Resonance-type effects in free radical electron spin lattice relaxation and electron spin echo dephasing due to a dynamics of a homogeneous-chain oligomeric system

Electron spin lattice relaxation and phase relaxation were measured by the electron spin echo (ESE) method in the temperature range 4.2-300 K for radicals appearing during the polycondensation reaction of poly(4-hydrazo-diphenylene disulfide) oligomers with N-methyl-2-pyrrolidone chain ends. The spin lattice relaxation rate 1/T-1 is governed by cross-relaxation to the fast relaxing radical pairs, with the relaxation rate rapidly increasing at low temperature and showing unusually weakly temperature dependent behaviour above 25 K. At 146 K the resonance peak from rotating CH3 groups (tau(0)=1.07x10(-12) s, E-a=441 cm(-1)) appears. Such a resonance-type enhancement has been not observed so far in electron spin lattice relaxation in solid-state materials. The ESE dephasing rate 1/T-M is temperature dependent with a pseudo-resonance minimum at about 100 K resulting from two different dephasing mechanisms. At low temperatures the dephasing rate decreases on heating due to quantum tunnelling motion of CH3 groups between nearly equivalent potential wells. Above 100 K the rate continuously increases with temperature as a result of thermally activated low frequency motions between shallow potential minima (tau(0)=2.6x10(-7) s, E-a=49 cm(-1)). Various processes leading to temperature variations of the dephasing rate are reviewed and discussed. The relaxation results are compared with those for radicals spontaneously occurring in partially cured phenol-formaldehyde resin.