Selective determination of elastomer distribution in multicomponent systems using proton-detected ¹³C imaging

Abstract

The application of the cyclic J cross-polarization (CYCLCROP) NMR imaging pulse sequence for the acquisition of proton-detected ¹³C NMR images of elastomeric materials is described. In CYCLCROP a series of two polarization transfers in the sense ¹H →¹³C → ¹H is applied before imaging in order to select a specific ¹H nucleus J-coupled to a ¹³C nucleus and filter out all other ¹H coherences. It is shown that in multicomponent systems this technique can be used to acquire selective images of one of the components by suitable selection of the cross-polarization transfer pair of CH_N nuclei. An important advantage of CYCLCROP imaging turns out to be its selectivity even in the case of completely unresolved ¹H lines, as they are often encountered in polymer proton spectra. By selecting the CH methine proton of PI for the cross-polarization transfer filter, we successfully recorded edited ¹H images of commercial natural abundance cis-polyisoprene, na-PI, in the presence of a second elastomeric material, whose ¹H coherences were completely edited out. With ¹³C-enriched polyisoprene, ¹³C-PI, synthesized in the laboratory, CYCLCROP was employed to record images of the spatial distribution of PI in mixtures of PI with polybutadiene, PB, and poly(hydroxyoctanoate), PHO. Two different mixing sequences for performing the cyclic J cross-polarization, MOIST and PRAWN, were examined. 2D NMR coherence transfer spectra of the single and the cyclic polarization 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_ρeff, reduces the efficiency of CYCLCROP compared to theoretical predictions. However, an at least 8-fold reduction in the experimental time required for the acquisition of images with the same S/N is obtained by using CYCLCROP instead of directly detected one-pulse ¹³C NMR imaging.