Chromatography Mass Spectroscopy in Polymer Analysis [1 ed.] 1847354823, 184735484X, 9781847354822, 9781847354846

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Chromatography Mass Spectroscopy in Polymer Analysis T.R. Crompton

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.ismithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder.

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ISBN: 978-1-84735-4822 (Hardback) 978-1-84735-4839 (Softback) 978-1-84735-4846 (ebook)

Typeset by Argil Services Indexed by Argil Services Printed and bound by Lightning Source

P

reface

A technique which is extremely powerful for the analysis and characterisation of polymers is one based on the use of controlled chromatography–mass spectroscopy of polymer decomposition techniques such as pyrolysis, followed by chromatography to separate any breakdown product, and, finally, mass spectroscopy, to achieve an unequivocal identification of such pyrolysis products to assist in the elucidation of details of the polymer structure and composition. Detail that can be obtained by such methods includes structural detail of the polymer backbone, branching, end groups, isomeric detail and fine detail in the structure of copolymers. This is a very interesting area of activity which is being increasingly used by polymer chemists in their attempts to improve existing polymers and to discover new ones with specific physical properties such as thermal stability and retention of properties over a long service life. The first three chapters discuss the various chromatographic and mass spectroscopic techniques now available. In Chapters 3–8 complementary methods, based on the combination of mass spectroscopy with various chromatographic techniques such as high performance liquid chromatography, gas chromatography and supercritical fluid chromatography are discussed. Pyrolysis chromatography–mass spectroscopy is a method of studying the structure of polymers that involves subjecting the polymer pyrolysis products to a chromatographic technique to simplify subsequent analysis and, finally mass spectroscopy to identify the pyrolysis products with the possibility of deducing finer details of polymer structure than were previously attainable by classical methods (Chapters 9–11). In addition to polymer structural details such techniques are proving very useful in polymer decomposition studies, also in the determination of monomers, oligomers and additives.

iii

Chromatography Mass Spectroscopy in Polymer Analysis By providing a thorough up to date review of work in this field it is hoped that the book will be of interest to those engaged in polymer research and development, and those who manufacture products from polymers. Roy Crompton November 2010

iv

C

ontents

1

Chromatographic Techniques ...................................................... 1 1.1

Gas Chromatography ........................................................ 1

1.2

High Performance Liquid Chromatography ....................... 1 1.2.1

Post-column Derivatisation: Fluorescence Detectors ................................................................ 4

1.2.2

Diode Array Detectors ........................................... 4

1.2.3

Electrochemical Detectors ...................................... 4

1.2.4 1.3

1.4

1.2.3.1

The determination of Monomers ............ 4

1.2.3.2

Determination of Oligomers .................... 7

Fractionation/Microstructure Studies ..................... 7

Size Exclusion Chromatography ...................................... 10 1.3.1

Characterisation Studies ....................................... 10

1.3.2

Branching ............................................................. 11

1.3.3

Compositional Analysis ........................................ 11

1.3.4

Molecular Weight ................................................. 11

1.3.5

Polymer Blends ..................................................... 11

1.3.6

Polymer Additives ................................................ 12

Supercritical Fluid Chromatography ................................ 13 1.4.1

Polymer Additives ................................................ 15

1.5

Thin Layer Chromatography ........................................... 16

1.6

Thermal Field Flow Fractionation ................................... 16

v

Chromatography Mass Spectroscopy in Polymer Analysis

2

Mass Spectroscopic Techniques ................................................. 29 2.1

2.2

Time-of-Flight – Secondary Ion Mass Spectroscopy ......... 29 2.1.1

Adhesion Studies .................................................. 31

2.1.2

Polymer Interface Studies ..................................... 31

2.1.3

Vulcanisation Studies............................................ 31

Matrix Assisted Laser Desorption Ionisation Mass Spectroscopy .................................................................... 32 2.2.1

3

2.3

Matrix Assisted Laser Desorption Ionisation Post Source Decay ................................................................... 41

2.4

Electrospray Ionisation Mass Spectroscopy ..................... 41

2.5

Field Desorption Mass Spectroscopy ............................... 44

2.6

Tandem Mass Spectroscopy ............................................. 44

2.7

Fourier-transform Ion Cyclotron Mass Spectroscopy ....... 45

2.8

Fast Atom Bombardment Mass Spectroscopy .................. 47

2.9

Radio Frequency and Glow Discharge – Mass Spectroscopy .................................................................... 48

