The Fractionation and Characterisation of Propylene-ethylene Random Copolymers

8/19/2019 The Fractionation and Characterisation of Propylene-ethylene Random Copolymers

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The fractionation and characterisation

of propylene-ethylene random

copolymers

by

Gareth Harding

Thesis presented in partial fulfilment of the requirements for the degree

of Master of Science at the University of Stellenbosch

Study leader Stellenbosch

Dr. AJ van Reenen December 2005


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I, the undersigned, hereby declare that the work contained in this thesis is

my own original work and that I have not previously in its entirety or in

part submitted it at any university for a degree.

Signature:…………………..

Date:………………………


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Abstract

This study involves the fractionation and characterisation of three propylene-

ethylene random copolymers. The fractionation technique used in the study was

temperature rising elution fractionation (TREF). The TREF fractions were

subsequently analysed offline by crystallisation analysis fractionation (CRYSTAF),

differential scanning calorimetry (DSC), 13C NMR, high-temperature gel-permeation

chromatography (HT-GPC), and wide-angle x-ray diffraction (WAXD). The effect of

the ethylene comonomer on the crystallisability of the propylene was investigated,

along with the effect of the comonomer on the type of crystal phase formed during the

crystallisation. The results show that the comonomer inhibits the crystallisation of thecopolymer and that as the ethylene content increases, the crystallisation and melting

points decrease. It was also shown that the higher the ethylene content, the more of

the γ-phase crystal type is formed. The distribution of the comonomer throughout the

copolymers was also investigated. The results show that there is an uneven

distribution of the comonomer with most of the comonomer accumulating in the

amorphous areas, and very little actually being incorporated in the crystalline regions.

It was also observed that the fractions eluting at the highest temperatures had

considerably higher polydispersities and lower molecular weights than the fractions

eluting just before them. The highest temperature fractions also have lower melting

and crystallisation temperatures than the preceding fractions. This has been attributed

to a nucleation effect by the sand support used during the TREF fractionation.


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Opsomming

Hierdie navorsing behels die fraksioneering en karakterisering van drie

propileen-etileen statistiese kopolimere. Die fraksioneering tegniek wat gebruik is in

die navorsing is temperatuurstyging elueering fraksioneering (TREF). Die TREF

fraksies was toe geanaliseer deur kristallisasie analise fraksioneering (CRYSTAF),

differensiële skandeer kalorimetrie (DSC), 13C kern magnetise resonans spektroskopie

(NMR), hoë-temperatuur jel-permeasie kromatografie (HT-GPC), en wye-hoek x-

straal diffraksie (WAXD). Die effek van die etileen ko-monomeer op die kristallisasie

van die propileen word geondersoek, asook die effek van die ko-monomeer op die

tipe kristal wat gevorm is gedurende die kristallisasie. Die resultate dui aan dat die ko-

monomeer kristallisasie van die kopolimeer inhibeer en dat as die etileen inhoud

verhoog word, dan daal die smelting and kristalisasie temperature. Dit is ook bewys

dat hoe hoor die etileen inhoud, hoe meer van die γ-tipe kristal word gevorm. Die

verspreiding van die ko-monomeer in die ko-polimere word ook ondersoek. Die

resultate dui aan dat daar ‘n oneweredige verspreiding van die ko-monomeer is en dat

die meeste van die ko-monomeer versamel in die amorfe gedeeltes van die

kopolimere, met baie min wat eintlik in die kristallyn omgewing is. Dit was ook

waargeneem dat die fraksies wat elueer by die hoogste temperature aansienlike hoë

polidispersiteite en laer molekulêre massas het as die fraksies wat voor hulle geelueer

is. Die fraksies van die hoogste temperature het ook lear smeltpunte en kristallisasie

temperature as die vorige fraksies. Dit kan toegeskryf word aan ‘n kernvorming

proses.


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Acknowledgements

I would like to thank the following people for their help and support in getting

me through this study.

Dr. AJ van Reenen - for his guidance throughout the study

Valerie Grumel - for all the HT-GPC work

Derick Mcauley - for all the CRYSTAF work

Remy Bucher at Ithemba Labs for all the WAXD work

Elsa Malherbe - for the NMR work

The Olefins research group

My parents - for their continued support throughout my studies


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Abbreviations

TREF Temperature rising elution fractionation

P-TREF Preparative temperature rising elution fractionation

A-TREF Analytical temperature rising elution fractionation

CRYSTAF Crystallization analysis fractionation

DSC Differential scanning calorimetry

NMR Nuclear magnetic resonance

HT-GPC High-temperature gel permeation chromatography

WAXD Wide-angle x-ray diffraction

LDPE Low density polyethyleneLLDPE Linear low density polyethylene

HDPE High density polyethylene

PP Polypropylene

MAO Methylaluminoxane

IR Infrared

RI Refractive index

o-DCB ortho-Dichlorobenzene

TCB Trichlorobenzene

TMB Trimethylbenzene

SEC Size-exclusion chromatography

GFC Gel filtration chromatography

DGMBE Diethylene-glycol-monobutylether

DMP Dimethyl phthalate

PD Polydispersity


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I

CONTENTS

List of figures...............................................................................................................III

List of tables............................................................................................................... VII

Chapter 1. Introduction............................................................................................1 1.1 General introduction ......................................................................................1

1.2 Aims...............................................................................................................2

1.3 References......................................................................................................2

Chapter 2. Background............................................................................................4

2.1 Polyolefins: A brief historical overview........................................................4

2.2 Polymerisation chemistry: An overview........................................................5

2.2.1 General mechanism of transition metal catalysed polymerisation ........5

2.2.2 Polymerisation control mechanisms and stereochemistry .....................8

2.2.3 The evolution of the transition metal catalysts ....................................11

2.3 Commercial polypropylene..........................................................................13

2.3.1 Varieties of polypropylene manufactured............................................13 2.3.1.1 Polypropylene homopolymer...........................................................13

2.3.1.2 Impact copolymers...........................................................................14

2.3.1.3 Random copolymers ........................................................................15

2.3.2 Crystallinity types ................................................................................16

2.4 Fractionation techniques ..............................................................................18

2.4.1 Fractionation by crystallinity ...............................................................18

2.4.1.1 Fractionation mechanism and crystallisation theory........................18

2.4.1.2 TREF................................................................................................20

2.4.1.3 CRYSTAF........................................................................................26

2.4.2 Molecular weight fractionation............................................................27

2.4.2.1 Analytical techniques.......................................................................27 2.4.2.2 Preparative techniques .....................................................................28

2.4.3 Solvent extraction ................................................................................28

2.5 Concluding remarks and methodology ........................................................29

2.6 References....................................................................................................30

Chapter 3. Experimental ........................................................................................39

3.1 TREF............................................................................................................39

3.1.1 The crystallisation step ........................................................................39

3.1.2 The elution step....................................................................................40

3.2 High-temperature GPC ................................................................................42

3.3 CRYSTAF....................................................................................................42

3.4 DSC..............................................................................................................43

3.5 NMR ............................................................................................................43

3.6 WAXD.........................................................................................................43

3.7 References....................................................................................................44

Chapter 4. Results and Discussion ........................................................................45

4.1 The unfractionated samples .........................................................................45

4.1.1 Molecular structure analysis ................................................................45

4.1.2 Crystallisation and melting ..................................................................48

4.1.3 Crystal phase analysis ..........................................................................50

4.2 Optimising the TREF fractionation .............................................................53

4.3 The fractionated material .............................................................................58 4.3.1 TREF analysis......................................................................................58


