DEVELOPMENT OF HIGH-RESOLUTION SOLID-STATE NMR

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In the last 20 years, the combination of cross polarization for sensitivity and magic-angle spinning (with dipolar decoupling) for resolution has made 13C NMR a standard tool for the characterization of solid polymers. Local structure is revealed by 13C chemical shifts and local dynamics from a variety of relaxation measurements in both the laboratory and rotating reference frames. Some success has been achieved in relating these microscopic parameters to macroscopic properties of the polymers. Nevertheless, this important connection is hampered by the fact that conventional 13C NMR spectra must be interpreted primarily in terms of a single chain with the effects of neighboring chains inferred indirectly. However, an accurate, direct characterization of the interchain packing in solid, glassy polymers is crucial to the understanding of the mechanical properties of the polymers. Today, the distances between packed chains can be determined by the same types of rotational-echo double resonance (REDOR) NMR experiments developed for proteins and protein complexes. REDOR NMR for an I-S spin pair involves the dephasing of transverse S-spin magnetization by rotor-synchronized I-spin pi pulses. Coupling to abundant protons is removed using a third radio frequency channel. The results of REDOR experiments lead directly to the strength of the heteronuclear I-S dipolar coupling and hence I-S internuclear distances. In applications on glassy polymers, polycarbonate chains with 13C labels have been mixed with chains having 2H labels. The resulting distances determined by 13C-2H REDOR NMR define local packing geometry on a system for which diffraction experiments are impossible. Interfaces of blends of heterogeneous polymers like poly(p-fluorostyrene) and 13C-labeled polycarbonate can also be characterized quantitatively and accurately by REDOR NMR as shown in the figure below. The residual protons in perdeuterated polymers behave as a rare spin, and REDOR between isolated 1H-13C pairs in perdeuterated poly-carbonate reveals the average location of the ubiquitous 0.3% (by weight) water in glassy polycarbonate.