In this presentation we explore the water transport in a series of epoxy networks with systematic variations in both their structural rigidity and cross-link density. Previously we performed positron annihilation lifetime spectroscopy measurements on these epoxies to characterize the packing defects, loosely referred to as “free volume,” as a function of temperature and correlated the free volume parameters to both the equilibrium moisture uptakes [1] and effective diffusion coefficients [2]. These measurements revealed a positive correlation between the free volume fraction of the material at absolute zero (static packing defects in the absence of thermal motion) and the equilibrium uptake for networks of nominally similar polarity across a wide temperature range. However, the polar group content turned out to be more important in establishing equilibrium uptake than the free volume fraction. These measurements revealed a mild dependence on the effective diffusion coefficient of water on the average positron packing defect cavity size at temperatures below 50 C, but a cavity size independence for temperatures above 50 C. What’s surprising is that these observations seem to contradict the conventional free volume concepts of transport where the diffusivity increases exponentially with free volume. To address this we explore the relationship between polymer dynamics and water transport by quantifying the mean square atomic displacement (MSD) fluctuations of these same epoxy networks with fixed window scans on the NIST High Flux Back Scattering Spectrometer. These measurements reveal a stronger correlation between the MSD and the transport kinetics than the positron free volume fractions. We quantitatively contrast the correlations between the positron free volume parameters and the moisture transport properties with the direct MSD measurements of matrix dynamics at the ns to ps time scale. These comparisons reveal a stronger correlation between polymer dynamics and moisture transport that seems to challenge previous theories suggesting that free volume governs transport. The nuances of these differences are discussed in detail. 1. Soles, C.L., et al., Journal of Polymer Science Part B: Polymer Physics, 1998. 36(17): p. 3035-3048. 2. Soles, C.L., et al., Journal of Polymer Science Part B: Polymer Physics, 2000. 38(5): p. 776-791.

Polymer dynamics, free volume, and transport in cross-linked polymer networks

251st National Meeting (2016)

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