2H NMR and Simulation Studies of Chain Segment Orientation in PDMS Bimodal Networks

We present a systematic study through H-2 NMR and molecular simulations of chain segment orientation in model PDMS bimodal networks. Estimates of the average segment orientation order parameter in model PDMS bimodal networks compare well to those of equivalent simulated networks. We find that line shapes of short and long chains are different due to the dissimilar degrees of motional restrictions. Short-chain conformations are perturbed from random coiled states when incorporated into bimodal networks whereas long-chain conformations are unaffected. Although experiments and simulations both indicate that the overall segment orientation normalized by the elastic modulus is fairly constant across all bimodal network compositions, short-chain normalized segment orientation displays a nonmonotonic trend with the network composition (related to changes in network microstructure and mechanical properties). Finally, we show that the unusual outer splittings observed in spectra of highly extended unimodal and bimodal samples correspond to segments in long chains able to be exceptionally stretched and oriented along the strain axis.