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II

4.3.2 Molecular structure analysis ................................................................61

4.3.3 Crystallisation and melting ..................................................................67

4.3.4 Crystal phase analysis ..........................................................................71

4.4 References....................................................................................................74

Chapter 5. Conclusions..........................................................................................78

5.1 Conclusions..................................................................................................78 5.2 Future work..................................................................................................79

Appendix A HT-GPC data .......................................................................................80

Appendix B 13C NMR data ......................................................................................82

Sample A..................................................................................................................82

Sample B..................................................................................................................83

Sample C..................................................................................................................84

Appendix C CRYSTAF data....................................................................................85

Sample A..................................................................................................................85

Sample B..................................................................................................................92

Sample C................................................................................................................100

Appendix D DSC data ............................................................................................108 Sample A................................................................................................................108

Sample B................................................................................................................115

Sample C................................................................................................................123

Appendix E WAXD data .......................................................................................131

Appendix F DSC data of the samples analysed by WAXD...................................133

Original samples ....................................................................................................133

Fractions of sample A ............................................................................................134

Fractions of sample B ............................................................................................136

Fractions of sample C ............................................................................................137


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III

List of figures

Figure 2.1 The Ziegler-Natta polymerisation mechanism. ............................................8 Figure 2.2 Polymerisation control mechanisms.............................................................9

Figure 2.3 Catalyst active sites on 1,0,0 and 1,1,0 cuts of the MgCl2 crystal..............10

Figure 2.4 Coordination of internal donors ensuring isospecific active sites.... ..........10

Figure 2.5 Types of polypropylene tacticity................................................................14

Figure 3.3.1 Setup used for the crystallisation step of preparative TREF. ..................39

Figure 3.3.2 Temperature profile used for the slow cooling of the samples used for

TREF............................................................................................................................40

Figure 3.3.3 An illustration of the elution column packing method.... ........................41

Figure 3.3.4 The TREF elution setup...........................................................................41

Figure 4.1 13C NMR spectrum of original sample A in the region between 10 and 55

ppm. .............................................................................................................................46 Figure 4.2 13C NMR spectrum of original sample B in the region between 10 and 55

ppm. .............................................................................................................................46

Figure 4.3 13C NMR spectrum of original sample C in the region between 10 and 55

ppm, with peak assignments. .......................................................................................46

Figure 4.4 The structure of isotactic polypropylene with a single ethylene unit inserted

between the regioregular propylene units. ...................................................................47

Figure 4.5 CRYSTAF curves of all three original samples A, B, and C.....................49

Figure 4.6 DSC melting curves of the three original samples A, B, and C. ................49

Figure 4.7 WAXD analysis of the three original samples A, B, and C after melt

pressing and slow cooling of the samples....................................................................51

Figure 4.8 TREF results of the first fractionation of sample A. The fractionationtemperatures were 25, 50, 75, 95, and 120°C. .............................................................53

Figure 4.9 TREF results of the second fractionation of sample A. The fractionation

temperatures were 25, 50, 75, 95, 120, and 140°C......................................................54

Figure 4.10 TREF results of the third fractionation of sample A. The fractionation

temperatures were 25, 50, 75, 85, 95, 105, 120, and 140°C. .......................................54

Figure 4.11 TREF results of the fourth fractionation of sample A..............................56

Figure 4.12 The TREF results of the fifth, and final, fractionation of sample A. .......57

Figure 4.13 Comparison of eluting 200 mL v. 400 mL for sample B. ........................58

Figure 4.14 A comparison of the fractionation of all three samples, A, B, and C.... ...60

Figure 4.15 HT-GPC molecular weight results for the fractions of sample A

illustrating the weight average molecular weight and polydispersity of the fractions.61

Figure 4.16 HT-GPC molecular weight results for the fractions of sample B


Figure 4.17 HT-GPC molecular weight results for the fractions of sample C


Figure 4.18 13C NMR spectrum of the 25°C (C1) fraction of sample in the region

between 10 and 55 ppm. ..............................................................................................64

Figure 4.19 Suggested chain structures for the room temperature fraction of sample C.

......................................................................................................................................64

Figure 4.20 13C NMR spectra of fractions C7, C9, and C11 in the region between 10

and 55 ppm...................................................................................................................65


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IV

Figure 4.21 A waterfall plot of the DSC melting endotherms of the first 8 fractions of

sample A. .....................................................................................................................67

Figure 4.22 A waterfall plot of the DSC melting endotherms of the last 7 fractions of

sample A. .....................................................................................................................68

Figure 4.23 CRYSTAF curves of selected fractions of sample A...............................68

Figure 4.24 DSC melting endotherms of fraction A12 obtained at different heatingrates. The rates were 5, 10, and 20°C/minute. .............................................................70

Figure 4.25 WAXD results for fractions A7, A9, and A11 of sample A.....................72

Figure 4.26 DSC melting endotherms of the first and second heating cycles of fraction

A7 of the sample which was slow-cooled for WAXD analysis...................................73

Figure B.1 13C NMR spectrum of the 25°C fraction of sample A (A1). .....................82

Figure B.2 13C NMR spectra of the selected fractions of sample A (A7, A9, and A11).

......................................................................................................................................82

Figure B.3 13C NMR spectrum of the 25°C fraction of sample B (B1).......................83

Figure B.4 13C NMR spectra of the selected fractions of sample B (A6, A8, and A10).

......................................................................................................................................83Figure B.5 13C NMR spectrum of the 25°C fraction of sample C (C1).......................84

Figure B.6 13C NMR spectra of the selected fractions of sample C (A7, A9, and A11).

......................................................................................................................................84

Figure C.1 CRYSTAF results for the 25°C fraction of sample A (A1).......................85








Figure C.9 CRYSTAF results for the 100°C fraction of sample A (A9).....................89

Figure C.10 CRYSTAF results for the 105°C fraction of sample A (A10).................89






Figure C.16 CRYSTAF results for the 25°C fraction of sample B (B1). ....................92

Figure C.17 CRYSTAF results for the 45°C fraction of sample B (B2). ....................93Figure C.18 CRYSTAF results for the 65°C fraction of sample B (B3). ....................93






Figure C.24 CRYSTAF results for the 100°C fraction of sample B (B9). ..................96

Figure C.25 CRYSTAF results for the 105°C fraction of sample B (B10). ................97



Figure C.28 CRYSTAF results for the 120°C fraction of sample B (B13). ................98Figure C.29 CRYSTAF results for the 125°C fraction of sample B (B14). ................99


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V


Figure C.31 CRYSTAF results for the 25°C fraction of sample C (C1). ..................100




Figure C.35 CRYSTAF results for the 80°C fraction of sample C (C5). ..................102Figure C.36 CRYSTAF results for the 85°C fraction of sample C (C6). ..................102



Figure C.39 CRYSTAF results for the 100°C fraction of sample C (C9). ................104

Figure C.40 CRYSTAF results for the 105°C fraction of sample C (C10). ..............104






Figure D.1 DSC data for the 25°C fraction of sample A (A1). .................................108








Figure D.9 DSC data for the 100°C fraction of sample A (A9). ...............................112

Figure D.10 DSC data for the 105°C fraction of sample A (A10). ...........................112






Figure D.16 DSC data for the 25°C fraction of sample B (B1).................................115





Figure D.21 DSC data for the 85°C fraction of sample B (B6).................................118Figure D.22 DSC data for the 90°C fraction of sample B (B7).................................118


Figure D.24 DSC data for the 100°C fraction of sample B (B9)...............................119

Figure D.25 DSC data for the 105°C fraction of sample B (B10).............................120






Figure D.31 DSC data for the 25°C fraction of sample C (C1).................................123

Figure D.32 DSC data for the 45°C fraction of sample C (C2).................................123Figure D.33 DSC data for the 65°C fraction of sample C (C3).................................124


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VI






Figure D.39 DSC data for the 100°C fraction of sample C (C9)...............................127Figure D.40 DSC data for the 105°C fraction of sample C (C10).............................127

Figure D.41 DSC data for the 110°C fraction of sample C (C11).............................128





Figure E.1 WAXD results for the selected fractions of sample A (A7, A9, and A11).

