moduli over 100
GPa could be obtained1.2.
In the past decade various attempts have been made to apply the principles of
the gel-spinning technique to other flexible polymers3A. In the case of PVOH
interesting results have been obtained, e.g. tenacities up to 2.3 GPa and moduli
of about 60 GPa5, even based on polymers with moderate molecular weights
(Mw appr. 102 kgjmole). The PVOH fibers have some advantages in compari
son with UHMW-PE fibers: higher melting temperature (ca 250 oe), potentially
Iower creep levels and better adhesion to (polar) matrices. A major drawback
however, of the PVOH fibersistheir sensitivity towards moisture: mechanica}
properties decrease drastically with increasing R.H.6•
As was explained befare in chapter 1, the incorporation of ethylene in the
PVOH chain could probably solve this problem7• EVOH copolymers are quite
unique because they are co-crystallizable over the whole range of composition8•9
•
This results in a, relatively, small decrease of melting temperature and
crystallinity of the EVOH copolymers with changing composition.
In this chapter, explorative results will be presented concerning bath the
synthesis and properties of EVOH fibers in comparison with PVOH and PE
fibers.
95
6.2 Experimental
6.2.1 Materials
EVOH copolymers a-g were preparedas described in chapter 2. PVOH powder
was obtained from Hoechst (Mowiol66-l 00) and is characterized by a viscosity
of 66 ± 4 cP ( 4% water solution at 20 oe).
6.2.2 Fiber-spinning/drawing
Solutions of PVOH and EVOH in ethyleneglycol (20% (w/w)) were spun at 160
oe into a liquid cooling bath (20 oe). In the case of EVOH copolymers with an
ethylene content below 26 mole %, methanol was used as cooling liquid, and
with EVOH copolymers with ethylene content z.. 26 mole %, dichloromethane
was used as cooling liquid. This procedure was used to prevent premature L
L demixing, which is detrimental for drawing4.I
0, before gelation/crystallization
in the as-spun EVOH fibers. '
After complete removal of the solvent by extraction, the filaments were dried
at 20 oe under vacuum and subsequently drawnat temperatures (T 0 ) well below
the melting temperature of the polymers. The draw ratio (A) was determined by
measuring the displacementsof ink-marks. The initial modulus (E) of the fibers
was obtained using a Zwick 1445 Tensile Tester. The length (between clamps)
of the fibers, which have been conditioned 24 hours at 22 oe and 6.5 % R.H.,
was 50 mm and a tensite speed of 5 mm/min was used.
The moisture sensitivity of the fibers was determined using:
where
.6.E = E6SRH
(1)
E100RH = modulus after conditioning in waterbath for 24 hours at
100% R.H ..
E6SRH = modulus after conditioning at 65 % R.H. for 24 hours.
96
6.2.3 Characterization techniques
DSC measurements were performed using a Perkin-Elmer DSC-7. A standard
heating rate of 5 oc;min was adopted.
200 MHz 1H NMR spectra were recorded with a Varian XL-200 spectrometer
at 50 oe. Sample concentration was approximately 3% (w/v) with perdeuteria
ted dimethyl sulfoxide (Me~O-d6, Merck) as solventand internallocking agent.
Using the Mw/Mn and Mn valnes of the reacetylated EVOH copolymers
(EVOH(R)), obtained by GPC and osmometry respectively, the weight average
degree of polymerization (DPw) was calculated (see also chapter 2):
Mw...JEVOH(R)) DPw=
28 Fa + 86 (1 - Fa) (2)
where FB is the molar ethylene content eH NMR).
The Mw valnes of the EVOH copolymers (MwC!lc(EVOH)) are obtained using
equation (3):
Mw.,.JEVOH) = 28 DPw FE+ 44 DPw (I -FE) (3)
6.3 Results and discussion
6.3.1 Synthesis of EVOH copolymers
In order to investigate the intrinsic possibilities of EVOH copolymers for the
production of high-strengtb/high-modulus fibers, polymerization conditions
were adopted to produce copolymers of maximum attainable molecular weight.
TBA was used as solvent because of its low chain transfer constant11• During the
polymerization processes the ethylene pressure and V Ac concentration we re
kept constant to ensure a narrow distribution with respect to molar composition
97
and molecular weight (see chapter 2). Table 6.1 shows the characteristics of
synthesized EVOH copolymers as a function of ethylene pressure using the
following reaction conditions: 480 ml TBA, 160 ml V Ac, 400 mg AIBN, T = 55 oe.
Table 6.1
Properties of synthesized EVOH copolymers
peth DPw Mwca~c(EVOH)
(kgf/cm2) (kg/mole)
EVOH-a 2 3170 135 EVOH-b 5 2640 111 EVOH-c 10 1910 77 EVOH-d 15 1400 55
* 1H NMR
0.08 0.13 0.22 0.30
It can be concluded that the degree of polymerization decreases with increasing
ethylene pressure. Changing initiator concentration, reaction time, reaction
temperature and V Ac concentration did hardly result in an increase of the DPw
values of the EVOH copolymers. The decrease in molecular weight is probably
caused by the higher propensity of the ultimate polymerie ethylene radical for
chain transfer reactions to both monomeric species (see also chapter 2).
Consequently, the conclusion can be made that the degree of polymerization in
the radical copolymerization of ethylene and V Ac is intrinsically limited.
Another synthesis route may surpass this problem.
6.3.2 Properties of EVOH fibers
In Table 6.2 the properties of solution-spun and isothermally drawn EVOH and
PVOH fibers are shown. The drawing temperature T 0 was chosen to provide
maximum efficiency of draw.
