Lower critical solution temperature behavior of poly(N-tetrahydrofurfuryl(meth)acrylamide) in water and alcohol-water mixtures.

The temperature responsiveness of poly(N-tetrahydrofurfurylacrylamide) (PTHFA, phase transition temperature, T(p) = 38 °C) and poly(N-tetrahydrofurfurylmethacrylamide) (PTHFMA, T(p) = 43 °C) in water and alcohol-water mixtures was investigated by infrared and Raman spectroscopy. Their T(p) increased monotonically with increasing concentration of added methanol but exhibited a re-entrant behavior in 1-propanol-water mixtures. Their amide I bands consist of three components because of doubly, singly, and zero hydrogen-bonding amide carbonyls in these mixtures, and the average number of hydrogen bonds per one C═O decreased with increasing concentration of the alcohols. A red shift of the amide II band also indicates decrease of H-bonding to the N-H group. The reduction then destabilizes the solution and lowers T(p). On the other hand, red shifts of the ν(C-H) bands indicate replacement of hydrophobically hydrating water molecules by the alcohols, which may stabilize the solutions. The latter and the former effect may be more effective in the methanol-water and the 1-propanol-water mixtures, respectively. The behaviors of the copolymers of N-isopropylacrylamide and N-tetrahydrofurfurylacrylamide or N-tetrahydrofurfurylmethacrylamide in methanol-water mixtures gradually changed from a monotonous behavior to a reentrant one with an increasing content of N-isopropylacrylamide, suggesting that the interaction between each individual monomer unit and solvent molecules cooperatively determines their behaviors as a whole.