....................................................................................................................................131

Figure E.2 WAXD results for the selected fractions of sample B (B6, B8, and B10).

....................................................................................................................................131Figure E.3 WAXD results for the selected fractions of sample C (C7, C9, and C11).

....................................................................................................................................132

Figure F.1 DSC melting endotherms of the first and second heating cycles of sample

A of the samples which were slow cooled for WAXD analysis................................133


B of the samples which were slow cooled for WAXD analysis................................133


C of the samples which were slow cooled for WAXD analysis................................134

Figure F.4 DSC melting endotherms of the first and second heating cycles of fraction

A7 of the sample which was slow cooled for WAXD analysis.................................134


A9 of the sample which was slow cooled for WAXD analysis.................................135


A11 of the sample which was slow cooled for WAXD analysis...............................135


B6 of the sample which was slow cooled for WAXD analysis. ................................136


B8 of the sample which was slow cooled for WAXD analysis. ................................136


B10 of the sample which was slow cooled for WAXD analysis. ..............................137Figure F.10 DSC melting endotherms of the first and second heating cycles of fraction

C7 of the sample which was slow cooled for WAXD analysis. ................................137


C9 of the sample which was slow cooled for WAXD analysis. ................................138


C11 of the sample which was slow cooled for WAXD analysis. ..............................138


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VII

List of tables

Table 2.1 Recent work carried out in the field of analytical TREF.............................22Table 4.1 HT-GPC molecular weight data for the unfractionated samples A, B, and C

......................................................................................................................................45

Table 4.2 13C NMR Chemical shift data for sample C ................................................47

Table 4.3 The percentage of ethylene included in each of the original random

copolymers...................................................................................................................48

Table 4.4 Crystallisation and melting data for the original samples as obtained by

DSC and CRYSTAF analysis ......................................................................................50

Table 4.5 Crystallinity percentages calculated from DSC endotherms, and the amount

of γ-phase crystals present in the original samples as determined from the WAXDspectra after slow cooling the samples.........................................................................52

Table 4.6 HT-GPC molecular weight and CRYSTAF data for the third fractionationof sample A..................................................................................................................55

Table 4.7 HT-GPC molecular weight data for the fourth fractionation of sample A..56

Table 4.8 TREF fractionation data for the fractions of samples A, B, and C..............59

Table 4.9 Ethylene content percentages for selected fractions of all three samples as

determined by 13C NMR ..............................................................................................66

Table 4.10 Summary of all the DSC and CRYSTAF data for all fractions of sample A

......................................................................................................................................69

Table 4.11 Crystallinity percentages calculated from DSC endotherms and the amount

of γ-phase crystals present in selected fractions of all samples, as determined from theWAXD spectra after slow cooling the samples ...........................................................72

Table A.1 HT-GPC data for the fractions of sample A. ..............................................80

Table A.2 HT-GPC data for the fractions of sample B................................................80

Table A.3 -GPC data for the fractions of sample C.....................................................81


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1

Chapter 1. Introduction

1.1 General introduction

Propylene copolymers have received a great deal of attention in recent times

due to the excellent properties that have been obtained by the introduction of a

comonomer. These copolymers have become increasingly competitive in a variety of

areas in which the polypropylene homopolymer was not, such as in flexible films [1].

The introduction of a comonomer has resulted in copolymers being developed with a

lower degree of crystallinity than the propylene homopolymer, allowing the use of the

copolymer in a broader spectrum of applications [2]. Polypropylene is also an

extremely interesting polymer in that it can crystallise in a variety of crystal forms,

each with its own properties [2]. This has meant that this material has become a viable

option and a serious commercial commodity in certain areas of use traditionally

dominated by other materials such as polyethylene. The effect of the introduction of a

comonomer on the macroscopic properties of the material must be explained on a

molecular level if the full benefits of this development are to be harnessed. This is

vitally important and is the fundamental basis of material science. Without this

knowledge, further development becomes far more difficult and much more of a

lottery.

This study looks at three commercial propylene-ethylene random copolymers.

The materials are predominantly polypropylene with a small degree of ethylene

included as comonomer. The material which comes out of the reactor during the

polymers’ synthesis contains a variety of chains of varying lengths and with varying

degrees of comonomer inclusion. Due to the complex nature of the manufactured

copolymer it is necessary to fractionate the material before a full characterisation is

possible. The technique employed during this study is temperature rising elution

fractionation (TREF). This is an excellent technique for the fractionation, i.e.

separation, of a semi-crystalline material into a number of fractions. The TREF

technique is based on the separation of material according to its ability to crystallise

[3]. The crystallisation temperature of a semi-crystalline polymer depends on a

number of factors such as the molecular weight, molecular weight distribution,

chemical composition, chemical composition distribution, tacticity, and the type of


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2

internal ordering of the crystal unit cell. The internal ordering, or crystal phase, of

polypropylene plays a large role in the properties of the polymer. Polypropylene can

crystallise in different crystal forms, the formation of which is influenced by various

internal factors such as chain defects, and external factors such as crystallisation

temperature and pressure [4]. Comonomer inclusion in a random copolymer can also

affect the type of crystal phase formed by acting as a chain defect.

1.2 Aims

The aims of this study are therefore as follows:

• The fractionation of three different propylene-ethylene random copolymers.

• The full characterisation of the fractions as well as the original samples.

• The determination of the effect of the inclusion of a comonomer on the ability

of the chains to crystallise.

• An investigation into the distribution of the ethylene comonomer in the

copolymers, and the effect of the distribution on the properties of the

copolymers.

• The determination of the effect of the ethylene comonomer on the crystal

phase formed in the original samples as well as the isolated fractions.

• An examination of the effect of the crystal phase on the melting characteristics

of the copolymer fractions.

1.3 References

1. Moore, E.P., Jr., & Larson, G.A., Introduction to PP in business, in

Polypropylene handbook , E.P. Moore, Jr., Editor. 2002, Hanser: Munich. p.

257-285.

2. Phillips, R.A., & Wolkowicz, M.D., Structure and morphology, in

Polypropylene Handbook , E.P. Moore, Jr., Editor. 2002, Hanser: Munich. p.

113-176.

3. Wild, L., Temperature rising elution fractionation. Advances in Polymer

Science, 1990. 98(1): p. 1-47.


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4. Foresta, T., Piccarolo, S., & Goldbeck-Wood, G., Competition between alpha

and gamma phases in isotactic polypropylene: effects of ethylene content and

nucleating agents at different cooling rates. Polymer, 2001. 42: p. 1167-1176.


