Dj. Xu et al., ROTATIONAL DIFFUSION AND ORDER PARAMETERS OF A LIQUID-CRYSTALLINE POLYMER STUDIED BY ESR - MOLECULAR-WEIGHT DEPENDENCE, Journal of physical chemistry, 100(39), 1996, pp. 15867-15872
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.