The dependence of yield behavior on temperature, pressure, and strain rate for linear polyethylenes of different molecular weight and morphology

Abstract

The temperature, strain‐rate, and pressure dependences of the yield stress have been determined for two polyethylene homopolymers of differing molecular weights and for a polyethylene copolymer. Samples were prepared by slowly cooling from the melt, and also by quenching in order to assess the effects of morphology on the yield behavior. The data have been analyzed on the basis of two Eyring processes acting in parallel. Comparison of all data sets reveals the existence in general of three distinct activated processes each with its own temperature, strain‐rate, and pressure dependences. The relative contribution of each process is dependent on the molecular weight, morphology, etc. Also discussed is the relation of these three yield processes to the well‐known loss processes of linear viscoelasticity.

The fracture toughness of tough polyethylenes by a novel high pressure technique

“…It is probable that Equation 2 is not completely general in that the linear dependence of OF on pressure may not hold at very high hydrostatic pressures. Chart and Williams [4] have reported a non-linear dependence of Ke on temperature for polyethylene in the region of the 3' relaxation and we have reported changes in the yield behaviour of polyethylene which can be associated with the mechanical relaxations of the polymer [5]. The 7 relaxation which normally occurs at "~--100~ at atmospheric pressure would be moved to higher temperatures with increasing hydrostatic pressure but since the mechanical relaxations are moved to higher temperatures at a rate of "~ 15~ per 100MNm -2 hydrostatic pressure, no non-linearity in the pressure dependence of K e due to the 3' relaxation would be expected up to the limit of hydrostatic pressure used in this work (i.e.…”

“…Since the stress-strain curves did not show a maximum which could be considered as the yield stress, a failure strain of 5% was taken below which the curves were considered as brittle failure. In previous work [2,5], a 2% offset stress has proved a reasonable measure of the yield stress and the 2% offset stress was Strain reached at strains greater than 5% for all pressures used in this work. At hydrostatic pressures below 250 MN m -2 , Rigidex 50 failed only after a degree of ductility and for these cases, the fracture mechanics approach could not be applied.…”

Polymer ultradrawability: the crucial role of α-relaxation chain mobility in the crystallites

“…This corresponds to the classical Eyring model of cold flow in glasses, which is based on stress-aided thermally activated mobility [113,114]. There is good evidence that tie molecules between crystallites play an important role in the plastic deformation of semicrystalline polymers (see Fig.…”

Temperature and strain-rate dependence of yield stress of polyethylene

“…Previous work 8 has considered that the yield stress in polyethylene can be modelled as a propagation-controlled process, following Eyring theory. For a propagation controlled yield process, the yield stress is considered as the stress required to propagate preexisting defects within the materials.…”