Influence of pressure on yield and fracture in polymers

Some new data on the effects of pressure on a poly(p-oxybenzoyl) polymer are presented and existing data on various other polymers are reviewed and analyzed. It is demonstrated that the effect of pressure on the elastic response of a polymer depends on the location of T_g relative to room temperature, and that the modulus-pressure data can be used to estimate the pressure shift of T_g. Also, the pressure coefficient of the modulus increase can be deduced from considerations of finite strain elasticity theory. There is a marked increase of tensile and compressive yield strengths with pressure and this can be interpreted in terms of a Mohr-Coulomb type of yield criterion. In some polymers, hydrostatic pressure inhibits cold drawing and reduces the elongation to fracture. However, in other polymers which at atmospheric pressure fracture prior to yielding, increasing the pressure above some critical value can cause significant increases in ductility. This effect is utilized to show that even rigid high-temperature polymers, like polymide, can be successfully cold extruded at room temperature if a proper high-pressure environment is present. The nature of the changes occurring near the brittle-ductile transition pressure have been investigated by use of the scanning electron microscope and the possible influence of the pressure medium has been examined. SEM pictures of fracture surfaces and fracture modes of various polymers will be presented.