Molecular simulation of polymer gels synthesized by free radical copolymerization: Effects of concentrations and reaction rates on structure and mechanical properties

The effects of polymerization concentrations and elementary reaction rates on the network structure and mechanical properties of gels synthesized by free radical copolymerization of monomers and cross-linkers are studied by a coarse-grained molecular dynamics simulation. The shear modulus, the number of elastically effective chains, and the number of trapped entanglements are calculated with varying the concentrations. The results reveal that the experimentally reported high cross-linking efficiency of the gels formed at high monomer concentrations and low cross-linker concentrations is due to the large number of trapped entanglements. The molecular mechanisms of changes in the network structure due to changes in the concentrations are elucidated by analyzing the detailed structure of network strands connecting adjacent cross-linking points. The results with systematically varying the elementary reaction rates indicate that the decrease in the propagation rate suppresses the formation of cyclic structures and reduces the influence of the trapped entanglements, i.e., improves the structural uniformity of the gels.