Saonti Chakraborty

Properties of the broad-range nematic phase of a laterally linked H-shaped liquid crystal dimer

In search for novel nematic materials, a laterally linked H-shaped liquid crystal dimer has been synthesised and characterised. The distinct feature of this material is a very broad temperature range (about 50oC) of the nematic phase, which is in contrast with other reported H-dimers that show predominantly smectic phases. The material exhibits interesting textural features at the scale of nanometers (presence of smectic clusters) and at the macroscopic scales. Namely, at a certain temperature, the flat samples of the material show occurrence of domain walls. These domain walls are caused by the surface anchoring transition and separate regions with differently tilted director. Both above and below this transition temperature, the material represents a uniaxial nematic, as confirmed by the studies of defects in flat samples and samples with colloidal inclusions, freely suspended drops, X-ray diffraction and transmission electron microscopy.

Nanostructures of Nematic Materials of Laterally Branched Molecules

The synthesis and small-angle X-ray scattering (SAXS) characterization is reported for 20 laterally branched mesogenic molecules, which are derived from the common rod-shaped 2,5-bis([4-(octyloxy)phenyl]carbonyloxy) benzoic acid unit. These compounds have a varying degree of flexibility, in that their lateral branch is formed upon conversion of the acid to either an ester or an amide, and most laterally branched molecules exhibit relatively wide nematic liquid-crystal phases with a direct nematic-to-crystal transition at lower temperatures. SAXS studies reveal the presence of smectic-like nanostructures (clusters) with short-range order in the nematic phase, with characteristic correlation lengths from 3 to over 10 nm. The smectic layers that are contained in these clusters are tilted with respect to the nematic director by angles ranging from 0° (i.e. untilted) to 55°. In some compounds, the intensity of the SAXS peak corresponding to the smectic layer spacing depends strongly on temperature. The main features of the nanostructures can be understood based on the molecular structure; therefore, guiding future synthetic work towards more precisely controlled and technologically useful nanostructures in nematics.

Shape-independent lateral force calibration

Current methods for lateral force calibration are often time-consuming, expensive, or cause significant wear of the tip. A quick and simple alternative is presented in which the linear relationship between force and voltage is exploited. The technique is independent of the shapes of the sample and cantilever and eliminates common problems, while maintaining better than 10% precision. This advance will facilitate quantitative comparisons between experiments.

Viscoelastic properties of a branched liquid crystal in the nematic phase

We report results from a comprehensive suite of measurements, including small angle X-ray scattering (SAXS), polarising optical microscopy, dynamic light scattering, elastic properties and dielectric and diamagnetic susceptibilities on macroscopic liquid crystalline properties of branched compound, 2-(octyloxycarbonyl)-1, 4-phenylene bis(4-(octyloxy)benzoate). SAXS studies reveal very distinct, although short-range smectic-C-type clusters, leading to increased viscosities, very similar to that been reported in many bent-core compounds. The measured elastic constants and their relative values are similar to calamitic liquid crystals, presumably because the clusters are not polar, unlike in bent-core liquid crystals, which (together with the director tilt) result in a layer chirality.