Thermoelastic behaviour of amorphous polymers above and through the glass transition interval - I. Polystyrene

The thermoelastic behaviour of an unfractionated polystyrene was studied in the temperature interval 353-453 K in the regimes of isobaric cooling and of isothermal quasi-adiabatic loading, respectively. The main experimental results can be summarized as follows. 1. In the temperature interval far above the glass transition temperature T -g, both the temperature and volume relaxations of the polystyrene melt aft er sudden pressure jumps were completely reversible and proved to be simple exponential functions of the time. Therefore, by a straightforward applica tion of Eqs (1) and (2) to the relevant thermoelastic data obtained in a si ngle experimental run one can arrive at the reasonable values of the specif ic volume, specific heat capacity, thermal diffusivity and heat conductivit y of the polymer in the equilibrium melt state. 2. In the temperature interval close to T-g, both the temperature and volum e relations of the supercooled polystyrene melt in compression/expansion cy cles became markedly asymmetric and non-exponential. The low values of the exponent beta in the fractional-exponent Eq. (5) for the volume relaxation suggest a broad spectrum of relaxation times indicating the high degree of coupling between different mechanisms of the molecular motions involved.