Preparation and salt-resistivity of poly(vinyl alcohol)-4-hydroxyl phthalate hydrogels

A series of poly(vinyl alcohol)-4-hydroxyl phthalate (PVA-HP) polymers with different esterification degrees (ED: 98, 50, and 26 mol% for PVA-HP1.0, PVA-HP0.5, and PVA-HP0.25, respectively) were synthesized, and their structures were confirmed by FTIR and ¹H NMR. Using ethylene glycol diglycidyl ether (EGDGE) as a crosslinker, the PVA-HP hydrogels were prepared to examine their swelling properties and temperature dependence in a variety of salt solutions. As a typical kosmotrope, the sulfate anion showed a significant salting-in effect to PVAHP1.0, whereas thiocyanate and chloride showed no obvious effect, demonstrating the anti-Hofmeister-Series (anti-HS) property of PVA-HP1.0. This extraordinary behavior was attributed to the scission of hydrogen bonds (HBs) involving -COOH and -OH groups linked to benzene rings under the effect of a sulfate anion. The scission of HBs became enhanced at elevated temperature, leading to thermo-swelling of the PVA-HP1.0 gels in Li₂SO₄ solutions. Regarding PVA-HP0.5 and HP0.25, the nature of the unesterified PVA part, which is subject to ordinary HS and thermo-deswelling properties, more or less counteracted the anti-HS by the substituting side groups, resulting in the performance of PVA gels. [Figure not available: see fulltext.]