miscibility of linear low density polyethylene (lldpe)-graft-poly(methyl methacrylate) with...

Die Angewandte Makromolekulare Chemie 241 (1996) 77-93 (Nr: 4189)

Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China

Miscibility of linear low density polyethylene (LLDPE)-graft-poly(methy1 methacrylate) with

poly(viny1idene fluoride) (PVF2) and its application as compatibilizer LLDPE/PVF2 blends

Lin Li”, Tao Tang*, Baotong Huang

(Received 5 January 1995; revised 28 April 1995)

SUMMARY: Differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy

(FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used to study the miscibility of blends of a graft copolymer of poly- (methyl methacrylate) on linear low density polyethylene (LLDPE-g-PMMA, G-3) with poly(viny1idene fluoride)b (PVF,) and the compatibilization of blends of LLDPEPVF,. The specific interaction between PMMA side chains and PVF, in G- 3/PVF, binary blends is weaker than that between the homopolymers PMMA and PVF,. There are two states of PVF, in the melt of a G-3/PVF2 (60/40, w/w) blend, one as pure PVF, and the other interacting with PMMA side chains. The miscibility between PMMA side chains and PVF, affects the crystallization of PVF,. LLDPE- g-PMMA was demonstrated to be a good compatibilizer in LLDPEPVF, blends, improving the interfacial adhesion and dispersion in the latter. Diffusion of PMMA side chains into PVF, in the interfacial region reduces the crystallization rate and lowers the melting point (T,) and the crystallization temperature (T,) of PVF, in the blends.

ZUSAMMENFASSUNG: Die Vertraglichkeit von Blends eines mit Polymethylmethacrylat gepfropften

linearen Polyethylens niedriger Dichte (LLDPE-g-PMMA, G-3) mit Polyvinyliden- fluorid (PVF,) wurde mittels Differentialkalorimetrie, Fourier-Transform-Infrarot- spektroskopie, Rasterelektronenmikroskopie und Rontgen-Photoelektronenspektros- kopie untersucht und mit der von LLDPEPVF,-Blends verglichen. Die spezifischen Wechselwirkungen zwischen den PMMA-Seitenketten und PVF, sind in den bintiren G-3PVF2-Blends schwacher als zwischen den Homopolymeren PMMA und PVF,.

* Correspondence author. ” Present address: Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023. Systematic name: poly( 1,l-difluoroethylene).

0 1996, Hiithig & Wepf Verlag, Zug CCC 0o03-3146/96/$07.00 77

L. Li, T. Tang, B. Huang

PVF, liegt in der Schmelze des G-3/PVF2 (60/40 w/w)-Blends sowohl in reiner Phase als auch in Wechselwirkung mit den PMMA-Seitenketten vor. Die Mischbar- keit zwischen den PMMA-Seitenketten und PVF, beeinflufit die Kristallisation von PVF,. LLDPE-g-PMMA wirkt als guter Phasenvennittler in LLDPE/PVF,-Blends, wobei es die Grenzflachenadhasion und die Dispersion in letzterem verbessert. Die Diffusion der PMMA-Seitenketten in PVF, an der Grenzflache veningert die Kri- stallisationsgeschwindigkeit und fiihrt zu einer Erniedrigung des Schmelzpunktes und der Kristallisationstemperatur des PVF, in den Blends.

I Introduction

Blending is an efficient method in improving polymer properties and creating new materials. Much work has appeared on blends of non-crystalline block copolymers/hornopolymers, for example, polystyrene-block- polyisoprene/polystyrene (SIPS)', polystyrene-block-polybutadiene-block- polystyrene/polybutadiene (SBS/PB)2,3, and SBS/PS3. For these blends, the molecular weight of the homopolymer is an important factor in determining the miscibility. The architecture of the copolymer also plays an important role"7. Jiang' studied the miscibility of polyisoprene-graft-polystyrene/ polyisoprene (PI-g-PS/PI) blends and found that PI-g-PS is immiscible with PI due to conformational restrictions of the PI backbone. Zhao's results showed that the location of the corresponding blocks has an important effect on the miscibility of 4-arm star-like PI-block-PS/PS blends'. Paul et a1.I'. studied the miscibility of SBS/poly(phenylene oxide) (SBS/PPO) and polyisoprene-block-polystyrene-block-polyisoprene/PPO (ISIPPO) blends. Dif- ferent from the above blends, there are specific interactions between PS segments and PPO". In this case, the molecular weight of PPO and the location of PS segments in the copolymer did not affect the miscibility of SBSRPO (or ISIPPO) blends.

