Water vapor transport in structurally varied polyurethans

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Abstract
A study has been made on four chain-extended polyurethan (PU) elastomers differing only in the nature of the flexible segment, namely, poly(butylene adipate) (PBA), poly(tetramethvlene oxide) (PTMO), poly(propylene oxide) (PPO), and poly(ethylene oxide) (PEO). The saturation water concentration increases from 1.6 to 3% in the first three samples and jumps to 113% in PEO-PU. The flux normalized to 1 mil thickness is nearly the same in the first three samples, whereas an increase of 70% is expected in PPO-PU on the basis of the higher water solubility and a 24-fold increase is expected in PTMO-PU on the basis of the 27°C lower glass transition. For PBA-PU, the upward curvature of the isotherm at higher activities, zero heat of mixing, and linear decrease in diffusion constant D with concentration indicate that immobilization of water in clusters plays an important role in the transport mechanism. Nearly identical behavior is observed in PTMO-PU, suggesting that the lower than expected D in this polymer (and in PPO-PU) must be attributed to domain structure and to the inaccuracy of the universal form of the Williams-Landell-Ferry (WLF) equation for the estimation of diffusion constants rather than to increased levels of water clustering. In PEO-PU the flux is 100-fold higher than the other PU as expected from the greater swelling; the sorption isotherm rises exponentially near saturation, and D drops sharply with concentration as measured by the sorption method and to a lesser extent from transmission measurements. However, these results are inconsistent with the liquid transmission rate apparently due to temperature effects, and it is concluded that D increases with swelling despite pronounced clustering.

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