Chemical Reaction Gas Chromatography .................................. 61 3.1

4

vi

Applications ......................................................... 34

Applications .................................................................... 61 3.1.1

Saponification Procedures .................................... 61

3.1.2

Zeisel Procedures ................................................. 62

3.1.3

Alkali Fusion ........................................................ 63

3.1.4

Reactive Hydrolysis – Methylation – Pyrolysis – Chromatography .................................................. 65

Complementary High Performance Liquid Chromatography – Mass Spectroscopy .................................................................... 73 4.1

Theory ............................................................................. 73

4.1

Applications .................................................................... 78

Contents

5

4.1.1

Polymer Characterisation ..................................... 78

4.1.2

Polymer Extractables ............................................ 82

4.1.3

Determination of Polymer Additives..................... 86

4.1.4

High Performance Liquid Chromatography – Infrared Spectroscopy ........................................... 87

Complementary Size Exclusion Chromatography – Mass Spectroscopy .................................................................... 91 5.1

Applications .................................................................... 92 5.1.1

Molecular Weight ................................................. 92 5.1.1.1

Polyesters............................................... 92

5.1.1.2

Poly(N-methyl Perfluoro – octylsulfonamido Ethyl Acrylate)........... 98

5.1.1.3

Polymethylmethacrylate....................... 100

5.1.1.4

2-Benzothiozolon-3-yl Acetic Acid-telechelic Polyethylene Oxides (PEG Esters) ........................................ 100

5.1.1.5

Polyesters............................................. 101

5.1.1.6

Polyethers ............................................ 107

5.1.1.7

Hydrocarbon Types ............................. 111

5.1.1.8

Nitrogen Containing Polymers ............ 112

5.1.1.9

Silicon Containing Polymers ................ 113

5.1.1.10 Miscellaneous Polymers ....................... 115

6

5.2

Polymer Degradation Studies ......................................... 115

5.3

End-group Analysis ....................................................... 116

Complementary Gas Chromatography – Mass Spectroscopy ... 125 6.1

Applications .................................................................. 125 6.1.1

Polymer Characterisation ................................... 125 6.1.1.1

Sulfur Containing Polymers ................. 125

vii

Chromatography Mass Spectroscopy in Polymer Analysis

6.1.2

6.1.1.2

3-Glycidoxyproply-tri-methoxysilane sols ...................................................... 126

6.1.1.3

Fluorine Containing Polymers ............. 127

Polymer Degradation Studies ............................. 128 6.1.2.1

Low Molecular Weight Compounds or Degradation Products .......................... 128

6.1.2.2

Molar Mass Changes during Degradation Analysed by Size Exclusion Chromatography and/or Matrix Assisted Laser Desorption Ionisation ............................................ 129

6.1.2.3

Polybutylene Adipate and Polybutylene Succinate ........................ 131

6.1.2.4

Rubbers ............................................... 131

6.1.2.5

Polystyrene Peroxide............................ 132

6.1.2.6

Polypropylene Hydroperoxides............ 134

6.1.2.7

Polystyrene .......................................... 135

6.1.2.8

Polyethylene Oxide – Polypropylene Oxide Copolymers............................... 141

6.1.3

Food Packaging Applications ............................. 148

6.1.4

Miscellaneous Polymers ..................................... 149

7

Complementary Supercritical Fluid Chromatography – Mass Spectroscopy .................................................................. 159

8

Headspace Analysis – Mass Spectroscopy ................................ 165

9

Pyrolysis Gas Chromatography – Mass Spectroscopy .............. 171 9.1

Applications .................................................................. 172 9.1.1

viii

Polyolefins .......................................................... 172 9.1.1.1

Polyolefin Homopolymers ................... 172

9.1.1.2

Polypropylene Carbonate .................... 172

Contents

9.1.1.3

Polyolefin Copolymers ......................... 176

9.1.1.4

Polystyrenes ......................................... 176

9.1.1.5

Polyesters............................................. 184

9.1.1.6

Chlorine Containing Polymers ............. 186

9.1.1.7

Rubbers ............................................... 187

9.1.1.9

Nitrogen Containing Polymers ............ 194

9.1.1.10 Sulfur Containing Polymers ................. 200 9.1.1.11 Silicon Containing Polymers ................ 207 9.2

Polymer Additives .......................................................... 207

9.3

Miscellaneous ................................................................ 209 9.3.1

