Kinetics of deformation of PTFE at high pressure

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(∂ lnε̇/∂ lnσ)T,P(∂ lnσ/∂P)T,ε̇. 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.