self-nucleated crystallization of a branched polypropylene
TRANSCRIPT
University of Massachusetts Amherst University of Massachusetts Amherst
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Masters Theses 1911 - February 2014
2011
Self-nucleated Crystallization of a Branched Polypropylene Self-nucleated Crystallization of a Branched Polypropylene
Dhwaihi Alotaibi University of Massachusetts Amherst
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SELF-NUCLEATED CRYSTALLIZATION OF A
BRANCHED POLYPROPYLENE
A Thesis Presented
By
DHWAIHI ALOTAIBI
Submitted to the Graduate School of the
University of Massachusetts Amherst in partial fulfillment
Of the requirements for the degree of
MASTER OF SCIENCE IN CHEMICAL ENIGINEERING
SEPTEMBER 2011
Chemical Engineering Department
© Copyright by Dhwaihi Alotaibi 2011
All Rights Reserved
SELF-NUCLEATED CRYSTALLIZATION OF A
BRANCHED POLYPROPYLENE
A Thesis Presented
By
Dhwaihi Alotaibi
Approved as to style and content by:
______________________________________
H. Henning Winter, Chair
______________________________________
Jonathan P. Rothstein, Member
______________________________________
Surita Bhatia, Member
________________________________________
Dimitrios Maroudas, Graduate Program Director
Department of Chemical Engineering
DEDICATION
To my Mom and Dad and to my wife who lift her family and relatives to support me
during this education journey.
v
ACKNOWLEDGMENTS
On the 15th
of January, 2009, the weather was very cold and the temperature 20oC
below zero it was a new weather experience for me and my wife. However, all
transferring and moving issues went away as ice cube under the sun when I met Professor
Henning Winter. He was supporting and helping me in every single issue I had faced
during my stay in Amherst. He made me self confident when giving me the opportunity
to participate in conference and workshops. I have learned from him that quality of work
is greatly important than quantity.
I was thinking how he became successful and reached higher levels in his career.
Then, I remembered this saying that “Behind every successful man, stands a woman”.
Yes, Mrs. Karen Winter is a very nice, kind, friendly and respectful woman. I will not
forget her close interest to help my wife during her stay. Professor Henning and Mrs.
Karen, please accept our sincerest thank and deep appreciation.
Thanks to all my family members and relatives. They were excited that I’m
pursing higher education. The death of my grandmother was a shock for me during my
education. She was happy before I left and wishing the success in my life (Allah bless
her). Thanks to my Father and my mother who never stay far from contacting me and
asking about my school and family despite of her health issue and recent diagnoses of
advanced stage of colon cancer.
Thanks to my wife for supporting me and taking care of the children and reduce
the load during my study. I would like to thank Professor Dimitrios Maroudas T.J.
(Lakis) Mountziaris for their help to resolve my careers’ issues. I’m thankful to
Professor Jonathan Rothstein for his valuable suggestions and recommendations.
vi
Thanks to Professor Surita Bhatia for her help. I’m thankful to Professor Alan
Lesser and his group (Jared Archer, Andrew Detwiler, Jan Kalfus, Naveen Singh, Zhan
Hang and Sinan Yordem for helping me in using their lab to conduct my experiments. I
would like also to thank Deepak Arora for his usual help in doing the experiments and
data analysis. Thanks to Marco Dressler for his help in writing, Huimin from Shu group,
my lab mates and colleagues Ahmed Khalil, Fei Li, Erica Bischoff Craig, Huang, Brian
momni ,Ankit, Katie, Niva, Omkar, Sunzida, Ferdos, Marcos, .I am also thankful to Ms.
Elisa Campbell from OIT for the thesis writing sessions. My sincere gratitude goes to
University of Massachusetts Amherst, Department of Chemical Engineering, Polymer
Science and Engineering, the International Programs Office and my employer Saudi
Basic Industrials Corporation (SABIC) for sponsoring my education through a distinct
education program provided for Technology and Innovation employees.
vii
ABSTRACT
SELF-NUCLEATED CRYSTALLIZATION OF A BRANCHED POLYPROPYLENE
SEPTEMBER 2011
DHWAIHI ALOTAIBI, B.S.Me.E., KING SAUD UNIVERSITY
M.S.Ch.E., UNIVERSITY OF MASSACHUSETTS AMHERST
Directed by: Professor H. Henning Winter
Long chain branched polypropylene (LCBPP) crystallizes rapidly and with high
nucleation density. The origin of this fast crystallization process is not well understood. It
has been attributed to its complicated molecular architecture. In this research, we explore
isothermal crystallization of LCBPP, 5%LCBPP and linear polypropylene (LPP) through
rheological, thermal, microscopy and optical measurements at different experimental
temperatures. The time resolved mechanical spectroscopy technique was used to predict
the liquid-to-solid transition (gel point) at different crystallization temperatures
(supercooling rates) in order to understand the structure during the crystallization process.
The crystallization process of LCBPP was completed in time scale less than that
of 5%LCBPP and LPP at different supercooling rates. This has been observed in all
crystallization experiments using DSC, SALS and Rheometery. LCBPP exhibit stiff
behavior at gel point compared to 5%LCBPP and LPP which imply that the small
spherulites observed under polarized microscopy are stiff. Understanding of the
rheological behavior during crystallization process will help to develop polymer with
different processing conditions and applications.
