Synthesis and Characterization of Liquid Crystalline Polyesters Containing α,β-unsaturated Ketone Moiety in the Main Chain Derived from 2,6-bis(4-hydroxybenzylidene)cyclohexanone

A series of polyesters containing a photosensitive benzylidene chromophore in the main chain were synthesized from 2,6-bis(4-hydroxybenzylidene)cyclohexanone with various aliphatic and aromatic diacid chlorides by interfacial polycondensation technique. The intrinsic viscosity of the synthesized homo and copolymers determined by Ubbelohde viscometer and found to be 0.13–0.18 dL/g. The molecular structure of the monomer and polymers was confirmed by Fourier Transform infrared, 1H NMR and 13C NMR spectroscopic techniques. These polymers were studied for their thermal stability and photochemical properties. Thermal properties were evaluated by thermogravimetric analysis and differential scanning calorimetry. It was found that the polymers were stable up to 238°C and start degrading thereafter. Increase in acid methylene spacer length decreases the thermal stability. The self-extinguishing property of the synthesized polymers was studied by calculating the limiting oxygen index value using Van Krevelen equation. The influence of the length of methylene spacer on phase transition was investigated and it was found that isotropic temperature decreases on increasing the acid methylene spacer length. Hot-stage optical polarizing microscopic study showed that synthesized polymers exhibit grainy texture in the temperature range of 115–127°C. The photolysis of liquid crystalline polyesters revealed that α,β-unsaturated ketone moiety in the main chain dimerizes through 2π+2π cycloaddition reaction to form a cyclobutane derivative that leads to crosslinking.