SIMULTANEOUS X-RAY DSC STUDY OF MESOMORPHISM IN POLYMERS WITH A SEMIFLEXIBLE MESOGEN

In order to investigate the limits of macromolecular chain flexibility tolerated by the nematic state, a new group of polyethers has recently been synthesized, where not only the spacer but also the mesogen provide a degree of flexibility. This is achieved by introducing a rotationally mobile ethylene group linking the two phenyl rings in the l-(4-hydroxyphenyl)-2-(2-methyl-4-hydroxyphenyl)ethane (MBPE). These “mesogens” are separated by flexible -O(CH2)nO- spacers, where n is either a single value (homopolymers) or has two different values (copolymers). Results of X-ray diffraction studies of MBPE polymers using the simultaneous X-ray diffraction and DSC technique (XDDSC) conclusively prove the existence of the nematic phase in most homopolymers and all copolymers. Depending on the polymer, the phase is either mono- or enantiotropic. While in a few cases the phase is thermodynamically stable, in most cases it is metastable. The weak first-order transition below the I-N transition temperature appears not to be the nematic-smectic transition as previously suspected, and its nature is still being investigated. Current X-ray evidence shows further that the molecular packing density in the nematic phase is considerably higher for polymers with even spacers than it is in polymers with odd spacers. © 1990, American Chemical Society. All rights reserved.