Dynamic Monte Carlo Simulation Of Macromer Polymerization And Associated Chemical Gelation

The latest decade has witnessed the progress in studies of the structure, properties and applications of gels, especially hydrogels. Despite the processing of researches in gel theory and application, the present theories have certain limitations, and it is usually hard to detect the inner structure of gels in experiments. So, computer simulation has been regarded as the third research approach besides theory and experiment, and becomes a powerful tool in the study of gel structure.In the hydrogel formation through chemical gelation, the polymerization of amphiphilic macromers proceeds unexpectedly fast. Meanwhile, the hydrogels synthesized from amphiphilic macromers show good potential for applications on the fields of Tissue Engineering and drug controlled release carriers. It is reasonable to assume that the super-molecular structure resulting from self-assembly of this kind of macromonomers in selective solvents may affect their polymerization and the resulting gel network. The computer simulation of associated polymerization and the subsequent chemical gelation is thus called for.Monte Carlo (MC) simulation is a widely used stochastic simulation method. But the conventional MC simulation of polymerization on the basis of the sampling of the unit reactions according to the rate constants can not obtain any direct configuration information related to macromolecular chains. Meanwhile, dynamic Monte Carlo (DMC) simulation has been widely used to study macromolecular chain configuration and the self-assembly of amphiphilic molecules, but its application in studies of polymerization is relatively rather limited.This thesis extends DMC with a coarse-grained bond-fluctuation lattice model todetect polymerization kinetics. As we know, this is the first try to use DMC to investigate polymerization of amphiphilic macromers, a special kind of micellar polymerization, and the associated chemical gelation. The main achievements of this Ph. D thesis are summarized as follows:1. DMC was extended to simulate a simplified free radical polymerization of amphiphilic macromers in a selective solvent, and the abnormal phenomenon in the literature that some macromers were polymerized very rapid in water. This thesis studied the kinetics of polymerization of coarse-grained amphiphilic macromers in a selective solvent and in an athermal solvent, and reproduced the interesting phenomenon experimentally reported in the literature, namely, polymerization of PEO-acrylate macromer proceeded unexpectedly fast in water. Macromers containing two "double bonds" at ends of them were modeled and their spatial structures in a selective solvent of different concentrations were examined. It has been confirmed that the enhancement of the local concentration of polymerizable and insoluble groups accounted for the acceleration of polymerization rate of the amphiphilic macromers in the selective solvent as compared to in the athermal solvent. Both microscopic chain configuration and mesoscopic supermolecular structures of the amphiphilic macromers have been changed with solvent and/or concentration. The polymerization kinetics of amphiphilic monomers are thus highly coupled to their self-assembled behaviors.2. The associated chemical gelation through the polymerization of macromers were further investigated, and the simulation outputs demonstrate the difference of the kinetics of chemical gelation and the gel network structure between in a selective solvent and in an athermal solvent. The both ends of the macromers we considered are polymerizable groups, so it is possible to form a gel network. As revealed by the simulation, the gelling kinetics is, compared to polymerization process, even more sensitive to medium types and macromer concentrations in the examined systems. Bridge content in the self-assembled state is very important for gel formation duringsuch a micellar polymerization. The possible gelling resulted from the polymerization of amphiphilic monomers are thus highly coupled to their self-assembled behaviors. Besides focusing on the chemical gelation, the effect factors of gel content were examined preliminarily. Our results revealed that the existence of the mono capped macromers and the trace of oxygen particles would reduce the gel content. The results would be helpful to the experimental works.3. A straightforward criterion to determine an infinite polymeric network in a finite computer simulation system was suggested, and gelling behaviors were then investigated on the basis of this criterion. Since any multi-particle simulation with a finite density should be modeled in a finite-size system in any computer simulation, the periodic boundary condition must be employed to define an infinite chain network in a limited space. The criterion of gel network becomes thus a challenging topic. We put forward an absolute method in this thesis, i.e. a mathematically rigorous and computationally available criterion for a cluster to be an infinite network in a finite system when the periodic boundary condition is employed. This characterization technique is suitable for not only the researches of this thesis, but also various percolation networks of chemical cross-linking and physical gelation in principle.4. The structure and properties of gels due to the macromer polymerization were investigated preliminarily. We made comparison between the gels resulting from different media, and examined preliminarily some factors to influence gel swelling. Our simulation showed that the divergence of swelling behavior of different gels was related to the change of their inner structures. On the other hand, the fractal structure of the infinite network near the gel point was investigated preliminarily by dynamic MC simulation. Near the gel point, we analyzed the structure of infinite network, validated that it belonged to geometrical fractal, and performed a scaling analysis.