Off-Lattice Approach to Simulate Radical Chain Polymerizations of Tetrafunctional Monomers

Abstract

Radical homopolymerization of multifunctional monomers leads to highly crosslinked networks that demonstrate structural heterogeneity. Spatially non-uniform diffusion limitations result in complex reaction behavior such as unequal functional group reactivity, radical trapping, and microgel formation. The heterogeneity that arises in these systems on the molecular and nanoscopic scales is very difficult to characterize experimentally. A unique off-lattice simulation approach is described to provide insight into structural evolution during multifunctional monomer polymerizations. This simulation approach combines simple Monte Carlo principles to incorporate specie mobility with the basic reaction framework of traditional kinetic gelation models. Simulation volumes containing up to 100 000 Lennard–Jones spheres, representing tetrafunctional monomer units, were randomly configured with the Metropolis Method. Rules for initiation, propagation, and termination were developed and implemented, as well as local and periodic particle relaxation schemes. The simulation captures more realistic particle dynamics and mobility than traditional, lattice-based kinetic gelation models. In this work, a detailed description of the simulation method and selected results are presented. Agreement of simulated and experimental trends related to radical trapping frequency as a function of mobility is shown. Visual images of microgel evolution and simulated information about functional group reactivity within microgels are set forth.