viii
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ............................................................................................... v
ABSTRACT ..................................................................................................................vii
LIST OF TABLES .......................................................................................................... x
LIST OF FIGURES ........................................................................................................ xi
CHAPTER
1. INTRODUCTION ............................................................................................... 1
1.1 Literature Review ..................................................................................... 1
1.2 Theoretical Basis ...................................................................................... 5
1.2.1 Gel Time and Small Amplitude Oscillatory Shear(SAOS) ............ .5
1.2.2 Small Angle Light Scattering (SALS) .......................................... 7
1.3 References ............................................................................................... .9
2. EXPERIMENTAL ............................................................................................. 11
2.1 Materials ................................................................................................ 11
2.2 Temperature Calibration ......................................................................... 11
2.3 Sample Preparation ................................................................................. 15
2.4 Blend Preparation ................................................................................... 15
2.5 Differential Scanning Calorimeter .......................................................... 16
2.5.1 Melting and Crystallization Peaks ............................................... 16
2.5.2 Isothermal Crystallization ........................................................... 16
2.6 Rheology ................................................................................................ 16
2.6.1 Master Curve Measurements ....................................................... 16
2.6.2 Gel Point Measurements ............................................................ 17
2.7 Small Angle Light Scattering (SALS) ..................................................... 17
2.8 Polarized Optical Microscopy ................................................................. 17
2.9 References .............................................................................................. 19
ix
3. POLYPROPYLENE STRUCTURE DURING CRYSTALLIZATION .............. 20
3.1 Introduction ............................................................................................ 20
3.2 Differential Scanning Calorimeter (DSC) .............................................. 20
3.2.1 Melting and Crystallization Peaks ............................................... 20
3.3 Rhelogical Measurements ....................................................................... 21
3.3.1 Master Curve Measurements ....................................................... 21
3.4 Isothermal Crystallization Measurements ............................................... 23
3.4.1 Gel Point Measurements ............................................................. 23
3.4.2 Differential Scanning Calorimeter (DSC) .................................... 25
3.4.3 Polarized Optical Microscopy ..................................................... 26
3.4.4 Small Angle Light Scattering ...................................................... 28
3.5 References .............................................................................................. 32
4. POLYPROPYLENE MECHANICAL PROPERTY AT GEL POINT ................ 33
4.1 Introduction ............................................................................................ 33
4.2 Gels Stiffness and Relaxation Exponent.................................................. 33
4.3 References .............................................................................................. 39
5. CONCLUSIONS AND FUTURE STUDIES ..................................................... 40
BIBLIOGRAPHY ......................................................................................................... 42
x
LIST OF TABLES
Table Page
3.1 Gel points of LCBPP, 5%LCBPP and LPP. ............................................................. 23
3.2 relative crystallinity at gel points of LCBPP, 5%LCBPP and LPP. .......................... 25
4.1 Relaxation exponent and gel stiffness values at gel point.......................................... 34
xi
LIST OF FIGURES
Figure Page
1.1 LCBPP by radical reaction below 80 oC (Ratzsch, 1999) ...................................... 2
1.2 LCBPP by monomer addition at temperatures below 150 oC (Ratzsch,
1999) .............................................................................................................. 2
1.3 Polarized optical microscopy images for LPP (a) and LCBPP (b)
crystallized non isothermally at 10K/min (Tabatabaei, Carreau, & Ajji,
2010). ............................................................................................................. 4
1.4 Liquid-solid transition (critical gel). ..................................................................... 7
1.5 SALS instrument setup......................................................................................... 8
2.1 Left, thermocouple positions between the shearing stage, light passes
through a small window(2.8mm). Thermocouples inside poly-1butene
sample, right................................................................................................. 12
2.2 Universal input meter connected to the shearing stage. ....................................... 12
2.3 Thermocouple readings for a)ATS rheometer and b)shearing stage. ................... 13
2.4 Thermocouple readings for a)ATS rheometer and b)shearing stage. ................... 13
2.5 Calibration curves for ATS rheometer and LINKAM shearing stage used
to set the instrument to the experimental temperature. .................................. 14
3.1 Heating peaks of LCBPP, 5%LCBPP, and LPP. ................................................. 21
3.2 Master curves of LCBPP, LPP, and 5%LCBPP merged at 147.9oC. ................... 22
3.3 Complex viscosity versus G* of LCBPP, LPP and 5%LCBPP samples
measured at 147.9oC. ................................................................................... 22
3.4 G* as a function of time of LPP (a,b), 5%LCBPP (c,d) and LCBPP(e,f) at
T=134.8oC and T=147.9
oC ........................................................................... 24
3.5 Polarized optical microscopy images during isothermal crystallization
process at T=145oC and t=100 min. a)LPP, b)5%LCBPP and
c)LCBPP. ..................................................................................................... 26
3.6 Relative crystallinity cry and normalized crystallinity cry/cry,∞ LCBPP
(a,b)5%LCBPP (c,d) and LPP (e,f)............................................................... 27
xii
3.7 Q , Qas a function of time measured at different
temperaturesLCBPP (a,b)5%LCBPP (c,d) and LPP (e,f) .................................... 30
3.8 Relative crystallinity from isothermal measurement using DSC and light
scattering. SALS data determined for temperatures 138.4oC, 143.0
oC
and 147.9oC. a)LPP b)5%LCBPP and c) LCBPP ......................................... 31
4.1 Meaning of gels stiffness and relaxation exponent values. .................................. 33
4.2 Gel stiffness and relaxation exponent measured at different crystallization
temperatures for (●)LPP, (▲)LCBPP and 5%LCBPP (■) ............................ 35
4.3 Semi-Log plot of gel time of (●)LPP, (▲)LCBPP and 5%LCBPP (■)
measured at different supercooling rates. ...................................................... 36
4.4 Loss tangent, tanvs frequency for different crystallization
temperatures. Interpolation times illustrated as different colors in the
plot. .............................................................................................................. 38
1
CHAPTER 1
INTRODUCTION
1.1 Literature Review
Linear Polypropylene (LPP) has been used for many decades in numerous
numbers of applications such as bottles, food trays, pipes, automotive, and for food
packaging. The extensional stress of the commercial LPP produced by Ziegler-Natta or
metallocene catalysis is very low to be used in processes that involve polymer stretching
such as blow molding, foaming and thermoforming.
In polymer foams, the cell size of the foam is important to control the final
product weight and properties. It is difficult to have large cell size with the linear polymer
because cells burst before developing a low density foam cell. However, foam cells can
be larger if they hold high stretching that can be achieved with the branched polymer
such as branched PP. Foam cells can expand more with the branched PP due to molecular
entanglements caused by the side branches that increase the extensional stress with time.
Introducing long chain branching (LCBPP) to the LPP backbone is one of the techniques
used to enhance the extensional stress of the polymer even further (Jahani & Barikani,
2005).
Grafting branched PP to the backbone of the LPP can be achieved by either
electron beam irradiation (cold process) or by monomer addition (hot process). Both
processes require adding peroxide as initiator during the chemical reactive extrusion
process (Borsig, van Duin, Gotsis, & Picchioni, 2008). Hydrogen abstraction can be
achieved by high energy electron beam irradiation at low temperature (solid state) which
2
fracture LPP chains directly and produce LCBPP followed by β scission that leads to
degraded LPP as in Figure 1.1 (Ratzsch, 1999).
Figure 1.1 LCBPP by radical reaction below 80 oC (Ratzsch, 1999)
Long chain branched PP can be produced by grafting monomers such as styrene
or methylmethacrylate. The reaction should start with peroxides that have very reactive
radical species such as oxy, methyl and phenyl radicals. β scission reaction stabilizes by
adding more monomer. Then reaction termination happens with styrene bonding between
two PP chains in the hydrogen form structure as in Figure 1.2.
Figure 1.2 LCBPP by monomer addition at temperatures below 150 oC (Ratzsch,
1999)
X2
3
The prices of Branched PP discourage their prevalent use in industrial end applications.
Therefore, researchers started studding the effect of blending branched with linear PP.
The blend generated a product with enhanced properties and low cost compared to the
pure branched PP (McCallum, Kontopoulou, Park, Muliawan, & Hatzikiriakos, 2007).
The rheological properties of LCBPP show higher zero viscosity at low
frequencies compared to LPP. Furthermore, a significant improvement in the strain
hardening was observed by adding 20 wt% only of LCBPP to the linear (McCallum et al.,
2007). Blends of LCBPP/LPP having similar melt flow rates, demonstrate better flexural
and tensile properties and blend miscibility compared to blends with different melt flow
rates (McCallum et al., 2007). Melt strength and strain durability can be improved
significantly by adding LCBPP to the linear PP (Wang, Zhang, Liu, Du, & Li, 2008).
