The investigation on states of water in different hydrophilic polymers by DSC and FTIR

The interaction between polymers and water in four hydrophilic polymer aqueous solutions were investigated by DSC and FTIR. DSC result shows that the different hydrophilic polymer/water mixtures have various water calorimetric behaviors in the melting temperature range of freezable bound water as well as free water. The melting temperature of freezable water and the amount of non-freezable water in the mixtures vary with the change of chemical structure of polymers. The melting point of the freezable bound water doesn't change with the water content, revealing that water bound weakly to polymer chains can form a stable crystalline structure at high water content. For the three hydrophilic polymer/water mixtures with C=O group, the weight ratio of non-freezable water to polymers is constant, but varies with polymer chemical structures. The FTIR spectra confirmed the formation of the hydrogen bonds and it was found that there exist different states of water based on various strengths of hydrogen bonds. The OH stretching bands indicated the fraction of strongly bound water decreases with increasing water content. It was concluded that at least in hydrophilic polymer aqueous solutions with polar sites in polymer chains, the formation of non-freezable water is ascribed to the hydrogen bonds between hydrophilic polymers and water molecules. Different strengths of hydrogen bonds can affect the thermal behaviors of water in the hydrophilic polymer/water mixtures.