Polyolefins (polyethylene (PE), poly(propy1ene) (PP)) are commodity plastics. Their blends with polar polymers could produce a series of new materials with impact also on the problems of recycling of waste plastics. Polyolefins are immiscible with polar polymers due to thermodynamic reasons. The morphology and properties of these blends are unstable, and these blends have no practical value. Adding compatibilizers is one of the meth- ods to solve these problemsJ3. For polyolefidpolar polymer blends, Ouhadi et al.I4 compatibilized PE/PVF2 blends using HPB-block-PMMA (HPB: hy- drogenated polybutadiene) as the compatibilizer. They thought that the compatibilization mechanism was, on the one hand, a miscibility between struc- turally similar HPB block and the bulk PE and, on the other hand, a specific

78

Miscibility of LLDPE-g-PMMA with PVF,

interaction between the PMMA block and the bulk PVF,. In LDPE/ABS (ABS: acrylonitrile-butadiene-styrene copolymers) blendsI5, HPB-block- PMMA is also an effective compatibilizer for the same reason. There are crystalline components in these blends. The effect of compatibilization on the crystallization behavior of the blends, however, was overlooked. Re- cently we found some new experimental evidence for effects of compatibilizers on the crystallization of poly(propylene)/polar polymer blends'&18.

Maleated poly(propy1ene) (PP-MA) and its graft copolymer with poly- (ethylene oxide) ((PP-MA)-g-PEO) are good compatibilizers in PP/polar polymer blends16* 17. Their compatibilization mechanism is that the PP backbone of the compatibilizers cocrystallizes with the bulk PP and the polar part of the compatibilizer is miscible with the polar polymer through specific interactions16.

In the present work, we studied the miscibility of LLDPE-g-PMMA, synthesized by free radical graft polymerization of methyl methacrylate (MMA) onto LLDPE, with PVF, and its effect on the compatibility of LLDPEPVF, blends. Because of the limitations of the free radical polymerization mechanism, the molecular characteristics of the graft copolymer are unknown. However, this kind of graft copolymer is practical and it is very important to study the application of such a graft copolymer for the compatibilization of immiscible polymer blends.

2 Experimental part

2. I Materials

Linear low density polyethylene (LLDPE) was a product of Union Carbide Cor- poration. PVF, (R = 60000) was purchased from Polysciences, Inc. Xylene, cyclohexanone, tetrahydrofuran and ethanol were all of A. R. grade.

2.2 Graft copolymerization

The graft copolymer of linear low density polyethylene (LLDPE) with poly- (methyl methacrylate) (PMMA) was synthesized by means of suspension polymerization. Before grafting, LLDPE powder was ground and sieved. The ground powder smaller than 40 mesh was collected for the grafting reaction. The ground LLDPE powder was immersed in heptane for 12 h at ambient temperature in a three-necked

79


flask to swell or partially dissolve. An aqueous solution of poly(viny1 alcohol) (PVA) and sodium alkyl sulfonate was dropped slowly into the three-necked flask at 60°C under stirring. After a stable dispersion system had been formed, MMA mono- mer with benzoyl peroxide was added. The reaction was kept for 24 h. Then the reaction mixture was poured into boiling water to eliminate heptane and PVA and sodium alkyl sulfonate. After washing several times with boiling water, an IR spectrum showed that the product was completely purified. Finally the product was extracted for 24 h in acetone to remove PMMA homopolymer and then dried in vacuum at 40°C. The ingredients of the reaction system were: LLDPE 10 g, MMA 9.4-18.8 g, BPO 0.32 g, PVA 1.2 g, sodium alkyl sulfonate 0.12 g, heptane 100 mL, water 200 mL. The content of grafted PMMA was 5.0-9.0 wt.-%.

