2-DIMENSIONAL PROTON MAGNETIZATION-EXCHANGE NMR-SPECTROSCOPY IN CROSS-LINKED ELASTOMERS

Solid-state two-dimensional proton magnetization-exchange NMR is used to investigate intergroup residual dipolar couplings in a cross-linkin g series of poly(styrene-cobutadiene) elastomers. The magnetization-ex change process between the CH and the CH2 group in the regime of short mixing time provides valuable insight regarding molecular order. A th ree-spin model is employed, in which the CH and CH2 protons are consid ered to be coupled by residual dipolar interactions. The spin-system r esponse reflects well-localized dipolar interactions. The time scale i n which the exchange process takes place justifies these assumptions a s well as the interpretation of various NMR relaxation experiments pre viously performed on this class of polymers. The residual intergroup d ipolar couplings are measured along the average polymer-chain directio n and correlated with the dynamic storage modulus. It is shown that th ey 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 [P-2]ap proximate to 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. (C) 1996 American Institute of Physics.