Py-GC-MS Methods ........................................... 209

9.3.2

Direct Pyrolysis – Gas Chromatography without Intervening Chromatographic Stage ...... 210

Abbreviations .................................................................................... 219 Index ............................................................................................... 225

ix

Chromatography Mass Spectroscopy in Polymer Analysis

x

1

Chromatographic Techniques

A variety of chromatographic techniques have been used for the examination of polymers, polymer volatiles, monomers, oligomers and volatile additives prior to their unequivocal identification in minute amounts by mass spectroscopy (MS) which is reviewed in Chapter 2. Regarding to the parent polymers themselves. Whilst gas chromatography (GC) is amenable for only some very low molecular weight (Mw) polymers (see Chapter 6), other techniques such as high performance liquid chromatography (HPLC), and, particularly, size exclusion chromatography (SEC) are applicable to the separation and/or fractionation of polymers prior to the detailed determination of their nature, amount and microstructure by MS. Another technique applicable to polymers is that based on controlled pyrolysis. Pyrolysis followed by GC, or liquid chromatography (LC) - MS provides much useful information on the composition and microstructure of polymers. Various aspects of chromatography–MS are discussed in Chapters 3-9. The various types of chromatographic techniques that have been used in conjunction with MS are now discussed.

1.1 Gas Chromatography For further discussion see Chapter 6.

1.2 High Performance Liquid Chromatography Modern HPLC has been developed to a very high level of performance by the introduction of selective stationary phases of small particle sizes, resulting in efficient columns with large plate numbers per metre. There are several types of chromatographic columns used in HPLC [1-5].

1

Chromatography Mass Spectroscopy in Polymer Analysis The most commonly used chromatography mode in HPLC is reversed-phase chromatography. Reversed-phase chromatography is used for the analysis of a wide range of neutral and polar organic compounds. Most commonly, reversed-phase chromatography is performed using bonded silica-based columns, thus inherently limiting the operating pH range to 2.0-7.5. The wide pH range (0–14) of some columns (e.g., Dionex Ion Pac NSI and NS 1-5 µm columns) removes this limitation, and consequently they are ideally suited for non-pairing and ion-suppression reversedphase chromatography: the two techniques that have helped extend reverse-phase chromatography to detection of ionisable compounds. Typically, reversed-phase ion-pairing chromatography is carried out using the same stationary phase as reversed-phase chromatography. A hydrophobic ion of opposite charge to the solute of interest is added to the mobile phase. Samples that are determined by reverse-phase ion-packing chromatography are ionic and thus capable of forming an ion pair with the added counter ion. This form of reversed-phase chromatography can be used for anion and cation separations and for the separation of surfactants and other ionic types of organic molecules. Ion suppression is a technique used to suppress the ionisation of compounds (such as carboxylic acids) so they will be retained exclusively by the reversed-phase retention mechanism and chromatographed as the neutral species. For further discussion see Majors [6]. Four basic types of elution are used in HPLC, namely, the isocratic system, the basic gradient system, the inert system and the advanced gradient system (see Figure 1.1). The most commonly used detectors are those based on spectroscopy in the region 185400 nm, visible-ultraviolet (UV) spectroscopy in the region 185-900 nm, post-column derivatisation with fluorescence detection (see next), conductivity [7] and multiple wavelength UV detectors using a diode array system detector (see next). Other types of detectors available are those based on electrochemical principles, refractive index, differential viscosity, and mass detection [8].

