The Compressibility of Crosslinked Polymers

Bulk compressibilities of seven crosslinked polymers have been measured at pressures as high as 20,000 atmospheres and over the temperature range 25 to 250°C. Volume changes of up to 20% were observed. In most cases the pressure vs. change in volume isotherms were nonlinear and the coefficients of compressibility at one atmosphere increased with increasing temperature. For many of the polymers, pressure‐induced glass transitions are observed. From data on the pressure and temperature dependence of the glass transition, glass‐rubber phase diagrams have been prepared. The nature of the compressibility process is qualitatively discussed in terms of the BUECHE theory. It is shown that the EHRENFEST relationship dTg/dP ≃ TVΔα/ΔCp holds for crosslinked polymers.