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.

Quasidivergent nematic surface electroclinic effect.

A polyimide coated substrate is treated so that vertical liquid crystal alignment (theta=0) obtains over the temperature range T(NA)< T < T(a), where T(NA) is the nematic-smectic-A transition temperature. When the cell is filled with a chiral liquid crystal whose helical pitch is unwound (surface stabilized), application of an in-plane electric field for T(NA)< T < T(a) induces a nonzero polar tilt theta proportional to E of the liquid crystal director at the surface, where the tilted orientation propagates elastically into the bulk. On heating toward T(a), this surface electroclinic response becomes large, corresponding to the onset of a surface tilt transition at T(a) from theta=0 to nonzero theta.

Effect of flexible bimesogen dopant on the stability of the anticlinic liquid-crystal phase.

Small quantities of the floppy bimesogen di(4PPB5)3Si were dissolved in an anticlinic liquid crystal consisting of a mixture of left-handed and right-handed TFMHPOBC, with enantiomer excess X=0.2 . The bimesogen dopant was found to promote anticlinic order with an anticlinic interaction coefficient per molecule udopant smaller than, but of the same order as, that of the rigid bent-core dopant P-7PIMB. For both dopants udopant was found to be much larger than that due to a pair of TFMHPOBC molecules in adjacent layers. The results are examined in terms of both the flexibility of the group linking the two legs of each dopant, as well as their chemical structure.

Bent-core dopant in a liquid crystal having a reentrant synclinic phase.

Small quantities of the bent-core mesogen P-7PIMB were dissolved in an anticlinic liquid crystal consisting of a mixture of left- and right-handed TFMHPOBC, with enantiomer excess X=0.2. The bent-core dopant promotes anticlinic order at higher temperatures, but becomes less effective in suppressing the synclinic phase at the reentrant synclinic transition due to an orientational transition of the dopant within the calamitic TFMHPOBC matrix. Measurements of the anticlinic-synclinic electric-field switching threshold as a function of temperature and dopant concentration facilitate a determination of the component of the anticlinic interaction coefficient U that is due to the bent-core dopant. It is found that the value of U per bent-core molecule is much larger than the corresponding value for a pair of TFMHPOBC molecules in adjacent smectic layers.

Twist elasticity and anchoring in a lamellar nematic phase.

Electro-optic measurements were performed on a lamellar nematic phase in which the mesogenic moieties lie in lamellae that are separated by partially perfluorinated side groups. The twist elastic constant K22, viscosity gamma(1), and the quadratic and quartic anchoring strength coefficients are reported. K22 and gamma(1) are found to be considerably smaller than that of typical three-dimensional nematics. The small K22 is due to the greatly weakened interactions between the spatially separated lamellae.