Effect of curing agent polarity on water absorption and free volume in epoxy resin studied by PALS

Microstructure and water absorption were systematically studied by positron annihilation lifetime spectroscopy (PALS), gravimetric measurements and Differential Scanning Calorimetry (DSC) for epoxy resins DER331 (E51) cured with three different kinds of amine curing agents DDS, DDM and MOCA. Experimental results indicated that the water absorption as a function of immersed time could be well fitted to Fick’s second law. Based on the gravimetric measurement, we found that the equilibrium water sorption M∞M∞ and the diffusion coefficients D of the epoxy resins have an order: E51-DDS > E51-DDM > E51-MOCA, which indicated that the curing agent plays an important role in determining the content of the water absorbed. Positron experimental results showed that the o-Ps lifetime dramatically decreased with the immersed time from 0 to 6 h, which suggested that water molecules were filled into free-volume holes and the interaction between the water–polymer decreased the mobility of molecular chains. In order to deeply discern the influence of water absorption upon the free volume, the continuous lifetime analysis, i.e. the maximum entropy lifetime method (MELT) was employed to obtain the distributions of the ortho-positronium (o-Ps) lifetime and the of the free-volume holes. From MELT analyses, we found the existences of two the long-lived components (τ3 and τ4), which indicated that two kinds of different o-Ps states exist. The shorter long-lived component (τ3) is related to the segmental packing density in local ordered region. Compared to dry sample, two peaks of the o-Ps lifetime and the free-volume hole in the wet samples all drift to low values, especially, this drifts is more marked for the water sorption occurring at higher temperature 75 °C. This fact suggested that when the epoxy resin is in the glassy state, the interaction between the water and matrix restricts the motion of segmental chains and prevents from the free-volume hole swelling.