Influence of water content on structure and mobility of polyvinyl alcohol: a molecular dynamics simulation

Fully atomistic molecular dynamics simulations have been performed on aqueous solutions of polyvinyl alcohol (PVA) to get some insight into the effect of water content on PVA structure and dynamics. Simulations were carried out on mixtures of water/atactic PVA at 300 K covering a composition range (water weight fraction X(w)=0.1-0.8. The analysis focuses on polymer segmental dynamics, which is monitored by the mean square displacement, Van Hove self correlation function, non-Gaussian parameter, and the intermediate incoherent scattering function. It was found that PVA dynamics is strongly affected by addition of water. Both, PVA carbon and hydrogen (hydroxyl and main chain hydrogens) motions are faster with further dilution. Differences on atom mobility (dynamic heterogeneity) decreases as water content increases allowing more isotropic segmental motions. Moreover, the non-Gaussian parameter shows a maximum at X(w)=0.25 followed by a continuous decrease with dilution. The non-Gaussian effects become more important as the water content decreases and dynamic heterogeneity is enhanced. These results are explained in terms of the plasticization effects of water on the polymer and the intrinsic dynamic heterogeneity observed on PVA dynamics.