However, the optimum composition that can increase the melt as well as the strain
durability was observed with blends 20-30 wt % of LCBPP (Wang et al., 2008). The
larger strain durability achieved at that specific compositions due to low molecules
mobility caused by high entanglement density. (Wang et al., 2008).
The effect of blending branched PP with linear PP on crystallization shows an
increase in the crystallinity measured by DSC when adding low weight fraction percent
of branched PP. The crystallinity decreases as the LCBPP amount increases over 20 wt%
compared to the pure branched PP. The previous crystallization studies show that the
nuclei sites were increased by adding LCBPP to LPP and, thus LCBPP increases the rate
of crystallization at the early stages. The growth is limited by the space of filing. The
interference of growth from the neighboring spherulite results in spherulites with high
4
density and small size as it was observed under polarized optical microscopy, see Figure
1.3.
(a) (b)
Figure1.3 Polarized optical microscopy images for LPP (a) and LCBPP (b)
crystallized non isothermally at 10K/min (Tabatabaei, Carreau, & Ajji, 2010).
It is believed that during the reaction process, some residual of the initiator is
remained in the LCBPP. This residual, could causes a coupling effect with the long
branching chains which then works as a nucleating agent during the extrusion
process.(Tabatabaei et al., 2010). In addition to that, any other implication of the small
spherulites is that the branches themselves work as nucleating agents (Seo, Kim, Kwak,
Kim, & Kim, 1999; Seo, Kim, Kim, & Kim, 2000; Tian, Yu, & Zhou, 2006). In fact,
there are no enough evidences of the first approach or suggestion about the effect of
residual initiator in the branched PP. Cleaning the commercial LCBPP from the residual
initiator, is one of the options to prove this suggestion.
Our central problem is the fast crystallization and the small size of the LCBPP
spherulites. The goal of this work is to compare the time scales of the rheology, optical
and isothermal crystallization using differential scanning calorimeter of linear PP,
5
branched PP and their blends, in order to understand how each system differs from the
other. Optical crystallization is measured by a built in Rheo-Optic instrument. The time
scale is then detected through small angle light scattering device.
1.2 Theoretical Basis
1.2.1 Gel Time and Small Amplitude Oscillatory Shear (SAOS)
The gel point is a liquid-solid transition stage in the polymer crystallization
process (Winter, 1987, 1991, 1993; Winter & Mours, 1997). It is stated that at the gel
point crystallizing polymers shows typical behavior of physical gelation. The gel point is
associated with the molecule connectivity. As the polymer crystallization moves on, the
material rheological properties must be affected. The relaxation processes slow down and
exhibit a unique rheological behavior at the gel point. It is proposed that the slow
dynamics of the critical gel is featured by a self-similar relaxation modulus and G(t) the
relaxation spectrum H(λ) (Winter and Mours1997)(Mours & Winter, 1996):
cn
cG S t
; for 0t
(1)
nSH
n
; for 0t
(2)
Where S is the gel stiffness, nc is the critical relaxation exponent, and λo is the relaxation
time denoting the crossover to some faster dynamics (entanglements, segments), and (n)
is the gamma function. Consequently, the gel time tgel is readily defined as the time
needed for a polymer to reach the critical physical gel sate. The small amplitude
oscillatory shear (SAOS) at low frequencies can be used to determine the gel time tgel.
The sample is deformed sinusoidally, the stress will also oscillate sinusoidally at the same
6
frequency but in general it will be shifted by a phase angel δ with respect to the strain
rate:
0 sin t
(3)
0 0 0sin ' sin " cost G t G t
(4)
In which, G’ is storage modulus, G” is the loss modulus. At the gel point, the storage
modulus also follows the power law behavior
"
' 1 costan 2
nG nG S n
for (5)
As a consequent of the power law dynamics, the loss tangent in the low frequency region
is predicted to be frequency independent at the gel point as in Figure 1.4:
"tan tan
' 2c
G nconst
G
for
(6)
Thus, the polymer sample can be applied a continuous sequence of frequency sweep, the
gel time tgel is determined as the time where tanδ is frequency-independent, at low
frequency region.
7
Figure1.4 Liquid-solid transition (critical gel).
1.2.2 Small Angle Light Scattering (SALS)
The small angle light scattering (SALS) analysis is conducted using the
conventional photographic techniques, which has been widely used in studying the
superstructure and its preferred orientation in a crystalline polymer sample. Scattering
patterns are obtained with the analyzer and polarizer parallel (VV and HH, in the quiescent
crystallization, are the same) and perpendicular (HV and VH). Figure 1.5 shows the SALS
instrument setup.
The crystallization kinetics is conveniently described in terms of the SALS
invariants Q and Q (Stein & Chu, 1970), which are defined as integrated scattering
intensities along a line (i.e. along scattering vector).
22 22
15VH nQ I q dqc
(7)
4
2 2 242
3V VV HQ I I q dqc
(8)
8
where VVI and
VHI are the polarized and depolarized scattering intensities,
is
the scattering vector, is the angular frequency, c is the speed of light in vacuum, 2
n
is the mean square orientation fluctuation, and 2 is the mean square density
fluctuation. Therefore, Q is analogous to the mean-square density fluctuation 2 and
Q parallels orientation fluctuations, i.e. mean-square optical anisotropy 2
n .
In SALS, the relative crystallinity is proportional to the square root of the normalized
orientation fluctuations invariant
rel ~ (9)
Figure1.5 SALS instrument setup.
polarized laser
He-Ne (632.8 nm)
IVV , I HV
IVH , IHH
linear polarizer
(LP1)
sample in device
beam splitter
Isource
I
3
2
1beam splitter
polarization
rotator
camera
LP3
scattering
pattern
VV,HV
control
LP1,LP2 and
analyzer sheet
LP3
Polarization
V
H
0 200 400 600 800
0.0
0.2
0.4
0.6
0.8
1.0
IHH
/IH
H0 (
Arb
it)
time (s)
(LP2)
Q
Q
Q
Q
9
1.3 References
Borsig E, van Duin M, Gotsis AD, et al.: Long chain branching on linear polypropylene
by solid state reactions. European Polymer Journal 44:200-212, 2008.
Jahani Y, Barikani M: Rheological and mechanical study of polypropylene ternary blends
for foam application. Iranian Polymer Journal 14:361-370, 2005.
McCallum TJ, Kontopoulou M, Park CB, et al.: The rheological and physical properties
of linear and branched polypropylene blends. Polymer Engineering and Science
47:1133-1140, 2007.
Mours M, Winter HH: Relaxation patterns of nearly critical gels. Macromolecules
29:7221-7229, 1996.
Ratzsch M: Reaction mechanism to long-chain branched PP. Journal of Macromolecular
Science-Pure and Applied Chemistry A36:1759-1769, 1999.
