In this work we present an improved approach for the analysis of (1)H double-quantum nuclear magnetic resonance build-up data, mainly for the determination of residual dipolar coupling constants and distributions thereof in polymer gels and elastomers, yielding information on crosslink density and potential spatial inhomogeneities. We introduce a new generic build-up function, for use as component fitting function in linear superpositions, or as kernel function in fast Tikhonov regularization (ftikreg). As opposed to the previously used inverted Gaussian build-up function based on a second-moment approximation, this method yields faithful coupling constant distributions, as limitations on the fitting limit are now lifted. A robust method for the proper estimation of the error parameter used for the regularization is established, and the approach is demonstrated for different inhomogeneous elastomers with coupling constant distributions.

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.

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A series of end-linked PDMS networks synthesized with different molar mass precursor chains is examined using 2H NMR spectroscopy. The resulting line shapes for networks in the undeformed state show clear differences with precursor chain molar mass. Furthermore, samples uniaxially extended to high extension ratio show a clear shoulder in the line shape and two characteristic splittings (two doublets). Comparison with spectra for deuterated free chains dissolved in an unlabeled network confirms that the inner doublet results from excluded volume interactions between segments whereas the outer doublet is due to more highly aligned chain segments from chains with conformations trapped by the cross-linking reaction.

We present a systematic study through 2H 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.