DISTRIBUTION IN CORRELATION TIMES FOR ROTATIONAL DIFFUSION OF SPIN PROBES IN POLYMERS

Electron spin resonance spectra of poly(vinyl acetate) (PVAc) and plasticized PVAc doped with nitroxide spin probes show a double-peak feature at temperatures immediately below T//5//0//G. This can be explained on the basis that probe tumbling occurs with a distribution of correlation times tau //R arising from a distribution of free volume within the polymeric matrix. The two-component e. s. r. spectra arise because the distribution in tau //R of the probes straddles the value (c. 3 multiplied by 10** minus **9s) which marks the change-over from fast- to slow-motion spectra. The implications of the existence of a distribution of tau //R values for calculations based on the assumption of a single value are examined.