The mechanical behaviour of polymers under high pressure

Abstract

The effect of pressure on the tensile deformation of amorphous polycarbonate and poly(ethylene terephthalate) and semi-crystalline polychlorotrifluoroethylene and polytetrafluoroethylene was investigated up to 8 kb. Tensile deformations of polycarbonate at atmospheric pressure at temperatures down to 116°K were also performed. The former three polymers showed increases of yield stress, yield strain and elastic modulus, and decreases of fracture strain. Polytetrafluoroethylene behaved in an analogous manner up to 4 kb, beyond which both the ‘yield’ stress and elastic modulus deviated systematically from the lower-pressure behaviour. This was found to correlate with changes of bulk modulus at a solid–solid phase transition near 5 kb.

The pressure dependence of yield stress was fitted by modified von Mises and Mohr–Coulomb yield criteria. A material parameter describing this pressure dependence was obtained for these and other polymers and was found to correlate qualitatively with the strength-limiting temperature, T _g or T _m, polytetrafluoroethylene being an exception. It was found that the pressure dependence of yield strain in polycarbonate could be accounted for by free volume considerations.

Correlation of high-pressure behaviour with low-temperature behaviour led to two conclusions: firstly, from a knowledge of bulk physical properties and a pseudothermodynamic equation, developed here, changes in low-temperature yielding phenomena can be used to predict similar changes with high pressure, and, secondly, the ductile–brittle transition of a normally ductile amorphous polymer may be closely related to a specific dynamic mechanical relaxation, the temperature of observation changing with pressure.