Kinetics of Free-Radical Crosslinking Polymerization: A Comparative Experimental and Numerical Study

Cross-linked polymer networks are materials of great technological relevance, such as rubbers, multi-component adhesives and hydrogels. Whereas rubbers and adhesives are the oldest examples of cross-linked polymer networks, the research on hydrogels has been more recently fueled by their growing importance in biology and biomedicine. The presence of cross-linkers confers to these networks unique properties, such as high mechanical and chemical resistance, thermal stability and even stimuli-responsiveness. A great number of network properties can be controlled by tuning the degree of cross-linking.

Therefore, modeling free radical polymerization processes in the presence of cross-linkers is a challenging problem that has been addressed for over more than half a century using numerous techniques. However, a model providing a comprehensive description of the phenomenon has not been proposed yet. In this work, we implement a simple free-radical polymerization scheme of a mono-vinyl (di-functional) monomer and a di-vinyl (tetra-functional) cross-linker in a Monte-Carlo (MC) scheme, which describes polymer dynamics using a bond-fluctuation model^[1]. MC simulations allow us to follow the entire polymerization kinetics and the formation of percolating network (gel phase) by realistically taking into account diffusion limitations, to extract scaling information at the percolation threshold and to recover the distribution of number of monomer units between two successive fully cross-linked units, from which the extent of swelling can be computed. The predictions of MC simulations are also successfully compared to a kinetic model based on numerical fractionation^[2], with kinetic constants used as fitting parameters. MC data and kinetic simulations are compared to some experimental data on the swelling behavior of polyacrylamide hydrogels and of polymethyl methacrylate (PMMA) gels, exhibiting good agreement. We conclude that the proposed MC simulation scheme represents a powerful tool from which precious and experimentally inaccessible information on polymerization processes in the presence of crosslinkers can be extracted.

[1] Carmesin, I.; Kremer, K. Macromolecules 1988, 21, 2819–2823

[2] Teymour, F.; Campbell, J. D. Macromolecules 1994, 27, 2460–2469