Effect of Precursor Molar Mass on the ²H NMR Line Shapes of End-Linked PDMS Elastomers

Abstract

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.

Microscopic Study of Chain Deformation and Orientation in Uniaxially Strained Polymer Networks: NMR Results versus Different Network Models

“…The affine prediction was found to disagree with early experimental findings from SANS, e.g., refs , , and . Still, the junction affine model has so far been the only model used to explain NMR results of strained elastomers. ,,− A reason for this might be the (apparent) agreement of the spectral line shape predictions of unstrained polymer networks with the affine Gaussian chain model. Below we also present a more extended analysis of literature spectra, which accounts also for nonelastic defects, and discuss the deviations from the AM.…”

“…Usually, “nematic interactions” on segmental orientations are invoked to explain the presence and the magnitude of the splitting, and these have also been claimed to possibly contribute to the overall elasticity. − Yet, we note that the splitting is usually rather small as compared to the full width of the spectra, so even though being prominent, its contribution to the integral observable is not large, as shown below. It should be mentioned that spectra for stretched PDMS model networks made of long ( M > M e ) precursors have been reported, in which also the network component appears to show a splitting. This contradicts the general interpretation and clearly deserves further studies, but again, the fact that nematic interactions may be more relevant for long and entangled chains in special cases does not mean that they exert a dominating influence at high cross-linking.…”

“…Thus, a tentative line shape analysis by spectral decomposition enables us to describe the spectra in terms of separate network and apparent defects responses and perform a comparison with our data. We reanalyzed published experimental data for PDMS , and PB networks, , and refer to the SI3 of the Supporting Information for details. Each individual spectrum was fitted by a superposition of one (or two) Lorentzian(s), which we relate to the apparent defect response of the relaxed (or stretched) polymer, and a network component, which was either approximated by a single Gaussian or by a combination of a Lorentzian and a Gaussian.…”

Monomer Fluctuations and the Distribution of Residual Bond Orientations in Polymer Networks

“…Recently, the group of Prof. Cohen analyzed RBOs in experiments on end-linked PDMS networks and using computer simulations [54,55,56]. The experimental results motivated a DE-compositon of the NMR spectra into a wide and narrow one, whereby it was shown that inelastic chains contributed only partially to the narrow spectrum.…”

“…Recently, the group of Prof. Cohen analyzed RBOs in experiments on end-linked PDMS networks and using computer simulations. − The experimental results motivated a decomposition of the NMR spectra into a wide and narrow one, whereby it was shown that inelastic chains contributed only partially to the narrow spectrum. This decomposition is therefore problematic since the authors cannot provide a striking physical explanation to justify the source of a second population of chains.…”