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4

Chapter 2. Background

2.1 Polyolefins: A brief historical overview

The term olefin is a derivative of the word “olefiant”, meaning oil-forming

gas, which was the term used by four Dutch chemists to describe the gas that

produced an oil (ethylene dichloride) by the addition of chlorine [1]. It was as early as

1858 that Goryainov and Butlerov managed to polymerise pentene by the addition of

boron trifluoride. This was followed soon after by Berthelot, in 1869, who managed to

polymerise propylene by a reaction with concentrated sulphuric acid [2]. The product

formed, a viscous oil, was of no industrial importance. In 1894 H. von Peckman

produced a linear, low molecular weight, polyethylene by the decomposition of

diazomethane, a technique also used for the production of polymethylene [1].

It was only much later, during the early part of the 1920’s, that the concept of

a high molecular weight polymer emerged, meeting considerable resistance in

scientific circles [3]. Taylor and Jones reported the polymerisation of ethylene in the

presence of diethylmercury in 1930 [4]. The concept of stereoregular polymerisation

was largely ignored and it was not until the stereoregular form of natural rubber was

observed in the 1940’s that stereoregularity as a concept became more readily

acceptable. It was in the 1950’s that real strides forward were taken in the

development of polyolefins [5], when in 1953 high-density polyethylene was

synthesised in the labs of Karl Ziegler. Early the following year Giulio Natta managed

to synthesise polypropylene with Ziegler following suite only a few months later [3].

Fontana managed the cationic polymerisation of propylene in 1952 [1], producing an

amorphous material which was useful as an additive for lubricating oil but lacked the

strength necessary for structural applications.

The first commercial production of a polyolefin was that of polyethylene by

ICI in 1939 [3]. Licenses for the production of polyethylene were subsequently

granted to Union Carbide and Du Pont [1]. Due to the fact that the branches in the low

density polyethylene (LDPE) significantly lowered the density of the polymer,

attempts were made to produce a polymer with a more linear structure, namely a

linear low density polyethylene (LLDPE). Copolymers of ethylene and 1-butene were


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5

synthesised in order to combat this problem although there was little demand for them

while the technology was still in its infancy [1]. The LLDPE which was eventually

produced was a superior polymer to LDPE for many applications. It was mainly

produced by Union Carbide’s Unipol process [6]. Crystalline polypropylene, on the

other hand, first went into commercial production in 1957 in the plants of a number of

companies, including Hercules, Montecatini, and Fabewerke-Hoechst [1].

The polyolefin industry took off from there, as new processes and products

were constantly being developed. Growth in the field has been phenomenal, with

production increasing at an excellent rate. Polypropylene production increased by

6.9% between 1993 and 2000, with the other thermoplastics also showing similar

strong trends (LDPE: 3.5%, LLDPE: 9.7%, HDPE: 6.1%) [7]. The sheer number of

applications and diverse fields of applicability of this class of materials is greatly due

to the continued and substantial research that is undertaken each year. New markets

for polyolefin materials are constantly being created through the designing of new

materials such as copolymers and blends, obtaining improved and more interesting

properties for so-called standard materials. Processing conditions have also changed

greatly over the years and the development of new catalyst systems has lead to an

exciting period in the lifetime of the industry, with new doors constantly being

opened.

2.2 Polymerisation chemistry: An overview

A discussion of catalyst technology and polymerisation processes is necessary

in order to understand why the polymers produced by heterogeneous catalysts have

their unique characteristics. The very nature of the catalyst is the reason for the

chemical composition distribution of the polymers produced. Consequently, the

necessity for fractionating a polymer in order to fully characterise it is directly due to

the polymerisation process itself.

2.2.1 General mechanism of transition metal catalysedpolymerisation

A Ziegler-Natta catalyst can be defined as a transition metal compoundincorporating a metal-carbon bond which is able to perform the repeated insertion of


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olefin units [8]. The active centres of Ziegler-Natta catalysts are basically formed due

to interaction between a transition metal compound and an organometallic cocatalyst

[5, 9]. The exchange of a halogen atom from the transition metal compound and an

alkyl group from the organometallic cocatalyst is a critical step in the formation of the

active centre [5, 10], as illustrated in Equation (1) for a TiCl3/AlEt3 system:

[TiCl3] + [AlEt3] → [Cl3TiEt] + AlEt2Cl (1)

The most important factor regarding the bond between the transition metal

atom and the carbon atom is that it has the ability to react with the double bonds of α-

olefins [5]. The monomer first coordinates to the transition metal before the actual

insertion occurs. This leads to the formation of a complex, four-member transition

state from which the monomer unit is inserted into the growing chain. This

mechanism has been proven by the presence of isobutyl chain-end groups formed in

the first step of the polymerisation reaction using 13C-enriched Al(CH3)3 [11]:

M-13CH3 + CH2=CH-CH3 → M-CH2-CH(CH3)-13CH3 (2)

The insertion of the α-olefin into the metal-carbon bond can occur in two different

ways [8]:

M-P + CH2=Ch-CH3 → M-CH2-CH(CH3)-Polymer (1,2 primary insertion) (3)

M-P + CH2=Ch-CH3 → M-CH(CH3)-CH2-Polymer (2,1 secondary insertion) (4)

where P represents the polymer chain. This defines the regiochemistry of the polymer

formed. Heterogeneous catalysts have extremely high regiospecificity, resulting in

mainly 1,2 insertions [8]. The polymer chain is then grown through the repeated

insertions of the monomer units. Secondary insertions can either be followed by a

primary insertion, leading to vicinal methyl groups, or by isomerization of the

secondary inserted unit, resulting in 1,3 insertion of the monomer [12]. The 1,3

insertions result in the following structure [2]:


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-CH2-CH(CH3)-CH2-CH2-CH2-CH2-CH(CH3)-CH2-

The isomerization is favoured by a higher polymerisation temperature [12].

Eventually each growing polymer chain is disengaged from the transition

metal atom. There are a number of ways in which this chain termination occurs. The

first method of chain termination is chain transfer to monomer [5]. This is the most

important chain termination process for the polymerisation of propylene with

heterogeneous catalysts (in the absence of hydrogen) [8, 9]. It involves the

replacement of a long alkyl chain at the transition metal atom with a short alkyl group

derived from the monomer as illustrated in Equation (6):

M-CH2-CHR-Polymer + CH2=CH-R → M-CH2-CH2-R + CH2=CR-Polymer (6)

A second reaction which can occur is the alkyl group transfer between the active

centre and the organometallic cocatalyst as illustrated in Equation (7):

M-CH2-CHR-Polymer + AlEt3 → M-Et + Et2Al-CH2-CHR-Polymer (7)

The Al-C bond decomposes on exposure to air and moisture, leaving a polymer

molecule [5]. The third way in which termination occurs is by means of a β-hydride

elimination Equation (8), although this process is not considered important in

propylene polymerisation with heterogeneous catalyst systems at normal

polymerisation temperatures [8]:

M-CH2-CHR-Polymer → M-H + CH2=CR-Polymer (8)

Equation (8) does however become a significant chain termination reaction in

metallocene-based catalyst systems [8]. There is also the β-methyl elimination

method of chain termination, although this process has never been observed during

the polymerisation of propylene with heterogeneous catalyst systems [8]. It is

however important during homogeneous polymerisations. The chain termination

reactions occur very infrequently compared to the chain growth reactions [5]. In order

to limit the molecular weight of the polymer formed, hydrogen is usually introduced


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to terminate a growing chain according to the so-called chain transfer to hydrogen

reaction [13], as illustrated in Equation (9):

M-CH2-CHR-Polymer + H

2 → M-H + CH

3-CHR-Polymer (9)

The chain transfer to hydrogen reaction is the most commercially important method of

controlling the molecular weight [5].

