A discussion of the molecular mechanisms of moisture transport in epoxy resins

A typical epoxy formulation can absorb several weight percent: of water, se riously degrading the physical properties of the resin. In two preceding pu blications (Soles, C. L.; Chang, F. T.; Bolan, B. A.; Hristov, H. A.; Gidle y, D. W.; Yee, A. F. J Polym Sci Part B: Polym Phys 1998, 36, 3035; Soles, C. L.; Chang, F. T.; Gidley, D. W.; Yee, A. F. J Polym Sci Part B: Polym Ph ys 2000, 38, 776), the role of electron density heterogeneities, or nanovoi ds (las measured through positron annihilation lifetime spectroscopy), in t he moisture-transport process is elucidated. In this article, the influence of these nanovoids is examined in light of both the specific epoxy-water i nteractions and the molecular motions of the glassy state to develop a plau sible picture of the moisture-transport process in an amine-cured epoxy res in. In this description, the topology (nanopores), polarity, and molecular motions act in concert to control transport. Water traverses the epoxy thro ugh the network of nanopores, which are also coincident with the polar hydr oxyls and amines. In this respect, the nanopores provide access to the pola r interaction sites. Furthermore, the sub-T-g (glass-transition temperature ) molecular motions coincident with the onset of the beta-relaxation proces s incorporate these polar sites and, hence, regulate the association of wat er with the epoxy. In effect, the kinetics of the transport mirror the dyna mics associated with the local-scale motions of the beta-relaxation process , and this appears to be the rate-limiting factor in transport. The volume fraction of the nanopores does not appear to be rate-limiting in the case o f an amine-cured epoxy, contrary to popular theories of transport. (C) 2000 John Wiley & Sons, Inc.