Molecular oxygen effects on the electron spin relaxation in a spin-labeled polystyrene

Abstract

The effects of molecular oxygen on the phase memory time TM and the electron spin–lattice relaxation time T1 of a spin-labeled polystyrene have been investigated over the temperature range 77–295 K. Two mechanisms whereby molecular motions can influence the phase memory times of free radical molecules have been identified. In a type 1 mechanism, molecular motions involving neighboring B spins contribute directly by dipolar interactions to the local field fluctuations at the A spin sites. In a type 2 mechanism, which is operative at fast motional frequencies close to the Larmor frequency, the molecular motions influence the spin–lattice relaxation rates and indirectly determine TM by spin-lattice induced radical B spin flips. With oxygen present in the samples, the temperature dependences of TM and T1 exhibit a minimum value at the same temperature. The minimum in T1 is attributed to a time dependent dipolar interaction between the molecular oxygen and the radical spin, whereas the minimum in TM is the result of a type 2 mechanism with the radical spins acting as B spins. Although the temperature dependence of the correlation time for the local field fluctuations at the radical site, τc, has not been evaluated, the TM and T1 values indicate that it is in the range τc<10−8 s at 77 K to τc<2×10−11 s for temperatures above the T1 minimum. The correlation times are determined either by localized motions of the oxygen molecules when they are restricted to the pre-existing holes present in the polymer or by the oxygen electron spin–lattice relaxation time. With the samples in a vacuum, the values of T1 obey a monotonically decreasing function with increasing temperature, whereas the TM values show a minimum at 140 K. We interpret the latter to indicate the onset of the molecular motions associated with the δ relaxation.