Fluorescence study of thermotropic liquid‐crystalline polyesters

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Abstract
Photoluminescence behavior (polarization, lifetime) related to liquid‐crystal (LC) formation was examined for the thermotropic liquid‐crystalline polyesters poly [(ethylene terephthalate)‐co‐(p‐oxybenzoate)] (PET40/OBA60) (OBA content: 60 mol %) and poly [(ethylene 2,6‐naphthalene dicarboxylate)‐co‐(p‐oxybenzoate)] (PEN50/OBA50) (OBA:50 mol %). The Growth of liquid‐crystalline (LC) phases of PET40/OBA60 proceeded during annealing. even at low temperature (e.g., 138°C) and were promoted by an increase in annealing temperatures Ta in the experimental temperature range 138–260°C. The concentration dependence of fluorescence spectra of PET40/OBA60 in solution suggested that the fluorescences at 325 and 395 nm can be attributed to monomer and ground‐state dimer, respectively. The increase in dimer fluorescence intensity and the decrease in the fluorescence anisotropy ratio r from 0.06 to –0.14 were observed with growth of LC phases. These effects are explained by an increase in the ground‐state dimer population and a slight change in the dimer configuration, respectively.PEN50/OBA50 showed monomer fluorescence at 395 nm due to naphthalenedicarboxylate segments and excimer fluorescence at 430 nm. The r value for the excimer fluorescence decreased from zero to about −0.14 with growth of the LC phase. Such an extraordinary phenomena, in comparison with the usual excimer fluorescences which occurs through energy migration, could be interpreted as the result of formation of high‐concentration excimer sites induced by chain orientation in LC domains. © 1993 John Wiley & Sons, Inc.