Showing papers by "S. Chandrasekhar published in 2003"

X-ray studies on the columnar structures of discotic liquid crystals

Abstract: Since the discovery of discotic liquid crystals in 1977 [1] the field has grown rapidly with an estimated 3000 compounds belonging to this category. Generally these compounds have flat or nearly flat cores and surrounded by four to eight long chain substituents. A vast majority of these compounds exhibit the columnar mesophase. This phase is characterized by a structure in which thedisc shaped molecules are stacked one on top of the other to form columns, the different columns constituting a two-dimensional lattice. Several variants of the columnar structure have been identified-upright columns, tilted columns, hexagonal lattice, rectangular lattice, etc. The stacking of the discs within the column could be on the one extreme, liquid-like or on the other have a true long range order. Although X-ray measurements on quite a few compounds exhibiting these different structures have been reported, the temperature dependence of the various structural parameters does not seem to have received much attention. We ...

Electrooptic and viewing angle characteristics of a display device employing a discotic nematic liquid crystal

Abstract: Recently we developed [1] a novel liquid crystal display device employing discotic nematic material. This device exhibits the following improvements over a conventional twisted nematic (TN) display device using a calamitic nematic material. The device (i) is simple to fabricate and (ii) has excellent viewing angle characteristics showing a wide and symmetric viewing angle profile, (iii) has much less difference in the pixel capacitance between the ON and OFF states resulting in reduced cross talk problem. However the response time of the device is slower than conventional TN devices. An obvious solution to this problem is to reduce the viscosity of the material. To achieve this we have doped, in small concentrations, a long chain alkane compound to the parent room-temperature discotic nematic material. Systematic studies show that both the switch ON and switch OFF response times show a significant decrease, i.e., the device switches faster, in the case of the mixtures. It should be mentioned that wide and symmetric viewing angle characteristics remain unaffected by the addition of the dopant material. However both the switch ON and switch OFF response times are still an order of magnitude slower compared to that of conventional TN devices. Considering the fact that these response times are not very different from those for the STN displays, the achievement of symmetric and wide viewing angle characteristics with a simple fabrication process makes this device quite interesting. To make them attractive enough to be considered for commercial applications new materials with faster response have to be developed. Efforts in this direction are underway in our laboratory.

Phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexyl thiotriphenylene (HHTT) under pressure

Abstract: The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 300MPa using wide-angle X-ray diffraction equipped with a high-pressure vessel. The reversible transitions of HHTT, i.e., crystal (Cr)-helical phase (H) -hexagonal columnar phase (Col h ) - isotropic liquid (I), were observed under pressures up to about 30MPa. The transition sequence changed significantly depending on the applied pressure. The H phase appeared monotropically in the pressure region between 50 and 180 MPa, indicating the Cr→Col h →I on heating and I→Col h →H→Cr transition sequences on cooling. Applying higher pressures altered further the phase behaviour: the enantiotropic Col h phase changed to monotropic one under high pressures above 180 MPa, indicating the Cr→I on heating and I→Col h →Cr transition sequences on cooling. The X-ray diffraction pattern of the H phase exhibited a sharp low-angle and a wide-angle reflections corresponding to the intercolumnar and the core-core distances, respectively. This feature is indicative of the three-dimensional ordering present in the H phase. On the other hand, the Col h phase showed only a low-angle reflection. It was found that the intercolumnar distance in the Col h phase decreases with increasing temperature at high pressures. This indicates that the negative coefficient of thermal expansion for the column packing in the Col h phase is held under pressure.