In the Laboratory
Studying Thermally Induced Chemical and Physical Transformations in Common Synthetic Polymers
W
A Laboratory Project Steven C. Hodgson, John D. Orbell, and Stephen W. Bigger* School of Life Sciences and Technology, Victoria University of Technology, Footscray Park Campus, P.O. Box 14428, Melbourne City MC, Melbourne, Victoria 8001, Australia; *
[email protected] John Scheirs ExcelPlas Australia, P.O. Box 163, Casula 2170, Australia
Rationale Despite the fact that polymer science constitutes a significant proportion of commercial activity in the chemical world, there is still a dearth of educational material in this area (1). Stevens (2) has compiled a list of articles in the educational literature up to 1988 that deal with polymer science.1 A close examination of this and the subsequent educational literature reveals that the most commonly produced synthetic polymers such as polyethylene (PE) and poly(vinyl chloride) (PVC) are sparingly represented, whereas more attention is given to less familiar polymers such as thermosetting resins (3, 4 ). In relation to thermal techniques, the literature is focused on their application to polymer characterization (5–7), polymer analysis (8), and the effects of thermal treatment on molecular orientation in polymers (6, 9, 10). Only a few papers, such as the ones by Bruck (11) and Burrows et al. (12), directly demonstrate to students techniques that characterize the thermal degradation of polymers. This paper was written in response to the need for simple educational experiments in polymer science that relate to common, everyday materials. The materials used in the project are inexpensive and comprise PE, poly(ethylene terephthalate) (PET), and PVC. These are arguably the most common of all synthetic polymers used in modern society and will be familiar to most students. Because many of the methods investigated in the project are simple, it is suggested that parts of it can be performed by junior undergraduate students. Furthermore, the experiments are not restricted to specialized polymer formulations and so the project could be readily extended to cover a wide range of commodity polymers. The recommended procedure involves the use of laboratory instruments such as UV–visible and infrared spectrophotometers for quantitatively measuring the effects. However, the experiments are designed so that the changes that occur can also be observed clearly by visual inspection. This enables the project to be performed qualitatively, if required, and eliminates the need for any sophisticated instrumentation if this is unavailable. Therefore the experiment can possibly be performed by advanced senior high school students. The project introduces students to some of the standard techniques that measure certain changes in synthetic polymers when the polymers are subjected to thermal treatment in an air-circulating oven. These changes are determined as a function of the time of treatment by measuring (i) the polymer hydroperoxide (POOH) content and carbonyl index (CI) of
low-density polyethylene (LDPE), (ii) the extent of whitening of PET, and (iii) the extent of discoloration or “yellowing” of PVC. The POOH content of LDPE is determined using a ferrometric method and the CI of this polymer is measured by both Fourier transform infrared (FTIR) spectroscopy and a staining technique involving 2,4-dinitrophenylhydrazine (2,4-DNPH). The project was designed in a modular way so that various principles and techniques can be extracted and adapted to suit a typical chemistry curriculum. For example, the concept of the functional group may be studied from the point of view of the infrared spectroscopy of polymeric materials, or students may be introduced to spectrophotometry within the context of polymer oxidation and subsequent staining. Other areas that can be explored individually or in combination are structural and organic chemistry, redox reactions, stoichiometry, reactions of free radicals, and kinetics. This is consistent with the approach of other educational researchers who have utilized polymer chemistry as a vehicle for basic training in chemistry (13–16 ). Experimental Procedure Samples of LDPE, PET, and PVC can be easily obtained from either commercial or post-consumer sources. Details of possible sources of samples, standard methods of film preparation (if required), and the thermal treatment procedure (which involves simply mounting the samples in an aircirculating oven set to an appropriate temperature, given below in the text) are given in the supplemental information.W Results and Discussion
Thermal Oxidation of LDPE Figure 1 shows the POOH level and the CI of LDPE film (90 ± 5 µm) as a function of the time of oven aging in air at 100 °C. The POOH level (Fig. 1䊊 a ) was determined by a ferrometric method described elsewhere (17, 18); it shows a gradual increase up to a maximum level, after which a decrease occurs as the carbonyl content of the film escalates. The CI (Fig. 1䊊 b ) was determined by an FTIR method where the intensity of the carbonyl peak is measured relative to the intensity of a methylene reference peak (19, 20). The data shown in Figure 1 are consistent with the notion that the oxidation of LDPE proceeds by an autooxidative mechanism (21) and demonstrate to students that POOH groups are
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In the Laboratory
Figure 1. Plots of 䊊 a polymer hydroperoxide (POOH) content and b carbonyl index (CI) of LDPE film as a function of the time of 䊊 oven aging (days) in air at 100 °C. The induction period of 21.5 days is shown. The error bars indicate the reproducibility that is possible for these measurements.
