NMR images of proton residual dipolar coupling from strained elastomers

Proton residual dipolar couplings of elastomer chains were used to generate contrast in NMR images of stretched elastomers. Dipolar contrast filters are discussed on the basis of dipolar encoded longitudinal magnetization as well as double-quantum and triple-quantum coherences. The associated contrast filter parameters were investigated by measurements of the line width in H-1 multiple-quantum filtered NMR spectra, the dipolar encoded longitudinal magnetization decay curves, and the double-quantum and triple-quantum buildup curves vs the extension ratio for a natural rubber band. Together with stress-strain measurements these results allow for correlation of H-1 residual dipolar couplings with stress values in stretched natural rubber. Two-dimensional H-1 maps of stress distributions are presented for a strained natural rubber band with a cut on one side based on dipolar encoded longitudinal magnetization and double-quantum contrast filters. Parameter maps of the stress distribution are shown following calibration with a sample from synthetic poly(isoprene). The advantages as well as the limitations of the H-1 NMR images as compared to H-2 images of deuterated oligomers incorporated in elastomers are also discussed.