98
Table 6.2
Properties of EVOH and PVOH fibers
FE Tm DPw To ).max E65RH E
CC) CC) (GPa) (-)
PVOH 0.00 237 3420 190 21.5 76 0.21 EVOH-e 0.11 224 3030 190 18.5 64 0.32 EVOH-f 0.26 196 1970 140 15.5 36 0.50 EVOH-g 0.34 182 1480 130 15 23 0.57
As can be observed the moisture sensitivity decreases with increasing ethylene
content caused by the incorporation of the hydrapbobic ethylene segments. The
EuJORH/E65RH (~E) ratio reduces from 0.21 for PVOH to 0.57 for EVOH
containing 34 mole % ethylene.
It is also clear that there is a decrease in ).max and maximum Young's modulus
(E) with increasing ethylene content. Since ).max is, among others, dependent on
the molecular weight of the polymers12, this can be attributed to a decrease in
chain length with increasing ethylene content, as is obvious from the decrease
of the DPw values (see Table 6.1 ).
6.3.3 Development of Young's modulus as a function of draw ratio ).
Previous studies13'14 on solid state drawing of flexible chain polymers, drawn
under conditions of optimum efficiency, pointed out that Young's modulus
depends uniquely on the absolute draw ratio and is independent of molecular
weight, polymer concentration and, within eertaio limits, draw temperatures of
the samples.
In order to study the development of Young's modulus with draw ratio for
PVOH and EVOH fibers, initial moduli of these fibers were determined at
various draw ratios.
The obtained results are shown in Figure 6.1. As a reference the literature data
for PE are also given15•
99
80 -t'G x / D. (!) / - / w
l /
/ /
/
60 / /
/
/ /
/
/ 40 /
+ / '/
7 I'
I' /+ 0
20 ///0/
J/{ '/0 /
0 10 20 30
.À
Figure 6.1 E65RH as tunetion of draw ratio for PVOH and EVOH copo/ymers:
X ""' 0 mole %, • ""' 11 mole %, + ""' 26 mole %, 8 • 34 mole % ethylene,
----- • PE
As can be observed, the moduli of PVOH, at comparable draw ratios, are higher
than those of PE. The leveland slope of the modulus/draw ratio curve of
100
EVOH copolymers decrease with increasing ethylene content; the curve of
EVOH containing 34 mole % ethylene is even below that of PE.
It is rather difficult to present a straightforward explanation for the observed
E vs >. curves of the various EVOH copolymers. Comparing PVOH with PE
fibers, and assuming a simpte series modeJ16.
17, the higher moduli values for
PVOH can be attributed toa higher value for the amorphous part (Ea) since the
glass transition temperature Tg of PVOH is above the testing temperature,
approximately 40 "C at 65% R.H. compared to the T8
of PE approximately -
100 "C. The crystal moduli (Ec) of PE and PVOH are comparable since both
polymers possess a planar zig-zag structure in the crystalline state. Literature
data for the Ec of PE are 240 to 340 GPa and 255 to 289 GPa for PVOH
respectively18•
In the case of EVOH copolymers two independent parameters have to be taken
into account:
! The Tg will drop with increasing ethylene content but the decrease is lower
than to be expected for a random copolymer due to favourable hydrogen
bonds between -OH side groups9• In the case of fibrous EVOH copolymers we
observed a slight decreasein Tg. For example, the Tg of sample EVOH-g (34
mole% ethylene) at 65% R.H. is approximately 10 to 15 "C below the T8
of
PVOH (appr. 40 oe, see above).
2. The overall crystallinity X., will decrease with increasing ethylene content but
EVOH copolymers remain crystalline due to co-crystallization9• However,
compared to PE and PVOH the overal crystallinity is lower, also due to alkyl
side ebains which are more abundant in EVOH copolymers. With increasing
ethylene content, the concentration of alkyl side ebains increases (see chapter
3) and consequently the packing of ebains should be more difficult.
At present, it is rather impossible to discriminate between the two parameters
mentioned above.
101
6.4 Conclusions
The incorporation of the hydrophobic ethylene segments in PVOH results in a
decrease of the moisture sensitivity. The drawback however, is that the
efficiency of draw, viz. the slope of the E->. curves, decreases with increasing
ethylene contentand equally important, the maximum obtainable draw ratio >.max decreases due to a reduction in chain length. Consequently, in order to produce
EVOH fibers combining a reduced moisture sensitivity and attractive
mechanical properties, an increase in molecular weight is a prerequisite. Due to
chain transfer reactions it is difficult to produce, on an economically and
technologically acceptable scale, EVOH copolymers possessing high molecular
weights.
102
6.5 References
1. Smith, P., Lemstra, P.J., Coll. Polym. Sci., ill, 7, 1980.
2. Smith, P., Lemstra, P.J., J. Polym. Sci., Polym. Phys. Ed., 19, 1007, 1981.
3. Peguy. A., St.Joho Manley, R., Polymer Comm., 25, 39, 1984.
4. Schellekens, R., Lemstra, P.J., DSM/Stamicarbon, Eur. Patent 114,983
(1984).
5. Tanaka, H., Suzuki, M., Ueda, F., Toray Industries, Eur. Patent 146,084
(1985).
6. Sakurada, 1., "Po1yvinyla1cohol Fibers", Marcel Dekker Inc., New York,
1985.