2.2.2 Polymerisation control mechanisms andstereochemistry

When dealing with a prochiral monomer such as propylene, the question of

stereospecificity as well as regiospecificity arises during the polymerisation with a

given catalyst.

Figure 2.1 shows the general mechanism of the polymerisation process. The

manner of the coordination during the first step of the reaction determines the stereo-

and regiospecificity of the monomer unit in the chain.

Figure 2.1 The Ziegler-Natta polymerisation mechanism.

The regioselectivity of the Ziegler-Natta catalysts is generally better than that

of the metallocenes catalysts [5, 8]. The majority of the monomer units will therefore

be inserted in the 1,2 insertion mode during polymerisation with heterogeneous

catalysts. This still leaves the question of preferential enantioface selectivity duringthe polymerisation wide open. Should alternating enantiofaces be inserted during


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Figure 2.3 Catalyst active sites on 1,0,0 and 1,1,0 cuts of the MgCl2 crystal.

During the development of the various catalyst generations (discussed in

Section 2.2.3), it was discovered that the addition of a Lewis base to the

heterogeneous catalysts resulted in an increase in catalytic activity and

stereospecificity. These Lewis bases subsequently became known as ‘internal donors’,

which were co-milled with the MgCl2 and TiCl4, and ‘external donors’ which were

combined with the cocatalyst [8]. The job of an internal donor such as ethyl benzoate,is to prevent the formation of atactic material by adsorbing onto the surface and

changing the aspecific site at the tetracoordinated Mg atom on the 1,1,0 plane to a

more isospecific site. An external donor, such as ethyl benzoate which can act as both

an internal and external electron donor, helps to prevent the extraction of the internal

donor as well as converting aspecific sites on the 1,0,0 crystal plane, as can be seen in

Figure 2.4.

Figure 2.4 Coordination of internal donors ensuring isospecific active sites.


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When dealing with copolymers, such as the propylene-ethylene random

copolymers used in this study, there is the added factor of the comonomer to be

considered when examining the polymerisation. The two different monomers have

completely different reactivity ratios [14]. The propylene reactivity ratio, r 1,

multiplied by the ethylene reactivity ratio, r 2, should be close to 1 for a random

copolymer (r 1r 2 ≈ 1). A blocky structure is present if r 1r 2 > 1 and an alternating

structure is present if r 1r 2 < 1 [8]. Ethylene monomer is far more reactive than

propylene monomer [14], although it is only present in very small amounts in the

random copolymers used in this study. Cheng and Kakugo [15] investigated ethylene-

propylene random copolymers and applied Bernoullian and first-order Markovian

models to the data they obtained, as well as the MIXCO.TRIAD and

MIXCO.TRIADX programs for analysis of the triad data. They found that due to the

heterogeneous catalyst, with three or four active catalytic sites, the polymer formed

was an in situ blend of three or four random copolymer components. It was also

observed that active sites with similarly high stereospecificities possess a broad

spectrum of reactivities towards the comonomer during a copolymerisation [8, 15]. It

is therefore clear that there are a number of factors that influence the nature of a

propylene-ethylene random copolymer during its polymerisation and that a

fractionation method is required for a full characterisation of the polymer.

2.2.3 The evolution of the transition metal catalysts

The development of the so-called Ziegler-Natta catalysts began around 1950

with Karl Ziegler’s work on the “Aufbau” reaction which involved the insertion of

ethylene into the Al-C bond of trialkyl aluminium and the subsequent growth of linearalkyl chains [1, 8]. It was in 1953 when the major breakthrough occurred with the

production of high-density polyethylene (HDPE) [8] in Ziegler’s laboratories, in

which Giulio Natta had placed three of his assistants. In the years following this

breakthrough, Ziegler and Natta, at the forefront of the polyolefin industry, were able

to make polypropylene and even define the stereo conformations of the

polypropylene. These original polypropylenes only contained up to approximately

40% isotactic material [8]. The development of the catalyst technology is best


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described by referring to the so-called generations of catalysts as they were

conceived.

The early work involving Ziegler-Natta catalysts involved a combination of

TiCl3 as the catalyst and AlEt2Cl as the cocatalyst. The productivity was relatively

low as was the isotacticity (around 90%). Removal of the atactic material as well as

the catalyst residues (a process known as de-ashing) was necessary [8]. It was

eventually realised that prolonged ball milling of TiCl3 and AlCl3 produced a more

active catalyst than pure TiCl3. This catalyst became known as AA-TiCl3 (Al-reduced

and activated) and is regarded as being the first generation of Ziegler-Natta catalysts.

One of the main problems associated with the first generation of catalyst was

the limited use of the titanium atoms as only those on the surface could take part in

the polymerisation. This led to the development of a second generation TiCl3 catalyst

with a much larger surface area [8], thus increasing the productivity and isotacticity.

De-ashing to remove catalyst residues and atactic material removal was still necessary

however.

Supported catalysts became known as the third generation and involved the

use of supports with functional groups onto which the TiCl4 could be attached. MgCl2

emerged as the main support used [5] and, with the aid of a Lewis base (benzoic acid

esters) acting as an internal and external electron donor, a highly active [9] and

stereospecific catalyst was born. The purpose of the electron donors is to aid in the

formation of highly isospecific active sites as well as to selectively poison the non-

stereospecific sites and convert them into isospecific sites [16]. The removal of atactic

material was still required though, and this led to further developments.

The fourth generation of catalysts (also known as super-active third generation

catalysts) was brought about by the use of alkylphthalates and alkoxysilanes as

internal and external electron donors respectively. There was thus a further

improvement in the catalyst performance, in terms of increased isotacticity and

productivity.

The latest development in the chain was the discovery that 1,3-diethers could

be used as internal electron donors, giving highly active sites and isotacticities

without an external Lewis base being required [16].

Homogeneous stereospecific catalysts gained importance when it was

discovered that metallocenes of Zirconium and Hafnium with methylaluminoxane


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(MAO) could be used to synthesise highly isotactic or syndiotactic polymers in very

high yields [8].

2.3 Commercial polypropylene

2.3.1 Varieties of polypropylene manufactured

Despite the fact that polypropylene already has such a huge number and

variety of applications, there are constantly more being developed. Due to the

competitiveness of the commercial polymer industry, companies are constantly

searching for new areas in which they can apply their products. This has led to

extensive research in the field of copolymerisation, including both new copolymers

and polymerisation conditions. One can differentiate between a statistical or random

copolymerisation and a sequential copolymerisation [8]. The versatility of

polypropylene is demonstrated when one looks at the different commercial types that

are manufactured, namely the homopolymer, random copolymers, and the so-called

impact copolymers.

2.3.1.1 Polypropylene homopolymer

The main structural factors that influence the properties of the polypropylene

homopolymer are tacticity, molecular weight and molecular weight distribution [17].

The different types of tacticity are illustrated in Figure 2.5.


