Molecular thermodynamic analysis for thermo-sensitive swelling behaviors of nanometer-sized hydrogel particles via lattice based theory

Abstract

Numerous amphiphilic polymers in aqueous solution have drawn attentions because they have both hydrophilic and hydrophobic functional groups. In water, they have diverse number of interactions such as hydrogen bonding, polar, ionic interaction with hydrophilic group of polymer, structure formation and entropic contribution by hydrophobic group of polymer. Affected by various interactions, they can form self-assembled structures. This phenomenon is controlled by temperature, pressure, pH, magnetic field, cosolvent and cosolute, and is subject to thermodynamic equilibrium conditions. These polymers are also called stimuli-responsive polymers. The combination of stimuli-responsive, self-assembly and water medium has a wide range of applications such as polymeric membrane, drug delivery systems, actuator, superabsorbent, enhanced oil recovery, and cosmetics. To wide applicable area of these polymer, numerous group have studied modification of polymer structure by copolymerization, crosslinking, core-shell, interpenetrating network, grafting, ionization, etc.

In this thesis, temperature, solvent and solute sensitivity of LCST-type polymer and hydrogels are investigated via molecular thermodynamic framework.

First, liquid-liquid equilibrium (LLE) of linear polymer and swelling ability of crosslinked polymer solutions are measured to detect equilibrium properties at various conditions. Thermo optical analysis (TOA) and dynamic light scattering (DLS) are utilized to determine cloud point of solutions and hydrodynamic radius of particles.

Second, molecular thermodynamic frameworks are developed for various stimuli on polymer solution. Multi-component closed-packed lattice models are used to account for entropic and enthalpic contribution of constituent components in mixing process. Elasticity of crosslinked network in swelling process is considered to osmotic equilibrium by Flory-Rehner approach. To explain co- and conon-solvency effect by organic solvent, association fraction is modified into temperature and pairwise dependency using donor-acceptor and competitive hydrogen bonding concepts. Effect of salt and surfactant on swelling behaviors of hydrogel are described by developing non-random contact model and generalized Langmuir isotherm, respectively. These modeling framework for various specific interactions can be applicable to wide-range system with diverse molecular models.

Predicting thermodynamic properties through reliable experimental data and great correlation with modeling plays a crucial role in the polymerization process and is significant for quality control of the polymers produced, optimization of the separation process, and operation. Therefore, this research is expected to make an important contribution to the chemical engineering industry related to polymer and hydrogel production.