Electron spin relaxation in solids-an application to spontaneously occurring radical centre in phenol-formaldehyde resins

An outline of electron spin-relaxation is briefly presented with mechanisms and processes of spin-lattice relaxation in ionic and amorphous solids. Electron spin resonance (EPR) and pulsed EPR measurements were performed in the temperature range 4-300 K on a radical occurring in a dried resol phenol-formaldehyde resin. The radical appears in a small concentration of about 1×10¹⁶ radicals/g in 30°C dried samples and the number of the radicals increases continuously with time up to 2×10¹⁷ radicals/g after 100 days. Computer simulations of the EPR spectrum show that breaking of the dimethylene ether linkage probably forms the radical during synthesis reactions. Electron spin-lattice relaxation rate increases relatively slowly on heating and at <80 K is determined by interactions with two-level tunnelling systems in the polymer structure. For higher temperatures the relaxation is governed by interactions with centres having energy levels split by 298 cm^-1. Phase relaxation is described by the phase memory time with rigid lattice limit T_M⁰=2 μs. This time is shortened above 100 K by thermally activated radical molecule motions with activation energy 85 cm^-1. The electron spin echo envelope modulation (ESEEM) spectrum shows unresolved peaks at ¹H indicating strong molecular dynamics and delocalisation of the unpaired electron.