Effect of para-substituents of azobenzene side chains tethered to poly(methacrylate)s on pretilt angle photocontrol of nematic liquid crystals

In this paper, we describe the effect of para-substituents of azobenzene side chains attached to poly(methacrylate)s on both photoinduced dichroism of the polymer films and pretilt angle photocontrol of nematic liquid crystals (LCs). Oblique exposure of the polymer films to non-polarized light gave rise to azimuthal photodichroism in parallel with the incident plane of actinic light. The photodichroism behavior was markedly influenced by the para-substituents of azobenzenes. Ellipsometric measurement revealed that the azobenzene moieties were inclined in optically biaxial manner probably because the polymer matrix is probably anchored at the substrate surface. When the cells were fabricated with nematic LCs, the polymer films generated homeotropic LC orientation, which was continuously transformed into homogeneous and even tilted alignment by oblique exposure to non-polarized 436 nm light. On the other hand, the transient pretilt angles of LCs were not measurable by oblique exposure to non-polarized 365 nm light though the tilted LC orientation was obtained at the photostationary state. This happened from the deterioration of LC orientation that takes place from the E-to-Z photoisomerization of azobenzenes at the outmost polymer surfaces. The photoinduced pretilt angles of nematic LCs were distinctly governed by the molecular structures of the azobenzene-containing polymers.