Seo Y, Kim B, Kwak S, et al.: Morphology and properties of compatibilized ternary
blends (nylon 6/a thermotropic liquid crystalline polymer/a functionalized
polypropylene) processed under different conditions. Polymer 40:4441-4450,
1999.
Seo Y, Kim J, Kim KU, et al.: Study of the crystallization behaviors of polypropylene
and maleic anhydride grafted polypropylene. Polymer 41:2639-2646, 2000.
Stein RS, Chu W: Scattering of light by disordered spherulites. Journal of Polymer
Science Part a-2-Polymer Physics 8:1137-&, 1970.
Tabatabaei SH, Carreau PJ, Ajji A: Polymer Engineering and Science 50:191-199, 2010.
Tian JH, Yu W, Zhou CX: Crystallization kinetics of linear and long-chain branched
polypropylene. Journal of Macromolecular Science Part B-Physics 45:969-985,
2006.
Wang XD, Zhang YX, Liu BG, et al.: Crystallization behavior and crystal morphology of
linear/long chain branching polypropylene blends. Polymer Journal 40:450-454,
2008.
Winter HH: Can the Gel Point of a Cross-Linking Polymer Be Detected by the G' - G''
Crossover. Polymer Engineering and Science 27:1698-1702, 1987.
Winter HH: Polymer Gels, Materials That Combine Liquid and Solid Properties. Mrs
Bulletin 16:44-47, 1991.
Winter HH: Effect of Phase-Transitions on the Rheology of Polymers. Makromolekulare
Chemie-Macromolecular Symposia 69:177-180, 1993.
10
Winter HH, Mours M: Rheology of polymers near liquid-solid transitions. Neutron Spin
Echo Spectroscopy Viscoelasticity Rheology 134:165-234, 1997.
11
CHAPTER 2
EXPERIMENTAL
2.1 Materials
Isotactic linear and branched polypropylene were used in this research work. The
linear PP is a thermoforming grade (HB205TF) a molecular weight (Mw) ≈ 600 kg/mol ,
molecular weight distribution (MWD) ≈4.2 and melt flow index (MFI) of 0.1g/ min. The
branched PP (WB140HMS) is low a density extruded foams grade (20-50 kg/m3) with
Mw≈350 kg/mol, MWD≈6 and MFI 0.21 g/min. The polymers were Ziegler-Natta type
supplied by Borealis Polyolefine GmbH together with their molecular characterization as
shown above.
2.2 Temperature Calibration
Temperature calibration is critical when comparing experiments at instruments
and different crystallization temperatures, since crystallization kinetics depends on the
super cooling temperature. Two K-type thermocouples (diameter 76μm and 0.92m
length) were used to calibrate the optical shearing stage used for SALS and the
rheometers. They were placed inside a poly-1butene disk (25 mm diameter, 2mm
thickness) using the shearing stage at two different positions (inner at 2.3 mm and outer
at 11.4 mm from the center) as shown in Figure 2.1.
The sample preparation explained in details by (Arora, Winter et al. 2011). The
thermocouples were calibrated using ice and boiling water and the readings taken after
one hour of stabilization to have accurate data. The reading results show a difference of
+0.01K for boiling and – 0.1K for ice water. The calibration disk was used for both
12
instrument calibration, optical shearing stage and ATS rhemometer Rheologica (Visco-
tech)
Figure2.6 Left, thermocouple positions between the shearing stage, light passes
through a small window(2.8mm). Thermocouples inside poly-1butene sample, right.
The temperature calibration conducted using advanced thermometer (Universal
input meter DP41-B by OMEGA), has the capability to measure up to 6 digits of reading
and two decimal, see Figure 2.2. K-type thermocouples merged inside poly-1-butene and
positioned
Figure2.7 Universal input meter connected to the shearing stage.
The thermocouple readings show a difference between the set and actual
temperature values for both ATS rheometer and shearing stage as depicted in Figure 2.3.
rw =7.5mm
R=15mm
top view
side view
sample
d = 2.8mm
Thermocouples
13
(a) (b)
Figure 2.8 Thermocouple readings for a)ATS rheometer and b)shearing stage.
The difference between the inner (TC inner) and outer (TC outer) thermocouple has
been plotted against the instrument temperature for both instrument as in Figure 2.4.
Figure 2.9 Thermocouple readings for a)ATS rheometer and b)Shearing stage.
The ATS rheometer has a large energy loss in comparison to SALS readings.
However, this is the reason of the decrease in the radial temperature. In the optical
shearing stage, the calibration thermocouples placed near the center of the operating
window (7.5mm) see Figure 2.1 left. Interpolation gives the value at the window. The
130 135 140 145 150 155-1
0
1
2
3
T
=T
Cin
ner-
TC
oute
r
ATS rheometer
LINKAM shearing stage
Tinstrument oC
130 135 140 145 150 155
130
135
140
145
150
155
160
TCinner
TCouter
Tinstrument
Tinstrument oC
Therm
ocouple
readin
g (
oC
)
(LINKAM shearing stage)
130 135 140 145 150 155
129
132
135
138
141
144
147
150
153
156
(ATS rheometer)
Therm
ocouple
readin
g (
oC
)
Tinstrument(oC)
TCinner
TCouter
Tinstrument
14
outer thermocouple reading is used for the temperature setting of the ATS rheometer . It
is considered as sample temperature to unify experimental temperature TX between the
shearing stage and rheometer. However, Torque =
( =Shear stress in
circumferential direction, r= radius) contributions of the outer region are higher than the
ones at the inner which make more reasonable to select the outer thermocouples reading
for the experiment.
Linear interpolation given are estimate of the temperature at the SALS window
(7.5mm) using the thermocouple readings of the inner and outer thermocouple as in
Figure 2.5. The DSC instrument was calibrated with an Indium sample. Since other types
of pans can be used for calibration. The calibration was conducted using standard pans
and the saved in a file and uploaded every time before starting a new experiment.
Figure 2.10 Calibration curves for ATS rheometer and LINKAM shearing stage
used to set the instrument to the experimental temperature.
130 135 140 145 150 155 160
130
135
140
145
150
155
160
Tinstrument oC
Tout(ATS rheometer)
Tx=7.5mm
(shearing stage)
Tx o
C
15
2.3 Sample Preparation
Hydraulic press (Carver) modified by installing a closed chamber connected to
vacuum pump to delay the oxidation and to reduce voids or air bubbles in the sample.
The material is placed in a mold ≈1.5 mm thickness covered from top and bottom with a
Kapton film to make sure that the sample is protected from any surface impurities. After
closing the chamber and applying the air vacuum, the sample is then heated to 180oC for
10min and the hydraulic pressure was applied to the mold (1000-1500 Psi). The
temperature was raised again to 200oC and left there for 10 min before cooling down to
room temperature. Such sample disk was used for rheology and light scattering
experiments.
