PREDICTION OF REENTRANT SWELLING BEHAVIOR OF POLY(N-ISOPROPYL ACRYLAMIDE) GEL IN A MIXTURE OF ETHANOL-WATER USING LATTICE FLUID HYDROGEN-BOND THEORY

The re-entrant volume phase transition of poly(N-isopropyl acrylamide) gel in ethanol-water mixtures has been predicted by using the extende d lattice-fluid-hydrogen bond (LFHB) theory. In our calculations we do not make any arbitrary assumptions for the polymer-solvent interactio n parameters. Instead, we determine the interaction parameters by fitt ing the LFHB theory to the swelling data of the gel in each of the sol vents. In addition to predicting the re-entrant transition, the theory predicts selective absorption of ethanol over water, particularly by the collapsed gel. Simultaneously, the hydrogen bonding between water and ethanol is predicted to be enhanced in the presence of the gel. Th e interpolymer hydrogen bonds increase during the gel collapse region. The polymer-ethanol hydrogen bonds increase and the polymer-water hyd rogen bonds decrease continuously with increasing ethanol composition in the outside phase. These predictions are in qualitative agreement w ith experimental observations and overcome the empiricism in previous theoretical work. A variety of qualitatively different swelling behavi ors of gels in mixed solvents is also predicted for varying hydrophili c-hydrophobic balance in the chemical structure of the gels. (C) 1997 American Institute of Physics.