HIGH-RESOLUTION SOLID-STATE C-13 NMR INVESTIGATION OF LOCAL MOTIONS IN A MESOMORPHIC MAIN-CHAIN COPOLYESTER

The local motions of a mesomorphic main-chain copolyester based on hydroquinone, hydroxybenzoic acid, and isophthalic acid units were investigated by using high-resolution solid-state C-13 NMR. The C-13 chemical shift tensor parameters were determined from the spinnig side band intensities. The rigid-lattice behavior was obtained from the study of model molecules in the crystalline state. At temperatures below the glass transition temperature, partial motional averaging of the chemical shift tensors of the aromatic carbons has shown the existence of oscillations for both para- and meta-substituted rings. The amplitude of these oscillations increases with increasing temperature. Results obtained from the chemical shift anisotropy of the carbonyl carbons have been interpreted in terms of correlated motions of the para-substituted phenyl rings and adjacent carbonyl groups. Above 425 K one also observes small-amplitude main-chain reorientations which are likely to be involved in the glass-transition phenomena.