Interaction of phenols with thermo-responsive hydrogels

Swelling and differential scanning calorimetry measurements are reported on the effect of three hydrotropic molecules, phenol, resorcinol and phloroglucinol, on poly(N-isopropyl acrylamide) (PNIPA) hydrogels. As the hydroxyl number in this molecular sequence increases the temperature range of the volume phase transition (VPT) decreases and broadens, reflecting nonuniformity in the distribution of the guest molecules inside the gel and the effects of steric hindrance. A mean-field model, based on the Curie–Weiss theory of magnetism, is used to illustrate the effect of the phenols on the gel swelling. Observations with other phenols (pyrocatechol, hydroquinone and pyrogallol) show that the depression of the VPT temperature is also governed by the relative position of the hydroxyl groups.