Identification of Synthetic Polymers and Copolymers by Analytical

Publication Date (Web): September 19, 2014 ... and performed in the polymer analysis courses for third-year undergraduate students of chemistry with m...
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Identification of Synthetic Polymers and Copolymers by Analytical Pyrolysis−Gas Chromatography/Mass Spectrometry Peter Kusch* Department of Applied Natural Sciences, University of Applied Sciences, Hochschule Bonn-Rhein-Sieg, D-53359 Rheinbach, Germany S Supporting Information *

ABSTRACT: An experiment for the identification of synthetic polymers and copolymers by analytical pyrolysis−gas chromatography/mass spectrometry (Py−GC/MS) was developed and performed in the polymer analysis courses for third-year undergraduate students of chemistry with material sciences, and for first-year postgraduate students of polymer sciences. In order to illustrate this analytical technique, polystyrene (PS) and poly(acrylonitrile-co-1,3-butadiene-co-styrene) (ABS) were selected for identification. The students were able to complete this experiment in a single 6 h (a 45 min) or in two 3 h (a 45 min) laboratory sessions. The success rate in the determining of PS was 100%. Because the task of identifying ABS is more demanding, the success rate was about 80−90%. Successful completion of the laboratory experiments and a laboratory report are required of students to obtain a permit for the examination in the polymer analysis course. KEYWORDS: Second-Year Undergraduate, Analytical Chemistry, Chromatography, Gas Chromatography, Mass Spectrometry, Qualitative Analysis, Polymer Chemistry, Hands-On Learning/Manipulatives

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Analytical pyrolysis−gas chromatography/mass spectrometry (Py−GC/MS) is a destructive analytical technique. Once the polymer/copolymer sample has been pyrolyzed, volatile fragments are swept from the heated pyrolysis unit by the carrier gas into the gas chromatograph, where they are separated into their components. Analytical pyrolysis combined with gas chromatography employing fused silica open tubular capillary columns and detection by mass spectrometry (Py−GC/MS) has extended the range of tools for characterization of synthetic polymers and copolymers.4−9 Typical fields of interest and application are identification of polymers/copolymers by comparison of pyrograms and mass spectra with known references, investigation of thermal degradation, and determination of monomers, volatile organic compounds (VOC), solvents, and additives in polymers/copolymers.10−15 In most cases of analysis of a polymer or composite material, the technique does not require any sample preparation, not even solubilization of the sample, which may be a difficult task for the type of materials analyzed.9 Starting at the University of Cologne (Germany) in the 1960s, Hummel and co-workers studied the process of thermal decomposition of polymers in detail using Py−GC/MS and Py−GC/FTIR. 14,15 In general, decomposition proceeds through radical formation, which, due to the high reactivity of radicals, initiates numerous consecutive and parallel reactions. Hummel and co-workers summarized the main pathways of

lthough synthetic polymers and copolymers are inherently difficult to analyze because of their high molecular weight and lack of volatility, various analytical techniques are used to characterize polymers/copolymers, including physical testing (rheological testing), thermogravimetric analysis (TGA), electron microscopy, Fourier transform infrared (FTIR) spectroscopy, size-exclusion chromatography (SEC)/gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), laser light scattering, ultraviolet−visible spectroscopy (UV/vis), and mass spectrometry (MS).1 These techniques have limitations, and often laborious and time-consuming sample preparation, including hydrolysis, dissolution, or derivatization, is needed before analysis. Pyrolysis has been used extensively over the past 20−30 years as an analytical technique for characterization of polymers/copolymers.2,3 The word pyrolysis, translated from the original Greek pyros = f ire and lyso = decomposition, means a chemical transformation of a sample by heating at a temperature above ambient temperature in an inert gas atmosphere, i.e., in the absence of oxygen. In pyrolysis, thermal fragmentation of nonvolatile macromolecules produces a complex mixture of volatile compounds suitable for gas chromatographic analysis. When a polymer/copolymer is pyrolyzed, bond fission processes are initiated. These may proceed by means of several temperature-dependent and competing reactions, which make the final fragment distribution highly dependent upon the pyrolysis temperature. Therefore, the essential requirements for an analytical pyrolysis apparatus are reproducibility of the pyrolysis temperature, rapid temperature rise, and accurate temperature control.4−7 © 2014 American Chemical Society and Division of Chemical Education, Inc.