7. Iwanami, T., Hirai, Y., Tappi Journal, 66(10), 85, 1983.
8. Bunn, C.W., Peiser, H.S., Nature, 159, 161, 1947.
9. Matsumoto, T., Nakamae, K., Ogoshi, N., Kawazoe, M., Oka, H.,
Kobunshikagaku (Polymer Chemistry), 28, 610, 1971.
10. Schellekens, R., Rotten, H.J.J., Lemstra, P.J., DSM/Stamicarbon, Eur.
Patent 212,757 (1987).
11. Bautl, H., U.S.P. 2,947,735 (1957).
12. Smith, P., Matheson Jr., R.R., Irvine, P.A., Polymer Comm., 25, 294, 1984.
13. Ward, I.M., Polym. Eng. Sci., 24( 1 0), 724, 1984.
14. Irvine, P.A., Smith, P., Macromo1ecules, 26, 240, 1986.
15. Smith, P., Lemstra. P.J., J. Mater. Sci., ll. 505, 1980.
16. Perepelkin, K.E., Chim. Vo1okna, ~. 3, 1966.
17. Arandakumaran, K., Roy, S.K., St.Joho Manley, R., Macromolecules, ~L
1746, 1988.
18. Ward, I.M., in "Mechanica! Properties of Solid Polymers", 2nd ed., Wiley,
New York (1983).
103
104
APPENDIX
SOLID STATE 13C NMR STUDY OF EVOH COPOLYMERS
A.l Introduetion
High-resalution 13C NMR spectra in solids have become readily measurable by
using the combined techniques1 of high-power proton decoupling2, cross
polarization(CP)3 and magie angle sample spinning(MAS}4• However, due to
anisatrapie magnetic susceptibility and chemical shift dispersion, line
broadening smears out the splitting resulting from differences in tacticity. Just
recently splitting due to tacticity differences was observed for the first time in
the 13C Solid State NMR spectra of PVOH5•
This appendix is concerned with 13C NMR spectra of solid EVOH copolymers.
The observed splitting in the methine carbon region could be explained by
taking into account bath sequence and tacticity effects.
A.2 Experimental
A.2.1 Materials
Five samples of EVOH copolymers (A-E) were preparedas described in chapter
2. Three commercially available EVOH copolymers (pellets) have been obtained
from Kuraray, coded EPL (F, Mn = 32 kg mote·1), EPF (G, Mn = 21 kg mole"
1) and EPG (H, Mn = 20 kg mole.1
). The EVOH copolymer (powder) with an
ethylene content of 90 mole % {I, Mn = 25 kg mole.1) was obtained by
hydrolyzing the corresponding EVA copolymer EL V AX-260 (Dupont). PVOH
powder (J) bas been obtained from Hoechst (Mowiol 66-1 00) and is characte
rized by a viscosity of 66 ± 4 cP ( 4 % water salution at 20 °C}.
105
A.l.l NMR measurements
The NMR data were acquired at room temperature on a Bruker CXP 200
spectrometer operatingat 50.3 MHz. Samples were loaded into an air driven two
component Beams-Andrew BN-POM rotor with a polyoxymethylene (POM)
cap. The samples were spun at 3.0-3.5 kHz. The chemical shifts were referenced
to the external chemical shift of the crystalline and amorphous POM-resonances
(both at 88.8 ppm).
Typical CP putse sequences used 4 ps 90° putse, 1 ms contact time and a 4 s
recycle time, collecting 2048 points in the time domain fora20kHz speetral
width. Depending on the sample, 1000-15000 FID's were collected for the
Dipolar-Decoupling/CP /MAS experiments1.6.
A.3 Results and discussion
Assignments
Figure A.l shows the 50 MHz 13C CP/MAS NMR spectra of PVOH and the
EVOH copolymers A-1. The complete assignment of the chemical shifts
together with the relative areas for the methine carbon resonances are given in
Table A. I and will be discussed now in some detail.
The CP/MAS spectra are a superposition of the spectra of crystalline and amor
phous material. However, crystalline material is generally better defined than
its amorphous counterpartand the "narrower" lines assigned bere are therefore
essentially due to the crystalline fraction. Sufficient decoupling power was used
to enable observation of relatively narrow lines from crystalline samples.
The spectra can be subdivided into a low field region (65-80 ppm) which is
assigned to all methine carbon resonances and a high field region (25-50 ppm)
for all methylene resonances. In the high field region of the spectra five
resonance patterns can be observed for the methylene carbons (see Fig. A.l). In
the nomenclature used in Fig. A.l each of the metbytenes (or secondary) carbon
atoms in the sequences is identified with S and two Greek letters denoting the
nearest methine carbon atoms (see chapter 3).
106
8
13
22
27
30
47
56
90
100 60
SAMPLE
J
A
B
c
D
E
F
G
H
20 ppm
Figure A.l 50 MHz 13C CP /MAS spectra of PVOH and EVOH
copolymers A-1. Peak 1-3 denote the three different methine
carbon resonances. Saa etc. denote the different methylene carbon
resonances
107
Table A.l
UC NMR assignments for PVOH and EVOH copolymers, recorded at 50 MHz, using CP/MAS methods*
% ethylene 2 3 Al A2 A3
PVOH(J) 0 77.3 71.2 65.4 0.14 0.50 0.36 A 8 76.9 71.1 65.4 0.13 0.48 0.39 B 13 76.8 71.2 65.4 0.17 0.51 0.32 c 22 15.6 70.5 65.2 0.18 0.52 0.30 D 27 74.6 70.4 65.5 0.29 0.52 0.19 E 30 74.9 70.4 65.0 0.31 0.50 0.20 F 34 74.8 70.4 65.4 0.30 0.51 0.19 G 47 74.0 69.5 65.2 0.40 0.43 0.17 H 56 73.7 69.0 65.1 0.43 0.42 0.15 I 90 73.3 69.3 0.79 0.21 -0
00
Sera Sa, Sn SBa SBB
J 45.7 A 46.0 27.3 B 45.9 27.2 c 45.9 42.2 33.1 29.8 27.1 D 45.3 40.5 33.6 31.0 26.3 E 46.3 40.7 33.9 29.8 26.3 F 46.1 40.7 33.7 30.9 26.9 G 49.7 39.7 33.2 29.9 26.2 H 45.2 40.0 33.5 29.9 25.9 I 40.4 33.5
• Chemica! shifts (in ppm) are relative to the 13C chemical shift of external POM (88.8 ppm).
