Effect of Free-Volume Hole Fraction on Dynamic Mechanical Properties of Epoxy Resins Investigated by Pressure–Volume–Temperature Technique

Abstract

Dynamic mechanical analysis experiments were carried out to investigate the mechanical properties of four types of chemically different epoxy resins. Pressure-volume-temperature (PVT) experiments were performed to determine the free-volume hole fraction (hPVT) of each epoxy resin using the Simha-Somcynsky lattice-hole theory. Using the Williams-Landel-Ferry equation, the correlations between the relative hole fraction (1 - hPVTTr/hPVT, where hPVTTr is the hole fraction at a reference temperature Tr) and four typical parameters reflecting dynamic mechanical properties [storage modulus (E'), loss modulus (E″), damping factor (tanδ), and complex viscosity (|η*|)] were studied in the temperature range from Tg(PVT) (the glass transition temperature determined by PVT data) to Tg(PVT) + 100 °C. In the temperature range from Tg(Eonset') (temperature corresponding to the intersection of the two tangent fitting lines in the E'(T) curve indicating the glassy-state and glass-transition stages) to Tg(PVT) + 100 °C, the variations in the four dynamic mechanical parameters with a relative hole fraction could be separated into two distinct categories: (i) log[E'(T)] and log[|η*|(T)] decreased linearly to their minimum values and then remained nearly unchanged with increasing relative hole fraction, and (ii) log[E″(T)] and log[tanδ(T)] first increased monotonically to their maximum values and then decreased linearly with the increasing relative hole fraction. This study demonstrates that the PVT technique is a feasible and reliable experimental method to determine the hole fractions of thermoset polymers.