LAYER STRUCTURES .8. POLY(BENZOXAZOLE-ESTER)S WITH A 4-LAYER OR A 6-LAYER REPEAT UNIT

Two isomeric polyesters (3a and 3b) were synthesized from 2-(4-hydroxy phenyl)benzoxazolecarboxylic acid, 1,12-dodecanediol, and tetraethylen e glycol, so that an alternating sequence of four subunits was obtaine d. Both polyesters differ in their internal orientation of the benzoxa zole units with the consequence that only one of them (3a) is liquid-c rystalline forming an enantiotropic smectic-C phase. Furthermore, a th ird polyester (4) was prepared with an alternating sequence of six sub units using 4,4'-dimercaptobiphenyl as a third stiff building block. B oth DSC and X-ray measurements confirm that all three polyesters form a crystalline solid state with a smectic type layer structure. Fiber p attern indicate that in the case of 3a and 3b the chains are tilted wi th an angle of about 45 degrees relative to the layer planes, and the solid state corresponds to a smectic-H structure. In the case of 4 the chains are in upright position, although four subunits are identical with the repeat unit of 3b. Therefore, the solidification of 4 may be understood as a self-assembling process. The thermal phase transitions were also characterized by synchrotron radiation measurements up to t he isotropization temperature.