Column

(a)

Detector

Pump

Sample injector

2

Recorder

Chromatographic Techniques

(b) Column Detector

Pump

Solvent conditioner

Sample injector

Integrator (c) Column oven Sample injector Solvent conditioner Mixer

Detector

A/D interface

Controller

(d)

Printer PC

Pump

Controller Detector Column

Mixing valve

Mixer

Pump

Sample injector Printer/ plotter

Fraction collector

Figure 1.1 Elution systems supplied by LKB, Sweden: (a) isocratic bioseparation system; (b) basic system; (c) advanced chromatography system; and (d) inert system. Reproduced with permission from LKB, Sweden. 3

Chromatography Mass Spectroscopy in Polymer Analysis

1.2.1 Post-column Derivatisation: Fluorescence Detectors The development of HPLC equipment has been built upon the achievements in column technology, but the weakest part is still the detection system. UV-visible and fluorescence detectors offer tremendous possibilities, but because of their specificity it is possible to detect components only at very low concentrations when using a specific chromophore or fluorophore. The lack of a sensitive all-purpose detector in LC like the flame ionisation detector (FID) in GC is still disadvantageous for LC for the detection of important groups of compounds. Consequently, chemical methods are increasingly used to enhance the sensitivity of detection. On-line post-column derivatisation started with the classic work of Spackman and co-workers [9] and has recently been of increasing interest and use [10]. With on-line post-column detection the complexity of the chromatographic equipment increases. An additional pump is required for the pulseless and constant delivery of the reagent.

1.2.2 Diode Array Detectors With the aid of a high-resolution UV diode array detector, the eluting components in a chromatogram can be characterised on the basis of their UV spectra. The detector features high spectral resolution (comparable to that of a high-performance UV spectrophotometer) and high spectral sensitivity. The high spectral sensitivity permits the identification of spectra near the detection limit, i.e., within the submilliabsorbance range. In the polychromator incorporated in the Perkin Elmer LC 480 diode array system the light beam is dispersed within the range 190–430 nm onto a diode array consisting of 240 light-sensitive elements. This affects a digital resolution of 1 nm, which thus satisfies the spectral resolution determined by the entrance slit.

1.2.3 Electrochemical Detectors Organic compounds, anions, and cations can be detected by electrochemical means. Majors [6] gives a very extensive review of HPLC columns now available and lists their characteristics in considerable detail also suppliers. Even before its use in conjunction with MS, HPLC found many applications in polymer analysis including the following.

1.2.3.1 The determination of Monomers [11-13, 15-17] HPLC using the reverse phase mode, has been used [12, 13] to determine acrylamide

4

Chromatographic Techniques monomer and related compounds, including methacrylonitrile, in polyacrylamide. By employing a low-wavelength UV detector, these compounds can be measured with high sensitivity. The relative precision of the 95% confidence level for acrylamide is ±7.5%. The retention times for acrylamide and related compounds are given in Table 1.1. No known impurities are observed at the retention time of acrylamide. Mourey and co-workers [15, 16] separated n-butyl lithium-polymerised polystyrene (PS) standards (general structure CH3(CH2)2(CH2CHPh)nCH2CHPh) with Mw values of 800, 2100, and 4800 anionically on silica gel with 1:3 v/v n-hexane–tetrahydrofuran (THF) or n-hexane–ethyl acetate or n-hexane–dichloromethane (Figure 1.2). THF and ethyl acetate gave separations according to the number of oligomer units, and dichloromethane separated the stereoisomers of individual oligomers.

Table 1.1 HPLC retention times* for acrylamide and related compounds Compound

Retention time (min)

Acrylic acid

1.4

β-Hydroxypropanamide

2.1

Aceatamide

3.0

Acrylamide

5.4

Propanamide

7.3

Acrylonitrile

11.8

Methacrylamide

18.0

Butanamide

20.8

Methacrylonitrile

46.0

*: Partisil-10 ODS-2, water: 2.0 ml/min, 208 nm Reproduced with permission from T.R. Crompton in Polymer Reference Book, Rapra Technology Limited, Shrewsbury, UK, 2006, p.201. ©2006, Rapra Technology Limited [14]

GC methods for determining unreacted terephthalic acid, ethylene glycol, and mono(2-hydroxyethyl) terephthalate (MHET) (Figure 1.3) are capable of determining only about 25% of the total prepolymer sample, as the higher Mw oligomers are not resolved. HPLC on a DuPont model 530 LC with (100:1 v/v) chloroform:reagent

5

Chromatography Mass Spectroscopy in Polymer Analysis alcohol eluent using a DuPont Zorbax SIL column and a 254 mm UV detector gives good separations of the seven polyethylene terephthalate (PET) prepolymer oligomers. The bis(2-hydroxyethyl) terephthalate (BHET) content determined by GC is used as the known standard for the HPLC separation. All chromatograms were run at room temperature. The solvent system was chloroform (1250 parts by volume) and reagent alcohol (12 parts by volume). The reagent alcohol consisted of 90 parts ethanol and 5 parts each of methanol and isopropanol. Approximately 15 mg of sample were placed in a three dram vial, and 5 ml chloroform/alcohol (9:1 v/v) were added. The sample was stirred on a magnetic stirrer until dissolved.

