The Surface Morphology Effects on Liquid Crystal Alignment After Photo-Agitation and Crosslinking of Polyimide

The photo-reactive activators are highly reactive radical generators upon the ultraviolet (UV) light illumination. The photo-reactive initiators produced nitrogen radical and alkyl radical after releasing carbon dioxide. The radicals could react with polyimide (PI) main chains. These reactions enforced the alignment layers and exhibited high azimuthal anchoring energy. The thickness of photo-irradiated PI alignment layers were reduced dramatically by photo-induced crosslinking, which induced surface wrinkling and roughness. The carbon dioxide gases released from the thin films produced many micro-pores, which provides tight anchoring of liquid crystal (LC) molecules. The azimuthal anchoring energy obtain by photo-alignment was better than that obtain by rubbing method with the same PI. The maximum value was 6.92×10−5 J/m2. Small aliphatic hydrocarbons, such as methane and propene, were released during photo-decomposition reaction from the PI surface. The polarity of film surface became more hydrophilic. The photo-alignment of LC was perpendicular to the polarization axis of UV light. On the basis of high anchoring energy, the rough surface, hydrophilic surface, and rapid photo-reactions, the photo-alignment mechanism is proposed.

Формирование режима ударного давления в форме бегущей волны в нематических твистовых ячейках

The paper presents a numerical study, within a nonlinear generalization of the classical Ericksen–Leslie theory, of a new nonlinear mechanism for the formation of the regime of shock pressure on the boun-ding walls of a microsized twisted nematic cell (TNC), realized in the form of the kinklike traveling wave $$\mathcal{P}$$ ( z – $${v}t$$ ) under the effect of the externally applied electric field and induced by a strong local distribance of the director field in the form of a Gaussian (normal) distribution. The mechanisms responsible for the formation of the traveling wave of shock pressure propagating in the TNC from one of its boundaries to another are studied, and it is shown how the magnitude of the electric field and the shape of the localized initial perturbation of the director field affect the similarity of the traveling wave to the kinklike wave. Studies of the dynamic relaxation of the director field in TNCs also showed that, at temperatures exceeding the temperature for the nematic–smectic A (NA) T _NA phase transition by several dozen mK, fluctuations in the order parameter of the emerging smectic phase suppress the effect of the electric field and favor the singular behavior of the azimuthal anchoring energy density as T → T _NA.

Mesogen-co-polymerized transparent polyimide as a liquid-crystal alignment layer with enhanced anchoring energy

This work shows the key role played by the mesogens side groups of polyimides in enhancing the azimuthal anchoring energy.

SURFACE AZIMUTHAL ANCHORING ENERGY BETWEEN THE GRATING SURFACE AND NEMATIC LIQUID CRYSTAL LAYER STUDIED BY USING A GEOMETRIC MODEL

A simple geometric model is proposed for estimating the azimuthal anchoring energy between the sinusoidal relief grating surface and nematic liquid crystal layer of a liquid crystal display (LCD) device as a function of the grating height and grating pitch. The model parameters are determined experimentally, and the model is then used to predict the surface azimuthal anchoring energy for gratings with various pitches and heights. It is shown that a good agreement exists between the predicted results for the surface azimuthal anchoring energy and the experimental data. Moreover, a good agreement is also observed between the estimated results and those obtained from Berreman’s expression and finite element method (FEM) simulations, respectively. Overall, the experimental and numerical results show that for the nematic liquid crystal considered in the present study (4-n-pentyl-4’-cyanobiphenyl (5CB)), the surface azimuthal anchoring energy increases with an increasing grating height or a reducing grating pitch.