VOLUME CHANGES DURING STRESS-RELAXATION IN POLYETHYLENE

This paper presents the results of an experimental study of the stress relaxation behaviour of PE where the focus was on determination of th e volume changes taking place during the relaxation process. The dimen sions of the samples were followed using a specially designed non-dist urbing extensometer. The extensometer data were confirmed in experimen ts where the volume was measured with a specially designed liquid stre ss dilatometer. The bulk of the results was obtained with LDPE and LLD PE. High density polyethylene was shown to behave similarly. The decre ase in volume, corresponding to an increasing Poisson's ratio, during relaxation was approximately linear with log time. Volume vs. stress d iagrams were linear; the values of the apparent bulk modulus calculate d from them were only slightly higher than those obtained from the str ess-strain curves. Gruneisen parameter was measured and compared with reported values. Possible similarities between the volume change durin g stress relaxation and that occuring during the process of physical a geing are discussed.