Peak 1-3 denote the three methine carbon resonances and A 1-A3 are the conesponding area intensities.
In the methylene carbon region no additional resonances can be observed in
comparison with the solution NMR results, so we have focussed our attention
to the methine carbon region, because, according to Terao5, hydragen bonding
certainly has more effect on the chemical shifts of these resonances.
The low field region shows three methine carbon resonances, peaks 1-3.
Referring to Terao5, and concentrating for the moment only at PVOH (J) and
excluding the minor resonances of anomalous structures, peaks 1-3 in the solid
state cannot simply be assigned to the tacticity induced splitting of the mm, mr
and rr triacts respectively: the relative areasof the three peaks in the solid state
are not consistent with the triact tacticity observed in solution5•
Moreover, a significant downfield shift of the resonances 1 and 2 occurs
compared with the chemica! shifts for mm and mr triad resonances observed in
solution, resulting in much larger chemica! shift differences between the
methine carbon resonances in the solid.
This downfield shift of peaks I and 2 is tentatively assigned5 to the action of
intramolecular hydragen bonds. This effect is larger in solicts than in solutions
because in the solid state many hydrogen bands cooperate to keep the molecule
in a certain conformation as opposed to solutions where frequent transitions
between conformations diminish the "time averaged" hydragen bond strength
as reflected by the chemical shifts.
In the case of the mm triad, the oxygen atom bonded to the central methine
carbon can farm two, one or zero intramolecular hydragen bond(s) and for the
mr triact one or zero intramolecular bond are possible. In the rr triact no intra
molecular hydragen bond is possible. By assuming that the line position shifts
downfield by about 6 ppm per intramolecular hydragen bond, Terao5 assigned
peak 1 in PVOH to the mm triact with two intramolecular hydragen bonds, peak
2 to the mm and mr triacts with one intramolecular hydragen bond and peak 3
to the mm, mr and rr triacts with no intramolecular hydragen bonds.
In the spectra of the EVOH copolymers peaks 1-3 can also be observed, see Fig.
A.I. In Table A.l the chemical shifts and the relative intensities of peaks 1-3
of the EVOH copolymers are shown with increasing ethylene content of the
EVOH copolymers. With increasing ethylene content an upfield shift of 4 ppm
is observed for resonance I, a smaller upfield shift for resonance 2, while no
109
effect has been observed for peak 3.
CP/MAS NMR measurements with varying contact times were carried out for
copolymers with 8 and 47 mole% ethylene (spectra A and G, Fig. A.l).
These experiments showed that the important CP parametersTuc and T 1p (H)
did not vary significantly with ethylene content for all resonances. This means
that the response factors in our UC CP /MAS measurements as presented in
Table A. I do notchange significantly. The areasof the three peaks also change
with different ethylene contents: the areas of peak 1 increases, the areas of
peaks 2 and 3 decrease with increasing ethylene content. At an ethylene content
of 90 mole % peak 3 has almost disappeared.
In the assignment of the methine carbon resonances in the EVOH copolymers
we have to take into account both tacticity effects, as in PVOH, il.ru! sequence
distribution effects. For the calculation of the 13C chemical shifts in the
sequence triads of~ EVOH copolymer, the 13C chemical shift additivity rules
for aliphatic alcohols, as calculated by Ovenale from solution NMR measure
ments, have been used:
a • 40.8 ppm 8 = 7.7 ppm 1 = -3.4 ppm
Assuming no conformation dependent chemica! shift effects to occur and using
the chemical shift of orthorhom bic polyethylene (33 ppmt we can now calculate
the chemical shifts of the methine carbon atoms in the three triads of the solid
crystalline EVOH copolymer, respectively 000 (67 ppm), OOE (70.4 ppm)
and EOE (73.8 ppm) where 000, OOE, EOE are abbreviations for (VOH,
VOH, VOH), (VOH, VOH, E) and (E, VOH, E) triads.
The chemical shift values presenled above are only meant to yield useful
assignments of the several methine carbon 13C NMR signals of EVOH
copolymers. These assignments are necessary because OvenaW did not report
dependable estimates for all three types of methines sustained by experimental
results. We areaware of chemical shift differences between liquids and solids.
Moreover, the choice of orthorhombic polyethylene as a basis for the shift
llO
calculations is rather arbitrary but this will cause the same uncertainty in each
of the three shifts. Of more importanceis the known sensitivity of substituent
induced shifts towards different conformational equilibria. From results
obtained by Möller9 for different poly(I ,2-dimethylbutane) polymers it can be
estimated that the uncertainties in our estimations amount to ca. 2 ppm. It is,
however, improbable that the order of the three methine carbon signals will be
misjudged.
