Influence of hydroxyl group for thermoresponsive poly( N -isopropylacrylamide) gel particles in water/co-solvent (1,3-propanediol, glycerol) systems

The influence of the addition of a co-solvent in an aqueous poly( N -isopropylacrylamide) (PNIPA) solution was investigated based on a thermodynamic framework, which considers the modified double lattice (MDL) theory of mixing and the Flory–Rehner (FR) chain model for the elasticity. The cloud points for the ternary system of PNIPA (1)/water (2)/propane-1,3-diol (1,3-PDO) and glycerol (3) were measured by thermo-optical analysis (TOA) and the swelling behaviors of nanometer-sized PNIPA gel particles were determined using photon correlation spectroscopy (PCS). Completely different results were obtained depending on the number of hydroxyl groups of the co-solvent. For glycerol, which has three hydroxyl groups, added in aqueous PNIPA, the volume transition temperature of the PNIPA gel particles continuously decreased. On the other hand, the addition of 1,3-PDO, which has two hydroxyl groups, resulted in an increased volume transition temperature of the gel. To determine the molecular interaction parameters, we utilized a step-by-step approach. First, a secondary lattice theory of specific interactions was employed to correlate the binary liquid–liquid equilibrium (LLE) of PNIPA in water and was directly applied to our ternary LLE data to obtain the remaining interaction parameters. Secondly, composition dependence of the energy parameter was adopted to describe the swelling behaviors of PNIPA gel particles in the water/co-solvent system. The method presented here provides an understanding of the thermodynamics of polymers and polymer gel in mixed solvents.