Unusually large hysteresis in temperature-responsive phase-transition behavior of aqueous solutions of isotactic copolymers comprising N-ethylacrylamide and N-isopropylacrylamide

Radical copolymerizations of N-ethylacrylamide (NEAAm) and N-isopropylacrylamide (NIPAAm) at various ratios were conducted in N-ethylacetamide at –40°C to prepare isotactic copolymers with mm triad contents of 48.7 % - 50.8 %. The temperature-induced phase transition behaviors of their aqueous solutions were investigated using light transmittance (500 nm) and dynamic light scattering. Reversible and sharp phase transitions were observed in the transmittance of aqueous solutions (1.0 wt%) of copolymers with NIPAAm compositions of ≤9.1 mol%. The phase transition temperatures in both the heating and cooling processes gradually decreased with increasing hydrophobic NIPAAm composition. However, copolymers with NIPAAm compositions of 13.9 to 28.0 mol% exhibited large hystereses (ca. 40 °C) in which the phase transition temperature in the cooling process dramatically decreased. Furthermore, the copolymer with NIPAAm composition of 37.8 mol% was insoluble in water. Syndiotactic poly(NEAAm-co-NIPAAm)s were soluble in water and showed reversible and sharp phase transitions regardless of the NIPAAm composition, which indicates that stereostructure plays an important role in the induction of unusually large hysteresis. Dynamic light scattering analysis suggested that the dehydrated polymers were swollen in the cooling process, even below the phase transition temperature in the heating process, likely because of the cross-linking domain formed by isotactic NIPAAm segments with hydrophobic interaction in the dehydrated state.

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