Published: September 19, 2014 1725

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mode with an ThermoQuest Xcalibur data system, and the NIST 05 mass spectral library. The pyrolysis products were introduced with the carrier gas through the injection block of the gas chromatograph into the fused silica capillary column, 30 m long, 0.25 mm i.d. with DB-5 ms (J&W Scientific Inc., Folsom, CA, U.S.A.) stationary phase, film thickness 0.25 μm. The column temperature was programmed from 60 °C (1 min hold) at 2.5 °C min−1 to 100 °C and then at 10 °C min−1 to 280 °C (20 min hold). Helium 5.0 grade (99.9990%, v/v) (Westfalen AG, Münster, Germany) was used as a carrier gas with programmed pressure from 70 kPa (1 min hold) at 1 kPa min−1 to 100 kPa (hold until the end of analysis). The temperature of the split/splitless injector was 250 °C, and the split flow was 75 cm3 min−1. The transfer line temperature was 280 °C. The EI ion source temperature was operated at a 250 °C and 70 eV. The emission current of the rhenium filament was 150 μA. The detector voltage was 350 V. Mass spectra and reconstructed chromatograms (total ion current [TIC]) were obtained by automatic scanning in the mass range m/z 35−455.

polymer decomposition by classification into four principal categories:14 1. Retropolymerization from the end of the polymer chain, predominantly forming monomers [e.g., poly(methyl methacrylate) (PMMA), poly-α-methylstyrene] 2. Statistical chain scission followed by a. Retropolymerization from radical bearing chain ends (e.g., polyisobutylene, polystyrene) b. Radical transfer and disproportionation (e.g., polyethylene, isotactic polypropylene) c. Stabilization of fragments by cyclization (e.g., polydimethylsiloxane) 3. Splitting off of side-chain leaving groups (e.g., polyvinyl chloride, polyacrylonitrile, polyacrylate) 4. Intramolecular condensation reactions with loss of smaller molecules (e.g., phenol-epoxide resins) This paper describes an experiment for the identification of synthetic polymers and copolymers by analytical Py−GC/MS. It was developed and performed to learn the analytical pyrolysis and data analysis methods in polymer analysis courses for third-year undergraduate students of chemistry with a materials science focus, and for first-year postgraduate students of polymer sciences. The inclusion of this experiment in a laboratory polymer analysis course has been made as an alternative and/or supplement to the most widely used FTIR spectroscopy and thermal analysis (TA) methods.

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HAZARDS There are no significant hazards. RESULTS AND DISCUSSION Evaluation and interpretation of pyrograms of polymers/ copolymers require a lot of time and experience. Several tools, such as mass spectral libraries, pyrogram databases, or polymer additive libraries, are commercially available and make the processing and interpretation of chromatograms easier.16 In this experiment students receive two unknown polymer/ copolymer samples (PS and ABS) for pyrolysis. Figure 2 shows the total ion chromatograms (the pyrograms) of pyrolysates of polystyrene (PS) and of poly(acrylonitrile-co1,3-butadiene-co-styrene) (ABS), obtained by students at 700 °C, respectively. The pyrograms in Figure 2 show the characteristic substances formed by thermal degradation of the investigated polymer/copolymer. The mass spectra of the compounds obtained by pyrolysis are presented in the Supporting Information. For the identification of peaks, the following resources were made available for students: the NIST 05 spectral library, a table with the characteristic pyrolysis products of synthetic polymers/copolymers (see the Supporting Information), and a selection of relevant literature data.4,5,9,10,17−20 The degradation products of samples identified by students are summarized in Table 1. As can be seen from Table 1, there are pyrolysate components that are common to the analysis of both polymers/copolymers (PS and ABS) that were studied. The main thermal decomposition product of polystyrene at 700 °C is the monomer of styrene (Figure 2, tR = 4.93 min). Literature data18 indicates that the styrene yield increases with pyrolysis temperature while amounts of styrene dimer (tR = 27.45 min) and styrene trimer (not identified) decrease. The pyrolysate of ABS contains all three monomers: 1,3-butadiene (tR = 1.37 min), acrylonitrile (tR = 1.51 min), and styrene (tR = 4.93 min), which indicates that the material is a terpolymer. Additionally, the pyrolysate showed evidence of hybrid dimers of acrylonitrile and styrene (tR = 21.37 and 23.04 min), styrene dimer (tR = 27.45 min), and styrene-acrylonitrile-styrene hybrid trimer (tR = 32.26 min) which were identified based on literature data.17

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EXPERIMENTAL PROCEDURE Commercially available samples of polystyrene (PS), CAS number 9003-53-6, and poly(acrylonitrile-co-1,3-butadiene-costyrene) (ABS), CAS number 9003-56-9, were used for investigations (Figure 1). Approximately 100−200 μg of the

Figure 1. Chemical structures of polystyrene (PS) and poly(acrylonitrile-co-1,3-butadiene-co-styrene) (ABS).

sample was inserted into the bore of the pyrolysis solidsinjector and placed in the quartz tube of the online furnace pyrolyzer Pyrojector II (SGE Analytical Science, Melbourne, Australia) operated at a constant temperature of 700 °C. The furnace pressure of helium carrier gas was 95 kPa. The pyrolyzer was directly connected to a ThermoQuest Trace 2000 gas chromatograph (ThermoQuest CE Instruments, Milan, Italy), interfaced to a ThermoQuest/Finnigan Voyager quadrupole mass spectrometer (ThermoQuest/Finnigan MassLab Group, Manchester, U.K.), operated in electron impact ionization (EI) 1726

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Figure 2. Total ion chromatograms (the pyrograms) of pyrolysates of polystyrene (PS) (top) and poly(acrylonitrile-co-1,3-butadiene-co-styrene) (ABS) (bottom), obtained by students at 700 °C in a helium carrier gas stream.

