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
illustrating the weight average molecular weight and polydispersity of the fractions.62
Figure 4.17 HT-GPC molecular weight results for the fractions of sample C
illustrating the weight average molecular weight and polydispersity of the fractions.63
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.2 CRYSTAF results for the 45°C fraction of sample A (A2).......................85
Figure C.3 CRYSTAF results for the 65°C fraction of sample A (A3).......................86
Figure C.4 CRYSTAF results for the 75°C fraction of sample A (A4).......................86
Figure C.5 CRYSTAF results for the 80°C fraction of sample A (A5).......................87
Figure C.6 CRYSTAF results for the 85°C fraction of sample A (A6).......................87
Figure C.7 CRYSTAF results for the 90°C fraction of sample A (A7).......................88
Figure C.8 CRYSTAF results for the 95°C fraction of sample A (A8).......................88
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.11 CRYSTAF results for the 110°C fraction of sample A (A11).................90
Figure C.12 CRYSTAF results for the 115°C fraction of sample A (A12).................90
Figure C.13 CRYSTAF results for the 120°C fraction of sample A (A13).................91
Figure C.14 CRYSTAF results for the 125°C fraction of sample A (A14).................91
Figure C.15 CRYSTAF results for the 140°C fraction of sample A (A15).................92
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.19 CRYSTAF results for the 75°C fraction of sample B (B4). ....................94
Figure C.20 CRYSTAF results for the 80°C fraction of sample B (B5). ....................94
Figure C.21 CRYSTAF results for the 85°C fraction of sample B (B6). ....................95
Figure C.22 CRYSTAF results for the 90°C fraction of sample B (B7). ....................95
Figure C.23 CRYSTAF results for the 95°C fraction of sample B (B8). ....................96
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.26 CRYSTAF results for the 110°C fraction of sample B (B11). ................97
Figure C.27 CRYSTAF results for the 115°C fraction of sample B (B12). ................98
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.30 CRYSTAF results for the 140°C fraction of sample B (B15). ................99
Figure C.31 CRYSTAF results for the 25°C fraction of sample C (C1). ..................100
Figure C.32 CRYSTAF results for the 45°C fraction of sample C (C2). ..................100
Figure C.33 CRYSTAF results for the 65°C fraction of sample C (C3). ..................101
Figure C.34 CRYSTAF results for the 75°C fraction of sample C (C4). ..................101
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.37 CRYSTAF results for the 90°C fraction of sample C (C7). ..................103
Figure C.38 CRYSTAF results for the 95°C fraction of sample C (C8). ..................103
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 C.41 CRYSTAF results for the 110°C fraction of sample C (C11). ..............105
Figure C.42 CRYSTAF results for the 115°C fraction of sample C (C12). ..............105
Figure C.43 CRYSTAF results for the 120°C fraction of sample C (C13). ..............106
Figure C.44 CRYSTAF results for the 125°C fraction of sample C (C14). ..............106
Figure C.45 CRYSTAF results for the 140°C fraction of sample C (C15). ..............107
Figure D.1 DSC data for the 25°C fraction of sample A (A1). .................................108
Figure D.2 DSC data for the 45°C fraction of sample A (A2). .................................108
Figure D.3 DSC data for the 65°C fraction of sample A (A3). .................................109
Figure D.4 DSC data for the 75°C fraction of sample A (A4). .................................109
Figure D.5 DSC data for the 80°C fraction of sample A (A5). .................................110
Figure D.6 DSC data for the 85°C fraction of sample A (A6). .................................110
Figure D.7 DSC data for the 90°C fraction of sample A (A7). .................................111
Figure D.8 DSC data for the 95°C fraction of sample A (A8). .................................111
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.11 DSC data for the 110°C fraction of sample A (A11). ...........................113
Figure D.12 DSC data for the 115°C fraction of sample A (A12). ...........................113
Figure D.13 DSC data for the 120°C fraction of sample A (A13). ...........................114
Figure D.14 DSC data for the 125°C fraction of sample A (A14). ...........................114
Figure D.15 DSC data for the 140°C fraction of sample A (A15). ...........................115
Figure D.16 DSC data for the 25°C fraction of sample B (B1).................................115
Figure D.17 DSC data for the 45°C fraction of sample B (B2).................................116
Figure D.18 DSC data for the 65°C fraction of sample B (B3).................................116
Figure D.19 DSC data for the 75°C fraction of sample B (B4).................................117
Figure D.20 DSC data for the 80°C fraction of sample B (B5).................................117
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.23 DSC data for the 95°C fraction of sample B (B8).................................119
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.26 DSC data for the 110°C fraction of sample B (B11).............................120
Figure D.27 DSC data for the 115°C fraction of sample B (B12).............................121
Figure D.28 DSC data for the 120°C fraction of sample B (B13).............................121
Figure D.29 DSC data for the 125°C fraction of sample B (B14).............................122
Figure D.30 DSC data for the 140°C fraction of sample B (B15).............................122
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.34 DSC data for the 75°C fraction of sample C (C4).................................124
Figure D.35 DSC data for the 80°C fraction of sample C (C5).................................125
Figure D.36 DSC data for the 85°C fraction of sample C (C6).................................125
Figure D.37 DSC data for the 90°C fraction of sample C (C7).................................126
Figure D.38 DSC data for the 95°C fraction of sample C (C8).................................126
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 D.42 DSC data for the 115°C fraction of sample C (C12).............................128
Figure D.43 DSC data for the 120°C fraction of sample C (C13).............................129
Figure D.44 DSC data for the 125°C fraction of sample C (C14).............................129
Figure D.45 DSC data for the 140°C fraction of sample C (C15).............................130
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
Figure F.2 DSC melting endotherms of the first and second heating cycles of sample
B of the samples which were slow cooled for WAXD analysis................................133
Figure F.3 DSC melting endotherms of the first and second heating cycles of sample
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
Figure F.5 DSC melting endotherms of the first and second heating cycles of fraction
A9 of the sample which was slow cooled for WAXD analysis.................................135
Figure F.6 DSC melting endotherms of the first and second heating cycles of fraction
A11 of the sample which was slow cooled for WAXD analysis...............................135
Figure F.7 DSC melting endotherms of the first and second heating cycles of fraction
B6 of the sample which was slow cooled for WAXD analysis. ................................136
Figure F.8 DSC melting endotherms of the first and second heating cycles of fraction
B8 of the sample which was slow cooled for WAXD analysis. ................................136
Figure F.9 DSC melting endotherms of the first and second heating cycles of fraction
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
Figure F.11 DSC melting endotherms of the first and second heating cycles of fraction
C9 of the sample which was slow cooled for WAXD analysis. ................................138
Figure F.12 DSC melting endotherms of the first and second heating cycles of fraction
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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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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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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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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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
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