2.4 Blend Preparation
Different weight percentages of branched PP were mixed manually with linear PP
to prepare them for the melt mixing or compounding process. C.W. Brabender D6/2
counter rotating twin screw extruder used to blend. Before using the extruder, screws and
die were cleaned with brush and copper knife at high temperatures to remove any
previous materials. The temperature setting was 160oC at the feeding zone, 170
oC for
zone1, 180oC for zone2 and 200
oC at the die.
The extruder was purged at high flow rate with PP supplied by SABIC-IP before
starting with a new blend. The extruder was first run empty and then was run for 5 min
before experimental sample were collected. The extrudate comes in strand shape which is
then cooled in a water bath at room temperature.
16
2.5 Differential Scanning Calorimetry (DSC)
Two experiments were performed with a TA Instruments-Q1000 Differential
scanning calorimeter (DSC) under nitrogen protection:
2.5.1 Melting and Crystallization Peaks
Samples of about 3.8-4.8mg were mounted in an aluminum standard pan. The
samples were heated to 200oC at a rate of 10K /min and cooled to -90
oC at the same rate.
Since the sample has a complex thermal history, the first heating cycle was eliminated
and the data are obtained from the second one.
2.5.2 Isothermal Crystallization
Single samples were used for isothermal crystallization measurements at different
experimental temperatures. samples were heated to 200oC, then cooled to the
experimental temperature at the same rate (10K/min), and held there for isothermal
crystallization for experimental observations for a long time.
2.6 Rheology
2.6.1 Master Curve Measurements
Small amplitude oscillatory shear frequency sweeps were performed on a stress -
controlled rheometer by Rheologica (Visco-tech) connected to dry Nitrogen to avoid
degradation. The disc prepared by the press (~1.5mm) was cut into circular shape and
placed between the parallel plates fixtures (diameter 25 mm). The assembly was heated
above melting point and held for 5 min and then compressed to the experimental
thickness (0.8mm) to ascertain homogeneous contact between the sample and rheometer
fixtures. The experiment performed at different temperatures ranging from 150-210oC
17
with increment of 10ok and constant stress. The loss modulus G
’’ and storage modulus G
’
were measured as a function of angular frequency ω ranging from 0.01–100 rad/s.
2.6.2 Gel Point Measurements
The approach of the gel point (liquid-to-solid transition) see (Schwittay, Mours, &
Winter, 1995) was studied rheologically with time-resolved rheometry experiments
(Mours & Winter, 1994) by heating the sample to 200oC, holding the temperature there
for 5 min to destroy any residual crystals, and then cooling it down to the experimental
temperatures. G’ and G’’ were recorded as a function of time and analyzed then using
IRIS Rheo-Hub software(Winter & Mours, 2006).
2.7 Small Angle Light Scattering (SALS)
A shearing stage (CSS 450 Linkam Scientific) with controlled heating and
cooling unit was used to do the small angle light scattering (SALS) and transmission
intensity measurements. The sample placed into the stage between two quartz windows
and heated to 200oC with a rate of 30K/min and held for 5 min to eliminate any thermal
history and dissolve residual crystals. The gap was then adjusted to 350m. The entire
stage was then inserted into the SALS instrument (Arora D et al., under review) and
cooled at 30K/min to the respective experimental temperature. Linearly polarized Laser
He-Ne (632.8 nm) light was shined through the sample to record scattered and
transmitted intensities on an analyzer sheet and with photodiodes for both parallel and
cross polar (Arora D et al., 2011).
2.8 Polarized Optical Microscopy
The Linkam shearing stage of the SALS experiment except is placed under an
optical microscope (Carl Zeiss Universal (ZPU01) under transmission mode) with cross-
18
polars. The sample placed into the stage between two quartz windows and heated to
200oC with a rate of 30K/min and held for 5 min to eliminate any thermal history and
dissolve residual crystals. The gap was then adjusted to 350m and the camera attached
to the microscope start capturing images every 10s. The stage was then cooled at
30K/min to the experimental temperature. To find more details about the microscope, see
(Pogodina, Lavrenko, Srinivas, & Winter, 2001)
19
2.9 References
Arora Deepak Winter HH, et al.: Network Formation in a Semicrystalline Polymer at the
Early Crystallization stages: From Nucleation to Percolation. Rheologica Acta
(under review)
Pogodina NV, Lavrenko VP, Srinivas S, et al.: Rheology and structure of isotactic
polypropylene near the gel point: quiescent and shear-induced crystallization.
Polymer 42:9031-9043, 2001.
Arora D, Nandi S, Winter H H (2011) A New Generation of Light Scattering Device with
Real-time Data Analysis for Rheo-optical Measurements. Applied Rheology 21:
41633
20
CHAPTER 3
POLYPROPYLENE STRUCTURE DURING CRYSTALLIZATION PROCESS
3.1 Introduction
In this chapter, we study the effect of structure on the crystallization process for
LPP, LCBPP and their blend using the three different experimental techniques which
were detailed in chapter 2. Furthermore, we study the basic characteristic properties of
these materials using DSC (melting temperatures) and rheology (master curve) before
starting the crystallization process.
3.2 Differential Scanning Calorimeter (DSC) Experiments
3.2.1 Melting and crystallization peaks
The results of the DSC shows that LCBPP melts at a lower temperature than LPP
and 5%LCBPP. For LCBPP, LPP and 5%LCBPP the melting peak temperatures were
measured as 158.31oC, 162.68
oC, and 164.66
oC, respectively. Moreover, the crystallinity
of LCBPP is lower than that of LPP and 5%LCBPP. The crystallinity increases when
adding 5%LCBPP to LPP see Figure 3.1. It was 50.60%, 52.58%, and 53.91% for
LCBPP, LPP, and 5%LCBPP respectively. Such increase has already been reported by
(Tabatabaei, Carreau, & Ajji, 2009) and (McCallum et al., 2007).
Since branches differ in molecular weight compared to the sections of
unperturbed backbone molecular weight, different size of lamellar crystals may grow
during cooling which then show as shoulder in the melting peak (Tian et al., 2006). In
addition of that, we attribute the different crystal size to the unknown or random distance
between the branching points that can cause different lamellae thickness.
21
Figure 3.1 Heating peaks of LCBPP, 5%LCBPP, and LPP.
3.3 Rhelogical Measurements
3.3.1 Master curve measurements
The dynamic mechanical data measured at different temperatures merged into a
single master curve at T=147.9oC using IRIS Rheo-Hub software (Winter and Mours,
2005), see Figure3.2. At low frequencies, LCBPP and 5%LCBPP exhibit larger storage
moduli G’ and large viscosity than LPP. The loss tangent (tan= G’’/G’), decreased
when adding 5wt% of LCBPP to LPP. This also expressed in the Winter plot (Winter,
2009), in which the complex viscosity as a function of complex modulus (G*) is
equivalent to the steady shear viscosity as a function of shear stress , see Figure
3.3. The complex viscosity of LCBPP is very high at low stresses compared to LPP
which is a favorable property for processes that involve slow stretching such as blow
molding and thermoforming. The strong shear thinning of LCBPP and its low viscosity
at high shear stress is favorable for mixing and shaping in the extrusion process. The
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
H
ea
t F
low
(W
/g)
80
130
180
Temperature (°C)
––––––– LCBPP
––––– · 5 % LCBPP
– – – – LPP
Exo Up
22
complex viscosity of LPP enhanced when adding only 5% of LCBPP (see sample
5%LCBPP in Figure 3.3).
