Molecular thermodynamic analysis for phase transitions of linear and cross-linked poly(N-isopropylacrylamide) in water/2-propanol mixtures

Ternary liquid–liquid phase transitions of linear poly(N-isopropylacrylamide) (PNIPA) and the swelling behavior of cross-linked PNIPA gels in water/2-propanol mixtures were investigated using thermo-optical analysis (TOA) and a photon correlation spectroscopy (PCS) technique, respectively. Closed immiscibility gaps in ternary phase diagrams were obtained below 35 °C although all binary mixtures involved in this system were completely miscible. At a fixed concentration of PNIPA (1.0 wt%), a decrease in the lower critical solution temperature (LCST) occurred first, and with an increasing 2-propanol fraction of the solvent mixture, the upper critical solution temperature (UCST) subsequently developed. Corresponding to the linear PNIPA solution behavior, swollen PNIPA gels in pure water first shrank and then reswelled with increasing 2-propanol content in a typical reentrant transition. For theoretical treatment of the mixtures, a multi-component lattice theory of mixing and Flory–Rehner chain model for elasticity were employed for molecular thermodynamic analysis. A secondary lattice concept of specific interactions was used to model binary water/PNIPA solutions, and a temperature dependence of the intermolecular interactions for 2-propanol/PNIPA was used to describe island type ternary phase diagrams. The swelling transitions of cross-linked PNIPA gels were calculated using model parameters obtained from the ternary phase diagrams of linear PNIPA or from experimental conditions. The predicted results were in good agreement with experimental data without the need for further adjustable parameters.