Abstract
Pulsed electron nuclear double-resonance (ENDOR) techniques and pulsed triple-resonance experiments are used to study radicals in a liquid-crystalline copolyester. In accordance with the commonly observed heterogeneity of polymers, the ENDOR spectrum is quantitatively analyzed in terms of a heterogeneous distribution of similar radicals with different hyperfine couplings. Furthermore, the nuclear longitudinal and transverse relaxation of hyperfine coupled protons has been studied by transient ENDOR and triple-resonance experiments. The nuclear longitudinal relaxation indicates that, above room temperature, thermally activated small-angular motions occur on a time scale of 100 μs. The nuclear phase-memory relaxation rate increases linearly with the hyperfine coupling. A quantitative description of a motional model is given, which explains the experimental results by small-angle fluctuations of the radicals on a microsecond time scale.
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