2.3 Preparation of LLDPE/PVF, blend samples

PVF, was dissolved in cyclohexanone at 135 "C, and then LLDPE, the compatibilizer, antioxidant 2,6-di-tert-buty1-4-methylphenol and xylene were added. The mixture was stirred at 135 "C until a homogeneous solution was formed. The blend, iso- lated by adding the solution to ethanol, was dried at 60 "C in vacuum.

Samples for measurement of the mechanical properties were prepared by pressing the blends in a mould in a hot press for 10 min at 18O0C/5O atm (5.07 MPa) and then cooled to ambient temperature in air. The thickness of the samples was 0.3 mm for dynamic mechanical analysis and 10-30 pm for FTIR.

2.4 Preparation of inteq5ace of polymers

Separate samples of LLDPE, PVF, and the compatibilizers as well as composite films for studying the interface of blends in different combinations were prepared in the manner described earlier".

2.5 Testing and characterization

Fractured surfaces for scanning electron microscopy (SEM) observation were prepared by fracturing the samples after dipping in liquid nitrogen for 5 min. The morphology of the fractured surfaces of LLDPE/PVF2 blends were observed with a JEOL-MAX-840 SEM after being coated with gold.

The DSC measurements were carried out under nitrogen with a Perkin-Elmer DSC-7C. The temperature program was as follows:

20 "C/min -20 "C/min 20 "C/min 40 "C 200 "C 40 "C 200 "C

(kept for 7 min)

80


Isothermal crystallization kinetics of the blends was studied using the same instru- ment. The crystallization curves were analysed according to the Avrami equation. The temperature program was:

40 "C/min -80 "C/min 40 "C 200 "C T, (PVF, crystallized isothermally)

(kept for 7 min)

The molecular state in the interfacial regions was observed using X-ray photoelectron spectroscopy (XPS) on an ESCALAB-MKII spectrometer with a monochro- mated MgKa X-ray source, voltage 13 kV, current 20 mA, operating pressure 5 pPa.

All peak binding energies were referred to the C-C peaks of polymers at 284.6 eV. The analyser pass energy was 50 eV.

Dynamic mechanical analysis of LLDPEPVF, blends was done from -150 to 150°C on a Rheovibron DDV-ILEA at a frequency of 3.5 Hz and a scanning rate of 1 "C/min.

Infrared spectra were obtained on a Nicolet 20DXB FT-IR spectrometer. The reso- lution was 0.5 cm-'.

3 Results and discussion

3.1 Miscibility of LLDPE-g-PMMA (G-3) and PVF,

3.1.1 Non-isothermal crystallization properties

Changes in T, and T, of a crystalline component are a criterion of miscibility of components in a blend". Since PVF, is miscible with PMMA21-24, T, and T, of PVF, in the blend gradually decrease with increasing content of PMMA2'. The specific interaction between PVF, and PMMA in the LLDPE matrix in a LLDPE/PMMA/PVF, (60/6/40, w/w) blend gave T, and T, of PVF, (Tab. 1, Fig. 1 and 2) lower than those in a simple binary LLDPE/PVF2 (60/40, w/w) blend (curve 1).

In case of a binary blend of LLDPE-g-PMMA (9.0 wt.-% PMMA)/PVF, of corresponding composition (60/40), where the only difference between the three-component system is that the PMMA in the latter has been fixed to the main LLDPE chain, the constrained motion of PMMA results in a lower PMMA concentration available for specific interaction with PVF,. In- deed, a higher T, and T, of PVF, in this binary blend is found than in the ternary blend where PMMA is free to move (Tab. 1, Fig. 1 and 2). There-

81


Tab. 1. T, and T, of different blends and component polymers.

