Selective measurement of heteronuclear 1H-13C dipolar couplings in motionally heterogeneous semicrystalline polymer systems

A pulse sequence for the selective recoupling of heteronuclear dipolar interactions in mobile amorphous phase of powdered semicrystalline polymers is described. H-1-C-13 dipolar interactions are selectively measured by PISEMA-type sequence. Selection of C-13 magnetization originating from amorphous phase is achieved by a train of saturation pulses followed by a short delay and a direct excitation pulse on C-13 spins. The development of undesired net C-13 magnetization during the recoupling sequence is prevented by the efficient "reverse" C-13 -> H-1 cross-polarization. The efficacy of the 2D method to measure H-1-C-13 dipolar couplings selectively for mobile components is demonstrated on powdered crystalline L-alanine, semicrystalline polyethylene, and nanocomposite polyamide-6/montmorillonite.