SELECTIVE DETERMINATION OF ELASTOMER DISTRIBUTION IN MULTICOMPONENT SYSTEMS USING PROTON-DETECTED C-13 IMAGING

The application of the cyclic J cross-polarization (CYCLCROP) NMR imag ing pulse sequence for the acquisition of proton-detected C-13 NMR ima ges of elastomeric materials is described. In CYCLCROP a series of two polarization transfers in the sense H-1 --> C-13 --> H-1 is applied b efore imaging in order to select a specific H-1 nucleus J-coupled to a C-13 nucleus and filter out all other 1H coherences. It is shown that in multicomponent systems this technique can be used to acquire selec tive images of one of the components by suitable selection of the cros s-polarization transfer pair of CHN nuclei. An important advantage of CYCLCROP imaging turns out to be its selectivity even in the case of c ompletely unresolved 1H lines, as they are often encountered in polyme r proton spectra. By selecting the CH methine proton of PI for the cro ss-polarization transfer filter, we successfully recorded edited 1H im ages of commercial natural abundance cis-polyisoprene, na-PI, in the p resence of a second elastomeric material, whose 1H coherences were com pletely edited out. With C-13-enriched polyisoprene, C-13-PI, synthesi zed in the laboratory, CYCLCROP was employed to record images of the s patial distribution of PI in mixtures of PI with polybutadiene, PB, an d poly(hydroxyoctanoate), PHO. Two different mixing sequences for perf orming the cyclic J cross-polarization, MOIST and PRAWN, were examined . 2D NMR coherence transfer spectra of the single and the cyclic polar ization transfer in the rotating frame are reported for PI in solution and in bulk. It is shown that the short effective relaxation time in the rotating frame, T-p,T-eff reduces the efficiency of CYCLCROP compa red to theoretical predictions. However, an at least 8-fold reduction in the experimental time required for the acquisition of images with t he same S/N is obtained by using CYCLCROP instead of directly detected one-pulse C-13 NMR imaging.