Molecular degradation of polymers

An understanding of the transmittal of force or stress through the molecular (or atomic) network of a body should not only be helpful in predicting the load carrying capability of materials but might also provide information on how structure can be altered to enhance properties. A molecular understanding of mechanical properties is particularly difficult in polymers where such factors as molecular weight, tacticity, crystallinity, presence of networks, orientation, etc., may have an influence. In no class of materials are intricacies of physical structure more important than in polymers. Polymers with very similar chemical structures but differing physical structure can have physical properties differing by orders of magnitude. It would obviously be helpful to have means to probe molecular and atomic events and occur during the loading and destruction of the polymer structure. Instruments that have been used for this purpose include: (1) electron spin resonance (ESR) to monitor free radical production resulting from homolytic chain scission, (2) Fourier transform infrared spectroscopy (FTIR) to measure new end groups resulting from molecular chain rupture, and (3) intrinsic viscosity or gel permeation chromotography (GPC) to detect molecular weight changes (MWC) accompanying molecular degradation. This presentation will discuss and compare research results from these methods. Particular emphasis will be placed on recent studies in which computer modeling has been used to identify ESR spectra and a comparison of the results obtained by ESR, FTIR, and MWC for polystyrene.