Blend (w/w) T, ("C) Tc ("C)

LLDPE PVF2 LLDPE PVF,

LLDPE 119.9 - 103.4 - LLDPEffVF2 (60/40) 120.5 163.6 107.5 137.9 G-3ffVF2 (66/40) 120.1 162.1 106.4 127.1 LLDPEPMMAPVF, (60/6/40) 121.6 159.8 105.2 119.3 PVF2 - 161.8 - 128.0

I LLDPE I

I I I I I

100 125 150 175

Temperature('C)

Fig. 1. Melting behavior of three different blends (1) LLDPEffVF, 60/40, (2) G-31 PVF2 (60/6)/40, (3) LLDPEffMMAPVF, 60/6/40 (w~w) .

the covalent links between LLDPE and PMMA cause a weaker specific interaction between PMMA side chains and PVF, than that between PMMA homopolymer and PVF,. In addition, the crystallization and melting peaks of PVF, in the G-3/PVF2 and LLDPEPMMAPVF, blends are broader than in the LLDPEPVF, blend.

82

Miscibility of UDPE-g-PMMA with PVF,

LLDPE \ 75 loo 125 150

Temperature('C)

Fig. 2. Crystallization behavior of the three different blends of Fig. 1.

3.1.2 Effects of interaction between grafted PMMA chains and PVF, on the isothermal crystallization rate of PVF,

The isothermal crystallization dynamics of a polymer is expressed by the Avrami equation2?

1 - X, = exp(-K, t") (1)

Taking logarithm on both sides of Eq. (1)

log 1-ln (1 - X,)] = log K, + nlog t

and plotting log[-ln(1 - X,)] against logt, the Avrami index (n) and rate constant (K,) can be obtained from the slope and the intercept (Fig. 3). The crystallization process of PVF, in the G-3/PVF2 blend can be resolved into two stages and the straight line part according to the Avrami equation refers to the primary crystallization. Two-stage crystallization, however, was not observed with LLDPE/PVF2 and LLDPE/PMMA/PVF, blends. In the G-3/ PVF, (66/40, w/w) blend, PMMA side chains were fixed and only a part of them could contact with PVF,. PVF, molecules not in contact with PMMA

83


0.2

0

: -0.2

I -0.4 - -0.6 *, x

c

r( ; -0.0 GI

- 1 .o

-1.2

-1.4

b 0.6

0.4 - 0.2 - 0 -

n n

x" -0.2 . I

c -0.4 - 7 -0.6 . GI -0.0

El

v bo

-1.0

-1.2

-1.4

1 2346

-0.6 -0.2 0 0.2 0.6

O*4r c 1 2346

0.2

0 h

h * x -0.2

I

c -0.4 n 7 -0.6

-0.8

v

v M

-1.0

-1.2

-1-& I - . -0.5 -0.1 0.3

Lg * Fig. 3. Avrami plots for PVF2 of the three blends: (a) LLDPEPVF, 60/40: (1)

136°C (2) 137"C, (3) 140°C (4) 138"C, (5) 139°C; (c) LLDPE/PMMA/ PVF, 60/6/40: (1) 131 "C, (2) 132"C, (3) 133"C, (4) 134"C, (5) 135°C.

147"C, (2) 148"C, (3) 149"C, (4) 150°C; (b) G-3/PVFz (60/6)/40: (1)

84


side chains underwent primary crystallization and those interacting with PMMA did not crystallize in time. After the PVF, bulk had crystallized completely, the part of the PVF, interacting with PMMA continued to crystallize to show the obvious secondary crystallization.

In the LLDPEPVF, blend, all PVF, molecules exist in the same environment. Rearrangement and defects of crystals in primary crystallization could form weak secondary crystallization, but this possible secondary crystallization was not observed in the G-3PVF2 blend. This might explain the broader crystallization peak of PVF, in the G-3/PVF2 blend than that in LLDPE/ PVF,. In the LLDPEPMMAPVF, blend, since PMMA homopolymer could form homogeneous blends with PVF,, all PVF, molecules also exist in the same environment. Their secondary crystallization was not observed in the G-3/PVF2 blend.

