ИСТИНА

Among a large number of modern melt-processed polymers, polyethylene terephthalate (PET) remains one of the most important large-scale polyester. Improving the thermal and mechanical properties of PET-based materials can be achieved through chemical modification. Novel thermotropic liquid crystal (LC) copolymers were synthesized by various methods: 1) melt polycondensation in a mixture of diglycol terephthalate and 4'-acetoxy-4-biphenylcarboxylic acid (ABCA); 2) acidolysis transesterification of PET with ABCA. Rather chaotic distribution of comonomer units has been proven by NMR method. Introduction into PET of 40-80 mol. % ABCA provides the production of copolyesters with increased thermal stability (glass transition point - up to 100 °C) and the ability to form anisotropic melts above 270 °C. In copolymers obtained by transesterification, the formation of the LC phase occurs at a lower content of ABCA in comparison with polycondensation copolyesters. In addition, these copolymers are more thermostable and have increased mechanical characteristics (due to the reduced content of ether bonds in macromolecules). According to XRD and polarization microscopy data, the “frozen” LC phase in solid copolymers coexists with crystalline regions of poly-(4'-hydroxy-4-biphenylcarboxylate), non-melting up to 400 °C. The viscoelastic properties of copolyester melts depend on their morphology. At a low content of ABCA units, copolymers form isotropic melts characterized by a quasi-Newtonian flow and a significant predominance of viscous characteristics over elastic ones. Rising the ABCA content is accompanied by an increase in viscoelastic characteristics of copolymers. With the formation of the LC phase in melts, a transition to a non-Newtonian flow with a yield point occurs, as well as an equalization of storage and loss moduli values. Thus, these novel copolyesters can be useful for obtaining heat-resistant materials with an ordered structure.