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

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 () 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 - /, where is the hole fraction at a reference temperature ) and four typical parameters reflecting dynamic mechanical properties [storage modulus ('), loss modulus (″), damping factor (tanδ), and complex viscosity (|η*|)] were studied in the temperature range from (the glass transition temperature determined by PVT data) to + 100 °C. In the temperature range from (temperature corresponding to the intersection of the two tangent fitting lines in the '() curve indicating the glassy-state and glass-transition stages) to + 100 °C, the variations in the four dynamic mechanical parameters with a relative hole fraction could be separated into two distinct categories: (i) log['()] and log[|η*|()] decreased linearly to their minimum values and then remained nearly unchanged with increasing relative hole fraction, and (ii) log[″()] and log[tanδ()] 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.