Dielectric studies of amorphous polymers at high pressures

Abstract

It is known that application of hydrostatic pressure causes an increase in Tg at low pressures; however, some question has arisen as to whether the increase will continue indefinitely. Two possibilities exists: (i) Tg may level off to a constant value at high pressures, or (ii) Tg may continue to increase at all pressures. The former is suggested by thermodynamic theories of the glass transition; the latter by free volume theories. In this study dielectric methods have been used to measure the pressure dependence of the α-transition temperature, Tα, for a number of amorphous polymers and to relate this to the pressured dependence of Tg. The results indicate that at low pressures dTα/dP may decrease with increasing pressure (i.e., Tα may appear to be levelling off), but at higher pressures (greater than ∼2 kbar) dTα/dP reaches a constant, non-zero value (i.e., Tα increases linearly with pressure). In addition, the high pressure limiting value of dTg/dP appears to be roughly proportional to the atmospheric pressure Tg of the polymer tested.