Dielectric studies of amorphous polymers at high pressures

摘要

It is known that application of hydrostatic pressure causes an increase inTgat low pressures; however, some question has arisen as to whether the increase will continue indefinitely. Two possibilities exists: (i)Tgmay level off to a constant value at high pressures, or (ii)Tgmay 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 agr;hyphen;transition temperature,Tagr;, for a number of amorphous polymers and to relate this to the pressured dependence ofTg. The results indicate that at low pressuresdTagr;/dPmay decrease with increasing pressure (i.e.,Tagr;may appear to be levelling off), but at higher pressures (greater than sim;2 kbar)dTagr;/dPreaches a constant, nonhyphen;zero value (i.e.,Tagr;increases linearly with pressure). In addition, the high pressure limiting value ofdTg/dPappears to be roughly proportional to the atmospheric pressureTgof the polymer tested.