Cooperative behavior of poly(vinyl alcohol) and water as revealed by molecular dynamics simulations

An aqueous poly(vinyl alcohol) (PVA) model has been extensively studied by using the molecular dynamics (MD) simulation method. The employed molecular and force field models are validated against the available data in the literature. In particular, the glass transition temperature (Tg) is determined from the specific volume versus temperature, which compares well with the experimental observations. The diffusion coefficients of water (H2O) through the PVA matrix follow the Arrhenius equations at both temperature regions separated by Tg, indicating the existence of free and bound water defined by hydrogen bonds (HBs). It has also been confirmed that HBs occur between PVA and H2O, between PVA and PVA, between H2O and H2O, and all of them play the key roles in the glass transition. The local dynamics suggested by the decorrelations of various bond vectors can be well described by the Williams-Landel-Ferry (WLF) equation. This work demonstrates the cooperative behavior of PVA and H2O which is responsible for the glass transition of the whole binary system.