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 $\mathrm{V*=kT(\partial \hspace{0.17em}}\ln \mathrm{\epsilon ̇/\partial \hspace{0.17em}}\ln {\mathrm{\sigma )}}_{\mathrm{T,P}}\mathrm{(\partial \hspace{0.17em}}\ln {\mathrm{\sigma /\partial P)}}_{\mathrm{T,\epsilon ̇}}$ . 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 ų at 0.7 kbar to about 160 ų at 3.5 kbar and 120 ų 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.