The flow of solid polymers under high pressure

Abstract

The deformation and flow properties of solid polymers under uniaxial tensile and compressive loading have been determined for a variety of both crystalline and amorphous polymers at hydrostatic pressures to about 7 , kb. For some polymers, like polytetrafluoroethylene, cold drawing is inhibited by increasing pressure, and scanning electron micrographs show that local plastic deformation is greatly reduced. However, the true flow stress and the true fracture stress both increase with pressure, as does, also, the elastic modulus. The increase is greatest in the low-pressure range below 2 kb. This behavior is attributed to the pressure-induced shifting of a lowtemperatureβ′-transition. The influence of polymer composition and of the initial elastic constants on the pressure dependence of the elastic modulus and the yield stress is discussed. The observed behavior can be analyzed in terms of finite strain effects and a pressure-dependent yield criterion. It is shown that some amorphous polymers undergo a pressure-induced brittle-ductile transition, and these findings are used to explore the hydrostatic extrusion of polyimide and polysulfone at room temperature.