The role of water in structural changes of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide) studied by FTIR, Raman spectroscopy and quantum chemical calculations

Temperature-induced phase transition in water solutions of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylmethacrylamide) (PNIPMAM) have been studied by ATR FTIR and Raman spectroscopy in combination with quantum chemical calculations. The presence or absence of the α-methyl group has a strong effect on the physical structure of water solutions. Although the hydrophobic interactions for PNIPMAM and PNIPAM are very similar, PNIPMAM with additional methyl group exhibits significantly weaker intermolecular interactions between the amide groups. That effect is the cause of the higher transition temperature T_t by about 8 °C for PNIPMAM compared to PNIPAM due to the formation of larger compact structures. The presence of the methyl group is significant for the reversibility of the temperature transition during the backward cooling as the dissolution of more stable compact PNIPMAM requires overcoming of a higher energy barrier and shows a strong hysteresis.