Besides these sequence effects, also effects similar as observed by Terao5 for
PVOH (i.e. hydrogen bond induced tacticity shifts), have to be taken into
account. In Table A.2 the theoretically calculated chemical shifts of the methine
carbon atoms are presented, adopting the same assumption as made by Terao5,
i.e. a downfield shift of 6 ppm per intramolecular hydrogen bond. The chemical
shift of 000 mm(2) can e.g. be calculated as 6a + a + 21 + 2(6) = 79 ppm.
Table A.2
Calculated 13C NMR chemica! shifts (in ppm) for the methine carbons
in the EVOH copolymers*
st ces te ces st te ces st te ces
000 67 mm(2) 79 000 000
mm(I) 73 mr(l) 73
mm(O) 67 mr(O) 67 rr(O) 67
OOE 70.4 OOE OOE
m (I) 76.4
m (0) 70.4 r (0) 70.4
EOE 73.8
* st = sequence triad, ces = calculated chemica! shift, te = tacticity effect.
111
Due to chemical shift dispersion, considerable overlap occurs, as is evident after
an inspeetion of the results in Table A.l and the experimentally observed
chemical shift patterns (Fig. A.l ). Therefore, only three peak regions can be
discerned and assigned:
Peak 1 = 000 mm (2) + OOE m (1) +EOE
Peak 2 = 000 mm (I)+ 000 mr (I)+ OOE m (0) + OOE r (0) (I)
Peak 3 = 000 mm (0) + 000 mr (0) + 000 rr (0)
where the figure in brackets indicates the number of intramoiecular hydrogen
bon ds.
From eq. (1) it is apparent that this resonance assignment includes the model
proposed by Terao for PVOH (one extreme, Fig. A. I: spectrum J). For ethylene
rich copolymer (the other extreme) this model prediets the observation of one
major resonance situated at 74 ppm. From an inspeetion of Fig. A.l (spectrum
I) this turns out to be true. Additional confirmation of the chemical shift
assignment can be obtained from relative area measurements, bearing in mind
that peak 3 ( eq. (I)) is only influenced by tacticity effects. This is obviously
not true for peaks 1 and 2 which are influenced by both tacticity Blli1. sequence
distribution effects.
Because (mm(O) + mr(O) + rr(O) + mm(l) + mr(I) + mm(2)] = I anq [m(O) + m(l)
+ r(O)] = I. eq. (1) can be transformed into:
A3 a•ooo A 1 + A2 + A3 = 000 + OOE + EOE
(2)
where a' = mm(O) + mr(O) + rr(O)
and A 1 - A3 being area intensities.
112
Furthermore, random Bernoullian statistics hold for EVA 10 and EVOH11
copolymers, so:
(3)
and:
[ A3 ]14
~ 1 + A2 + ~ = a(l - F J = aFvoH (4)
where FvoH and FE are the molar vinyl alcohol and ethylene contents
respectively of the EVOH copolymer and a = (a')14•
The value of a reflects the fraction of 000 triacts which is not influenced by
any hydragen bonding. In Figure A.2 this relation bas been plotted together
with a graphof eq. (4) assuming a= 1 (no effects of hydragen bonding at all).
The experimental results used in Fig. A.2 include the outcome of spectrum I
(Fig. A.l). As can be noticed, the contribution of peak 3 in this sample is very
low, it can be estimated at 0 to 2 %.
From this graphit can be evaluated that a 111$ 0.7 and consequently a',.. 0.5. So
over the whole range of copolymers, approximately 45 to 55 % of all 000
triacts being present in the crystalline part of these copolymers do not take part
in any intramolecular hydrogen bonding.
Summarizing, the results described above demonstrate clearly that in solid
EVOH copolymers with different ethylene contents, the combined effects of
sequence distribution and tacticity induced hydragen bonding have to be taken
into account much in the same way as in solutions12.
13• Moreover, the effect of
the hydragen honds on the 13C NMR chemical shifts is much larger for solicts
than for solutions5•
113
0.8
0.6
o~~~~_J~~~-L-L~~~~~~~~~
0 0.2 0.4 0.6 0.8 1
o : a=1 Mole fraction (1 - Fe:>
• : EVOH copolymer A - J
Figure A.2 Plot of (A3/( Al+ A2 + A3JYI2 against the
molar fraction ( 1-FE) (see eq. (4)). Al etc. denote the
area intensities of the methine carbons
A.4 Conclusions
Using Solid State 13C NMR an assignment of the chemical shifts of the methine
and methylene carbons could be made. In the assignment of the methine carbon
resonances two effects have to be taken into account: tacticity effects, as in
PVOH, arul sequence distribution effects. With these effects, the splitting of the
methine carbon resonances was discussed. A model was proposed which explains
this splitting.
114
A.S Keferences
1. Schaefer, J., Stejskal, E.O., J. Am. Chem. Soc., 2.8_, 1031, 1976.
2. Sarles, L.R., Cotts, R.M., Phys. Rev.,ll., 853, 1958.
3. Pines, A., Gibby, M.G., Waugh, J.S., J. Chem. Phys., i2, 569, 1973.
4. Andrew, E.R., Int. Rev. Phys. Chem., 1. 195, 1981.
5. Terao, T., Maeda, S., Saika, A., Macromolecules, 1§., 1535, 1983.
6. Schaefer, J., Stejskal, E.O., Buchdahl, R., Macromolecules, l.Q, 384, 1977.
7. Ovenall, D.W., Macromolecules, l1. 1458, 1984.
8. Kitamaru, R., Horii, F., Marayama, K., Macromolecules, 12., 636, 1986.
9. Möller, M., Cantow, H.-J., Polymer Bull.,~. 119, 1981.
10. Wu, T.K., Ovenall, D.W., Reddy, G.S., J. Pol. Sci. Polym. Phys. Ed., !l, 901,
1974.
