The glass transition behaviour and thermodynamic properties of amorphous polystyrene

The effect of molecular weight (MW) and thermal history on the heat capacity (c_p) of amorphous polystyrene over the temperature range from 200–440 K has been measured using differential scanning calorimetry (d.s.c.) General equations are given that show the influence of MW on c_p in both the glassy and liquid states. An enthalpic definition of the glass temperature reflects changes in the glass structure, due to thermal history, that are not found using conventional d.s.c. constructions. Thermodynamic and statistical mechanical theories give reasonable approximations to the observed T_g-MW curve but small systematic deviations are found. The effect of thermal history on T_g can be reproduced using a MW-dependent activation enthalpy.