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Figure 2.5 Types of polypropylene tacticity.

The main influence of the tacticity is on the crystallinity of the polymer.

Isotactic polypropylene homopolymer is highly crystalline. It has a correspondingly

high melting point of approximately 186°C, although this value has been a matter of

controversy for a number of years [18], and a Tg of approximately 0°C, which results

in a brittle polymer below this temperature [8]. The product is however extremely

versatile, which, coupled with the low monomer cost and efficient polymerisation

technology, makes it one of the most important commercial thermoplastics [19].

The homopolymer can be too rigid for certain applications. A lower melting point would improve weldability and improved impact resistance at low temperature

is also necessary for certain applications. Requirements such as this have led to the

development of the copolymers of polypropylene, tailor-made for specific

applications.

2.3.1.2 Impact copolymers

The fact that the polypropylene homopolymer has such a poor impact

resistance, especially at low temperature, has led to the development of the so-called

impact copolymers produced by means of a sequential polymerisation reaction. A

sequential polymerisation involves a two-stage process, whereby propylene is first

polymerised on its own, followed by a second stage where both propylene and

ethylene are polymerised in the presence of the originally polymerised material from

the first stage [8]. Two reactors are required, connected in series, for the production of

these heterophasic copolymers [20]. The rubber phase is usually an ethylene-


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propylene rubber although an ethylene-propylene-diene monomer elastomer is also

often used [21]. The result is an elastomeric poly(propylene-co-ethylene) copolymer

dispersed in a matrix of polypropylene homopolymer. These sequential

polymerisation reactions yield polymers with greatly improved impact strength [22];

hence they are often referred to as impact copolymers.

Various factors influence the performance of these copolymers, including the

amount of elastomer included in the polymer, the size of the rubber particles, the

chemical affinity of the elastomer for the polypropylene matrix, as well as the

distribution of the rubber particles [21, 23]. A homogeneous distribution of the rubber

particles provides the best dispersion of energy, giving the best stiffness-to-impact

balance [20]. An homogeneous distribution of the rubber particles is also necessary in

order to avoid reactor fouling [24]. An optimum rubber particle size often exists for a

given matrix/rubber system. The optimum size for the PP/EPR system is

approximately 0.4 μm [21]. The composition of the elastomer is also important [25];

for example, varying the ethylene/propylene ratio in an ethylene-propylene rubber can

have a large effect on the copolymers properties. A high propylene content would

result in poorer impact resistance, better interfacial adhesion and less shrinkage

stresses, due to the polypropylene crystallinity, than a rubber with a lower propylene

content [21]. Increasing the ethylene content would reduce the polypropylene

crystallinity while improving the polyethylene crystallinity, thereby improving impact

resistance up to a maximum value, after which the interfacial adhesion would

decrease too much, and reduce the impact strength [21]. The optimum ethylene

concentration for the best impact resistance is approximately 50 to 60 mol% [26].

2.3.1.3 Random copolymers

In order to harness the strength of polypropylene and to improve the properties

of the material for certain applications, it is necessary to reduce the crystallinity

slightly so as to improve properties such as flexibility and optical clarity [8]. This is

done by introducing a comonomer, such as ethylene or 1-butene, into the

polymerisation medium so as to create a discontinuity in the polymer chain,

disrupting the crystal structure of the polypropylene slightly, thereby altering the

morphology and structure in order to improve these properties [17]. The properties of

the copolymer are largely dependent on the amount of comonomer included as well as


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the distribution of the comonomer throughout the polymer [20]. Generally the

inclusion of the comonomer results in a reduction in the crystallisation rate, a lower

degree of crystallinity, and a lower melting point [17]. The lower melting point of the

copolymer is often required for the heat-sealable layer on a film, and it is the

comonomer content that has the greatest influence on the melting point. The crystals

produced are not as perfect as those of the homopolymer, which means that the

difference in the refractive index between the crystalline and amorphous areas is less.

Light is therefore not refracted as easily, resulting in lower haze and higher clarity for

the copolymer [17].

These propylene-ethylene random copolymers are synthesised using the so-

called statistical copolymerisation method, whereby the propylene is polymerised in

the presence of small quantities of ethylene [8] in a single reactor. The degree of

randomness of the polymerisation often varies due to factors such as the

polymerisation conditions, the catalyst system, and the reactivity ratio of the

comonomer relative to propylene [17]. The type of chains produced, or more

specifically the amount of extractables produced by a polymerisation system, is a

critical factor for food contact applications. For example, ethylene lowers the melting

point to a greater degree than a comonomer such as 1-butene but also produces a

higher level of extractables [17].

If the random copolymer is subjected to slow cooling from the melt then the

actual form of the crystals produced is altered. There are substantial amounts of the γ-

form crystals formed as well as the α-form. The specific crystal phase formed by

these random copolymers is discussed in more detail in the next section, as the

crystalline morphology (type of crystal) is of some importance in the application of

these materials.

2.3.2 Crystallinity types

With the crystallinity of a polymer being the key factor which influences the

physical properties of a polymer, information regarding the crystallinity becomes of

paramount importance when assessing a polymer’s applicability. The crystallinity of

polypropylene homopolymer is governed mainly by the tacticity of the chains [17].


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When investigating a propylene-ethylene random copolymer there is the added

factor of the comonomer to consider, as this also influences the degree of crystallinity.

The ethylene comonomer serves to reduce the crystallinity of the copolymer, thus

improving properties such as the flexibility and optical clarity [17]. It is possible to

look at the crystallinity of such a copolymer on various levels, from the skin-core

morphology on a visual scale to the spherulitic scale, lamellar scale, and finally the

crystallographic scale where the actual unit cell is examined [8, 27]. Polypropylene

has four crystal forms, namely the α-form (monoclinic), β-form (trigonal), γ-form

(orthorhombic) and a metastable mesomorphic form, often referred to as the smectic

form [27, 28]. The smectic form is formed by fast cooling of the polymer melt at low

temperatures [19] and represents a state of order intermediate between the amorphous

and crystalline states [29]. All forms of the crystal contain chains in the characteristic

31 helix conformation of polypropylene [28]. There are in fact two types of

monoclinic unit cells: the α1-form originally indexed by Natta and Corradini in the

C2/c space group, and the α2-form in the P21/c space group [29].

It is well known that an increase in the comonomer content increases the

number of ‘defects’ in the chains, thereby reducing the length of the isotactic

sequences [30, 31]. The amount of the γ-phase is proportional to the number of short

isotactic segments, caused by the interruption of the isotactic sequences by the

comonomer [20]. An increase in the comonomer (such as ethylene) content therefore

causes an increase in the growth of the γ-phase crystals [32]. A random terpolymer

with ethylene and 1-butene yields an even higher percentage of the γ-phase at the

same molar comonomer content as a similar copolymer [20]. The γ-phase is also

known to be enhanced by crystallisation at high pressures, low molecular weight, and

the presence of chain defects or chemical heterogeneity caused by atacticity [32, 33].

The α-phase is however the most stable and heating of the γ-phase results in

conversion to the α-phase [29]. The γ-phase has an epitaxial relationship with the α-

phase and either phase can grow onto the lamellae of the other phase [28, 32]. The γ-

phase crystals consist of bilayers in which the adjacent layers are at an angle of 80° to

each other as opposed to being parallel [29, 34, 35]. The presence of these non-

parallel chains in the crystal structure of γ-phase polypropylene is unique in

crystallisable synthetic polymers [29, 36]. The γ-phase also displays screwdislocations and nucleates on the α-form crystal on the (010) contact plane [37]. The


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γ-phase crystals are elongated in the b-axis direction and their chains are inclined at

an angle of 40° to the lamellar surface [38].

