Swelling Heterogeneities in End-Linked Model Networks: A Combined Proton Multiple-Quantum NMR and Computer Simulation Study

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The segmental order of monomodal and bimodal polymer networks is investigated by proton multiple-quantum NMR spectroscopy as applied to poly(dimethylsiloxane) model systems and by Monte Carlo simulations using the bond fluctuation model. The multiple-quantum method is sensitive to chain order parameter distributions and therefore probes heterogeneities in the extent of fast motional averaging of individual network strands subject to topological constraints. These data are in qualitative agreement with the simulations. We find a broadening of the chain order parameter distribution upon swelling accompanied by a nonaffine change of this distribution, indicating a heterogeneous swelling process. Comparing the simulated tensor order parameter with the autocorrelation function of segments, we observe major deviations in the swollen state but also for the long-chain fraction of the dry, bimodal network. These effects are attributed to the fluctuation dependence of the tensor order parameter which comprises information about both orientational order as well as fluctuation properties of fast segment reorientation processes. In general, our results indicate that segmental order is not simply related to the cross-link density. Fluctuations on larger scales such as reorientation of chain clusters, dynamical correlations between segments in the entangled state, and heterogeneities in the segmental fluctuations have an essential influence on the tensorial order. The spatial length scale of the heterogeneities is also investigated by self-diffusion measurements of the solvent molecules.