Figure 3. Plot of the optical absorbance at 350 nm versus the time of oven aging (days) in air at 100 °C of LDPE films that were treated with 2,4-DNPH after oxidation. The induction period of 21.5 days is shown.
with 2,4-DNPH (Fig. 3) has a shape similar to that of the b . Thus the staining technique can CI plot shown in Fig. 1䊊 also be used to monitor the rate of degradation of LDPE. Students can identify an induction period (23) for the thermooxidative process (Figs. 1 and 3); and they can investigate any variation that occurs in the induction period if commercial LDPE resins having different levels of thermal stabilizer are used in the project.
Thermal Whitening of PET The thermal treatment of PET (506 ± 7 µm) between heated plates at 250 °C for approximately 10 min results in the whitening of the polymer owing to a crystallization process in which molecular reorientation converts amorphous regions to crystalline domains (24). Samples of thermally treated PET are shown in Figure 2b along with an unaged sample for comparison. The extent of whitening is indicative of the degree to which molecular reorientation has occurred upon annealing. With reference to this example, students can be alerted to the importance of avoiding thermal whitening during the manufacture and processing of PET for optically clear packaging applications (24).
Figure 2. Samples of (a) LDPE aged in air at 100 °C and stained with 2,4-DNPH, (b) PET annealed at 250 °C, and (c) PVC aged in air at 180 °C. The time for which each sample was thermally treated is indicated below the sample.
important precursors in the thermooxidative process. The instructor and students may also qualitatively explore the presence of other peaks in the FTIR spectrum of oxidized LDPE that result from thermooxidation. Some of these peaks are listed in the supplemental information.W The increase in the CI of LDPE during thermal aging may be observed visually using a 2,4-DNPH staining technique (22) in which the intensity of the stain coloration increases with time of aging (Fig. 2a). A plot of the optical absorbance at 350 nm versus the time of aging of LDPE films stained 746
Thermal Yellowing of PVC Figure 4 shows the UV–visible absorption spectra in the 250–700-nm region of thick sections of PVC (288 ± 4 µm) aged in air at 180 °C for up to 5 h. The increase in intensity of absorption in this spectral region occurs as a result of the formation of conjugated polyene structures, produced by the dehydrochlorination of the polymer (25). The mechanism of polyene formation (26, 27) is discussed in the supplemental information.W The spectra in Figure 4 show that (i) the concentration of conjugated polyenes increases with time of aging and (ii) the wavelength of maximum absorption undergoes a bathochromic shift as the extent of polyene formation increases. The formation of conjugated polyene structures leads to a visible discoloration of PVC. Such discoloration can be seen in Figure 2c, where samples of thermally treated PVC are shown along with an unaged sample for comparison.
Journal of Chemical Education • Vol. 77 No. 6 June 2000 • JChemEd.chem.wisc.edu
In the Laboratory
Note 1. The list compiled by Stevens (2) comprises journal articles dealing with polymer science that were published in J. Chem. Educ. from 1956 to 1988 inclusive. A comprehensive and updated list of articles that also includes those published in Educ. Chem. may be obtained from the authors on request.
Literature Cited
Figure 4. Optical absorption spectra of PVC films oxidized in air a 2, 䊊 b 3, and 䊊 c 5 h. Unaged PVC was used as at 180 °C for 䊊 the reference.
