Computer simulation studies of phase behaviors in polymeric gels

Yimin Liu

Abstract

Computer simulation models are presented to study the sol-gel transition in irreversible and reversible athermal gelations and thermoreversible gelations in multicomponent system with bifunctional and tetrafunctional monomers. Attempts are made to consider the gel formation in solution with appropriate dynamics of macromolecules, interaction energies, and aggregation. The realistic features such as mobility, the rate of reaction, reversibility, and the quality of solvent are incorporated in the model to investigate the sol-to-gel transition by analyzing the evolution of the concentration of bonds, the volume fraction of the gel, the weight average degree polymerization, and the correlation length. These factors are found to have significant effects on the sol-to-gel transition, particularly on the critical point, universality, and the phase separation. For example, the mobility reduces magnitude of the critical point in irreversible gelation. The critical exponents for sol-to-gel transition depends on the rate of reaction and the degree of reversibility in athermal polymerization and thus, lead to different universalities. An energy parameter J is used to probe the effects of solvent, spanning through good, theta, and poor solvent conditions. The inhomogeneities in the gel network are also investigated by the static structure factors. The equilibrium properties of thermoreversible gels are studied by a thermodynamic model involving two energy levels for the bonds. Lowering the temperature leads to gelation and heating, to gel-melting, which is consistent with the experimental and theoretical phase diagram. The structural inhomogeneity in gel networks is shown to be caused by the microphase separation in the gelation processes. The results compare well with the experimental findings. Thus, the models presented in this thesis capture the most fundamental properties of the sol-gel transition.