Synthesis and characterization of thermotropic liquid crystalline polyesters with biphenyl unit in the main chain

Two new series of liquid crystalline polyesters containing central biphenylene moiety in the mesogenic unit with ester and ether linkages to the different lengths of flexible spacers (series I and II, respectively) were synthesized using melt polycondensation reaction. All the obtained compounds were characterized by conventional spectroscopic methods. The copolymer compositions were obtained from 1H NMR. The thermal behavior of the polymers has been characterized using polarized optical microscopy and differential scanning calorimetry. The effect of monomer structure and methylene spacer length has been investigated on the prepared polyesters and is also compared with that of the analogous polyester having central phenylene unit. Optical microscopy showed a highly threaded liquid crystalline texture to a high birefringent Schlierene texture characteristic of the nematic phase for all polyesters. DSC experiments were also found in accord with mesophase formation. The melting points and isotropization temperature values of series I polymers having a central 4,4′-biphenyldicarboxylate residue were significantly higher than those of the series II having a central 4,4′-biphenol moiety due to different molecular geometry. The glass transition temperature values of series I polyesters are also higher than those for series II because of increased chain rigidity. The effect of copolymerization on liquid crystallinity has been studied on the prepared copolymers from equimolar mixture of monomer with different lengths of spacers (n = 4, 6) and it is compared with that of the analogous homopolyesters. Our finding shows that the copolymers exhibit reduced melting point and extended liquid crystalline range.