Phase behavior of poly(N-isopropylacrylamide) in binary aqueous solutions

In this work, the phase behavior of linear poly(N-isopropylacrylamide) (PNIPA) in water–solvent mixtures was investigated. Several solvents, including low molecular weight alcohols, were selected and phase separation temperatures were determined through cloud point measurements. All the studied systems exhibited the cononsolvency effect, i.e. lower PNIPA compatibility within definite ranges of composition in water-rich mixtures. However, it was first detected that the coexistence of phase separation temperatures—a lower critical solution temperature (LCST) with an upper critical solution temperature (UCST)—at higher solvent concentrations in most systems, depend on the hydrophobic nature of the solvent. The change from a LCST to a UCST was correlated with the competition between polymer–water and polymer–solvent interactions mediated by compositional factors. The effects produced by the different solvents tested were qualitatively compared, considering aspects related to their particular molecular structures, such as the potential to form hydrogen bonds and the implications of the size and shape of non-polar groups for hydrophobic hydration.