11. Wu, T.K., J. Pol. Sci. Polym. Phys. Ed., 14, 343, 1976.
12. Moritani, T., Iwasaki, H., Macromolecules, ll, 1251, 1978.
13. Ketels, H., Beulen, J., v.d. Velden, G., Macromolecules, ll, 2032, 1988.
115
116
SUMMARY
This thesis describes the synthesis and characterization of ethylene vinylalcohol
(EVOH) copolymers as well as the utilization of these copolymers in high
harrier systems and as starting materials for high-performance fibers.
The pure homopolymer polyvinylalcohol (PVOH) is a well established polymer
used for the production of harrier materials and fibers. The -OH sidegroup in
PVOH is relatively small and consequently PVOH is crystallizable, despite its
atactic character, into a distorted monoclinic version of the polyethylene
structure. Due to its crystallinity but notably due to the relative high glass
transition temperature T, of approximately 80 oe (0 % relative humidity),
PVOH possesses excellent barrier properties for gases like oxygen and carbon
dioxide.
The similarity in crystal structure between polyethylene (PE) and PVOH
stimulated research efforts to produce high-modulus fibers. The PVOH fibers
have some potential advantages in comparison with PE fibers e.g. higher
melting temperature, approximately 250 oe (compared to 145 oe for PE),
potentially lower creep levels and better adhesion to (polar) matrices.
The major drawback however of the pure homopolymer PVOH is its intrinsic
moisture sensitivity and thermal instability. Melt-processing is virtually
impossible due to thermal degradation and the abundance of -OH groups in the
polymer promotes the absorption of water. The mechanical and barrier
properties deteriorate with increasing relative humidity.
A possible salution to this problem could be the incorporation of hydrophobic
ethylene units in the main chain. On a technological scale, PVOH is produced
via complete hydralysis (methanolysis) of polyvinylacetate (PV A). Direct
synthesis from the corresponding monomer "vinylalcohol" is impossible since
this species exists in its more stable, tautomerie, keto-form: acetaldehyde.
It is well known that ethylene and vinylacetate copolymerize readily into a
copolymer, ethylene vinylacetate (EVA) copolymer, in which the ethylene and
117
vinylacelate units are arranged at random along the molecule. Hydralysis of
these EVA copolymers results in ethylene vinylalcohol (EVOH).
EVOH copolymers are unique in the sense that they remain crystalline over the
entire composition range. The melting point decreases to some extent with
increasing ethylene content but far less than is to be expected for a random
copolymer due to co-crystallization of ethylene and "vinylalcohol" units.
Moreover, the Tg of EVOH copolymers, with lower ethylene contents, is
camparabie with that of PVOH.
A major advantage is that EVOH copolymers can be processed via the melt and
due to the presence of hydrapbobic ethylene units their moisture sensitivity is
strongly reduced in comparison with the homopolymer PVOH.
In order to onderstand and to exploit the unique properties of these EVOH
copolymers, well-defined and characterized samples are a prerequisite. The
copolymerization (in solution) of ethylene and vinylacelate was studied and the
polymerization conditions were optimized in order to obtain EVOH copolymers
in which both the compositional and molecular weight distributions are
minimized (chapter 2).
The microstructure of EVOH copolymers was studied using both 1H NMR and 13C NMR. An assignment of the resonances in the spectra was given and the
sequence distribution, alkyl chain branching, anomalous linkages and tacticity
were studied as a function of the overall ethylene content of the copolymers
(chapter 3).
An important observation is the decrease in overall molecular weight with
increasing ethylene content. This decrease in molecular weight is caused by the
higher propensity of the ultimate polymerie ethylene radical for chain transfer
to both monomeric species. As a consequence, EVOH copolymers are relatively
brittie due to their lower molar mass.
118
It was found however, that EVOH copolymers are miscible with polyamides
(Nylon-6) inthemelt (chapter 4). U pon cooling from the melt, phase separation
occurs via consecutive crystallization of the two constituents. Since Nylon-6
crystallizes first, a so-called "structured blend" is obtained with a specific
morphology. This structured blend possesses excellent harrier properties due to
an enhanced concentration of the EVOH copolymer in the amorphous/inter
spherulitic boundaries of the Nylon-6 matrix. The toughness of the system is
provided by the Nylon-6 matrix.
In contrast to the EVOH/Nylon-6 blends, the blends of EVOH with polyethy
leneterephthalate (PET) and polyolefines, which are discussed in chapter 5, are
immiscible. Using the so-called Multiflux static mixer, layer structures in pellets
of these blends could be obtained. Attempts were made to preserve the specific
morphology upon further processing (film blowing) because stratified films are
expected to possess excellent barrier properties.
Furthermore, the use of EVOH copolymers for the production of high-per
formance fibers was investigated (chapter 6). The moisture sensitivity decreases,
in comparison with PVOH fibers, with increasing ethylene content. However,
both the efficiency of draw and the maximum obtainable draw ratio .Àmax
decrease with increasing ethylene content. The decrease in .À"""' is due to a
reduction in overall chain length with increasing ethylene content as mentioned
above (chapter 2).
Consequently, the ultimate mechanica! properties of EVOH fibers are
intrinsically limited by the microstructure of the EVOH copolymers and by
chain transfer reactions occurring during the copolymerization of ethylene and
vinylacetate, as described in chapter 2.
