APPLICATION OF PULSED ENDOR TO THE STUDY OF RADICALS IN A LIQUID-CRYSTALLINE COPOLYESTER

Pulsed electron nuclear double-resonance (ENDOR) techniques and pulsed triple-resonance experiments are used to study radicals in a liquid-c rystalline copolyester. In accordance with the commonly observed heter ogeneity of polymers, the ENDOR spectrum is quantitatively analyzed in terms of a heterogeneous distribution of similar radicals with differ ent hyperfine couplings. Furthermore, the nuclear longitudinal and tra nsverse relaxation of hyperfine coupled protons has been studied by tr ansient ENDOR and triple-resonance experiments. The nuclear longitudin al relaxation indicates that, above room temperature, thermally activa ted small-angular motions occur on a time scale of 100 mu s. The nucle ar phase-memory relaxation rate increases linearly with the hyperfine coupling. A quantitative description of a motional model is given, whi ch explains the experimental results by small-angle fluctuations of th e radicals on a microsecond time scale. (C) 1995 Academic Press, Inc.