Contributions of the nanovoid structure to the kinetics of moisture transport in epoxy resins

Absorbed moisture can degrade the physical properties of an epoxy resin, je opardizing the performance of an epoxy-based component. Although specific w ater-epoxy interactions are known to be very important in determining trans port behavior, the role of network topology is not clear. In this article, a series of epoxies in which the topology is systematically varied (and the polarity held constant) is used to explore how topology influences the kin etics of moisture transport. The topology is quantified via the positron an nihilation lifetime spectroscopy technique in terms of the size and volume fraction of electron density heterogeneities 5-6 Angstrom in diameter, a di mension comparable to the 3-Angstrom kinetic diameter of a water molecule. Surprisingly, the volume fraction of such nanopores does not affect the dif fusion coefficient (D) of water in any of the resins studied. For temperatu res at and below 35 degrees C, there is a mild exponential dependence of D on the average nanopore size observed. Otherwise, the kinetics of moisture transport do not appear to depend on the nanopores. However, the initial fl ux of moisture into the epoxy does appear to correlate with the intrinsic h ole volume fraction. That this correlation persists only in the initial sta ges of absorption is partially understood in terms of the ability of the wa ter to alter the nanopore structure; only in the initial stages of uptake a re the nanopores, as quantified in the dry state, relevant to transport. Th e role of specific epoxy-water interactions are also discussed in terms of transport kinetics. The lack of a correlation between the topology and tran sport suggests that polar interactions, and not topology, provide the rate- limiting step of transport. (C) 2000 John Wiley & Sons, Inc.