Co-nonsolvency effect of thermosensitive N-isopropylacrylamide nanometer-sized gel particles in water–PEG systems

Abstract

Nanometer-sized N-isopropylacrylamide (NIPA) gel particles were prepared by precipitation polymerization, and their thermosensitive swelling behaviors in water–poly(ethylene glycol) (PEG) (MW = 200, 400) systems were investigated using photon correlation spectroscopy (PCS). The volume transition temperature decreased with increasing PEG concentration, and a completely unexpected swelling behavior was observed at high PEG concentrations. The cloud points of poly(N-isopropylacrylamide) (PNIPA) in water–PEG systems were also determined by thermo-optical analysis (TOA) in order to compare the volume transition temperatures of gel particles with the phase transition temperatures of linear PNIPA in given solvent mixtures. A lattice-based molecular thermodynamic framework was employed to calculate interaction parameters from binary liquid–liquid equilibria (LLE), and those parameters were then applied directly to describe the co-nonsolvency effects of the ternary solutions. By combining the molecular thermodynamic model for the mixing contribution with the Flory–Rehner (FR) model for the elasticity contribution, the reentrant swelling equilibria at various temperatures were calculated. Using only one adjustable parameter, we were able to describe the swelling behaviors at high temperature regions (above the LCST). Lastly, the predicted results were compared to PEG (MW = 200, 1000) selectivity data inside NIPA gel, and were found to be in good agreement with the proposed model without further adjustable parameters.