Distribution in correlation times for rotational diffusion of spin probes in polymers

Abstract

Electron spin resonance spectra of poly(vinyl acetate) (PVAc) and plasticised PVAc doped with nitroxide spin probes show a double-peak feature at temperatures immediately below T50 G. This can be explained on the basis that probe tumbling occurs with a distribution of correlation times τR arising from a distribution of free volume within the polymeric matrix. The two-component e.s.r. spectra arise because the distribution in τR of the probes straddles the value (c. 3 × 10−9 s) which marks the change-over from fast- to slow-motion spectra. The implications of the existence of a distribution of τR values for calculations based on the assumption of a single value are examined. It is concluded that when probe tumbling is slow the error in τR could be significant, but for rapid tumbling (motionally narrowed spectra) the error is likely to be insignificant.