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 (h(PVT)) of each epoxy resin using the Simha-Somcynsky lattice-hole theory. Using the Williams-Landel-Ferry equatiol, the correlations between the relative hole fraction (1 - h(PVT)(T)/h(PVT), where h(PVT)(T) is the hole fraction at a reference temperature T-r) and four typical parameters reflecting dynamic mechanical properties [storage modulus (E'), loss modulus (E "), damping factor (tan delta), and complex viscosity (vertical bar eta*vertical bar)] were studied in the temperature range from T-g(PVT) (the glass transition temperature determined by PVT data) to T-g(PVT) + 100 degrees C. In the temperature range from T-g(E'onset) (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 T-g(PVT) + 100 degrees 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[vertical bar eta*vertical bar(T)] decreased linearly to their minimum values and then remained nearly unchanged with increasing relative hole fraction, and (ii) log[E"(T)] and log[tan delta(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.