Fluorescence Study on Intermolecular Interactions between Mesogenic Biphenyl Moieties of a Thermotropic Liquid-Crystalline Polyester (PB-10)

Abstract

By applying fluorescence together with DSC and WAXD, structure analysis for a thermotropic liquid-crystalline (LC) polyester (PB-10), prepared from 4,4‘-dihydroxybiphenyl and sebacic acid, containing 10 methylene units, was carried out. The PB-10 shows two enantiotropic transitions: One is the crystal−smectic H transition at 207 °C and the other the smectic H−isotropic phase transition at 257 °C. By measuring fluorescence spectra at 25−283 °C as well as those of diacetoxybiphenyl and biphenyl as model compounds, a high excitation wavelength dependence of various intermolecular ground-state complexes was found to depend on different degrees of overlap between mesogenic biphenyl moieties. Fluorescence spectra can be divided into three groups according to excitation wavelength: monomer fluorescence of biphenyl group excited at 260−300 nm (emission peak at 343 nm), fluorescence from intermolecular ground-state complexes excited at 320−360 nm, and fluorescence excited at 380−400 nm. The fluorescence peak wavelength excited at 320 nm shifts from 365 to 429 nm during heating. This is due to the change of molecular interactions between mesogenic moieties from the interaction between two fully overlapping biphenyl groups to that between a biphenyl and an ester moiety. Temperature dependence of the fluorescence peak wavelength excited at 320 nm and Arrhenius-type plots of fluorescence intensity excited at various wavelengths show a common break at 175 °C, a temperature a little lower than the crystal−smectic H transition temperature. This suggests that the phase transition starts initially from the local change in microstructure (intermolecular interaction between mesogenic moieties) and then extends to a change in macrostructure. Fluorescence is an effective tool for providing molecular level information on LC polymers.