Water absorption in novolac epoxy resins studied by positron annihilation

Water absorption rates of four kinds of epoxy resins cured with phenol novolac (PH) were compared with the results of positron annihilation lifetime measurements (PAL). Three samples : cresol novolac (CR), tetra-methyl-biphenol (TMB) and bis-phenol A (BA), showed almost similar absorption rates, while tris-hydroxy-phenyl-methane (THPM) absorbed water twice as fast as others. Since THPM has a larger τ3 value (hence larger intermolecular-space holes) than the other samples, the correlation between τ3 and the absorption rate seems evident. For THPM, the lifetime distribution was shifted toward a shorter lifetime after absorbing water, which is interpreted in terms of the particular structure of three-dimensional holes