POLYMORPHISM IN LIQUID-CRYSTALLINE POLY[TETRAMETHYLENE TEREPHTHALOYLBIS(4-OXYBENZOATE)]

Two different crystalline forms have been observed in liquid crystalline poly[tetramethylene terephthaloylbis(4-oxybenzoate)]. Form I is produced when the sample precipitates from solution. When the sample is slowly cooled or quenched from the mesophase, form II is obtained. Form I shows an irreversible solid-solid transformation to form II upon heating, prior to the solid-mesophase transition. Once the thermal conversion of form I to form II is complete, it is not possible to recover form I without solving the crystals of form II. The existence of polymorphism and the transformations from the crystal to the mesophase and to the isotropic state have been studied by differential scanning calorimetry, X-ray diffraction, and thermooptical microscopy. Solid-state C-13 NMR spectroscopy has also been used in order to obtain information about the structure of these two crystalline forms. A gauche-trans-gauche conformation in the alkyl region is proposed for form I according to the chemical shifts of the methylene carbons. A more planar conformation in the flexible spacer, as well as a different orientation for the phenyl rings is suggested for form II.