119
120
SAMENVATTING
Dit proefschrift beschrijft de synthese en karakterisering van etheenvinylalco
hol (EVOH) copolymeren, alsmede de toepassing van deze copolymeren in
barrière systemen en als uitgangsmateriaal voor hoge modulus vezels.
Het pure homopolymeer polyvinylalcohol (PVOH) wordt al gebruikt voor deze
toepassingen. Omdat de -OH groep in PVOH relatief klein is, kan PVOH,
ondanks zijn atactisch karakter, kristalliseren. De kristalstructuur kan gezien
worden als een enigszins vervormde versie van monoklien polyetheen (PE).
Door deze kristalliniteit, maar nog meer door de relatief hoge glasovergangs
temperatuur T11 van ca. 80 "e (0 % relatieve vochtigheid), bezit PVOH zeer
goede barrière eigenschappen voor gassen zoals zuurstof en kooldioxyde.
De grote overeenkomst tussen de kristalstructuur van PE en PVOH heeft
onderzoek gestimuleerd op het gebied van de produktie van hoge modulus
vezels op basis van PVOH. Deze vezels hebben enkele voordelen in vergelijking
met PE vezels zoals: hogere smelttemperatuur (ca. 250 oe t.o.v. 145 oe voor
PE), mogelijk een lagere kruip en een betere adhesie aan (polaire) matrices.
Echter, de nadelen van het homopolymeer PVOH zijn de vochtgevoeligheid en
de thermische instabiliteit. Verwerking via de gesmolten toestand is onmogelijk
omdat degradatie optreedt. Het hoge gehalte aan -OH groepen stimuleert de
absorptie van water. Mechanische én barrière eigenschappen verslechteren met
toenemende relatieve vochtigheid.
Een mogelijke oplossing voor deze problemen is het inbouwen van hydrofobe
etheeneenheden in de hoofdketen. Op technologische schaal wordt PVOH
geproduceerd door een volledige hydrolyse (methanolyse) van polyvinylacetaat
(PV A). Een directe synthese via het "vinylalcohol" monomeer is onmogelijk
omdat dit monomeer alleen in zijn meer stabiele, tautomere, keto-vorm
voorkomt, namelijk aceetaldehyde.
Het is bekend dat etheen en vinylacetaat copolymeriseren tot het etheen
vinylacetaat (EVA) copolymeer, waarin de etheen- en vinylacetaateenheden at
121
random verdeeld zijn over het molekuul. Hydrolyse van deze EVA copolymeren
leidt tot etheenvinylalcohol (EVOH). EVOH copolymeren zijn in zoverre uniek
dat ze kristallijn zijn over het gehele samenstellingsgebied. De smelttemperatuur
daalt met toenemend etheengehatte, maar in veel mindere mate dan verwacht
kan worden voor een random copolymeer. Dit wordt veroorzaakt door het feit
dat de etheen- en "vinylalcohol" eenheden cokristalliseerbaar zijn: ze kristal
liseren in één kristalrooster. Bovendien is de glasovergangstemperatuur T8
van
EVOH copolymeren met lage etheenconcentraties ongeveer gelijk aan die van
PVOH.
Een groot voordeel is dat EVOH copolymeren via de gesmolten toestand
verwerkt kunnen worden en dat door de aanwezigheid van de hydrofobe
etheeneenheden de vochtgevoeligheid sterk verminderd is in vergelijking met
het homopolymeer PVOH.
Om de unieke eigenschappen van deze EVOH copolymeren te onderzoeken en
te begrijpen, zijn goed gedefiniêerde en gekarakteriseerde monsters nodig. De
copolymerisatie (in oplossing) van etheen en vinylacetaat werd bestudeerd en
de polymerisatie-omstandigheden werden geoptimaliseerd ten einde EVOH
copolymeren te verkrijgen met een smalle verdeling wat betreft samenstelling
en molekuulgewicht (hoofdstuk 2).
De microstructuur van de EVOH copolymeren werd bestudeerd met 1H NMR
en 13C NMR. De resonanties in de spectra werden toegekend aan de verschil
lende atomen. Tevens werden de sequentieverdeling, alkylketenvertakking,
anomale koppeling tussen de monomeren en tacticiteil onderzocht als functie
van het etheengehalte in het polymeer (hoofdstuk 3).
Een belangrijke constatering is de afname van het molekuulgewicht met
toenemend etheengehalte. Deze afname wordt veroorzaakt door een grotere
voorkeur van het etheenradicaal aan het polymeer voor ketenoverdrachtsreakties
naar beide monomeren. Door de lagere molekuulgewichten zijn de EVOH
copolymeren relatief bros.
122
EVOH copolymeren blijken in de smelt mengbaar met polyamiden (Nylon-6)
(hoofdstuk 4). Tijdens het afkoelen van de smelt vindt fasenscheiding plaats
door het niet gelijktijdig kristalliseren van beide componenten. Nylon-6
kristalliseert als eerste en een zogenaamde "gestructureerde blend", dit is een
blend met een specifieke morfologie, wordt verkregen. Deze gestructureerde
blend bezit zeer goede barrière eigenschappen door een verhoogde concentratie
van het EVOH copolymeer in de amorfe/interkristallijne gebieden van de
Nylon-6 matrix. De Nylon-6 matrix zorgt voor de taaiheid van dit systeem.