The specific interaction of PMMA and PVF, led to the reduction of T, and crystallization rates of PVF,. The crystallization temperature ranges in the three blends LLDPEPVF,, G-3PVF2 and LLDPEPMMAPVF, are different (Fig. 4). The crystallization temperature range of the LLDPEPVF, blend is the highest among the three blends and the next was that of G-3/ PVF, blend. In the G-3/PVF2 blend, the motion of PMMA side chains is ob- structed owing to covalent linking with the LLDPE backbone. At the same PMMA content, the effects of PMMA homopolymer on PVF, crystallization are different from tht of PMMA side chains. T, of PVF, in LLDPE/PVF,/ PMMA blend is the lowest of the three blends.

3.1.3 Effect of interaction between grafted PMMA chains and PVF, on spherulite growth rates

PVF, crystallizes in the form of spherulites in the G-3/PVF2 and LLDPE/ PVF, blends. The relationship of its growth rate (G) and crystallization temperature (T) can be expressed in the nucleation formula26:

G = Go exp [-AFdKT] exp [-Aw&T] (2)

where AF, is the activation energy of diffusion of the segments into the crystalline phase, Avo is the energy of crystal nucleus formation, Go is a constant, and K is the Boltzmann constant. AF, can be replaced by AFm,:

AFWF = 4 120 TJ(51.6 + T, - TJ (3)

85


1.4

1.2

1 .o h c

'4 0.8 E

'*O 0.4

0.2

0

v 0.6 c

Fig. 4. Crystallization rates of the three blends (same components as in Fig. 1); ( 1 ) LLDPEPMMAPVF,, (2) G-3PVF2, (3) LLDPEPVFZ 60/40.

where AFWF is the temperature-dependent free energy of activation. The term exp [-Ay$KT] can be replaced by exp [-k$ATT,] with AT = T", - T,, and T", being the equilibrium melting point. Eq. (2) can be transformed into:

(4) In G + AFdKT = A - k,/( A n , )

According to Eq. (4), the slope of a plot of (1nG + AFdKT) against (ATTJ' gives k, (Tab. 2).

Tab. 2. k, values of different blends.

PVF, Blend

T, T i ( oC)b k,

LLDPEPVF, -37.2 166.7 6 110 G-3PVFZ -7.4 164.5 16550

a Measured using a Rheovibron DDV-11-EA. Measured using a Perkin-Elmer DSC-7C.

T, of PVF, in the G-3RVF2 blend is higher than that in the LLDPERVF, blend. T, of PVF, in the former is lower than that in the latter (Tab. 1). Ac- cording to Eq. (3), AF, of PVF, in the former is larger than that in the latter.

86


When the spherulite growth rate is mainly determined by AFo, the spherulite growth rate of PVF, in the G-3PVF2 blend will be lower than that in LLDPEPVF, blend. When the spherulite growth rate is mainly determined by k,, the spherulite growth rate of PVF, in the G-3PVF2 blend will also be lower than that in the LLDPEPVF, blend since k, of PVF, in the former is larger than that in the latter (Tab. 2). The self-gathering of PVF, molecules and diffusion of PVF, molecules into the crystalline phase is difficult due to specific interaction between PMMA side chains and PVF,. This is reflected in the increases in k, and AFo. Therefore, the spherulite growth rate of PVF, in the G-3PVF2 blend is reduced.

3.1.4 Infrared spectrum evidence of specific interactions between grafted PMMA chains and PVF,

If the origin of miscibility between G-3 and PVF, results from the specific interaction between PMMA side chains and PVF,, a change will be reflected in the IR spectra of the blends. The differential spectra of the G-3/ PVF, and the LLDPEPMMAPVF, blends, respectively, from G-3 were compared with the IR spectrum of PVF,. The location of some absorption peaks changed (Tab. 3). Painter et al.27 pointed out that the change in the

Tab. 3. Effect of specific interaction between PVF, and PMMA on IR absorption peaks of PVF,.

