Upper critical solution temperature-type thermosensitive hydrogel phase equilibrium of poly(2-hydroxyethyl methacrylate)/water/n-alkanol mixtures

We investigated the swelling behavior of upper critical solution temperature (UCST)-type nano-sized poly(2-hydroxyethylmethacrylate) (PHEMA) particle gel with n-propanol and n-butanol in the presence of water. Because PHEMA is a biocompatible hydrogel and is highly water absorptive, it is worthwhile to investigate the phase behavior between PHEMA and water. The swelling behavior of the cross-linked PHEMA particle gel and the phase miscibility of the linear PHEMA mixed with the n-alkanol/water solution were evaluated. Our experimental results showed that increasing the water fraction in the mixed solvent gradually reduces the swelling temperature. Moreover, the two phase region of the PHEMA/water/n-butanol mixture showed sudden expansion at ∼15 vol% water in the ternary diagram. In these systems, oriented interactions among each component exist, which strongly affect the chemical association. The modified double lattice (MDL) and Flory-Huggins (FH) models were employed to describe the swelling and phase behaviors of the given ternary systems. The MDL model matched our data much better than the FH model because it was able to correctly represent the oriented interactions from hydrogen bonding or other specific forces encountered in the systems. To demonstrate the model compatibility to UCST-type swelling behavior, we also examined other polymer solutions. Polyvinyl acetate (PVAc) gel/2-propanol was studied as an associating system and polystyrene (PS)/cyclohexane was evaluated as a non-associating system.