Variation of glass temperature with pressure in polypropylene

Abstract

By measurement of the specific volume of polypropylene as a function of temperature at various pressures, the variation of glass temperature with pressure, dTg /dP, was determined. Within experimental error the magnitude of this quantity is the same as the value of T v ¯ Δ α / Δ C p , where Δα and ΔCp are the change in coefficient of expansion and specific heat respectively at the glass temperature. This is an indication that thermodynamics can be applied to the glass transition. The value of dTg /dP is the same as Δβ/Δα, where Δβ is the change in compressibility at Tg calculated from the data, but it is shown that this equality must follow as a consequence of the manner in which the experiments were carried out, quite independently of the application of thermodynamics.