Pressure dependence of elastic moduli

In a recent review [l] on the effects of hydrostatic pressure on the mechanical behavior of polymers, two of us noted that the pressure dependence of the Young's modulus is large if the value of the modulus at atmospheric pressure is low and that it is comparatively modest if the modulus at atmospheric pressure is high [l, p. 611. It was also noted that among the polymers listed [l, Tables 1 and 21, the amorphous polymers seemed to be less sensitive to pressure than many of the crystalline polymers. This difference in pressure sensitivity appears to lie in the location of glass transition temperature Tg of the amorphous region with respect to the testing temperature (room temperature, Tg). When Tg > Tr, as in many amorphous polymers (e.g., PS, PSU, PMMA), the rate of increase of modulus with pressure is moderate; when Tg < Tr, as in many crystalline polymers (e.g., PE, PP), the rate of increase is much higher. Furthermore, in a given polymer, hydrostatic pressure induces T, to shift to higher temperatures. In polymers with Tg < Tr, at a critical pressure when Tg is shifted to Tg, there appears a break in the slope of the modulus versus pressure curve [l].