- K. Saalwächter, F. Kleinschmidt, J.-U. Sommer.
Swelling Heterogeneities in End-Linked Model Networks: A Combined Proton
Multiple-Quantum NMR and Computer Simulation Study.
Macromolecules 37, 8556-8568 (2004).
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 non-affine change of
this distribution, indicating a heterogeneous swelling process. Comparing the
simulated tensor order parameter with the auto-correlation 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, as well as 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.