Differential spectra of Wavenumbers (cm-')

PVF, (as standard) 842.9 - 1 152.0 1213.4 1276.0 1401.2 G-3PVFz-G-3 844.2 1114.9 1153.3 1214.7 1279.5 1405.0 LLDPEPMMAPVFZ-G-3 845.8 1115.9 1154.0 1215.2 1280.4 1406.4

location of the 840, 1070 and 1200 cm-' peaks of PVF, resulted from specific interactions between PMMA and PVF,. It can be inferred that the change in 842, 1 152, 1 21 3, 1 276 and 1 401 cm-' absorption peaks in the differential spectra of the blends from G-3 resulted from the specific interaction of grafted PMMA chains and PVF,. This proves further the specific interaction between PMMA side chains and PVF, on a molecular level.

With the same PMMA content, the change in the location of the PVF, absorption peaks in the G-3PVF2 blend is different from that in the LLDPE/

87


PMMAPVF, blend. This also results from the difference in PMMA side chains and PMMA homopolymer. The shift of the PVF, absorption peaks in LLDPE/PMMA/PVF, blends is larger than that in the G-3PVF2 blend. This result is in accordance with conclusions obtained from studies of crystalline properties and crystallization dynamics.

3.2 Effect of LLDPE-g-PMMA on the compatibility of LLDPE/PVF, blends

LLDPEPVF, blends, where both components are crystalline, are immiscible due to thermodynamic reasons.

The adhesion at the interface is very weak. An effective way of improving the interfacial miscibility between the two components is to add a compatibilizer. For A/B polymer blends, if one of the segments of A-b-C (or A-g-C) is miscible with component A and the other with component B by specific interaction, then the copolymer A-b-C (or A-g-C) can be used as a compatibilizer for A/I3 blends. PMMA is miscible with PVF,. Because LLDPE-g- PMMA was synthesized by suspension polymerization, the PMMA chains are grafted mainly in the non-crystalline region of LLDPE, yielding T, and T, only slightly different from pure LLDPE (Tab. 4). WAXD results show that the crystalline structure of the LLDPE backbone in the LLDPE-g-

Tab. 4. T, and T, of LLDPE-g-PMMA.

LLDPE - 119.9 103.4 G- 1 4.8 121.1 104.8 G-2 7.0 121.4 105.8 G-3 9.0 121.3 105.2

PMMA is the same as that of pure LLDPE (Tab. 5) , implying that the PMMA side chains do not affect the crystalline structure of backbone LLDPE. Therefore, the LLDPE backbone of the graft copolymer is suscep- tible to cocrystallization with the bulk LLDPE. The LLDPE-g-PMMA copolymer thus should be an effective compatibilizer for LLDPEPVF, blends.

88


Tab. 5. Crystal structural parameters of LLDPE-g-PMMA, determined by wide- angle X-ray diffraction (WAXD).

Sample Lat$ice parameteis Cryztalline size I 2 d I 1 0

a (A) b (A) LllO (A) L, (A)

LLDPE 7.435 4.935 151.4 77.6 191100 G-3 7.425 4.925 147.9 66.1 171100 LLDPEPMMA 7.419 4.915 79.4 24.6 231100

3.2.1 Effect of LLDPE-g-PMMA (G-3) on the morphology of LLDPEPVF, blends

The fractured surface morphology was observed using SEM. After adding LLDPE-g-PMMA to the blend of LLDPEPVF, = 75/25, particles of the dis- persed phase were obviously reduced (Fig. 5). This shows that the interfacial tension between LLDPE and PVF, was decreased due to the miscibility between the compatibilizer and the components.

In order to confirm the effect of LLDPE-g-PMMA on interfacial adhesion of LLDPE/PVF, blends, the fractured surface of LLDPEPVF, = 75/25 blend was extracted with dimethylacetamide for 24 h. PVF, in the binary LLDPE/PVF, blend was completely dissolved by the solvent and cavities were formed on the fractured surface. This shows that the interfacial adhesion between LLDPE and PVF, is very poor. On addition of 10 wt.-% LLDPE-g-PMMA, however, still many PVF, particles remained on the extracted surface of the blend. This implies that LLDPE-g-PMMA acts as a compatibilizer of LLDPE and PVF,. The compatibilization makes it impos- sible to have all the PVF, particles extracted.

When the composition of LLDPEPVF, blend was 50/50 or 25/75, G-3 also had the effect of compatibilization.

