SEGMENTAL ORIENTATION IN AROMATIC POLYESTERS BY SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE AND MODELING

Proton spin-spin relaxation measurements and the corresponding second moments [DELTAH-2] of broad line spectra are reported for powder samples of homopolyesters of hydroxybenzoic acid (HBA) and hydroxynaphthoic acid (HNA) and for oriented tape samples of HIQ-35, a terpolyester composed of HBA, isophthalic acid, and hydroquinone. These are reviewed in relation to previous results reported for related copolyesters of HBA and HNA. In particular, the values of [DELTAH-2] for isotropic samples were predominantly composition dependent and of little practical value for studies of detailed molecular structure. Orientation-dependent second-moment anisotropies, on the other hand, provide indirect insight into molecular conformation and orientation through comparison with results from detailed molecular models. The orientations of monomers are well described by the distribution function proposed by Bower; a quantitatively accurate description of the orientation distribution in cylindrically symmetric geometries requires only the Legendre polynomial coefficient [P2]. Modeling results based on rotational isomeric state theory for chain segments of finite length and high degrees of orientation successfully reproduce the degrees of anisotropy observed in [DELTAH-2] for both HBA/HNA and HIQ polymers. The model provides a link between measures of orientation obtained at different molecular scales by mechanical analysis, wide-angle x-ray diffraction, and nuclear magnetic resonance spectroscopy.