A differential scanning calorimetry study of the states of water in swollen poly(vinyl alcohol) membranes containing nonvolatile additives

Abstract

To find a possible reason for the enhanced solubility and hence the permselectivity of O 2 over N 2 in various membranes, the states of water in poly (vinyl alcohol) membranes swollen to swelling ratios ( H) ranging from 0.3 to 0.7 (water/swollen membrane, g/g) with water or aqueous solutions of five nonvolatile substances were investigated by differential scanning calorimetry (DSC) over the temperature range from 170 to 320 K. The enthalpy of the phase transition of the water confirmed that at least three states of water exist in the membranes, namely free, freezing bound and nonfreezing water, as already postulated. The addition of substances with high affinities to the polymer was found to result in a large increase in the amount of both freezing bound water and nonfreezing water. The extra increase of the latter type of water, with the hydration of the polymer and the additives being taken into account, is attributed to interactions in the polymer chain/surrounding water region where the added substances are concentrated.