3.2.2 Effect of compatibilizer on the interfacial structure of LLDPE and PVF,

The above results show that LLDPE-g-PMMA (G-3) can improve the dispersion and the strength of the interfacial adhesion in LLDPEPVF, blends. Selective extraction experiments further illustrate this conclusion. However,

89


Fig. 5. Effect of G-3 on the fracture surface morphology of LLDPEPVF, 75/25 blend; G-3: (a) 0 wt-%, (b) 5 wt.-%, (c) 10 wt.-%, (d) 15 wt.-%.

because of the mutual inclusion of the two components, the effectiveness of the extraction is limited. A model experiment was therefore camed out to prove the compatibilizing effect of LLDPE-g-PMMA in LLDPEPVF, blends".

Composite films of LLDPEPVF, and LLDPE/G-3PVF2 were prepared according to the previously described method". The thickness of the G-3 layer was 2-5 pm. A LLDPEPVF, composite film was dipped in pure cyclohexanone at 110°C for 30 min to dissolve residual PVF, and the sample was washed with ethanol and dried. XPS showed that the exposed interface did not contain fluorine atoms (Tab. 6). That is, there was no PVF, on the exposed interface, as expected from the immiscibility between a polyolefin and a polar polymer.

90


Tab. 6 . PVF, composite films” (XPS measurements).

Composition of the interface (in mol-%) of LLDPEPVF, and LLDPE/G-3/

LLDPE/ LLDPE/G-3/PVF2 PVF2

Time for heat treatment (h) 0 0 4 Time for separationb (min) 1 8 not observed Interface composition (mol-%) LLDPE 100 69.4 20.2 PMMA 0 28.3 6.2 PVF, 0 2.3 73.6

” The interface was exposed by dipping the composite films in cyclohexanone at 110°C. Time needed for LLDPE and PVF, films to separate.

The same experiment was done with a LLDPE/G-3/PVF2 composite film. On dipping in cyclohexanone for 8 min, the PVF, film separated from the LLDPE film. After the PVF, film had dissolved completely, the original interface was examined using XPS (Tab. 6), revealing fluorine atoms. The results imply that interdiffusion of PMMA side chains in LLDPE-g-PMMA and PVF, had occurred in the interfacial region, which prevented some PVF, from being dissolved from the interfacial region by cyclohexanone. This shows that some specific interactions between macromolecules are too strong to be overcome by solvents. It was observed that, while there was only 9.0 wt.-% of PMMA in G-3 bulk, the interface had about 28 wt.-% of PMMA. This shows that the PMMA side chains of G-3 are enriched on the PVF, side in the interfacial region.

In another experiment, a LLDPE/G-3/PVF2 composite film was heated above the T, of PVF, before treatment with cyclohexanone in order to facil- itate interdiffusion of PMMA and PVF,. Since there was no immediate se- paration of LLDPE and PVF, in cyclohexanone for composite films that had been heat treated, heat treatment apparently promoted the interdiffusion of PMMA side chains and PVF, in the interfacial region. The entanglement strength of PMMA side chains and PVF, is so high that it is not disen- tangled by cyclohexanone. The PVF, side of the composite film was dissolved slowly with residence time in the solvent. The exposed interface was washed and dried. XPS showed that the content of fluorine atoms in the interfacial region was very high, the content of PVF, being as high as 73.6 wt.-% in the interfacial region (Tab. 6).

91


LLDPE

* I I 1

The above experiments fully proved that LLDPE-g-PMMA (G-3) could improve the interfacial adhesion of LLDPE and PVF, because it is compati- ble with both LLDPE and PVF,.

