Influence of physical aging and side group on the free volume of epoxy resins probed by positron

The effect of physical aging and side group on the free volume has been studied by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimeter (DSC) for a series of epoxy resins cured separately with novolac resin and esterified novolac resins, that is, the hydroxyl group of phenol novolac resin was placed by a CH3COO-group, a CH3CH2 CH2COO- and a C6H5 CH2COO-group, which were named EP, EPA, EPB and EPP, respectively. Positron experimental results show that the side group plays an important role in determining free volume properties of epoxy resins, i.e. the stronger the cross-linking of curing agent, the smaller the free volume. This is confirmed by DSC measurement. On the other hand, the free volume relaxation properties were investigated by the Kolrausch–Williams–Watt (KWW), the single exponential and the double exponential functions in aging course for all samples, respectively. We found that ortho-positronium (o-Ps) intensity decreased with increasing aging time, which meant that the free volume diminished resulting from the formation of the local order structure due to the chain segmental rearrangement. The relaxation time constants of both τ and the parameter β have been calculated, which indicated that the order of relaxation rate of free volume characterized by τ−1 measured by PALS was the same as the enthalpy relaxation rate characterized by ∂ΔH/∂(log t) measured by DSC.