High in-plane alignment of liquid crystalline methacrylate copolymers bearing photoreactive 2-styrylpyridine and mesogenic 4-methoxybiphenyl groups

This paper describes the thermally enhanced in-plane alignment and other properties of liquid crystalline (LC) copolymers (1, 2, and 3) consisting of a methacrylate with a 2-styrylpyridine side group as a photocyclodimerizable group and a methacrylate with a methoxy biphenyl side group as a mesogenic group. ¹H NMR spectroscopy indicated that the contents of the photoreactive monomer unit in copolymers 1, 2, and 3 were 0.65, 0.46 and 0.19 (mole fraction), respectively. The copolymers in the film showed a transmittance of 92% at wavelengths > 450 nm, even at 100 °C, as well as thermal stability up to approximately 360 °C. Copolymers 1 and 2 gave a nematic texture at 155 and 150 °C, respectively, while copolymer 3 showed a smectic texture at 130 °C. The copolymer films aligned parallel to the linearly polarized UV(LPUV) light after LPUV light irradiation and subsequent thermal annealing. The respective in-plane order parameters of copolymers 1, 2, and 3 in the film at 315 nm were 0.65 at 0.001 J/cm², 0.59 at 0.01 J/cm², and 0.48 at 0.025 J/cm² by the same treatment in the LC temperature ranges. This suggests that the photoreactive LC copolymers not only have high optical transparency and good thermal stability but also have controllable, high in-plane alignment with the contents of the photoreactive monomer unit.