DSC studies on bound water in polypropylene films plasma-grafted with 2-hydroxyethyl acrylate and acrylic acid or methacrylic acid

Abstract

Water sorbed on porous polypropylene films, grafted to various extents by plasma-graft copolymerization of 2-hydroxyethyl acrylate with acrylic acid or methacrylic acid, was examined using differential scanning calorimetry. From the heat of crystallization or melting of water in the grafted membrane, the non-freezing water content ( W nf) was calculated using the relationship W c = W f + W fb + W nf, where W t is the total water content in the membrane and W f is the free water content with phase transition temperatures similar to those of ordinary water and W fb is freezing bound water. The W nf values became larger with increase in the number of hydrophilic groups (graft ratio) in the membrane. The separation factor of water-ethanol was correlated with W nf value. A simplified static heterogeneous model is suggested to explain the dynamic process of membrane separation for water and ethanol.