Mechanical degradation of thin polystyrene films

Abstract

Thin films of polystyrene drastically degrade, on a molecular scale, when subjected to mechanical forces. An examination, by gel permeation chromatography, of the molecular weight distributions of the degraded polymers shows that the polymer molecules do not undergo the random scissions or bisections that have been postulated in the many prior theoretical treatments of polymer degradation, but rather degrade via non-random multiple scissions. The mode of degradation is such that while medium molecular weight polystyrene molecules degrade directly to a limiting low molecular weight species, high molecular weight polystyrene degrades both indirectly through an intermediate species and directly to the limiting species. An analysis of the degradation data from a wide molecular weight distribution polystyrene sample yields a rate equation which is a decreasing function of molecular weight. A model is proposed which predicts the form of the degradation rate equation on the basis of molecular size: the mode of degradation is explained by the concept of polymer chain entanglement. Possible degradation pathways are differentiated using a computer simulation.