A Discussion of the Molecular Mechanisms of Moisture Transport in Epoxy Resins

A typical epoxy formulation can absorb several weight percent of water, seriously degrading the physical properties of the resin. In two preceding publications (Soles, C. L.; Chang, F. T.; Bolan, B. A.; Hristov, H. A.; Gidley, 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 Phys 2000, 38, 776), the role of electron density heterogeneities, or nanovoids (as measured through positron annihilation lifetime spectroscopy), in the moisture-transport process is elucidated. In this article, the influence of these nanovoids is examined in light of both the specific epoxy–water interactions and the molecular motions of the glassy state to develop a plausible picture of the moisture-transport process in an amine-cured epoxy resin. In this description, the topology (nanopores), polarity, and molecular motions act in concert to control transport. Water traverses the epoxy through the network of nanopores, which are also coincident with the polar hydroxyls and amines. In this respect, the nanopores provide access to the polar interaction sites. Furthermore, the sub-Tg (glass-transition temperature) molecular motions coincident with the onset of the b-relaxation process incorporate these polar sites and, hence, regulate the association of water with the epoxy. In effect, the kinetics of the transport mirror the dynamics associated with the local-scale motions of the b-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 of an amine-cured epoxy, contrary to popular theories of transport. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 792–802, 2000