Table 1. Identification of Pyrolysis Products of PS and ABS at 700 °C PS Pyrolysis Product

Benzene Toluene Ethylbenzene Styrene (1-Methylethenyl)-benzene (α-methylstyrene) Indene (1-Methylenepropyl)-benzene (α-ethylstyrene)

1,2-Diphenylethylene (stilbene) 1,2-Diphenylethane (bibenzyl) 1,1′-(1-Methyl-1,2-ethanediyl)bis-benzene (α-methylbibenzyl) (3-Phenylbut-3-enyl)-benzene (SS, styrene dimer) 2,5-Diphenyl-1,5-hexadiene

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ABS Relative Abundance

Pyrolysis Product

0.9 3.3 1.0 100.0 4.7

1,3-Butadiene Acrylonitrile Methacrylonitrile Benzene Toluene Ethylbenzene Styrene (1-Methylethenyl)-benzene (α-methylstyrene)

6.9 16.6 3.2 2.3 7.3 2.3 100.0 8.8

1.37 1.51 1.63 1.94 2.75 4.20 4.93 7.53

−4.4 77.3 90.3 78.8 110.6 136.2 145.2 162.5

54.1 53.1 67.1 78.1 92.1 106.2 104.1 118.2

106-99-0 107-13-1 126-98-7 71-43-2 108-88-3 100-41-4 100-42-5 98-83-9

2.5 1.0

Indene (1-Methylenepropyl)-benzene (α-ethylstyrene)

3.7 1.9

9.92 10.47

181.6 184.8

116.2 132.2

95-13-6 2039-93-2

2-Methylenepentanedinitrile (2-methyleneglutarodinitrile) Benzenepropanenitrile 2-Methylene-4-phenylbutanenitrile (AS, acrylonitrile-styrene hybrid dimer) 4-Phenylpent-4-enenitrile (SA, styreneacrylonitrile hybrid dimer) 1-Naphthalenecarbonitrile (1-naphthonitrile)

2.7

10.83

277.9

106.1

1572-52-7

4.1 12.0

18.81 21.37

261.8 Not documented

131.2 157.2

645-59-0 Not documented

8.0

23.04

Not documented

157.2

Not documented

0.5 3.5 0.7

24.35 24.65 24.76 25.21

299.0 307.0 284.0 283.6

153.2 180.2 182.2 196.2

86-53-3 588-59-0 103-29-7 5814-85-7

4.8

27.45

208.3

25247-68-1

1.0 0.4

29.46 32.26

168−169 (16 Torr) 367.3 Not documented

234.3 261.4

7283-49-0 Not documented

0.2 7.0 0.9 8.9 1.6

Relative Abundance tR, min

1,2-Diphenylethane (bibenzyl) 1,1′-(1-Methyl-1,2-ethanediyl)-bis-benzene (α-methylbibenzyl) (3-Phenylbut-3-enyl)-benzene (SS, styrene dimer) 2,5-Diphenyl-1,5-hexadiene 2-Phenethyl-4-phenylpent-4-enenitrile (SAS, styrene-acrylonitrile-styrene hybrid trimer)

CONCLUSION The experiment shows that analytical pyrolysis in combination with capillary GC/MS is a powerful tool for the study and identification of synthetic polymers/copolymers. Students learn

bp, °C

MW

CAS Registry No.

and get experience in hyphenated pyrolysis−GC/MS technique with two devices in two small groups of 4−6 students in the group. The identification and interpretation of pyrograms and mass spectra for different classes of chemical substances such as 1727

