Heterogeneity in polymer networks formed by a single copolymerization reaction: I. Gelation and pre-gel structure

Coarse-grained molecular dynamics simulations of the formation of 3- and 6-functional polymer networks explicitly shows gelation via large dendritic clusters. The heterogeneous network structures are studied in detail; clusters have pendant structures and occupy significant volume fraction. Cluster fractal dimension and the Fisher exponent for cluster size are consistent with percolation theory and kinetic gelation models. Several criteria for determining gelation yield consistent results somewhat higher than the estimate for ideal networks on regular lattices. Almost all crosslink loop formation occurs after gelation just as traditional statistical theories assume. The presence and size of these clusters provide a natural explanation for density variations seen in microscopy studies of crosslinked polymers. The second paper in this series describes how the network structure develops after gelation. Large pendant structures, and more localized defects, persist and comprise a substantial fraction of material that is not contributing to network strength.