The importance of the crystal type comes into play when one considers the

applications of the polymer. The γ-phase crystal has a lower melting point than the α- phase, and also produces polymer with improved optical properties [17, 34, 39, 40].

2.4 Fractionation techniques

The necessity of a suitable fractionation technique has evolved from the need

to fully understand how the polymer chain architecture influences the physical

properties of the material. The use of fractionation enables one to obtain many

fractions of a much more narrow distribution than the unfractionated material, be it a

chemical composition or a molecular weight distribution. The three main techniques

used for fractionating a polymer are fractionation according to crystallisability,

molecular weight, and solubility. These shall be discussed separately in the following

sections.

2.4.1 Fractionation by crystallinity

Crystallinity is one of the most important characteristics of a polymer, greatly

influencing the physical properties, and is therefore a key basis for fractionating a

polymer. The fractionation reveals exactly how much material can crystallise and to

what extent. This information is vital for the development of new materials and

catalyst systems for subsequent better product performance.

2.4.1.1 Fractionation mechanism and crystallisationtheory

Fractionation of the propylene-ethylene random copolymer by temperature

rising elution fractionation (TREF) is based on separation according to

crystallisability [41-45]. In other words the actual molecular structure and

composition directly affects the ability of the chains to crystallise [43]. The longest

crystallisable isotactic sequence in the propylene-ethylene random copolymer will

therefore determine at what temperature the particular chain will crystallise. The


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effect of the comonomer upon the crystallisation and melting point of the copolymer

is complicated when examined at a molecular level [46]. The crystallisation and

melting point will be affected according to the degree to which the comonomer

disrupts the crystal lattice. The melting point of the copolymer will definitely be lower

than that of the homopolymer [46]. The chain ends and the diluent also contribute to

the lowering of the melting point [46]. An approximation for the depression of the

melting point has been given by Flory and is shown here as Equation (1).

( )211'0

11vv

V

V

H

R

T T f mm χ −⎟

⎠

⎞⎜⎝

⎛ ⎟⎟ ⎠

⎞⎜⎜⎝

⎛

Δ=− (1)

In this equation Tm0 represents the melting point of a perfect crystal, Tm is the melting

point of the polymer-diluent mixture, V and V’ are the molar volumes of the polymer

repeat unit and diluent respectively, R is the gas constant, χ is a polymer-solvent

interaction parameter, and ν is the volume fraction of diluent [41, 46]. According to

Flory if the non-crystallisable comonomer causes the depression of the melting point

then for a comonomer unit randomly distributed along a polymer chain the melting

point becomes:

A

f mm

N H

R

T T ln

11

0⎟⎟ ⎠

⎞⎜⎜⎝

⎛

Δ−=− (2)

where N A is the mole fraction of comonomer units in the random copolymer [41, 46].

It has been found that the degree of melting point depression is actually greater than

that predicted by the theory [41]. Shirayama, Kita, and Watabe [47] discovered an

almost linear relationship between the melting point and the percentage ofcomonomer. Zhang, Wu, and Zu [48] assumed that the melting temperature of a

copolymer, Tm, is close to that of the homopolymer, Tm0, such that Tm x Tm

0 ≈ (Tm0)2,

and that ΔH is constant in that temperature range. They thus reduced Equation (1) to

Equation (3) and obtained a relationship between melting temperature and

comonomer content.

( ) E mmm X H T RT T Δ−≅20

0 (3)


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The effect of molecular weight on the fractionation was also considered by Wild [49].

The data obtained by Wild indicated that if the polymer chain ends are considered to

be the equivalent of a branch point then the molecular weight dependence on the

fractionation mostly disappears. They also showed that the molecular weight

dependence falls away as soon as the molecular weight reaches approximately 104

g/mol. Zhang, Wu, and Zu [48] also noted that there were two types of chains with a

low melting point, those with a low molecular weight and those with a high

comonomer content (ethylene in their case). They also noted that for a chain to have a

high melting point it must have a high molecular weight as well as a low ethylene

content. It is therefore clear that although molecular weight effects cannot be ignored,

the fractionation of a copolymer such as the propylene-ethylene random copolymers

is dependant on the ability of the chains to crystallise.

The two main techniques that are used to fractionate semi-crystalline polymers

according to crystallisability are temperature rising elution fractionation (TREF) and

crystallisability analysis fractionation (CRYSTAF).

2.4.1.2 TREFThe ability to fully characterise a polymer material in order to fully understand

where it gets its macroscopic properties from has been the goal of many researchers

over the past fifty years. Much of the early work in this field was focused on ways to

establish molecular weight distributions. Desreux and Spiegels [50] were the first to

realise that a semi-crystalline polymer could be fractionated according to solubility at

a given temperature, and that this fractionation was based on the ability of the

polymer to crystallise and not simply on its molecular weight. Their pioneering work

involved the elution of fractions of polyethylene at successively higher temperatures.

Further development and refinement occurred in the field, but it was not until

Shirayama et al. [51] described the method of fractionating low density polyethylene

according to the degree of short chain branching that the term “temperature rising

elution fractionation” was born. At this time See and Smith [52] were investigating

the effect of different solvent/non-solvent mixtures of varying compositions on the

elution of linear polyethylene and isotactic polypropylene. Their experimental setup

was essentially the same as that used for TREF, with the exception that they


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maintained a constant elution temperature and varied the strength of the eluting

solvent. This was similar to the work of Guillet et al. [53] on polyethylene. With the

development of size exclusion chromatography as an excellent method for

determining molecular weight distributions, fractionation according to crystallisability

became the new area of interest.

The development of the TREF experimental setup occurred for very practical

reasons. It is far easier to dissolve polymer off a support than it is to collect fractions

which crystallise at successively lower temperatures. The TREF technique separates

material on the basis of molecular structure or composition [41]. Changes at a

molecular level influence the crystallisability of the chains and therefore the solubility

at a given temperature. The general TREF technique can be divided into two main

steps, namely a crystallisation step and an elution step.

During the crystallisation step, the semi-crystalline polymer that is being

analysed is first dissolved at high temperature, and then allowed to cool slowly under

the control of a programmed temperature profile. According to Wild [41] the

maximum cooling rate that should be used for achieving a good separation is

2°C/hour. Various media have been utilised for the crystallisation step of TREF with

the most common being a temperature controlled oil bath [49, 54]. Alternatives do

exist such as the oven from a GPC setup [55], although in this case heat transfer is not

as good. One advantage of an oven however is the decreased cycle rotation time due

to the fact that the oven can be cooled far quicker than an oil bath, in preparation for

the next fractionation [41]. Problems associated with temperature gradients in the

column as well as poor heat transfer have been noted by Wild [41]. A single medium

can be used for both the crystallisation and elution steps as in the setup of Bergstrom

and Avela [56] and Nakano and Goto [57]. It is often the case that two separate media

are used [49, 54], enabling the simultaneous crystallisation of a number of samples,

seeing as this is the time-limiting step of TREF [41]. The operations utilising a

separate step usually use an oil bath for the crystallisation step followed by either

another oil bath or an oven for the elution step. The importance of the crystallisation

step was not fully recognised at first, although it gradually gained importance as it

was eventually recognised as the critical step necessary to obtain good reproducible

separations [45]. The cooling step can either be done in the presence of a support [48,

49, 55, 58], or simply in solution [54, 59, 60], which is then later slurried with a


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support before the elution step. The addition of 0.1% of an antioxidant is advised in

order to prevent polymer degradation [61].

