Evaluations of Mesogen Orientation in Thin Films of Polyacrylate with Cyanobiphenyl Side Chain

The orientation behavior of mesogens in a polyacrylate with cyanobiphenyl (CB) side chain in thin films was investigated in detail by UV-vis absorption spectroscopy and grazing incidence small-angle X-ray scattering (GI-SAXS) measurements using both high-energy X-rays of Cu Kα line (λ = 0.154 nm) and low-energy synchrotron X-rays (λ = 0.539 nm). By changing the film thickness ranging 7-200 nm, it is concluded that the planar orientation is predominant for thin films with thickness below 10-15 nm. This planar mesogen orientation near the substrate surface coexists with the homeotropically aligned CB mesogens in films thicker than 30 nm. For the thinnest 7 nm film, the planar orientation is unexpectedly lost, which is in consort with a disordering of smectic layer structure. Peculiar orienting characteristics of CB mesogen are suggested, which probably stem from the tendency to form an antiparallel arrangement of mesogens due to the strong dipole moment of the terminal cyano group.