Universality of Dynamic and Thermodynamic Behavior of Polymers near their Glass Transition

Describing the dynamics and thermodynamics of amorphous materials near the glass transition is a major challenge in soft-matter physics and polymer engineering. The relaxation time changes many orders of magnitude as the temperature approaches the glass transition, Tg. The glass transition, in turn, depends on the cooling rate (in DSC or dilatometry experiments) and the pressure. While there are several empirical rules relating the thermal expansion, density, relaxation time, temperature, and pressure (Boyer-Spencer, Simha-Boyer, and the Casalini-Roland “thermodynamical scaling”), there has been no unified theoretical model linking them together. Here, we apply the principle of corresponding states within the dynamic Sanchez-Lacombe two-state, two-(time)scale (SL-TS2) theory to derive a universal polymer equation of state (EoS). The proposed EoS describes the α-relaxation time (“dynamic EoS”) and specific volume of amorphous polymers (“volume EoS”) using five material parameters – dynamic fragility (m), glass transition temperature, elementary relaxation time, bulk modulus, and specific volume at the glass transition point. The model is compared to experimental data for nine amorphous polymers with varying values of Tg and m, and a good qualitative and quantitative agreement is found. The new EoS could serve as a bridge to connect experimental data (measured at low cooling rates) with computer simulations (done at very high cooling rates).