H-1-NMR RELAXATION STUDY OF CHAIN MOTIONS AND SOLID-STATE ORDERING INSTIFF, RANDOM COPOLYMERS

Fully-aromatic, thermotropic, liquid crystalline random copolyesters o f 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA) wer e studied at elevated temperatures with H-1 NMR. Proton T-1 measuremen ts provide high-frequency characterization of the glass transition. Pr oton rotating frame relaxation time (T-1 rho) measurements near room t emperature are sensitive to HBA rotational motions; these motions are expected when there is monomer length-scale chain conformational disor der. In melt-quenched samples, this disorder is spatially homogeneous (i.e., the material solidifies without crystallization). Annealing bel ow the nominal solidification temperature (T-s) results only in a slig ht, uniform lessening of this disorder. For samples annealed above T-s , proton T-1 rho measurements show a spatially distinct component in w hich monomeric rotational motions are suppressed. Spin-diffusion exper iments indicate this component has a mean thickness of 10 +/- 2 nm. H- 1-C-13 (cross polarization-magic-angle spinning) CP-MAS measurements i ndicate that these ordered regions are enriched in HBA. The fraction a nd HNA content of the ''cocrystallites'' decrease at higher annealing temperatures. We discuss a new conceptual model of solid ordering in h ighly sequence-frustrated, stiff HBA/HNA copolyesters that is consiste nt with these and previously reported data.