In tegenstelling tot de EVOH/Nylon-6 blends, zijn de blends van EVOH met
polyetheentereftalaat (PET) en polyolefinen niet mengbaar (hoofdstuk 5). Met
behulp van de zogenaamde Multiflux menger konden lagenstructuren in het
granulaat van deze blends gerealiseerd worden. Pogingen werden ondernomen
om de specifieke morfologie te behouden tijdens verder verwerken (foliebla
zen), omdat gelaagde films goede barrière eigenschappen bezitten.
Tenslotte werden de mogelijkheden van EVOH copolymeren voor de produktie
van hoge modulus vezels onderzocht (hoofdstuk 6). De vochtgevoeligheid daalt,
in vergelijking met de PVOH vezels, met toenemend etheengehalte. Echter, de
verstrekeffectiviteit en de maximale verstrekgraad >.max dalen ook met
toenemend etheengehalte. De afname van ).mal< wordt veroorzaakt door een
afname van de ketenlengte met toenemend etheengehalte zoals eerder
beschreven (hoofdstuk 2).
Dit heeft tot gevolg dat de mechanische eigenschappen van EVOH vezels
intrinsiek beperkt worden door de microstructuur van de EVOH copolymeren
en door de ketenoverdrachtsreakties die optreden tijdens de copolymerisatie van
etheen en vinylacetaat, zoals beschreven in hoofdstuk 2.
123
124
CURRICULUM VITAE
Harm Ketels, geboren 23 oktober 1960 te Tegelen, behaalde het diploma
Gymnasium-,8 in 1979 aan het St.-Thomascollege te Venlo.
In datzelfde jaar begon hij met de studie Scheikunde aan de Katholieke
Universiteit te Nijmegen. Het kandidaatsexamen (SI) werd afgelegd in mei
1982. Het doctoraal examen met als hoofdvakken Organische Chemie (Prof. B.
Zwanenburg) en Farmacochemie (Prof. J.M. van Rossum) werd in juni 1985
behaald.
Vanaf september 1985 tot en met augustus 1989 was hij als wetenschappelijk
medewerker verbonden aan de Technische Universiteit te Eindhoven en
werkzaam in de vakgroep Polymeerchemie en Kunststoftechnologie van de
faculteit Scheikundige Technologie.
In het kader van het onderzoek verbleef hij enkele maanden aan de Science
University of Tokyo (Prof. M. Ito), voor het verrichten van specifieke NMR
metingen.
Per 1 oktober 1989 treedt hij als wetenschappelijk medewerker in dienst bij
General Electric Plastics in Bergen op Zoom.
125
STELLINGEN
De thermodynamische interactie-parameters van polymeermengsels bepaald uit
smeltpuntsdepressie metingen, zijn minder nauwkeurig dan vaak wordt
gesuggereerd.
Kwei, T.K., Patterson, G.D., Wang, T.T .. Macromolecules. ~. 780. 1976.
2
De verklaring die Amano et al. geven voor de afname van de sonische modulus
van EVOH copolymeren in vergelijking met PVOH is onvolledig.
Amano, M., Nagakawa, K .. Polymer Comm., 28. 119. 1987.
3
Dat Prevorsek et al. een memory-effect van 20 40 min meten voor poly(car
bonaat-co-tereftalaat) in de gesmolten toestand is onwaarschijnlijk, aangezien
de langste relaxatietijd (op basis van groepsbijdragen) ca. 5 sec. bedraagt.
Prevorsek, D.C .. de Bona, B.T .. J. Macromol. Sci., Phys. Ed.. 4 ). 515. 1986.
Prevorsek, D.C., de Bona, B.T .. J. Macromol. Sci .. Phys. Ed.. ). 605. 1981.
4
Voor het bepalen van spin-spin relaxatietijden mogen de meetsignalen
verkregen met de solid-echo methode en de spin-echo methode niet gekoppeld
worden.
Tanaka, H., Nishi, T., Phys. Rev. B. 33( 1 ). 32. 1986.
5
Uit het feit dat Amiya et al. concluderen dat voor EVOH copolymeren, emulsie
polymerisatie in het algemeen een bredere molekuulgewichtsverdeling geeft dan
polymerisatie in oplossing, mag geconcludeerd worden dat onvoldoende
rekening is gehouden met de conversie en de samenstellingsverdeling.
Amiya, S., lwasaki, H .. Fujiwara. Y .. J. Chem. Soc. Japan. LL 1698. 1977.
6
Gezien het feit dat in korte tijd drie publicaties met vergelijkbare inhoud en
conclusies verschijnen waarbij de auteurs niet naar elkaar refereren, mag
geconcludeerd worden dat ook tussen wetenschappers nog Europese grenzen
moeten verdwijnen.
Wendisch, D., Reiff, H., Dieterich, D., Angew. Makromol. Chemie, 141, 173.
1986.
Au/ der Heyde, H., Hübel, W., Boese, R., Angew. Makromol. Chemie. 153, I,
1987.
Gerard, J.-F .. Le Perchec, P .. Tho Pham. Q., Makromol. Chem., 189, 1719. 1988.
7
De experimenteel vastgestelde concentratie-afhankelijkheid van de X-parameter
moet reeds in de partitiefunc-tie in rekening worden gebracht.
8
Het is onrechtvaardig dat managers geprezen worden om hun vermogen tot
delegeren, terwijl deze eigenschap voor wetenschappers als een nadeel wordt
beschouwd.
9
Uit de recente ontwjkkelingen in Japan kan geconcludeerd worden dat de
invloed van de vrouw op de samenleving groter is dan tot nu toe door de
westerse wereld werd aangenomen.
10
Pas na het einde van de promotiezitting is de druk van de ketels.
H.H.T.M. Ketels
Eindhoven, 12 september 1989