MOLECULAR-DYNAMICS OF A LIQUID-CRYSTALLINE POLYMER STUDIED BY 2-DIMENSIONAL FOURIER-TRANSFORM AND CW ESR

Two-dimensional Fourier transform (2D-FT) and CW-ESR experiments at X- band frequencies were performed over a broad range of temperatures cov ering the solid and melt states of a liquid crystalline (LC) polymer. The CW-ESR experiments were analyzed by conventional motional models. The nematic phase was macroscopically aligned in the de magnetic field , whereas the solid state showed microscopic order but macroscopic dis order (MOMD). An end-label on the polymer showed smaller ordering and larger reorientational rates than that of the cholestane (CSL) spin pr obe dissolved in the same polymer, since the former can reorient by lo cal internal chain modes. It was demonstrated that the 2D-FT-ESR exper iments provide greatly enhanced resolution to the ordering and dynamic s of the end-label, especially when performed as 2D-ELDOR (electron-el ectron double resonance) experiments as a function of mixing time. The conventional model of Brownian reorientation in an orienting potentia l was unsuccessful in interpreting these results. Instead the model of a slowly relaxing local structure (SRLS), which enables differentiati on between the local internal modes experienced by the end-label and t he collective reorganization of the polymer molecules around the end l abel, yielded much improved fits to the experiments in the nematic pha se. These nonlinear least squares fits showed that the polymer ''cage' ' relaxes more than 2 orders of magnitude slower than the local end-ch ain modes, and there is a moderate orientational potential coupling th e local end-chain motion to the ''cage'' with axial and nonaxial local order parameters of about 0.14 and 0.29 at 100 degrees C. In the soli d state the 2D-ELDOR spectra were fitted to the MOMD model, which is a limiting case of the SRLS model when the cage relaxation becomes very slow.