SOME PROBLEMS IN THE NONLINEAR VISCOELASTICITY OF AMORPHOUS GLASSY-POLYMERS

The non-linear viscoelastic behaviour of glassy amorphous polymers is briefly reviewed. Conventional explanations such as frictional heating, strain dilatation, stress-activated flow, etc. are shown to be inadequate. The same holds for the rejuvenation hypothesis which says that large deformation processes create free volume. This hypothesis leads to a reasonable qualitative description but a quantitative one seems to be impossible, mainly because free volume is an ill-defined quantity. It is finally considered whether non-linear viscoelasticity and yielding might be explained from the non-linear force fields between the molecules. It turns out that the small-strain moduli and the yield stress can be predicted quantitatively while a qualitative explanation is obtained for some hitherto unexplained experimental facts. The implication is that the behaviour close to yield basically differs from that at small strains; so, flow at the yield point is not simply an acceleration of small-strain creep.