Mobile phase: Column: Flow Rate: Sample:

THF PE PLgel 5µm mix - bed C 1mL/minute NBS Standard 705 Polystyrene

Precision of MW calculations (20 runs)

Recorder response

MW 0.76%CV MN 0.72%CV MZ 0.78%CV

Eight replicate runs 5.2

5.6

6.0 Time (minutes)

6.4

6.8

Figure 1.2 Curves of n-butyl lithium polymerised curves of PS standards on Perkin Elmer PL-Gel mixed gel column. Reproduced with permission from Perkin Elmer.

6

Chromatographic Techniques

HOOC

COOH

+

HOCH2CH2OH

HOOC

COOCH2CH2OH

HOCH2CH2

MHET mono(2-hydroxyethyl) terephthalate

OCO

COOCH2CH2OH

BHET bis(2-hydroxyethyl) terephthalate dimer also trimer, tetramer, pentamer, hexamer and heptamer

Figure 1.3 Formation of MHET and BHET from ethylene glycol and terephthalic acid. Reproduced with permission from T.H. Mourey, G. Smith and L.R. Snyder, Analytical Chemistry, 1984, 56, 1773. ©1984, ACS [15]

1.2.3.2 Determination of Oligomers Non-MS methods have been described for the determination of oligomers in polyvinylchloride, chlorinated polyethylene [18], styrene [15, 16, 19] and ethylene glycol [20], styrene–methyl methyl methacrylate [21], alkyl/phenol formaldehyde [22], and PET [22].

1.2.4 Fractionation/Microstructure Studies A good example of this application is the work of Lyons and co-workers [23] on the use of solvent gradient adsorption LC in a study of interchain compositional inhomogeneities in PS-co-ethylene random copolymers. Eluent gradient adsorption chromatography is one of the main HPLC techniques used in polymer fractionation including deposition–dissolution, chromatography under critical conditions, temperature gradient adsorption LC and precipitation dissolution LC [24–30]. Figure 1.4 contains overlaid chromatograms obtained by elution on a C18 column using a two part acrylonitrile–tetrahydrofuran elution system of ethylene–styrene

7

Chromatography Mass Spectroscopy in Polymer Analysis copolymers which contain between 49% and 100% styrene. Separating the same polymers using a nitrophenyl column using a hexane - THF mixture as eluent, gave chromatograms of a much higher resolution.

Absorbance @ 254 nm 1.5 Polystyrene 100% STY 72.1 % STY

1

69.1% 77.2% STY 63.6% STY

0.5 40.9% STY

0 5

10

15

20

25

Retention Time (Minutes) Figure 1.4 Overlaid chromatograms of selected electrospray ionisation (ESI) samples on a C18 column. Reproduced with permission from J.W. Lyons, D. Poche, F.C-Y. Wong and P.B. Smith, Advanced Materials, 2000, 12, 23, 1847. ©2000, Wiley [23]

In the case of the nitro method, the microstructure of the copolymer seems to have an effect on the retention time–composition relationship. The nitro column composition–retention time relationship is not a straight line for copolymers. Instead the relationship could be said to consist of two straight lines. One of the straight lines covers the composition range of 40 to 78 wt% styrene (Figure 1.5b).

