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 hyd roquinone, hydroxybenzoic acid, and isophthalic acid units were invest igated by using high-resolution solid-state C-13 NMR. The C-13 chemica l shift tensor parameters were determined from the spinnig side band i ntensities. The rigid-lattice behavior was obtained from the study of model molecules in the crystalline state. At temperatures below the gl ass transition temperature, partial motional averaging of the chemical shift tensors of the aromatic carbons has shown the existence of osci llations for both para- and meta-substituted rings. The amplitude of t hese oscillations increases with increasing temperature. Results obtai ned from the chemical shift anisotropy of the carbonyl carbons have be en interpreted in terms of correlated motions of the para-substituted phenyl rings and adjacent carbonyl groups. Above 425 K one also observ es small-amplitude main-chain reorientations which are likely to be in volved in the glass-transition phenomena.