Kinetics of deformation of PTFE at high pressure

摘要

The dependence of the deformation behavior of PTFE on hydrostatic pressure is characterized by determination of the activation volume for plastic flow, V*, which is defined as V*=kT(∂ lnЄ˙/∂ lnσ)T,P(∂ lnσ/∂P)T,Є˙. At a given strain the first partial derivative in this equation, a measure of the strain-rate dependence of the flow stress, is observed to increase with pressure, but at a slower rate than the second partial derivative, a measure of the pressure dependence of the flow stress, decreases with pressure. Hence V* decreases from a value on the order of 300 Å3 at 0.7 kbar to about 160 Å3 at 3.5 kbar and 120 Å3 at 7.59 kbar. The decrease in V* occurs discontinuously near 2.4 kbar and we believe this to be due to a change in the rate-limiting mechanism for deformation. This transition has a marked similarity to that previously observed by the authors in polyethylene; it appears possible that the same deformation mechanisms are operative in both materials.