We report on stress−strain and swelling results for polydimethylsiloxane bimodal networks, studied both experimentally and via Monte Carlo simulations. These end-linked networks were formed with negligible extent of soluble fractions, and reasonable agreement is found between experimental results and simulation data. We examine the variation in microstructure for networks with different concentrations of short chains. When the concentration of short chains is low, these chains aggregate during the end-linking process and lead to a heterogeneous network structure, while networks formed with higher short chain concentrations are more homogeneous. Short chains that are long enough to initially have a Gaussian conformation also produce the characteristic stress upturn and enhanced toughness previously reported in bimodal networks with non-Gaussian short chains. We find that it is the limited extensibility of the short chains at high concentrations and not the cluster formation of short chains at low concentrations that leads to the enhanced mechanical properties of these elastomers.

Monte Carlo simulations of an entanglement-free cross-linked polymer network of semiflexible chains reveal a peculiar stepwise elastic response. For increasing stress, step jumps in strain are observed that do not correlate with changes in the number of aligned chains. We show that this unusual behavior stems from the ability of the system to form multiple ordered chain domains that exclude the cross-linking species. This novel elastomer shows a toughening behavior similar to that observed in biological structural materials, such as muscle proteins and abalone shell adhesive.

The effect of chain stiffness and entanglements on deformation properties of end-linked networks was investigated using Monte Carlo simulations. Tetrafunctionally cross-linked monodisperse networks were prepared in the framework of the bond fluctuation model (BFM). The degree of entanglement in these networks was tuned by changing the initial polymer concentration at curing (Φ0). The chain stiffness was controlled by using an adjustable bond angle bending potential. Continuum-space simulations of isotropic swelling and uniaxial deformation were carried out in isobaric and isostress ensembles, respectively. Both equilibrium swelling and stress−strain data indicate that the stiffer chain networks are more entangled, confirming previous simulation results on polymer melts. For such entangled networks, stiffer chains are associated with a higher elastic modulus and smaller equilibrium swelling; the opposite is true, however, for entanglement-free “diamond” networks. The elastic modulus determined from low-strain uniaxial deformations agrees with semitheoretical predictions for moderate chain stiffness, but not for very stiff chains. For the realistically cross-linked (and hence entangled) networks studied here, the theoretically predicted strain-induced discontinuous ordering transition was not observed, although the transition region becomes narrower for the least entangled networks made from the stiffest chains. Entanglement-free diamond networks do exhibit a discontinuous transition from a disordered state to a nematic-like state under uniaxial extension.

The effect of chain stiffness and entanglements on the elastic behavior and microscopic structure of cross-linked polymer networks was studied using Monte Carlo simulations. We investigated the behavior of entangled and entanglement-free networks at various degrees of chain stiffness and densities. Based on previous results that indicated that trapped entanglements prevent strain-induced order-disorder transitions in semiflexible chain networks, we prepared the entangled networks by end-linking the chains in very dilute conditions so as to minimize the extent of trapped entanglements. We also considered the entanglement-free case by using a "diamond" structure. We found that the presence of even a very small amount of trapped entanglements is enough to prevent a discontinuous strain-induced transition to an ordered phase. In these mildly entangled networks, a nematiclike order is eventually attained at high extensions but the elastic response remains continuous and the cross-links remain uniformly distributed through the simulation box. The entanglement-free diamond networks on the other hand show discontinuities in their stress-strain data. Networks at higher densities exhibit a more stable ordered phase and show an unusual staircaselike stress-strain curve. This is the result of a stepwise extension mechanism in which the chains form ordered domains that exclude the cross-links. Extension is achieved by increasing the number of these ordered domains in the strain direction. Cross-links aggregate in the spaces between these ordered domains and form periodic bands. Each vertical upturn in the stress-strain data corresponds to the existence of an integer number of ordered domains. This stepwise elastic behavior is found to be similar to that exhibited by some tough natural materials.