Investigation on the Heat Capacity of Water in Poly(Acrylic Acid)/Water Mixtures Through Stepscan Method

Abstract

The heat capacity of poly(acrylic acid)/water mixtures with low water contents were measured by the stepscan differential scanning colorimetry (DSC) method and the Cp of the bound water in the mixtures were obtained based on simple equations with an assumption that the effect of interaction between polymer and water molecules could be ignored. The results indicated that the water decreased the glass transition temperature of poly(acrylic acid) (PAA), evidently as a plasticizer, and PAA/water mixtures were thermodynamically homogeneous blends. It was found that the Cp of nonfreezable water in PAA/water mixtures was higher than those of glassy water and cubic ice. The Cp increased with the water content, showing that average interactions between polymer and water decreased with water content. The Cp of bound water in the mixtures in the liquid state was lower than that of bulk water and also increased with water concentration, since the water molecules’ regular structures were broken. It showed that the Cp of bound water in the mixtures was lower than that of free water. The Cp of glassy water was calculated based on experimental Cp values of the PAA/water mixtures, which provided an easy way to obtain the Cp values of glassy water through a simple experiment. The values, however, were higher than those obtained by other researchers.