A. Rastegar

Mechanism of liquid crystal alignment on submicron patterned surfaces

The alignment mechanism of liquid crystals on polymeric surfaces that were patterned using an atomic force microscope (AFM) tip was studied by polarizing optical microscopy. Depending on the thickness of the polymer, polymer chain alignment or grooves appear to be responsible for the liquid crystal alignment. In thick polymer films (above 100 nm) the polymer chains will align in the direction of the scan due to the large lateral force exerted by the tip. In thin polymer films (5–20 nm) the polymer chains are strongly fixed to the surface and will not realign by the tip; instead grooves will be formed. The azimuthal surface anchoring the energy of thick polymer films does not depend on the scan line separation and scan force, is of the same order of magnitude as for conventionally rubbed polyimide surfaces, but increases by the number of scans. AFM patterned pixels do not show a pretilt angle.

A shear cell for aligning and measuring birefringence of bow-shaped (banana) liquid crystals

We present a special shear cell for aligning unconventional liquid crystals and performing high temperature optical studies. Sample thicknesses of 1–100 μm in the temperature range of 20–200 °C with a shear amplitude as large as 1 mm are achievable with this cell. The shear cell was used to align banana-shaped liquid crystals and to measure their optical anisotropy in the B2, B3, and B4 phases which was not possible before. The optical anisotropy of the B2 phase slowly increases by cooling and jumps to a low value at the transition to the B3 phase. The optical rotation is almost constant in the B2 phase and increases sharply at the transition to the B3 phase. The B3 and B4 phases are distinguishable by their optical anisotropy.

Surface versus bulk anisotropy of photo-sensitive poly(vinyl cinnamate) alignmet layers

Abstract Reaction kinetics and orientational anisotropy of poly(vinyl cinnamate) (PVCN) alignment layers photo-co-polymerized with linearly polarized UV light were probed by measurements of linear dichroism (LD) and surface optical second-harmonic generation (SSHG). The results show that the bulk and the surface properties of PVCN films are very similar and suggest that in PVCN there are no specific surface photochemical processes, which could presumably reduce the anchoring of liquid crystals on PVCN substrates. The azimuthal surface anchoring energy coefficient of nematic liquid crystal 5CB in contact with our substrates was of the order of 10−6 J/m 2 and strongly depended on the procedure used to measure it. This can be explained by a presence of various memory effects on the LC/PVCN interface, which strongly affect the alignment ability of the PVCN layers.

Alignment of nematic liquid crystals on an electrically poled photopolymer film

We have observed liquid crystal alignment on an electrically poled polyvinylcinnamate film in which a polar symmetry was introduced by an in-plane electric field and stabilized by unpolarized ultraviolet light. The azimuthal anchoring energy and pretilt angle was measured in a twisted nematic or planar cell and is explained via the molecular structure of the unreacted cinnamates and of the photoproducts β-truxinates. Cinnamates mainly contribute via a strong flow alignment, while β-truxinate produces the anisotropic properties of the film such as its polar anisotropy, azimuthal anchoring, and pretilt angle.

Electrically Aligned Photo-Polymer Films for Liquid Crystal Alignment

Abstract The photo and electric field alignment of thin film of polyvinylcinnamate (PVCN) is shown to be considerably enhanced by using a combination of both methods. ITO-coated glass substrates were covered with a PVCN film using spin or dip coating and then heated above the glass transition temperature of PVCN. The alignment was induced by applying an electric field and stabilized by [2+2] cycloaddition of the polymer using a linearly polarized UV light. The anisotropy of the aligned polymer films was checked by measuring linear birefringence. Without crosslinking, the anisotropy of the electrically aligned films is very weak. Crosslinked PVCN films using polarized UV light in the absence of any external fields lead to a better anisotropy than the aligned film by an electric field. The anisotropy is however increased substantially when the aligned PVCN films were crosslinked in the presence of a field.

Alignment of nematic liquid crystals on poled photo-polymer films

Abstract We have studied the alignment of nematic liquid crystals on Electrically Poled Photopolymer (EPP) films. Polarization microscopy studies show different textures in the planar and Twisted Nematic (TN) cells that were made of an EPP substrate and a rubbed-polyimide (PI) substrate. The EPP films exhibit a small pretilt angle of 3 to 5 degrees depending on the cell configuration. In contrast to the small anisotropy of the EPP film compared to Linearly Photo Polymerized (LPP) films, the azimuthal anchoring strength of the EPP films is larger than that of LPP films and depends on the UV exposure time. The alignment of EPP films in the TN cells is stable for months whereas the alignment of the LPP films is destroyed within three days.

Electric field aligned photo-polymer films for liquid crystal display

Abstract Oriented photopolymer films were obtained by using electric field alignment of the cinnamoyl side groups near the glass transition temperature of poly(vinyl)cinnamate (PVCN) followed by photo-polymerization using unpolarized UV light. In this way the induced anisotropy could be fixed and was subsequently shown to lead to a well aligned liquid crystal texture. The combination of electric field and UV light offers a solution to the pretilt angle problem for photo-aligned substrates.

FINITE SIZE EFFECTS IN THE DYNAMIC BEHAVIOR OF SMECTIC C* PHASE STUDIED BY LIGHT SCATTERING

Abstract The dynamic behavior of ferroelectric liquid crystals in thin cells with planar boundary conditions and the helix unwound, was studied by the quasielastic light scattering. The autocorrelation function of the scattered light is non-exponential. From the angular dependence of the measured relaxation rate, the lowest orientational mode eigenvalue was determined. From this value the surface anchoring energy for SCE9 on nylon surface was determined to be 5×10−5 J/m2

Observation of twist nematic liquid-crystal lines

We have observed that a single groove made by an atomic force microscope (AFM) tip in a polyimide layer strongly aligns nematic liquid crystals locally and have used this phenomenon for studying twist nematic lines. We have measured the intensity profile of light transmitted across a single line and the azimuthal surface energy as a function of the spatial separation of grooves. From these measurements, we have determined the azimuthal surface anchoring energy of the AFM structured and of the untreated polyimide. We find that the twist coherence length, which determines the width of TN lines, is approximately proportional to the cell thickness, while the surface anchoring energy can change it for a factor of 2 at maximum.

Collective excitations in the vicinity of the SmA-SmC alpha* phase transition

Abstract Using photon correlation spectroscopy we have analyzed the order parameter dynamics in the SmA and SmC*α phases of 4-(1-methylheptyloxy-carbonyl) phenyl 4′-octyloxy biphenyl-4-carboxylate (MHPOBC) liquid crystal. In the SmA phase we observe two modes: a very fast soft mode that slows down with decreasing temperature and a very slow mode. In the SmC*α phase, another mode appears at intermediate relaxation rates, and is identified as the phase mode. The dispersion of the soft and phase modes is nearly flat in the SmC*α phase, indicating high chirality of this phase.