Phase transition in polymer hydrogels investigated by swelling, DSC, FTIR and NMR

Abstract The swelling and thermal behavior of single (SN) and double network (DN) hydrogels containing polyacrylamide (PAAm) and temperature sensitive poly( N , N -diethylacrylamide) (PNNDEAAm) were investigated by gravimetry and differential scanning calorimetry (DSC). Changes of the hydration of carbonyl groups and mobility of polymer network chains in the hydrogels with temperature were studied by Fourier-transformed infrared and proton nuclear magnetic resonance ( 1 H NMR) spectroscopy. The enthalpy of dissociation of the hydrophobic interaction, minimum and maximum numbers of water molecules per monomer unit in PNNDEAAm SN hydrogel involved in the hydrophobic hydration were determined by DSC. The volume phase transition accompanied by expulsion/uptake of water and heat absorption/release in heating/cooling was exhibited by all DN hydrogels containing PNNDEAAm. In these hydrogels, above the phase transition temperature, the population of hydrated carbonyls is enriched with the free and single hydrated ones in the same way as in the aqueous solution of linear PNNDEAAm and mobility of the PNNDEAAm chains is strongly reduced. Presence of the PAAm network does not influence the phase transition temperature but strongly reduces the enthalpy of the phase transition, promotes higher degrees of hydration of carbonyls, and increases mobility of the PNNDEAAm chains in the mixed hydrogels above the phase transition temperature.