Combined liquid‐crystalline polymers: Rigid‐rod type main‐chain polyester with lateral mesogenic groups

Abstract

AbstractA series of combined main‐chain side‐group liquid‐crystalline polymers was synthesized by melt polycondensation from trans‐1,4‐cyclohexanedicarboxylic acid, chloro‐1,4‐phenylene diacetate and 6‐(4‐methoxy‐4′‐biphenylyloxy)hexyl‐1,4‐phenylene diacetate. These polyesters combine the features of rigid‐rod main‐chain LC‐polymers and of side‐group LC‐polymers. Polymers with small fractions of mesogenic side‐groups (5 and 10 mol‐%) have lower melting points than the parent polymer poly(chloro‐1,4‐phenylene 1,4‐cyclohexanedicarboxylate) but are still liquid crystalline up to their decomposition at T ≥ 400°C. In polymers with larger fractions of mesogenic side‐groups (50 and 100 mol‐%) partial isomerization of the trans‐1,4‐cyclohexanedicarboxylate units to the cis‐form occurs. The cis units represent kinks which disrupt the rigid‐rod main‐chain structure, leading to non‐crystalline polymers with clearing temperatures below 350°C. The clearing temperatures of a series of polyesters with 100 mol‐% mesogenic side‐groups and trans contents ranging from 23 to 97% suggest that some interaction between mesogenic side‐groups and residual mesogenic segments in the main chain is required for the formation of a liquid‐crystalline phase. X‐ray diffraction patterns obtained from melt‐spun fibers show that the polymers with 5 and 10 mol‐% mesogenic side‐groups are highly crystalline and have a triclinic unit cell. The mechanical properties of the combined polymers were studied by means of dynamic mechanical thermal analysis (DMTA). Three different mechanical loss processes were observed. The glass relaxation was found to be more pronounced in the polymers with larger fractions of mesogenic side‐groups, suggesting that the latter have a plasticizing effect.