Studying Thermally Induced Chemical and Physical Transformations in Common Synthetic Polymers:

• Steven C. Hodgson ^[1] ; John D. Orbell ^[1] ; Stephen W. Bigger ^[1] ; John Scheirs ^[2]
  1. [1] Victoria University

     Victoria University

     Australia

     
  2. [2] Melbourne City MC
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 77, Nº 6 (June), 2000, págs. 745-745
• Idioma: inglés
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• Resumen

  • A simple project is described for introducing students to some experimental procedures commonly used to measure the effects of thermal treatment on synthetic polymers. The thermally induced changes that occur in the commodity polymers low-density polyethylene (LDPE), poly(ethylene terephthalate) (PET), and poly(vinyl chloride) (PVC) are examined as a function of the time of thermal treatment in an air-circulating oven. In particular, simple procedures are described for determining (i) the polymer hydroperoxide (POOH) content and carbonyl index (CI) of LDPE, (ii) the extent of whitening of PET, and (iii) the extent of discoloration or "yellowing" of PVC, all of which change during thermal treatment. The POOH content of LDPE is determined using a ferrometric method and the CI of this polymer is measured by both Fourier transform infrared spectroscopy and a staining technique involving 2,4-dinitrophenylhydrazine. The thermal oxidation of LDPE and the kinetics of formation of its POOH and carbonyl species are discussed with reference to the accepted mechanism for the autooxidation of polyolefins. The whitening of PET and the yellowing of PVC during thermal treatment are explained by means of a crystallization process and a "zip" dehydrochlorination reaction, respectively.