ROTATIONAL DIFFUSION AND ORDER PARAMETERS OF A LIQUID-CRYSTALLINE POLYMER STUDIED BY ESR - MOLECULAR-WEIGHT DEPENDENCE

The microscopic rotational dynamics of a main chain liquid crystalline (LC) poly(ether) in its nematic phase is studied in detail by nonline ar least squares analysis of ESR spectra in the slow motional regime. This complements results reported in an accompanying paper, which focu ses on macroscopic translational diffusion using the DID-ESR (dynamic imaging of diffusion by ESR) technique. Far infrared 250 GHz ESR spect roscopy is used to determine the magnetic g and A tensors of the 3-car boxy-PROXYL spin label attached to the LC polymer. ESR spectra of the labeled polymers of varying molecular weights are analyzed to yield th e rotational diffusion coefficients and orientational order parameters . Different cases of the degree of macroscopic alignment are observed in these samples and accounted for in the simulations. For molecular w eights lower than 11 000 (for both tracers and matrices), the rotation al diffusion coefficient R is found to correlate with the molecular we ight of the polymer matrix and to be independent of the molecular weig ht of tracer, suggesting the importance of free volume for end-chain m otion. Macroscopically aligned samples, corresponding to lower molecul ar weight LC polymers, show an inverse correlation of R with order par ameter, consistent with observations previously reported for nonpolyme ric LCs, which were associated with free volume effects.