  We consider the mesoscopic model for the liquid crystalline brush that might
serve as a photoswitchable aligning surface for preorientation of low molecular
weight liquid crystals in a bulk. The brush is built by grafting the polymer
chains of a side-chain molecular architecture, with the side chains terminated
by a chromophore unit mimicking the azobenzene unit, to a substrate. When
irradiated with ultraviolet light, the chromophores photoisomerize into a
non-mesogenic cis state and the whole system turns into an ordinary polymer
brush with no orientational order and two states: the collapsed and
straightened one, depending on the grafting density. When irradiated with
visible light, the chromophores photoisomerize into a mesogenic trans state,
resulting in formation of a transient network between chains because of a
strong attraction between chromophores. Spontaneous self-assembly of the brush
in these conditions results in an orientationally isotropic polydomain
structure. The desired uniaxial planar ordering of chromophores within a brush
can be achieved at certain temperature and grafting density intervals, as the
result of a two-stage preparation protocol. An external stimulus orients
chromophores uniaxially at the first stage. The system is equilibrated at the
second stage at a given temperature and with the external stimulus switched
off. The preoriented chromophores either keep or loose their orientations
depending on the strength of the memory effect inherent to a transient network
of chains that are formed during the first stage, similarly to the case of the
liquid crystalline elastomers, where such effects are caused by the covalent
crosslinks.