Figure 3.2 Master curves of LCBPP, LPP, and 5%LCBPP merged at 147.9oC.
Figure 3.3 Complex viscosity versus G* of LCBPP, LPP and 5%LCBPP samples
measured at 147.9oC.
23
3.4 Isothermal Crystallization Measurements
3.4.1 Gel point measurements
The time resolved technique employed by interpolating the data at different times
allows determining the approximate time were the material transits from melt to solid
state (physical gel point). G’ and G
’’ were measured as a function of time and analyzed
using IRIS Rheo-Hub software(Winter & Mours, 2006). The physical gel point exhibits
the typical scaling behavior of G’ and G” which results in a constant tan=G”/G’ (Figures
and more explanation are in the next chapter about this part). Table 3.1 lists the gel point
time tgel at different crystallization temperatures for all three different samples. The
transition was observed earlier with LCBPP and 5%LCBPP samples in comparison to
LPP. The complex modulus G*=(G
’2+G
’’2)
1/2 was plotted as a function of time for each
temperature measured at different frequencies to observe crystallization process and
measure the time where the G* eventually reaches a constant values indicating that the
sample crystallinity is approaching its maximum and the crystallization process is
completed. The evolution of G*
with time plotted in Figure 3.4 for LPP, 5%LCBPP and
LCBPP at T= 138.4oC and T=147.9
oC. However, the effect of the degree of super cooling
on the saturation time and the effect of branches on that time was also, observable from
the plots.
Table3.1 Gel points of LCBPP, 5%LCBPP and LPP.
Tx (oC)
tgel(s)
LPP 5%LCBPP LCBPP
138.4 5480 861 540
143.0 11760 2632 1457
147.9 33360 5055 3705
24
Figure 3.4 G* as a function of time of LPP (a,b), 5%LCBPP (c,d) and LCBPP(e,f) at T=134.8oC and T=147.9oC
(a)
(b)
(c)
(d) (f)
(e)
25
3.4.2 Differential scanning calorimeter (DSC)
Multiple base lines were used to determine the running integrals as a function of
temperature. The relative crystal mass fraction cry was calculated by applying the ideal
heat of melting for 100% crystalline PP (209J/g) (Tabatabaei et al., 2009) and plotted as a
function of time for each sample at different temperatures (Figure 3.6 a,c and e). The data
were normalized with respect to their maximum values of crystallinity cry,∞ and plotted
against time for each temperature and sample, see Figure 3.6 b,d and f.
As a result of high nucleation sites the LCBPP reaches its maximum crystallinity
at higher rate and the crystallization process completed quickly compared to the
5%LCBP and LPP. Table 3.2 listed the values of relative crystallinity at the gel point for
each temperature and sample. The relative crystallinity at gel point increases with higher
degree of super cooling with the LPP sample. This was observed previously studied by
(Schwittay et al., 1995) on iPP. However, LCBPP behaves the opposite of that. The
crystallinity was increasing with low degree of super cooling.
Table3.2 relative crystallinity at gel points of LCBPP, 5%LCBPP and LPP.
Tx (oC)
cry
LPP 5%LCBPP LCBPP
138.4 19.2 44.7 16.8
143.0 17.58 44.6 20.78
147.9 30.47 24.5 21.4
Moreover, the overall crystallinity was decreasing with LCBPP and increasing in
the case of LPP when increasing the degree of super cooling. The reason for that could be
attributed to the structure of long chain branches polypropylene where the molecules
26
require more time to order themselves in the lamella form. Therefore, the low degree of
super cooling will have larger lamella thicknesses and the secondary crystallization will
take place which eventually give higher crystallinity.
3.4.3 Polarized optical microscopy
Long chain branches effect was obvious during the isothermal measurement of
LCBPP and 5%LCBPP samples in comparison with LPP. The crystallization process of
LCBPP was the fastest among other samples when the same temperature and isothermal
crystallization time. Small spherulites were observed in the LCBPP sample attributed to
the high nucleation density which prevents the neighboring spherulites from growth.
Long chain branches, are one of the reasons of high nucleation density since they limit
the molecules movement and cause the disordered structure as in Figure 3.5 c).
Moreover, residual initiator and some degraded molecules during the reaction process can
influence increase the nucleation sites. (Tabatabaei et al., 2009; Tian et al., 2006). The
effect of adding branched PP to the linear was significant as if we compared to LPP in
Figure 3.5 a) and b)
(a) (b) (c)
Figure 3.5 Polarized optical microscopy images during isothermal crystallization
process at T=145oC and t=100 min. a)LPP, b)5%LCBPP and c)LCBPP.
27
Figure 3.6 Relative crystallinity cry and normalized crystallinity cry/cry,∞ LCBPP (a,b)5%LCBPP (c,d) and LPP (e,f).
0 10000 20000 30000 40000 50000 60000
0.0
0.2
0.4
0.6
0.8
1.0
LPPcry
/cry
,
time (s)
138.4oC
147.9oC
143.0oC
0 5000 10000 15000
0.0
0.2
0.4
0.6
0.8
1.0
138.4o
C
143.0o
C
147.9o
C
time (s)
cry
/cry
,
5%LCBPP
0 5000 10000 15000
0.0
0.2
0.4
0.6
0.8
1.0
cry
/cry
,
time (s)
LCBPP
138.4o
C
143.0o
C
147.9o
C
0 10000 20000 30000 40000 50000 60000
0.0
0.2
0.4
0.6
0.8
tgelt
gel
cry
138.4oC
143.0oC
147.9oC
time (s)
LPP
tgel
0 5000 10000 15000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
5%LCBPP
tgeltgel
138.4o
C
143.0o
C
147.9o
C
cry
time (s)
tgel
0 5000 10000 15000
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
cry
tgelt
gel
138.4o
C
143.0o
C
147.9o
C
tgel
LCBPP
time (s)
(a)
(b)
(c)
(d)
(e)
(f)
28
3.4.4 Small Angle Light Scattering
The small spherulites size observed in LCBPP polarized optical microscopy
images, was a good reason for us to use small angle light scattering experiment. This is
because there are less empty spaces (melt region) during the crystallization process with
the LCBPP compared to LPP which then make light shine through large number of
spherulites. The orientation fluctuation invariant Q and density fluctuation invariant Q
were calculated according to Stein-Wilson theory for random orientation correlations
(Koberstein, Russell, & Stein, 1979). Q was normalized to its steady value Q and then
plotted as a function of time as at different temperatures. The growth of Qwith the time,
follow the same sigmoid shape of G* and cry/cry,∞. As expected, Qreaches the
maximum in short time for the lower temperature and vice versa. In some temperatures,
we do see some peaks in the normalized Q data which can be attributed to the different
crystal shape (disk or rod like shape). The growth of crystallinity X(SALS) was plotted
(Figure 3.7,a,band c solid lines) following Stein’s suggestion X(SALS)~
∞ (Pogodina
et al., 2001). The absolute crystallinity Xabs was estimated by multiplying the maximum
relative crystallinity from DSC data into X(SALS)
The values of Q were normalized (0-1) and plotted in Figure 3.7 b,d and e.
