Configurational Enthalpy Model for Describing Temperature and Pressure Dependence of the Relaxation Time of Amorphous Polymers above Tg

Recently we reported [1] that the super-Arrhenian temperature dependence of the relaxation time for 21 molecular glass formers is quantitatively described by a single-parameter model based on the excess enthalpy, where the excess entropy based model of the Adam-Gibbs form fails to describe the data. For these molecular glass formers the liquid/crystalline enthalpy/entropy were obtained by straightforward integration of the heat capacity data, where excess enthalpy/entropy was obtained by subtracting the crystalline value from the liquid value and the independently measured heat of fusion fixes the value of integration constant. For polymeric glass formers the crystalline values are unavailable; consequently, the glassy heat capacity was used in place of the crystalline heat capacity to obtain the configurational (vs. excess) enthalpy/entropy, and the glassy PVT surface is used to determine these quantities at elevated pressures. We show that the resulting configurational enthalpy model describes the mobility data for 12 amorphous polymers, including both the temperature and pressure dependence of the relaxation time above Tg. The configurational entropy model in the Adam-Gibbs form fails to describe the mobility data.
1. Caruthers and Medvedev, Phys. Rev. Mat. 2, 055604 (2018)