8

Chromatographic Techniques 120

(a) C18 column

Weight % STY in ESI

100

80

(b) Nitro column

60

40

20

(c) Silica column

0 0

5

10 15 20 25 Retention Time (Minutes)

30

35

Figure 1.5 Composition versus retention time relationships for ESI copolymers. Reproduced with permission from J.W. Lyons, D. Poche, F.C-Y. Wong and P.B. Smith, Advanced Materials, 2000, 12, 23, 1847. ©2000, Wiley [23]

The following separations on a silica gel column have been reported: styrene–methyl acrylate copolymers on a silica gel column [31], styrene–acrylonitrile copolymers by precipitation LC [32], styrene–butadiene copolymers on a polyacrylonitrile gel column [33], styrene–methyl methacrylate copolymers on a silica gel column [34–36], styrene– methyl methacrylate block copolymers by column adsorption chromatography using a 50 mm id cylindrical column [37], and styrene-n-butyl methacrylate copolymers by orthogonal chromatography [22], PS [38-42], polyester resins [39], polyesters [43], polyethylene–glycol [44], polyethylene glycol ethers [45] and polytetrahydrofuran – polymethylmethacryalate (PMMA) copolymers [45], polyarylamide [46], polyethylene glycol [44, 46], polyalkylmethacrylate [47], polyelectrolytes [48] and oligo-L-lactides [49], polyoxymethylene [50], polyoxypropylene [50], polytetrahydrofuran [51], polyepichlorohydrin [50], oligobutadienes [50], hydroxy terminated polybutadiene [51, 52], carboxy butadienes [53, 54] and hydroxybutadienes [53, 54].

9

Chromatography Mass Spectroscopy in Polymer Analysis

1.3 Size Exclusion Chromatography In older references SEC is sometimes referred to as gel permeation chromatography. Applications of SEC are many and include: •

Polymer characterisation studies

•

Determination of type of branching

•

Compositional analysis

•

Determination of Mw

•

Separation of polymer blends

•

Determination of polymer additives

•

Fractionation/characterisation studies

Clarke [55] has described an advanced form of SEC which uses several detectors together in a single instrument thereby enabling measurements of both Mw and molecular structure to be measured directly. Brun [56] has published a comprehensive analysis of data reduction in triple detector SEC. Robert and co-workers [57] have reported the results of an SEC interlaboratory statistical evaluation of a high temperature technique for polyamides 6, 11 and 12. Laboratory robotics have been devised to automate the sample preparation procedure for high temperature SEC analysis [58]. Prokai and Simonsick [59] coupled SEC with Fourier transform–mass spectroscopy (FT–MS) through a Finnigan UltraSource interface. Sodiated molecular ions of the SEC effluent are produced using ESI. ESI/FT-MS combines the size separation-based technique of SEC with one of the most powerful mass spectroscopic techniques of FT–MS offering high mass accuracy (ppm), ultra-high resolving power (greater than 106), and the capability to perform tandem MS.

1.3.1 Characterisation Studies Characterisation studies have been carried out on the following polymers. Low density polyethylene [60], high density polyethylene [60, 61], t-butyl methacrylate4-vinyl pyridine copolymer [62], styrene butadiene copolymer [63], PS [64-70], polyethylene oxide [64], polypropylene oxide [71, 72], glycidyl methacrylate [73],

10

Chromatographic Techniques butyl methacrylate [73], PMMA [74], polyacrylic acid [74], PET [75], polyamide-6 [75] and oligolactones [76], poly(2-hexene) [77], phenol formaldehyde resins [78], Novolac resins [79], anionic polyelectrolytes [80], poly(n-vinyl pyrolidone–maleic acid copolymer [81], carboxy-terminated dendritic polymers [82], polyurethanes [83], polybutyl acrylate [65], polycarbonate [65], polyesters [65], methacrylate– methacrylic acid copolymer [65], poly(dimethyl siloxane) [84, 85], epoxy copolymers [86], copolyesters of butylene adipate [87], copolyesters of butylene succinate [87], copolyesters of butylene sebacate [87], hydroxy terminated poly(ether sulfone) oligomer [88], PS–styrene co-methyl acrylate blends [89], PMMA [89, 90], carboxy methyl cellulose [91], star branched polystyrene-b-butadiene-b-styrene [92], poly(phenyl methyl silane) [93] and PS-b-PMMA copolymers [94].

1.3.2 Branching Branching studies have been conducted on polyolefins, including short chain branching studies [95] and branching ratios in low density polyethylene [96].

1.3.3 Compositional Analysis Trathnigg and co-workers [97] have quantitated copolymers and blends using a dual concentration detector. Xu and co-workers [98] used UV absorbance and differential refractive index detectors in conjunction with SEC to study the copolymer composition of chlorinated butyl rubber/PS comb graft copolymers as a function elution volume.

