LCST behavior of copolymers of N-isopropylacrylamide and N-isopropylmethacrylamide in water

Abstract

The lower critical solution temperature (LCST) behavior of copolymers of N-isopropylacrylamide (NiPA) and N-isopropylmethacrylamide (NiPMA) in water was studied as a function of the copolymer composition, using a combination of turbidity measurements and differential scanning calorimetry (DSC). The copolymers were prepared by free radical polymerization using N,N-dimethylformamide as a solvent and α,α′-azobis(isobutyronitrile) as an initiator. The copolymer composition was determined by elemental analysis. It was found that the temperature (T c) at which the copolymer undergoes a phase transition, i.e., LCST, increases linearly with increasing the mole fraction (f m) of NiPMA in the copolymer, within the T c range from 32 °C (at f m = 0; NiPA homopolymer) to 42 °C (at f m = 1; NiPMA homopolymer). Also found from heating DSC thermograms were the linear dependencies of the enthalpy (ΔH) and entropy (ΔS) changes at T c upon f m. However, the ΔH (5.5 kJ/unit-mol) at f m = 1 was slightly smaller than that (5.7 kJ/unit-mol) of poly(N-n-propylacrylamide) but considerably smaller than that (7.8 kJ/unit-mol) of poly(N-n-propylmethacrylamide). The same trend was observed in the f m dependence of ΔS. These results were discussed in terms of the structural effects of the NiPMA monomer unit on the heat-induced phase transition in water of poly(NiPA-co-NiPMA)s. It was suggested that a strong interaction of water with the amide group in the NiPMA would raise the transition temperature, but a local dehydration which occurs around the isopropyl side chain would not lead to large changes in the enthalpy and entropy at T c.