Zero Poisson's ratio PTFE in uniaxial extension

A polytetrafluoroethylene (PTFE) paste was extruded using a capillary rheometer at various temperatures through dies of different reduction ratios. Uniaxial tensile experiments have been performed on the collected extrudates using the Sentmanat Extensional Rheometer (SER) at different temperatures and Hencky strain rates. A nonlinear viscoelastic model is used to model the transient tensile results. A model is also developed to predict the tensile ultimate strength as a function of processing conditions such as temperature and die reduction ratio, as well as the testing operating conditions i.e. temperature and Hencky strain rate. The results show that the ultimate strength increases with extrusion temperature and die reduction ratio and decreases with testing temperature. Moreover, the relative density of the extrudates upon stretching is studied and it is found that it decreases by increasing the level of stretching. A simple mathematical model which accounts for the Poisson's ratio and elastic strain recovery is also developed to model the data. It is shown that the experimental findings are in agreement with the fibril structure/morphology obtained from SEM micrographs.