1.3.4 Molecular Weight SEC has been used in the determination of a range of polymers including polyepichlorohydrin [99], polybis(carboxylato phenoxy phosphazine [100], polyarylene ethynylene [101], PMMA - polydimethyl siloxane grafts [102], surfactants [103], polycarbonate [104], PMMA [58], polybutadiene [105], polyelectrolytes [106], aliphatic oligoamide [107], PET [108], and polyvinyl alcohol [109].

1.3.5 Polymer Blends Nguyen and co-workers [110, 111] used adsorption/desorption chromatography to physically separate binary component blends of PS with PMMA and PS with polyvinyl acetate (PVAc). This technique was coupled to SEC in order to separate the blend components by size. Full adsorption/desorption chromatography has been applied 11

Chromatography Mass Spectroscopy in Polymer Analysis to three- and four-component blends comprising combinations of PS, PMMA, PVAc, and polyethylene oxide. Blends of poly(styrene-co-acrylonitrile) with polyethylene-co-propylene-co-diene have been characterised by SEC with UV and refractive index detectors, which allowed for the determination of average blend composition as a function of elution volume, and by precipitation LC, which allowed for complete separation of the blend components [112].

1.3.6 Polymer Additives

Irganox 1010

Polymer Additives

700

Irganox 1076

Figure 1.6 shows a chromatogram, obtained using SEC, of a mixture of additives extracted from polypropylene (PP). The mobile phase was water (channel A) and acetonitrile (channel B). The analysis, performed using a liquid chromatograph equipped with a diode array detector, shows the presence of methylated hydroxytoluene, methylated hydroxyethylbenzene, Amide E (erucamide), Irganox 1010, and Irganox 1076 (both sterically hindered phenols).

600

BH–impurities?

BH–impurities?

100

DLTDP BH–impurities?

200

BH–impurities?

Amide E

300

BH–impurities?

BHEB

mAU

400

BHT

500

0 2

4

6 8 Time (minutes)

10

12

Figure 1.6 Analysis of polymer additives. Source: Author’s own files

12

14

Chromatographic Techniques

1.4 Supercritical Fluid Chromatography Capillary SFC offers unprecedented versatility in obtaining high-resolution separations of difficult compounds. Beyond its critical point, a substance can no longer be condensed to a liquid, no matter how great the pressure. As the pressure increases, however, the fluid density approaches that of a liquid. Because solubility is closely related to density, the solvating strength of the fluid assumes liquid-like characteristics. Its diffusivity and viscosity, however, remain. SFC can use the widest range of detectors available to any chromatographic technique. As a result, capillary SFC has already demonstrated a great potential in applications to polymer additives. SFC is now one of the fastest growing analytical techniques. The first paper on the technique was by Klesper and co-workers [113], but SFC did not catch analysts’ attention until Novotny and co-workers [114] published the first paper on capillary SFC. Most SFC use carbon dioxide as the supercritical eluent, as it has a convenient critical point of 31.3 °C and 7.3 MPa. Nitrous oxide, ammonia, and n-pentane have also been used. This allows easy control of density between 0.2 and 0.8 g/ml and the utilisation of almost any detector from LC or GC. A capillary column has been used with a splitter (Figure 1.7), which was placed after the valve to ensure that a smaller volume was introduced onto the column. This method works well for compounds that are easily soluble in carbon dioxide at low pressures. Good reproducibility is attained for capillary SFC using a direct injection method without a split restrictor. SFC uses detectors from both LC and GC. A summary of detection systems used in SFC has been documented [115]. One of the most commonly used detection systems is electron capture detector. A sensitivity to about 50 pg (minimum detection limit) on a column is obtainable [116]. The photoioniation detection is to a certain extent specific in that only compounds that can be ionised by a UV lamp will give a response [117]. A sulfur chemiluminescence detector (Sievers Research Inc., Colorado, USA) has been investigated. Good sensitivities and chromatograms have been shown for standards and real samples. This detector shows no response to carbon dioxide and gives low pictogram sensitivities for a wide range of sulfur compounds.