Numbers indicate the maximum value of Q for each temperature. The time to reach peak
values, increases with temperature. However, this was expected since the Q reflect the
spherulites density. Furthermore, the peaks are results of light transmission and scattering
through the spherulites. The density fluctuation was at early times for LCBPP and
5%LCBPP compared to LPP
29
Figure 3.7 Q Q , Qas a function of time measured at different temperatures LCBPP (a,b)5%LCBPP (c,d) and LPP (e,f)
0 5000 10000 15000 20000 25000
0.00
0.25
0.50
0.75
1.00
138.4oC
142.0oC
143.0oC
145.0oC
147.9oC
Q
(A
rbit)
LPP
time (s)
tgel 143.0
0Ct
gel 138.40C
0 2000 4000
0.0
0.2
0.4
0.6
0.8
1.0
Q
(A
rbit
)
tgel 147.9
0C
tgel 138.4
0C
tgel 143.0
0C
138.4oC 143.0
oC
144.0oC 145.0
oC
147.9oC
LCBPP
time (s)
1 23
4 5
0 5000 10000 15000 20000 25000
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
138.4oC
142.0oC
143.0oC
145.0oC
147.9oC
Q/Q
rbit)
time (s)
LPP
0 5000 10000 15000 20000 25000
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
138.4oC
142.0oC
143.0oC
145.0oC
147.9oC
Q/Q
rbit)
time (s)
LPP
0 5000 10000 15000 20000 25000
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
138.4oC
142.0oC
143.0oC
145.0oC
147.9oC
Q/Q
rbit)
time (s)
LPP
0 1000 2000 3000 4000 5000 6000 7000 8000-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
5%LCBPP
138.4oC
143.0oC
145.0oC
147.9oC
time(s)
Q/Q
rbit)
0 2000 4000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
138.4oC
143.0oC
144.0oC
145.0oC
147.9oC
Q/Q
rbit)
time (s)
LCBPP
(a)
(b)
(c)
(d)
(e)
(f)
30
In Figure 3.8 a,b and c, we plotted the data for three temperatures and compared
them to the relative crystallinity from DSC for all three samples. The light scattering
predicts a slightly faster crystal growth than the DSC measurement. The light scattering
seems to be more sensitive at early stages of crystallization. The crystal imperfections
and imperfect alignment of crystal optical axes results in variance between crystallinity
from DSC and light scattering data using the orientation fluctuation invariant. Previous
research work done by (Arora D et al., 2011) showed the same different between the two
expermeints.
31
Figure 3.8. Relative crystallinity from isothermal measurement using DSC and light
scattering. SALS data determined for temperatures 138.4oC, 143.0
oC and 147.9
oC.
a)LPP b)5%LCBPP and c) LCBPP
cryQQ
,δδ
0 20000 40000 60000-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
cry
time (s)
2
LPP
SALS DSC
cry
138.4o
C 138.4o
C
143.0o
C 143.0o
C
147.9o
C 147.9o
C
1
3
0 5000 10000 15000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
3
1
DSC
cry
time (s)
cry
138.4o
C 138.4o
C
143.0o
C 143.0o
C
147.9o
C 147.9o
C
SALS
5%LCBPP
2
1 10 100 1000 10000
5.0x10-6
1.0x10-5
1.5x10-5
2.0x10-5
2.5x10-5
time (s)
Qd (
Arb
it)
0.0
0.1
0.2
0.3
0.4
0.5
cr
y
138.4o
C
143.0o
C
147.9o
C DSC
SALS
(a)
(b)
(c)
32
3.5 References
Koberstein J, Russell TP, Stein RS: Total integrated light-scattering intensity from
polymeric solids. Journal of Polymer Science Part B-Polymer Physics 17:1719-
1730, 1979.
McCallum TJ, Kontopoulou M, Park CB, et al.: The rheological and physical properties
of linear and branched polypropylene blends. Polymer Engineering and Science
47:1133-1140, 2007.
Pogodina NV, Lavrenko VP, Srinivas S, et al.: Rheology and structure of isotactic
polypropylene near the gel point: quiescent and shear-induced crystallization.
Polymer 42:9031-9043, 2001.
Schwittay C, Mours M, Winter HH: Rheological expression of physical gelation in
polymers. Faraday Discussions 101:93-104, 1995.
Tabatabaei SH, Carreau PJ, Ajji A: Rheological and thermal properties of blends of a
long-chain branched polypropylene and different linear polypropylenes. Chemical
Engineering Science 64:4719-4731, 2009.
Tian JH, Yu W, Zhou CX: Crystallization kinetics of linear and long-chain branched
polypropylene. Journal of Macromolecular Science Part B-Physics 45:969-985,
2006.
Winter HH, Mours M: The cyber infrastructure initiative for rheology. Rheologica Acta
45:331-338, 2005.
Winter HH: Three views of viscoelasticity for Cox-Merz materials. Rheologica Acta
48:241-243, 2009.
Arora D, Nandi S, Winter HH, Applied Rheology, in press (2011)
33
CHAPTER 4
POLYPROPYLENE MECHANICAL PROPERTY AT GEL POINT
4.1 Introduction
In this chapter, we will study more the Dynamic Mechanical Spectroscpy (DMS)
results and foucus on the structure at the gel point. Gel stiffness and relaxation exponent
are two important parameters to identify the material stiffenss during the crystallization
process using rheometry.
4.2 Gels Stiffness and Relaxation Exponent
If we recall the relaxation modulus relation to the gel stiffeness from chapter1,
G(t)=St -n
. The values of the relaxation exponent n and gel stiffness S can be used to
compare between the material stiffenes or softness at gel point. The stiff materilal has
small n and large S values while the soft material will exhibit large n and small S values
as shown in Figure 4.1. Morever, material behaviour can be understood directly from data
analyzed by IRIS and using Time Resolved Mechanical Spectropy (TRMS) technique
IRIS Rheo-Hub software (Winter & Mours, 2006).
Figure 4.1 Meaning of gels stiffness and relaxation exponent values.
34
The loss tangent tanat gel point will have constant values above or below one.
However, the value of tandepends on the material loss modulus G” and storage
modulus G’. The material behaves softer when G” is greater than G’ and gets stiffer when
G’ is greater than G’’.
