Molecular Thermodynamic Analysis for Assessing the Relationship Between Reentrant Swelling Behavior and Ternary Liquid-Liquid Equilibrium Nanometer-Sized Gel Particles in Mixed Solvents System

Abstract

This present study performed the calculation and analysis for multi-component liquid-liquid equilibrium (LLE) of polymer solutions using molecular thermodynamic models. In chapter 1, the influence of phase separation on swelling behavior was investigated based on the thermodynamic framework of re-swelling phenomena. The cloud-point for a ternary system of water(1)-tetrahydrofuran (THF)(2)-poly (N-isopropylacrylamide)(3) was examined by thermo-optical analysis (TOA). Nanometer-sized N-isopropylacrylamide (NIPA) gel particles were prepared by precipitation polymerization and their swelling behaviors were determined using photon correlation spectroscopy (PCS). NIPA gel particles underwent re-swelling when the ratio of water to THF was varied. First, the modified double lattice model (MDL) was employed to determine ternary interaction energy parameters for the LLE of linear poly NIPA in a water-THF cosolvent system. The reentrant swelling equilibria of the NIPA gel in the water-THF system were then calculated using the interaction energy parameters. Expanding upon a previous study, the phase transition behaviors of PNIPA water/ N,N-dimethylformamide (DMF) mixtures were examined by TOA, and and their swelling behaviors were investigated via PCS in chapter 2. Both the lower critical solution temperature (LCST)-type and the upper critical solution temperature (UCST)-type phase transitions were observed at different solvent weight fractions. Under LCST conditions with a low DMF fraction, the phase transition temperatures were convex curves with increasing DMF content. In contrast, under UCST conditions with a high DMF fraction, the phase transition temperatures sharply decreased with increasing DMF content. These phase transition composition boundaries correspond to the type of swelling for the NIPA gel solution. For model calculations, the MDL model was employed to determine interchange energy parameters for the LLE of linear PNIPA in a water/DMF mixed solvent system. The swelling equilibria of the NIPA gel in water/DMF were then calculated using the pre-determined interchange energy parameters. In chapter 3, This study aims to demonstrate not only that a PNIPA shows the behavior of thermal phase transition in water/organic solvent and to calculate the interaction between each component in LLE modeling, but also that applying a swelling model using the parameters obtained from corresponding ternary PNIPA system. The focusing on observation concerning the phase separation of ternary polymer solution is the range of specific temperature within which closed immiscibility loop phase can exist. The transmitted light intensity method reveals that the PNIPA in mixed solvents with different ratio of water and second solvent exhibits the LCST or UCST-type phase separation. A molecular thermodynamic model which can be used to describe the ternary phase behavior and swelling behavior of hydrogels in mixed solvents has been established based on the multi-component lattice model. The LLE of water/organic solvent/PNIPA were first calculated, and the corresponding swelling behaviors were subsequently calculated using predetermined model parameters from linear PNIPA solutions.