Grazing incidence X-ray reflectivity of coumarin side-chain polymers used for liquid crystal photoalignment layers

Abstract

Specular X-ray reflectivity measurements are used to examine photoinduced surface roughness of side-chain coumarin polymers that form liquid crystal photoalignment layers. Thin films were irradiated with polarised or unpolarised ultraviolet light and the X-ray reflectivity at grazing incidence was measured for samples irradiated with different ultraviolet fluences. Kiessig fringes are observed over a large dynamic range and a comparison of the measured and calculated reflectivity gives a value for the surface roughness. The photoinduced roughness increases with ultraviolet fluence, from 5.5 to 20 Å. Negligible photoinduced roughness is detected for a film, which was previously irradiated using conditions that give strong planar liquid crystal alignment. Specular reflectivity data taken with the plane of incidence of the X-ray parallel and perpendicular to the polarisation direction of the ultraviolet light are identical within experimental uncertainty. Hence in-plane anisotropic surface roughness is not observed.