During the elution step the polymer is dissolved off the support at successively

higher temperatures. Columns thus became an integral part of the experimental setup

as they provided a simple medium in which to perform the fractionation. Initially

constructed from glass [62, 63] and later from stainless steel [64, 65], the columns

developed were of many different sizes. There are a few good reviews in the literature

that cover all aspects of TREF [41, 42, 44, 45, 61, 66].

As the experimental techniques were improved and refined a distinction could

be drawn between the technique involving an on-line detector for continuous signal

detection (analytical TREF), and the technique involving the collection of much larger

fractions for subsequent offline analysis (preparative TREF). These techniques are

now discussed separately.

Analytical TREF (A-TREF)

Analytical TREF is a relatively recent development in the experimental setup

of TREF, with workers such as Usami, Gotoh, and Takayama [55] being among the

first to describe their systems in detail. Analytical TREF involves the same slow,

controlled crystallisation step as in the preparative version of the fractionation, but

instead of collecting the fractions for offline analysis the eluent is sent to an RI/IR

detector which constantly monitors the polymer being eluted. Recently the trend has

been to use an IR detector set at 3.41 μm (C-H stretch), as this presents less of a

problem when compared to an RI detector, with respect to with baseline noise [43,

45], due to the relative insensitivity of IR to temperature fluctuations [41, 42]. Table

2.1 contains a detailed list of various analytical TREF systems and their

corresponding variables that have been utilised recently.

Table 2.1 Recent work carried out in the field of analytical TREF

Polymer

type

Sample

size

(mg)

Support

materialSolvent

Cooling

rate

(°C/h)

Heating

rate

Flow

rate

(ml/min)

Reference


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Polymer

type

Sample

size

(mg)

Support

materialSolvent

Cooling

rate

(°C/h)

Heating

rate

Flow

rate

(ml/min)

Reference

LLDPE - - o-DCB 5 4°C/min - [60]HDPE/EVA - - - 5 4°C/min - [60]

PP-co-PE 2-5Diatomaceous

earthXylene 5 4°C/min 3 [54]

LLDPE 2 - o-DCB 1 [55]

LLDPE - - TCB0.05 -

0.50.1 - 1 0.2 [67]

LLDPE - Diatomaceousearth

Xylene 5 4°C/min 3 [68]

HDPE -Diatomaceous

earthXylene 5 4°C/min 3 [68]

LLDPE 2Diatomaceous

earthTCB 5.6 4°C/min 3 [65]

PE - Glass beads TCB 1 20°C/h 1 [69]

C104H210 - Glass beads TCB 1 20°C/h 1 [69]LLDPE 100 Chromosorb-P TCB 1.5 20 4 [49]

LLDPE - Chromosorb-P TCB 1.5 20 2 [58]

LDPE - Chromosorb-P TCB 1.5 20 2 [58]

HDPE - Chromosorb-P TCB 1.5 20 2 [58]

LLDPE - Glass beads o-DCB 1.5 1 1 [70]

LDPE - Chromosorb-P o-DCB 1.5 20 0.3 [71]

Analytical TREF development has advanced quite prodigiously in recent years due to

the possibility for automation, reducing the manpower required to obtain results.

Preparative TREF (P-TREF)

Similar to analytical TREF in many ways, the preparative variation of the

technique is a means to obtain the greatest amount of information regarding the

composition of a semi-crystalline polymer. There is less possibility for automation in


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this case however, as the elution is step-wise, requiring the collection of fractions at

successive temperatures as designated by the operator. These fractions are then

isolated from the eluent and sent for further offline analysis by 13C nuclear magnetic

resonance spectroscopy (NMR), differential scanning calorimetry (DSC), wide-angle

x-ray diffraction (WAXD), gel permeation chromatography (GPC), and CRYSTAF to

name but a few. This allows a complete molecular picture to be drawn up regarding

the polymer under investigation. Larger sample sizes are necessary for P-TREF, as

sufficient material must be collected at each fractionation temperature in order to

perform further analysis. Table 2.2 summarises the preparative TREF systems

recently described in the literature and their corresponding variables.

Table 2.2 Recent work carried out in the field of preparative TREF

Polymer

type

Sample

size (g)

Support

materialSolvent

Cooling

rate

(°C/h)

Analysis of

fractionsReference

LLDPE 8 Chromosorb-P Xylene 5 - [59]

LDPE 8 Chromosorb-P Xylene 5 - [59]

HDPE 8 Chromosorb-P Xylene 5 - [59]

PP 8 Chromosorb-P Xylene 5 - [59]

VLDPE 8 Chromosorb-P Xylene 5 - [59]

PP-co-PE 4Diatomaceous

earthXylene 5 NMR, GPC [54]

LLDPE 2.5 - o-DCB - NMR, DSC,

FTIR[55]

PP-co-PE 2 Sea sand Xylene 1.5 DSC [64]

PP-co-PE 15 Glass beads TMB ±1.7

CRYSTAF,

NMR, FTIR,

GPC, DSC

[48]

PP 15 Glass beads TMB ±1.7

CRYSTAF,

NMR, FTIR,

GPC, DSC

[48]

PP 1 Sea sand Xylene 1.5 NMR [72]


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25

Polymer

type

Sample

size (g)

Support

materialSolvent

Cooling

rate

(°C/h)

Analysis of

fractionsReference

PP-co-PE 1 Sea sand Xylene - NMR, FTIR,DSC

[73]

PP-co-

PE/B15 Glass beads TMB ±1.15

NMR, GPC,

CRYSTAF,

DSC

[62]

LLDPE 4 Chromosorb-P Xylene 1.5 SEC [49]

LDPE 4 Chromosorb-P Xylene 1 IR, DSC [56]

LDPE 3.5 Glass beads o-DCB 100 GPC [57]HDPE 3.5 Glass beads o-DCB 100 GPC [57]

LLDPE - Chromosorb-P TCB 1.5IR, NMR,

GPC, DSC[58]

LDPE - Chromosorb-P TCB 1.5IR, NMR,

GPC, DSC[58]

HDPE - Chromosorb-P TCB 1.5IR, NMR,

GPC, DSC[58]

LLDPE Glass beads o-DCB 1.5GPC, DSC,

IR[70]

ULDPE - Chromosorb-P TCB 1.5 NMR. DSC [74]

PP-co-

EPR- Ballotini Xylene 5

DSC, GPC,

FTIR[75]

PP 10 Sea sand Xylene - NMR, DSC [76]

PP-co-PE Sea sand Xylene 1.5 NMR, DSC [77]

PP-co-PE 3-5 Silica sand Xylene 6.5

NMR, FTIR,

WAXD,

DSC

[63]

Development of preparative TREF has now reached a point where the quality

of the fractionation is more important than the convenience of the technique [41]. It is

better to spend a little more time on the fractionation if this provides a better picture

of the molecular co

Documents

The Fractionation and Characterisation of Propylene-ethylene Random Copolymers