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(8) Vergne, M. J.; Hercules, D. M.; Lattimer, R. P. A Developmental History of Polymer Mass Spectrometry. J. Chem. Educ. 2007, 84, 81− 90. (9) Moldoveanu, S. C. Analytical Pyrolysis of Synthetic Polymers; Elsevier: Amsterdam, 2005. (10) Applied Pyrolysis Handbook, 2nd ed.; Wampler, T. P., Ed.; CRC Press: Boca Raton, FL, 2007. (11) Haken, J. K. Pyrolysis Gas Chromatography of Synthetic PolymersA Bibliography. J. Chromatogr. A 1998, 825 (2), 171−187. (12) Rial-Otero, R.; Galesio, M.; Capello, J.-L.; Simal-Gándara, J. A Review of Synthetic Polymer Characterization by Pyrolysis−GC−MS. Chromatographia 2009, 70 (3−4), 339−348. (13) Polymer Spectroscopy; Hummel, D. O., Ed.; Verlag Chemie: Weinheim, Germany, 1974. (14) Atlas der Polymer- und Kunststoffanalyse, Hummel, D. O., Scholl, F., Eds.; Hanser: München, Germany, 1988. (15) Hallensleben, M. L.; Wurm, H. Polymeranalytik. Nachr. Chem., Tech. Lab. 1989, 37 (6), M1−M45. (16) Potyka, U. Identifying and Characterizing Polymers using Pyrolysis-GC/MS. Column 2007, June, 20−23. (17) Tsuge, S.; Ohtani, H.; Watanabe, C. Pyrolysis-GC/MS Data Book of Synthetic Polymers; Elsevier: Amsterdam, 2011. (18) Hummel, D. O.; Düssel, H. J.; Rübenacker, K. Feldionen- und Elektronenstoß-Massenspektrometrie von Polymeren und Copolymeren. II. Polyisobuten, Polystyrol, Polypropen, Polystyrolperoxid, Polyvinylchlorid. Makromol. Chem. 1971, 145 (1), 267−287. (19) Hodgson, S. C.; Bigger, S. W.; Billingham, N. C. Studying Synthetic Polymers in the Undergraduate Chemistry Curriculum. A Review of the Educational Literature. J. Chem. Educ. 2001, 78 (4), 555−556. (20) Levy, E. J.; Wampler, T. P. Identification and Differentiation of Synthetic Polymers by Pyrolysis Capillary Gas Chromatography. J. Chem. Educ. 1986, 63 (3), A64−A68.

aliphatic and aromatic hydrocarbons and nitriles give them an understanding of the processes that take place in a pyrolyzer, a gas chromatograph, and a mass spectrometer. By means of the identified pyrolysis products, the students can easily recognize the retropolymerization mechanism by decomposition of PS and ABS. Additionally, students learn common names and systematic names of identified substances. The use of the mass spectral database NIST 05 allows them to estimate advantages and limitations of this tool. The third-year undergraduate students of chemistry with material sciences and the first-year postgraduate students of polymer sciences in the polymer analysis course are able to complete this experiment in a single 6 h (a 45 min) or in two 3 h (a 45 min) laboratory sessions. The success rate in the determining of PS is at 100%. Because the task in the identification of ABS is more demanding, the success rate is at about 80−90%. The successful completion of the laboratory experiments and a laboratory report are required of students to obtain a permit for the examination in the polymer analysis course.

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ASSOCIATED CONTENT

S Supporting Information *

Instructions for students and instructors; mass spectra obtained from pyrolysis products of PS and ABS; table of typical pyrolysis products of synthetic polymers and copolymers at 700 °C. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I would like to thank my daughter Maria Kusch, M.A., for her critical reading of the manuscript. REFERENCES

(1) Characterization and Analysis of Polymers; Seidel, A., Ed.; WileyInterscience: Hoboken, NJ, 2008. (2) Sobeih, K. L.; Baron, M.; Gonzales-Rodriguez, J. Recent Trends and Developments in Pyrolysis−Gas Chromatography. J. Chromatogr. A 2008, 1186, 51−66. (3) Bower, N. W.; Blanchet, C. J. K. Analytical Pyrolysis− Chromatography: Something Old, Something New. J. Chem. Educ. 2010, 87, 467−469. (4) Kusch, P.; Knupp, G.; Morrisson, A. Analysis of Synthetic Polymers and Copolymers by Pyrolysis-Gas Chromatography/Mass Spectrometry. In Horizons in Polymer Research; Bregg, R. K., Ed.; Nova Science Publishers: New York, 2005; pp 141−191. (5) Kusch, P. Pyrolysis-Gas Chromatography/Mass Spectrometry of Polymeric Materials. In Advanced Gas ChromatographyProgress in Agricultural, Biomedical and Industrial Applications; Mohd, M. A., Ed.; InTech: Rijeka, 2012; pp 343−362. (6) Kusch, P. Identification of Organic Additives in Nitrile Rubber Materials by Pyrolysis-GC-MS. LCGC North Am. 2013, 31 (3), 248− 254. (7) Kusch, P.; Obst, V.; Schroeder-Obst, D.; Fink, W.; Knupp, G.; Steinhaus, J. Application of Pyrolysis−Gas Chromatography/Mass Spectrometry for the Identification of Polymeric Materials in Failure Analysis in the Automotive Industry. Eng. Failure Anal. 2013, 35, 114− 124. 1728

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