Multiaxial elongations of polyisobutylene and the predictions of several network theories

Abstract

The experimental investigation of the rheological behaviour of polymer melts is mostly performed by simple shear and elongational tests. In former communications we described the performance of more general flows, viz. multiaxial elongations with different ratios of the principal components of the strain rate tensor. In this paper the results obtained with polyisobutylene at 23°C are compared with the predictions from different constitutive equations for these types of flows in order to examine whether these equations can describe the general nonlinear deformation behaviour. Network theories are known to do so for simple shear and simple elongation. Therefore, four types of network theories are taken into consideration. Their predictions agree only qualitatively with the measured data, and this means that large differences may exist. It is astonishing that for tests with a variation of the test mode during the deformation period, out of all equations taken into consideration here the linear viscoelastic equation predicts the best description of the experimental data. The conclusion is that the generalization of the material behaviour known from simple test programs to the behaviour in more general deformations may lead to very wrong predictions.