Figure 4.4 shows tanas a function of frequency measured during crystallization
process of LCBPP, 5%LCBPP and LPP at different experimental temperatures. The
frequency independent loss tangent values of LPP at the gel points were above 1 for all
three temperatures. However, the LCBPP behaves the opposite, the values of loss
tangent were below 1. This indicates that the LCBPP is a stiff material at gel point
compared to the LPP which is softer. The effect of 5% LCBPP combine both material
stiffness and softness behavior influenced by experimental temperature. Since we didn’t
observed different loss tangent values when changing the experimental temperatures in
the case of LPP and LCBPP, the loss tangent values are not enough to conclude and
attribute the reason to the crystallization temperature. Further analysis required in order
to understand the effect of temperature on the material stiffness. The relaxation exponent
and gel stiffness can be calculated and identified at the gel point using the IRIS software.
Table 4.1 show the relaxation exponent and gel stiffness values at gel point for LPP,
LCBPP and 5%LCBPP at different experimental temperatures.
Table 4.3 Relaxation exponent and gel stiffness values at gel point.
LCBPP 5%LCBPP LPP
T oC
tgel
(s)
S
(kPa.sn)
n tgel
(s)
S
(kPa.sn)
n tgel
(s)
S
(kPa.sn)
n
138.4 540 100 0.45 861 53 0.56 5480 75 0.75
143.0 1457 74 0.38 2632 274 0.37 11760 51 0.69
147.9 3705 168 0.21 5055 66.7 0.62 33360 55 0.61
35
Figure 4.2 Gel stiffness and relaxation exponent measured at different
crystallization temperatures for (●)LPP, (▲)LCBPP and 5%LCBPP (■)
LCBPP has low relaxation exponent and higher gel stiffness values compared to
LPP at all experimental temperatures. This show that LCBPP is a much stiff material
compared to soft LPP. Furthermore, 5% LCBPP effect has influenced the LPP
mechanical properties by increasing the stiffness at gel point. In addition of the structure
effect, the crystallization temperature has increased the values of n in all three samples.
Lower super cooling decreases relaxation exponent values (Horst & Winter, 2000a,
2000b).
LCBPP stiffens can be attributed to thinner lamellae thickness associated with the
small spherulites connectivity function. Previous studied has shown soft gels due to
thicker lamellae of by (Horst & Winter, 2000a), (Arora D et al., 2011). Figure 4.3 shows
the effect of temperature on critical gel time for the three samples. between the critical
0.2 0.3 0.4 0.5 0.6 0.7
50
100
150
200
250
3
2
1
3
2
32 1
1 138.4oC
143.0oC
147.9oC
LPP
5% LCBPP
LCBPPS
Pa
.sn
/1000
n
1
2
3
36
gel time and super cooling temperature were following apparent linear fit(Schwittay et
al., 1995). We can see from the plot that gel point was influenced by experimental
temperature where it was detected earlier at higher degree of supercooling (lower
crystallization temperatures).
However, this is expected, since the crystallization temperature is the driving
force. The crystallization process will be completed in short time resulting in small
spherulites. Moreover, gel point was affected by the polymer structure as it was faster in
LCBPP and 5%LCBPP compared to LPP. The long chain branches can hinder the growth
and produce small spherulite in short crystallization process time.
Figure 4.3 Semi-Log plot of gel time of (●)LPP, (▲)LCBPP and 5%LCBPP (■)
measured at different supercooling rates.
2.37 2.38 2.39 2.40 2.41 2.42 2.43100
1000
10000
100000
LCBPP
LPP
5% LCBPP
1000/Tx[K]
t ge
l (s
)
High supercooling
37
(a)
(b)
(c)
(d) (f)
(e)
38
Figure 4.4 Loss tangent, tanvs frequency for different crystallization temperatures. Interpolation times illustrated as
different colors in the plot.
The mutation number
is used as indication of the sample stability during experiment. The larger values of Nmu (>0.9),
the less sample stability and meaningless rheological data(Mours & Winter, 1994; Winter, Morganelli, & Chambon, 1988). The value
of Nmu for LCBPP sample at low frequency (0.015rad/s) was very high (1.75) when conducted at temperature 138.4oC as in Figure
3.4e in chapter 3 and Figure 4.4b.
(g) (h) (i)
39
4.3 References
Horst RH, Winter HH: Stable critical gels of a copolymer of ethene and 1-butene
achieved by partial melting and recrystallization. Macromolecules 33:7538-7543,
2000a.
Horst RH, Winter HH: Stable critical gels of a crystallizing copolymer of ethene and 1-
butene. Macromolecules 33:130-136, 2000b.
Schwittay C, Mours M, Winter HH: Rheological expression of physical gelation in
polymers. Faraday Discussions 101:93-104, 1995.
Winter HH, Mours M: The cyber infrastructure initiative for rheology. Rheologica Acta
45:331-338, 2006.
Arora D, Nandi S, Winter HH, Applied Rheology, in press (2011)
Winter HH, Morganelli P, Chambon F: Stoichiometry Effects on Rheology of Model
polyurethanes at the gel point. Macromolecules 21:532-535, 1988.
Mours M, Winter HH: time-resolved rheometry. Rheologica acta 33:385-397, 1994.
40
CHAPTER 5
CONCLUSIONS AND FUTURE STUDIES
The study confirms that LCBPP crystallizes very fast and at high nucleation
density. The long chain branches and the residual initiator might be the reason for
creating the nucleation sites. The samples of this study do not contain nucleating agent or
other additives as far as we know. The shoulder observed in the melting peak of LCBPP
is attributed to branches of varied molar mass. Furthermore, the random distance between
the branching points that can cause different lamellae thickness which eventually causes
more defects in the crystal structure.
LPP crystallinity was increasing when increasing the degree of supercooling
when comparing temperature 138.4oC and 143.0
oC. On the other hand, it was decreasing
with LCBPP. Fast crystallization at 138.4oC leads to low final crystallinity for LCBPP
and 5%LCBPP. Low compared to the crystallinity after crystallization at the higher
temperatures. The Low supercooling can slow the crystal growth and allow high crystal
perfection and formation of the secondary crystallization. In addition of the high
nucleation density at higher crystallization temperature and vice versa in the case of LPP.
The evolving modulus G* reaches its maximum at time close the maximum DSC
for all temperatures. This implies that the two experiments are controlled by the same
structural detail. SALS was found to be most sensitive to early crystallization stages while
the entire crystallization process was best observed with DSC. Time resolved mechanical
spectroscopy is very useful technique to understand the material behavior at critical gel
point. Gel stiffness and relaxation exponent were two important parameters to compare
between the polymer stiffness and softness. LCBPP and 5%LCBPPat gel point were
41
stiffer than LPP at different supercooling rates. This implies that LCBPP and 5%LCBPP
has small and stiff spherulites compared to large and soft ones with LPP. Understanding
crystallization at gel point will help to design different semi crystalline polymers suitable
for processes based on the process conditions.
Further studies on the LCBPP can be done to investigate and clarify the reasons of
high nucleation density. Moreover, measuring the spherulites size and number at gel
point will help to understand and relate that to the material stiffness. Furthermore
crystallization of other different weight percentages of LCBPP blends can be studied.
42
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