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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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