Deformation of Polyethylene at High Pressure

The influence of pressure, deformation rate, and pressure fluid on the mechanical properties of high-molecular-weight polyethylene is examined. A miniature motor-driven testing machine which operates completely enclosed in a high-pressure vessel is employed. This device allows for the first time in any material measurement of the sensitivity of the flow stress σ to an instantaneous change of strain rate under superposed high hydrostatic pressure. From these data and the pressure dependence of σ it is possible to calculate the activiation volume V* for plastic flow as a function of pressure; it is suggested that V* is the appropriate parameter to describe the pressure dependence of a kinetic process such as plastic deformation. It is found that samples deformed in n-pentane generally exhibit a reduced flow stress relative to those tested in water. In addition, it is observed that V* for specimens tested in pentane is significantly less than for those tested in water (∼230 Å at 1 atm as opposed to ∼266 Å3). In both cases V* is observed to decrease moderately with increasing pressure.