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It is reported on precision measurements of the hydrostatic extrusion rate of polyethylene, polyoxymethylene, and their blends in dependence on pressure and temperature. It turns out that the extrusion rate change after a sudden pressure change consists of an immediate response and a second one which relaxes toward its new equilibrium value with a time constant of several minutes. Both relaxation components have different sign. Increasing pressure causes a decrease of the extrusion rate by the slow relaxation step. Relaxation strength and inverse time constant of this relaxation component exhibit a maximum at a certain pressure. The overall relaxation behaviour is assumed to be caused by a superposition of the competing effects of free volume relaxation in the amorphous parts in these semicrystalline polymers, of compressibility relaxation, of volume recovery in the extruded strands, and of viscosity changes in the sample materials during the pressure induced plastic deformation of both the amorphous and the crystalline phases.
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Schaffner et al. (1991) studied this question.
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