Two-dimensional proton magnetization-exchange NMR spectroscopy in cross-linked elastomers

Abstract

Solid‐state two‐dimensional proton magnetization‐exchange NMR is used to investigate intergroup residual dipolar couplings in a cross‐linking series of poly(styrene‐cobutadiene) elastomers. The magnetization‐exchange process between the CH and the CH₂ group in the regime of short mixing time provides valuable insight regarding molecular order. A three‐spin model is employed, in which the CH and CH₂ protons are considered to be coupled by residual dipolar interactions. The spin‐system response reflects well‐localized dipolar interactions. The time scale in which the exchange process takes place justifies these assumptions as well as the interpretation of various NMR relaxation experiments previously performed on this class of polymers. The residual intergroup dipolar couplings are measured along the average polymer‐chain direction and correlated with the dynamic storage modulus. It is shown that they are sensitive to both cross‐link points and nonpermanent geometric constraints of the chain motion. The dynamic order parameter along the chains is evaluated. It exhibits the remarkably high value of 〈P₂〉≊0.13 for the carbon–carbon bond connecting the CH and the CH₂ groups in polybutadiene in the uncross‐linked copolymer melt and only slightly increases with the cross‐link density.