De. Demco et al., 2-DIMENSIONAL PROTON MAGNETIZATION-EXCHANGE NMR-SPECTROSCOPY IN CROSS-LINKED ELASTOMERS, The Journal of chemical physics, 105(24), 1996, pp. 11285-11296
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.