Conclusions This project provides a simple means of demonstrating a number of standard experimental techniques used routinely to investigate the effects of thermal treatment on three of the most common synthetic polymers. The versatility of the experiments enables them to be conducted in a quantitative or qualitative manner, depending on the seniority of the students and the spectroscopic facilities that are available. The project also (i) highlights how the technique of FTIR can be used as both a qualitative and quantitative tool in polymer analysis and (ii) illustrates that a given quantity such as the CI of a polymer may be measured using more than one procedure, thereby strengthening confidence in the experimental data. Chemicals Required The following polymers and chemicals are needed for the project: low-density polyethylene (LDPE) film, poly(ethylene terephthalate) (PET), poly(vinyl chloride) (PVC), ammonium iron(II) sulfate hexahydrate, 2,4dinitrophenylhydrazine, methanol, ethanol, toluene, concd hydrochloric acid. Acknowledgments We are grateful to the ARC Small Grants Scheme for providing funding to investigate the degradation of synthetic polymers. SWB would like to dedicate this paper to the memory of his father, William S. Bigger, who gave him unrelenting encouragement and support throughout his education and career to date. His strong belief in the need for one to achieve a sound education will always be remembered. W
Supplemental Material
Supplemental material for this article is available in this issue of JCE Online. It includes instructor notes, student materials, experimental procedures, and various other information needed to implement the project.
1. Ford, W. T.; Krause, S.; Sperling, L. H. ACS Div. Polym. Chem. Newslett. 1996, June, 10. 2. Stevens, M. P. In Polymer Chemistry—An Introduction, 2nd ed.; Oxford University Press: New York, 1990; pp 612–617. 3. Peng, W.; Riedi, B. J. Chem. Educ. 1995, 72, 587. 4. Toorkey, R. F.; Rajanna, K. C.; Sal Prakash, P. K. J. Chem. Educ. 1996, 73, 372. 5. Cloutier, H.; Prud’homme, R. E. J. Chem. Educ. 1985, 62, 815. 6. Marentette, J. M.; Brown, G. R. J. Chem. Educ. 1993, 70, 539. 7. Blumberg, A. A. J. Chem. Educ. 1993, 70, 399. 8. Williams, K. R. J. Chem. Educ. 1994, 71, A195. 9. Clough, S. B. J. Chem. Educ. 1987, 64, 42. 10. Billmeyer, F. W.; Geil, P. H.; Van Der Weg, K. R. J. Chem. Educ. 1960, 37, 460. 11. Bruck, S. D. J. Chem. Educ. 1993, 65, 18. 12. Burrows, H. D.; Ellis, H. A.; Odilora, C. A. J. Chem. Educ. 1995, 72, 448. 13. Ferry, J. D. J. Chem. Educ. 1959, 36, 164. 14. See recommendations of the ACS Physical Chemistry Course Subcommittee. J. Chem. Educ. 1985, 62, 780. 15. See recommendations of the ACS Physical Chemistry Course Subcommittee. J. Chem. Educ. 1985, 62, 1030. 16. Stucki, R. J. Chem. Educ. 1984, 61, 1092. 17. Zeppenfeld, G. Mackromol. Chem. 1966, 90, 169. 18. Scheirs, J.; Carlsson, D. J.; Bigger, S. W. Polym.-Plast. Technol. Eng. 1995, 34, 97. 19. Grassie, N.; Scott, G. Developments in Polymer Degradation & Stabilization; Cambridge University Press: Cambridge, 1985; p 15. 20. Scheirs, J.; Delatycki, O.; Bigger, S. W. Eur. Polym. J. 1991, 27, 1111. 21. Bolland, J. L.; Gee, G. Trans. Faraday Soc. 1946, 42, 236; 1946, 44, 669. 22. Scheirs, J.; Delatycki, O.; Bigger, S. W.; Billingham, N. C. Polym. Int. 1991, 26, 187. 23. American Society for Testing and Materials. ASTM Method D 3895-95; ASTM: West Conshohocken, PA. 24. Milgrom, J. Plastics Recycling; Products & Processes; Hanser: New York, 1992, Chapter 3, p 48. 25. Andrady, A. L.; Fueki, K.; Torikai, A. J. Polym. Sci. 1990, 39, 763. 26. Rabek, J. F. Polymer Photodegradation. Mechanisms and Experimental Methods; Chapman and Hall: London, 1995; pp 151– 162. 27. Rabek, J. F.; Rånby, B.; Skowronski, T. A. Macromolecules 1985, 18, 1810.
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