Contributions of the Nanovoid Structure to the Kinetics of Moisture Transport in Epoxy Resins

摘要

Absorbed moisture can degrade the physical properties of an epoxy resin, jeopardizing the performance of an epoxy-based component. Although specific water-epoxy interactions are known to be very important in determining transport 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 kinetics of mosture transport. The topology is quantified via the positron annihilation lifetime spectroscopy technique in terms of the size and volume fraction of electron density heterogeneities 5-6 A in diameter, a dimension comparable to the 3-A kinetic diameter of a water molecule. Surprisingly, the volume fraction of such nanopores does not affect the diffusion coefficient (D) of water in any of the resins studied. For temperatures at and below 35degC, 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 flux of moisture into the epoxy does appear to correlated with the intrinsic hole volume fraction. That this correlation persists only in the initial stages of absorption is partially understood in terms of the ability of the water to alter the nanopore structure; only in the initial stages of uptake are the nanopores, as quantified in the dry state, relevant to transport. The role of specific epoxy-water interactions are also discussed in terms of transport kinetics. The lack of a correlation between the topology and transport suggests that polar interactions, and not topology, provide the rate-limiting step of transport.