Interactions between Water and Pharmaceutical Polymers Determined by Water-Vapor Sorption Measurement and Differential Scanning Calorimetry

Interactions between water molecules and various pharmaceutical polymers were studied by water vapor sorption and differential scanning calorimetry (DSC). Water-vapor sorption isotherms at 30°C were determined by gravimetric technique and the monolayer values and moisture content at a relative humidity of 79% were estimated for each sample. The state of the water molecules in moistened polymers was investigated using DSC and one or two exothermal peaks of water crystallization were observed in the DSC cooling curves. The content of non-freezing water, which is restricted by hydrophilic groups in polymer molecules, was determined from the amount of freezing and total water in the polymer. The amount of water sorbed at high relative humidity and non-freezing water in DSC measurement correlated to some extent. In hydrophilic polymers having numerous ether linkages, the amount of non-freezing water was larger than that predicted from the small amount of sorbed water in the vapor phase.