PULSED ELECTRON-NUCLEAR DOUBLE-RESONANCE STUDY OF MOLECULAR MOTIONS OF RADICALS IN A HIGH-STRENGTH LIQUID-CRYSTALLINE POLYMER

Radicals in a liquid-crystalline copolyester are studied by pulsed ele ctron paramagnetic resonance (EPR) and pulsed electron nuclear double resonance (ENDOR). For the first time transient ENDOR techniques are u sed to investigate molecular motion of radicals in a disordered solid. The nuclear phase memory relaxation of hyperfine coupled protons is m easured in a temperature region between 250 and 350 K in the solid sta te. Remarkably, the relaxation rate increases linearly rather than qua dratically with the strength of the hyperfine coupling \A(zz)\. This r esults from a thermally activated process, characterized by small-angl e fluctuations of radicals with an amplitude of the order of 1 degrees . The corresponding activation energy of 24 kJ/mol indicates that the radicals are chain ends in the polymer.