(447a) Simulation Studies on Bimodal Polymer Networks with Heterogeneous Microstructure

Coarse-grained molecular models of polymer networks were used to study how kinetic, and entropic effects can be harnessed to manipulate the network architecture and the tensile properties. We investigated first the impact of chain length polydispersity and its kinetic implications on the mechanical properties of bimodal end-linked networks, which are formed by endlinking two sets of telechelic linear polymer chains of different molar mass that are chemically identical. Molecular simulations of the end-linking reaction and network deformation have been used to elucidate, in particular, the origin of the enhancement of the mechanical properties (like toughness) seen experimentally in end-linked bimodal polymer elastomers. The impact of the system composition (mol% of short chains) on the network microstructure was monitored by obtaining detailed network topological characterizations that include the identification of network imperfections (i.e., pendant chains and loops), elastic chains, and short-chain clusters. The responses upon deformation of these topological chain types and their contribution to the network elasticity were also quantified through their segment orientation, end-to-end distances and gyration radii. Our results showed that, at relatively low concentrations of short chains (relative to their overlap concentration), end-linking reaction kinetic limitations become important and lead to the formation of a significant amount of network structural inhomogeneities and defects (mostly short-chain loops), which did not seem to contribute to the elastic behavior. Although the network that exhibited the best mechanical performance had a slight degree of heterogeneity, we also found that it is mainly the limited extensibility of the short chains at high concentrations and not the cluster formation of short chains at lower concentration that leads to the enhanced mechanical properties of these elastomers. Our results further identify the conditions at which optimal toughness can be expected. Uni-axial stretch experiments, and network swelling of bimodal Poly(dimethylsiloxane) (PDMS) networks compared well with the simulation results.