3.2.3 Effect of compatibilizer on crystallization of the blends

The components in LLDPEPVF, blends are crystalline. Compatibiliza- tion must be somehow reflected in the crystalline properties of the blends (Tab. 7, Fig. 6). In the LLDPEPVF, (50/50, w/w) blend, there was no change of T, and T, of LLDPE after addition of 10 wt.-% of G-3. This re-

Tab. 7. Changes in T, and T, of the LLDPEIPVF, (50150, wlw) blend

LLDPEIPVF, G-3 T m ("C) Tc ("C) (WW (wt.-%)

LLDPE PVF, LLDPE PVF,

103.4 - 10010 - 119.9 - 50150 0 119.6 160.3 104.0 136.7 50150 10 120.0 159.7 104.7 133.0

- 161.8 - 128.0 01100 -


sults from the same source of LLDPE in the backbone of G-3 and in LLDPE bulk. For the PVF, component, T, had slightly changed in the blends, but T, changed much more (Fig. 6). T, of PVF, in the LLDPEPVF, blend is 8.7"C higher than that of pure PVF,; after addition of 10 wt.-% of G-3, T, of PVF, in the compatibilized blend is 3.7"C lower than that in the corresponding binary blend.

In the LLDPEPVF, binary blend, the LLDPE serves as nucleation agent for the crystallization of PVF,. This nucleation facilitates undercooling of PVF, in crystallization, and T, increases. Thus, the difference between T, of PVF, in LLDPEPVF, and compatibilized LLDPEPVF, blends reflects the interaction of PVF, and PMMA.

T. Inoue, T. Soen, T. Hashimoto, H. Kawai, Macromolecules 3 (1970) 87 L. Toy, M. Niinomi, M. Shen, J. Macromol. Sci.-Phys. B 11 (1975) 281 T. Kotata, T. Miki, K. Arai, J. Macromol. Sci.-Phys. B 17 (1980) 303 M. Jiang, J. Xie, T. Yu, Polymer 23 (1982) 1557 M. Jiang, X. Huang, T. Yu, Polymer 24 (1983) 1259 M. Jiang, X. Cao, T. Yu, Polymer 27 (1986) 1917 M. Jiang, X. Cao, T. Yu, Polymer 27 (1986) 1923 M. Jiang, Physical Chemistry of Polymer Alloys, Sichuan Education Press, Chengdu, 1988, p. 217 W. X. Zhao, Master thesis, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 1987

lo P. S. Tucker, J. W. Barlow, D. R. Paul, Macromolecules 21 (1988) 1678 'I P. S. Tucker, J. W. Barlow, D. R. Paul, Macromolecules 21(1988) 2794 l2 P. S. Tucker, D. R. Paul, Macromolecules 21 (1988) 2801 l 3 D. R. Paul, S. Newman, Polymer Blends, Vol. 2, Academic Press, New York

l4 T. Ouhadi, R. Fayt, R. Jerome, Ph. Teyssie, J. Appl. Polym. Sci. 32 (1986) 5647 l5 R. Fayt, Ph. Teyssie, Macromolecules 9 (1986) 2077 l6 T. Tang, X. B. Jing, B. T. Huang, J. Macromol. Sci.-Phys. B 33 (1 994) 287 l7 T. Tang, B. T. Huang, J. Polym. Sci., Part B: Polym. Phys. 32 (1994) 1991 l8 T. Tang, H. Y. Li, B. T. Huang, Macromol. Chem. Phys. 195 (1994) 2931 l9 T. Tang, H. Chen, X. Q. Zhang, L. Li, B. T. Huang, Polymer 35 (1994) 4240 2o T. Nishi, T. T. Wang, Macromolecules 8 (1975) 909 21 T. T. Wang, T. Nishi, Macromolecules 10 (1977) 421 22 Y. Hirata, T. Kotaka, Polym. J. (Tokyo) 13 (1981) 273 23 J. H. Wendorff, J. Polym. Sci., Polym. Lett. Ed. 18 (1980) 439 24 G . Hadziioannou, R. S. Stein, Macromolecules 17 (1984) 567 25 M. Avrami, J. Chem. Phys. 9 (1941) 177 26 D. J. Walsh, S. Rostami, Adv. Polym. Sci. 70 (1985) 136 27 M. M. Coleman, J. Zarian, D. F. Vamell, P. C. Painter, J. Polym. Sci., Polym.

1976, Chap. 12

Lett. Ed. 15 (1977) 745

93

miscibility of linear low density polyethylene (lldpe)-graft-poly(methyl methacrylate) with...

Documents