Kinetics of radical decay. 4. Polycrystalline p-azoxyanisole

Abstract

Free radical decay in x-irradiated powder samples of p-azoxyanisole (PAA) studied between 60/sup 0/C and the liquid crystal phase transition at 118/sup 0/C was observed to occur in a ''stepwise'' manner with no evidence of radical conversion. Of several models tested the data were best fit by a kinetic equation due to Waite which involves a random initial spatial distribution of radicals, an activation energy for radical combination, E/sub comb/, which dominates at early time, and an activation energy for diffusion, E/sub diff/, which dominates at late times. Application of the model to the data yields E/sub comb/ = 9.6 and E/sub diff/ = 19.3 kcal/mol. The activation energies correspond well to those reported by other workers from NMR measurements of rotational and diffusional motion in PAA. The model also allows estimation of the diffusion coefficient provided r/sub 0/ in the kinetic equation can be approximated. Taking r/sub 0/ as the largest lattice dimension (20.5 A) one obtains a value of D approximately equal to 1 x 10/sup -16/ cm/sup 2//s at 0.96 of the phase transition temperature. Such considerations lead to the assignment of E/sub diff/ to an extrinsic, defect controlled diffusion process and E/sub comb/ to a rotationmore » of the radical about the long molecular axis necessary to reach a favorable orientation for combination with another radical. The general role of radical shape and ease of rotation in influencing the decay kinetics is discussed.« less