Reversible Changes in Solution pH Resulting from Changes in Thermoresponsive Polymer Solubility

TL;DR: Pendant groups on polymers that have lower-critical solution temperature (LCST) properties experience a water-like environment below theLCST where the polymer is soluble but are less hydrated above the LCST when the polymer phase separates from solution, which can change the bulk solution pH.

Abstract: Pendant groups on polymers that have lower-critical solution temperature (LCST) properties experience a water-like environment below the LCST where the polymer is soluble but are less hydrated above the LCST when the polymer phase separates from solution. When these pendant groups are amphoteric groups like carboxylate salts or ammonium salts, the change in solvation that accompanies the polymer precipitation event significantly changes these groups’ acidity or basicity. These changes in acidity or basicity can lead to carboxylate salts forming carboxylic acid groups by capturing protons from the bulk solvent or ammonium salts reverting to the neutral amine by release of protons to the bulk solvent, respectively. When polymers like poly(N-isopropylacrylamide) that contain a sufficient loading of such comonomers are dissolved in solutions whose pH is near the pKa of the pendant acid or basic group and undergo an LCST event, the LCST event can change the bulk solution pH. These changes are reversible. These...