H NMR Relaxation Study of Chain Motions and Solid-State Ordering in Stiff, Random Copolymers

Abstract

Fully-aromatic, thermotropic, liquid crystalline random copolyesters of 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA) were studied at elevated temperatures with 1 H NMR. Proton T 1 measurements provide high-frequency characterization of the glass transition. Proton rotating frame relaxation time ( T 1 ρ ) measurements near room temperature are sensitive to HBA rotational motions; these motions are expected when there is monomer length-scale chain conformational disorder. In melt-quenched samples, this disorder is spatially homogeneous (i.e., the material solidifies without crystallization). Annealing below the nominal solidification temperature (T s ) results only in a slight, uniform lessening of this disorder. For samples annealed above T s, proton T 1 ρ measurements show a spatially distinct component in which monomeric rotational motions are suppressed. Spin−diffusion experiments indicate this component has a mean thickness of 10 ± 2 nm. 1 H- 13 C (cross polarization-magic-angle spinning) CP-MAS measurements indicate that these ordered regions are enriched in HBA. The fraction and HNA content of the “cocrystallites” decrease at higher annealing temperatures. We discuss a new conceptual model of solid ordering in highly sequence-frustrated, stiff HBA/HNA copolyesters that is consistent with these and previously reported data.