Coarse-grained molecular models of polymer networks were used to study how energetic, 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 suggest that, at low concentrations of short chains (relatively to their pervaded volume), the kinetics of the end-linking reaction leads to the formation of a significant amount of network structural inhomogeneities (i.e., short-chain clusters) and defects (mostly short-chain loops), which considerably decrease the toughness and modulus of the material. Our results further identify the conditions at which optimal toughness is expected. Uni-axial stretch experiments, and network swelling of bimodal Poly(dimethylsiloxane) (PDMS) networks compared well with the simulation results.

Additionally, the tensile properties of idealized networks with highly regular topologies (corresponding to bi-continuous phases) were also simulated. These networks, formed by regularly connected chains with few or no entanglements, are expected to have tunable elastic modulus and toughness—depending upon network topology, chain stiffness, and molecular weight—and high ultimate strain and stress values due to their structure regularity. These regular networks exhibit a particular step-wise elastic response and multiple ordering transitions (as seen in some extremely tough natural materials such as the organic adhesive inside the abalone shells, muscle protein “titin”, and spider silk). In addition, possible ways of obtaining these tri-dimensional regularly connected networks—inspired in recent experimental works on the fabrication of bi-dimensional regular networks—are being explored through simulations.