Orientations of side chains and adsorbed liquid crystal molecules on a rubbed polyimide surface studied by optical second harmonic generation

Study of the molecular orientation of steroidal side chains at polyimide surfaces using sum frequency generation vibrational spectroscopy

The molecular orientation of steroidal side chains at rubbed polyimide (PI) surfaces is studied by sum frequency generation (SFG) vibrational spectroscopy. The main objective is to find a correlation between the molecular structure of the PI film and the liquid crystal alignment on the polymer. Analysis of the SFG spectra shows that rubbing of the polymer film appears to cause conformational changes in the methyl group of the polymer side chain near the steroidal structure. However, rubbing does not significantly influence the orientation of the isopropyl group at the end of the polymer side chain. This shows that the liquid crystal alignment is not correlated with the orientation of the isopropyl group.

Rubbing-induced anisotropy of long alkyl side chains at polyimide surfaces

Molecular organization at polyimide surfaces used as alignment layers in liquid crystal displays was investigated using vibrational sum frequency generation (SFG) spectroscopy. We focus on the orientation of the long alkyl side groups at the polymer surface using polarization-selected SFG spectra of the CH(3)- and CH(2)-stretch modes of the side chain. Mechanical rubbing and baking, an accepted industrial procedure used to produce pretilt of the liquid crystal, was found to induce pronounced azimuthal anisotropy in the orientational distribution of the alkyl side chains. Orientational analysis of the SFG vibrational spectra in terms of the azimuthal and tilt angles (in and out of plane, respectively) of the alkyl side chains shows their preferential tilt along the rubbing direction, with the azimuthal distribution narrower for stronger rubbed polymer samples.

Proton driven vase-to-kite conformational change in cavitands at an air-water interface monitored by surface SHG

The conformational changes of quinoxaline-bridged cavitands deposited as Langmuir films were monitored at different pH values of the subphase using surface second harmonic generation during the compression of the monolayer at the water surface. A quantitative analysis of the susceptibility tensor elements was performed for methylene (MeCav)- and quinoxaline (QxCav)-bridged cavitands for pH values varying between 5.7 and 0.1. For MeCav (reference compound), no significant changes were observed for different pHs, confirming that the cavity does not undergo protonation or a drastic conformational change. For the QxCav, however, the results suggest a partial opening of the cavity on the basis of analysis of the compression curves.

Mapping molecular conformation and orientation of polyimide surfaces for homeotropic liquid crystal alignment by nonlinear optical spectroscopy

Surface-specific sum-frequency vibrational spectroscopy and second-harmonic generation were used to study the structures of polyimide (PI) surfaces for homeotropic liquid crystal (LC) alignment and the molecular orientation of LC adsobates on these surfaces. The imide ring was perpendicular to the surface with one of CO bonds protruding out of the surface and the other pointing into the bulk rather than flat on the surface. The ester CO bond in the side chain was sticking out of the surface with a tilt angle of about 45 degrees -55 degrees from the surface normal, indicating that the rigid side chain core was, more or less, along the surface normal. The part of alkyl chain on the top of the side chain followed the orientation of the side chain core and protruded out of the surface with some gauche defects. The cyano biphenyl LC molecules were adsorbed on the PI preferentially with the terminal cyano group facing the PI surface.

Dynamic light scattering measurements of azimuthal and zenithal anchoring of nematic liquid crystals

Dynamic light scattering is used to obtain the anchoring energy coefficients of nematic liquid crystal (4-n-pentyl-4(')-cyanobiphenyl) on rubbed nylon and photoactive poly-(vinyl-cinnamate). The anchoring coefficients are determined by measuring the relaxation time of the fundamental director fluctuation mode in a homogeneously aligned wedge cell as a function of cell thickness. The method is nonperturbative as no external torque is applied to the liquid crystal during the measurement. We show that by using two different scattering geometries, both azimuthal and zenithal anchoring energy coefficients can be measured on the same sample. The obtained zenithal anchoring coefficient is found to be, in contrast to previously reported results, approximately only two times larger than the azimuthal one. The effect of higher order fluctuation modes on the detected autocorrelation function is in good agreement with numerical calculations.