Investigation of microstructure in poly(p-hydroxybenzoic acid)-co-(ethylene terephthalate) using nuclear magnetic resonance spectroscopy

The microstructure and dynamics of liquid crystalline copolymers of p-hydroxybenzoic acid (HBA) and ethylene terephthalate (ETP) were investigated by proton and exp 13 C solid-state NMR spectroscopy. The HBA/ETP 80/20 (80 mol% HBA) copolymer contains poly(HBA)-like crystallites with a mass fraction of 4-7% and a smallest linear dimension of approx 80 A. The dominant crystal structures in HBA/ETP 60/40 (60 mol% HBA) have a similar mass fraction but resemble poly(ethylene terephthalate) crystallites. NMR spin-lock relaxation indicates cooperative motion in both copolymers > 130 deg C involving small angular fluctuations of the local polymer chain axis about the local nematic director. A minority "rigid" component in HBA/ETP 80/20, which is 22-28% abundant at 150 deg C and approx 15% abundant at 290 deg C, does not exhibit cooperative motion. HBA/ETP 60/40 displays similar behavior. Static exp 13 C NMR line shapes indicate that aromatic rings exhibit free rotation about the polymer chain axis within the component that exhibits cooperative motion, while aromatic rings exhibit small-angle ( approx 20 deg ) fluctuations about their substituted axes in the rigid component. exp 13 C MAS NMR shows that the compositions of the rigid components in both copolymers > 130 deg C are nearly the same as the bulk compositions. The observed behavior is consistent with the presence of nonperiodic layer crystallites. Graphs, Spectra. 32 ref.--AA