13

Chromatography Mass Spectroscopy in Polymer Analysis

Inject

(a) Injection port

Load

Sample waste reservoir Pump On/off valve Oven Split restrictor

Fused silica capillary column

Split vent

(b) Inject Injection port Load Sample waste reservoir Pump Fused silica capillary Oven Figure 1.7 Sample injectors: (a)single split valve injector; (b) times split and direct valve injector. Source: Author’s own files

14

Chromatographic Techniques

1.4.1 Polymer Additives The possibilities of SFC for determining polymer additives have been demonstrated [118–129]. The low elution temperature and high resolution of capillary fluid chromatography makes this technique very attractive. Other advantages are that a FID can be used and that interfacing with spectroscopic detectors is somewhat easier than with HPLC. Quantitation in capillary SFC has been found to be more difficult than in HPLC and capillary GC, however, because of lack of precision in injection [40]. Polymer additives - mould release agents, plasticisers, antioxidants, and UV absorbers, with Mw extending beyond 1000 - are generally unsuitable for GC or LC analysis because of their low volatility, lack of chromophore, or thermal instability. SFC is now the method of choice for the analysis of such compounds. Figure 1.8 shows the chromatogram of a polymer containing Tinuvin 1130.

HO N N N (CH2)2COO(CH2CH2C )n H HO N N N (CH2)2COO(CH2CH2O)

n/2 2

Recorder response

Average molecular weight Mw> 600

0 100

10 150

20

30 200

40 50 Time (minutes) 250 Pressure (atm)

60

70 300

80

90

350

Figure 1.8 Supercritical fluid chromatogram of polymer additive, Tinuvin 1130. Conditions: column 10 m x 50 µm, SB-methyl; mobile phase, carbon dioxide at 150 °C with pressure programming detection; FID. Source: Author’s own files

15

Chromatography Mass Spectroscopy in Polymer Analysis

1.5 Thin Layer Chromatography Thin layer chromatography (TLC), like GC, comes into its own when dealing with mixtures of substances. TLC using plates coated with 250 µm absorbent is an excellent technique for separating quantities of up to 20 mg of additive mixtures into their individual components and provides enough of each to prepare a recognisable infrared (IR) or UV spectrum which can be compared with spectra of authentic known compounds. However, this technique does not conveniently handle larger quantities and in these cases separation on the 50–500 mg scale can be carried out with only a small loss of resolution by chromatography on a silica gel or aluminium packed column. The separated bands are marked under UV light, removed from the plate, and extracted with diethyl ether. Separations on this scale provide sufficient of each fraction for full characterisation by MS and IR spectroscopic techniques. Thin-layer LC has been used for the determination of alkylated cresols and amine antioxidants [130, 131] in polybutadiene, phenolic antioxidants in polyethylene [132–134] and PP [134], dilauryl and distearyl thiodipropionate antioxidants in polyolefins, ethylene–vinyl acetate copolymer, acrylonitrile–styrene terpolymer and PS, UV absorbers and organotin stabilisers in polyolefins [130], and accelerators such as guanidines, thiazoles, thiurams, sulfenamides, dithiocarbamides, and morpholine disulfides in unvulcanised rubber compounds.

1.6 Thermal Field Flow Fractionation Kassalainen and Williams [135] coupled thermal field flow fractionation (ThFFF) and matrix-assisted laser desorption/ionisation time-of-flight mass spectroscopy (MALDI-ToF-MS) to yield a powerful combination of techniques for the analysis of polydisperse PS. ThFFF high Mw selectivity and sensitivity to chemical composition were used to separate polydisperse polymers and polymer mixtures into the narrow polydispersity and homogeneous chemical composition fractions essential for MALDIToF-MS analyses. On the other hand, because it is possible to measure Mw directly using MALDI-ToF-MS, it alleviates the need for polymer standards for ThFFF. Kassalainen and Williams [135] address the coupling of ThFFF and MALDI-ToF-MS and identify compatibility issues. Optimum conditions were determined and developed to maximise the capabilities of the combined technique. Depending on the polymer Mw and the method of matrix-assisted laser desorption/ionisation (MALDI) sample deposition, fractions from 1–10 ThFFF runs were combined for MALDI-ToF-MS analysis. Binary solvents are used to enhance ThFFF retention and resolution of low Mw (