UV-A and UV-B banning highly soluble photocrosslinkable thermotropic liquid crystalline copolyesters with ether linkage in the backbone

Abstract

Four random copolyesters were synthesized by the polycondensation of a chalcone diol 3-(4-hydroxy-3methoxyphenyl)-1-(4-hydroxyphenyl)prop-2-en-1-one (HMPP) with the common diacid, namely, 4,4′-oxybis(benzoic acid) with two varying diacids (terephthalic acid and isophthalic acid) and two varying diols (1,4-dihydroxyanthraquinone and 1,5-dihydroxyanthraquinone). The chalcone diol was synthesized by acid-catalyzed Claisen-Schmidt reaction. These four random copolyesters were synthesized by solution polycondensation technique and were characterized by qualitative solubility tests and viscosity measurements. The monomeric moieties incorporated in the copolyester backbone was established by FT-IR, 1H NMR and 13C NMR spectroscopic techniques. Tg, Tm and Tiso were determined by DSC thermograms and found to be in the range of 59–97 °C, 123–143 °C and 160–252 °C, respectively. The copolyesters displayed nematic phase without involving methylene-flexible spacers in the polymeric backbone. The phase transition was found to be reproducible on heating and cooling above Tm. The photocrosslinking ability was ascertained by recording UV spectra at different time intervals of irradiation of UV light from 150-watts, 365-nm mercury lamp at room temperature in DMF. Herein, for the first time, we report the synthesis and characterization of a series of novel photosensitive liquid crystalline chalcone polymers without spacer chains. In addition to these, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been performed to ascertain the cross-linking nature of the synthesized polymer from computed absorption spectra. All these polyesters exhibit a good photocrosslinking property by absorbing UV-A light. They continue to absorb UV-B light (which is more dangerous to human skin and eyes) after the crosslinking. As these polymers filter UV-A and UV-B rays one after another, a switching system can be evolved from these materials. These polymers exhibit high thermal stability and UV filtering ability at both UV-A and UV-B regions and have good solubility ensuring their application-oriented nature.