Pulsed electron nuclear double resonance study of molecular motions of radicals in a high-strength liquid-crystalline polymer

Abstract

Radicals in a liquid-crystalline copolyester are studied by pulsed electron paramagnetic resonance (EPR) and pulsed electron nuclear double resonance (ENDOR). For the first time transient ENDOR techniques are used to investigate molecular motion of radicals in a disordered solid. The nuclear phase memory relaxation of hyperfine coupled protons is measured in a temperature region between 250 and 350 K in the solid state. Remarkably, the relaxation rate increases linearly rather than quadratically with the strength of the hyperfine coupling | A zz |. This results from a thermally activated process, characterized by small-angle fluctuations of radicals with an amplitude of the order of 1°. The corresponding activation energy of 24 kJ/mol indicates that the radicals are chain ends in the polymer.