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

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 have used an image plate detector to carry out detailed X-ray studies of several compounds exhibiting the columnar phase. Analysis of the X-ray patterns yields, apart from information on the structural symmetry in terms of the planar space groups, quantitative data on (i) the ordering of the chains and the cores of the mesogens, (ii) the core-core correlation length along the column axis and (iii) the intercolumnar spacing. In particular, it is seen that the extent of ordering of the discs within the columns has significant temperature dependence. Data collected as a function of temperature will be presented and questions regarding ordering within the columns addressed, particularly in two compounds, one of which exhibits a columnar to a plastic phase transition and the other, a columnar to a helical phase transition

X-ray, dielectric and high pressure studies on a compound exhibiting ferro-, ferri- and antiferroelectric smectic phases

We report X-ray layer spacing, dielectric constant and high pressure measurements on a compound that exhibits the following sequence of phase transitions: smectic A-ferroelectric smectic C∗-ferrielectric smectic Cy∗-antiferroelectric smectic CA∗ -hexatic antiferroelectric smectic IA∗-crystal K. The pressure-temperature diagram shows two three-phase meeting points, which are topologically consistent with critical end points, and reentrant behavior of the C∗ and C∗ phases as the pressure is varied at constant temperature

Wide viewing angle and fast responding TN LCD

A new type of TN display device has been fabricated by sandwiching a dual frequency addressable nematic liquid crystal material between two unrubbed polyimide coated glass plates. Between crossed polarizers, the bright state is observed by applying a high frequency electric field, while the dark state by applying a low frequency electric field of similar strength. This dual frequency addressed rubbing free device shows very wide and highly symmetric viewing angle characteristics, with contrast ratio comparable to that of a rubbed TN device, and with very fast response time. Addressing the device in a "dual frequency dual amplitude pulsed mode" leads to only a small increase (~1.5 times) in power consumption in comparison with the conventional single frequency addressed device

High precision density studies near the smectic A-nematic tricritical point

We have carried out very accurate density measurements (with a precision of ± 5x10− 5g cm− 3) near the smectic A-nematic transition in binary mixtures of 4-n-nonyl-4'-cyanobiphenyl (9CB) and 4-n-decyl-4'-cyanobiphenyl (10CB). The transition crosses over from second to first order as the temperature range of the nematic phase decreases. For mixtures with the shortest nematic range the data deviate noticeably from a single power law behaviour. Such a deviation is an indication of the first order nature of the transition. Very good fits to a single power law have been obtained for pure 9CB and the x=0.04 mixture where x is the mole fraction of 10CB in 9CB. The critical exponent obtained from the power law fitting has enabled us to locate the tricritical point to be very close to x=0.04, which is in agreement with the results obtained previously by high resolution calorimetric[1] and X-ray scattering studies[2]

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

The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 300 MPa 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 30 MPa. 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

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

The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 500 MPa using high pressure optical and DTA measurements. The known enantiotropic phase transitions of HHTT, i.e. crystal (Cr)-helical phase (H), H-hexagonal columnar phase (Colh) and Colh-isotropic liquid (I) were observed up to 32 MPa. Application of hydrostatic pressures above 32 MPa results in the H and Colh phases becoming monotropic, depending upon the applied pressure. The H phase was observed as a monotropic phase in the pressure region between 32 and about 180 MPa. Thus, the I MColh MH MCr transition sequence appeared only on cooling under these pressures, while the Cr MColh MI transition occurred on heating. Further increases in pressure above a second limiting value leads to the Colh phase becoming monotropic. Thus the I MColh MCr transition sequence appeared on cooling, while the Cr MI transition was observed on heating. The T vs. P phase diagram based on the data obtained in the heating mode contains two triple points; one is estimated as 40 MPa, 77.2° C for the Cr-H-Colh triple point and the other is extrapolated as 285 MPa, 118.3° C for the Cr-Colh-I triple point. These triple points define the upper limits for the appearance of the stable H and Colh phases, respectively

Schlieren textures in free-standing nematic films: evidence of biaxiality

Conoscopic studies reveal that the nematic phase of 4,4-( p -terphenyl)-bis\[2,3,4-tri(dodecyloxy)benzal]imine is biaxial, the biaxial angle increasing with schlieren textures exhibited by the nematic phase have been between glass plates, as well as free-standing consisting entirely of s or 2-brush disclinations, confirming an earlier suggestion that the absence of 4-brush disclinations is evidence of biaxiality. decreasing temperature. The examined. Films sandwiched films, exhibit a nematic schlieren texture

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

Recently we developed 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

Isospectrality in Chaotic Billiards

We consider a modification of isospectral cavities whereby the classical dynamics changes from pseudointegrable to chaotic. We construct an example where we can prove that isospectrality is retained. We then demonstrate this explicitly in microwave resonators.Comment: 5 pages, 7 figure