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Deb R.,Assam University | Laskar A.R.,Assam University | Sarkar D.D.,Assam University | Mohiuddin G.,Assam University | And 4 more authors.

Based on the chemical incompatibility between the aromatic mesogenic cores and the flexible end aliphatic (methylene units) chains, which can lead to the formation of polarization modulated layered undulated phases, four-ring bent-core compounds have been designed and synthesized. The designed compounds are thermally and hydrolytically stable due to intramolecular hydrogen bonding, highly fluorescent and exhibit 2D-polarization modulated layer undulated (PMLU) smectic phases. The phase transition temperatures have been confirmed by differential scanning calorimetry and the phases are confirmed by polarizing optical microscopy and X-ray diffraction studies. For compounds 1-10-11 and 1-15-11 the peaks are indexed to tilted columnar lattice (Colob), while for 2-12-11 the peaks are indexed to the rectangular lattice belonging to the PMLU family The influence of the direction of ester linking group played an important role in the mesophase as well as the evaluated parameters of the dipole moment, the bending angle and the asymmetry parameter by spin-restricted DFT calculations using the B3LYP exchange-correlation functional and the 6-311G(d,p) basis set. © 2013 The Royal Society of Chemistry. Source

Madhuri P.L.,Center for Soft Matter Research Jalahalli | Prasad S.K.,Center for Soft Matter Research Jalahalli | Nair G.G.,Center for Soft Matter Research Jalahalli
RSC Advances

Dielectric spectroscopic investigations are reported on an antiferroelectric liquid crystal in its pristine form and when stabilized by a polymer network without and with application of a DC bias field. While the in-phase mode associated with the helix distortion is hardly affected, the high frequency mode connected with the antiphase fluctuations shows a large decrease with increasing concentration of the photoactive monomer used to generate the polymer stabilization. With the help of X-ray data, we rule out any inter-layer nanophase segregation of the polymer strands, and argue that this strong effect is due to the large elastic interaction between the liquid crystal molecules and the polymer strands. Employing expression from a Landau theory we extract several relevant parameters, including the viscosities associated with the different relaxation modes, the elastic constant, and importantly the antiferroelectric interaction, which exhibits a substantial decrease upon polymer stabilization. Source

Bhargavi R.,Center for Soft Matter Research Jalahalli | Nair G.G.,Center for Soft Matter Research Jalahalli | Prasad S.K.,Center for Soft Matter Research Jalahalli | Kumar N.,Jawaharlal Nehru Centre for Advanced Scientific Research | Sundaresan A.,Jawaharlal Nehru Centre for Advanced Scientific Research
Soft Matter

Combining the magnetic properties of metal particles and the orientational anisotropy of a liquid crystal has been of interest owing to potential technological and bioengineering applications. The target is to achieve the ferromagnetic state while retaining the fluid environment of a nematic liquid crystal (NLC). Such a realization in a low molecular weight material which undergoes physical gelation is all the more advantageous since it can exploit the electrical switching, the concomitant birefringence change, etc. in the sol state as well have the structure frozen in the gel state. We report here such an attempt on a NLC doped with FePt nanoparticles and an organogelator by performing calorimetry, XRD, dielectric, elastic, rheological and magnetic characterization in the anisotropic sol and gel states. While the XRD data suggest that the orientational correlations of NLC are slightly strengthened by the presence of the particles and the gelator, the Frank bend elastic constant increases by two orders of magnitude indicating the mechanical rigidity of the gels. The magnetic measurements reveal that the superparamagnetic feature of the FePt particles is weakened in these ferrogels. This is surprising since it has recently been shown that a polymer ferronematic retains the ferromagnetic characteristics of the doped metal particle. These results suggest that the local environment of the particles has an important role in quenching the thermal fluctuations which in turn influences their magnetic interaction. Despite the bulk viscosity of the gelated system being high due to the trapping of NLC by the gelator, the immediate neighbourhood of the particles is still a fluid with enough orientational and translational freedom. This results in an ineffective quenching of the thermal fluctuations and consequently weak magnetic interactions. © 2011 The Royal Society of Chemistry. Source

Shankar Rao D.S.,Center for Soft Matter Research Jalahalli | Vijay Kumar M.,Center for Soft Matter Research Jalahalli | Krishna Prasad S.,Center for Soft Matter Research Jalahalli | Hiremath U.S.,Center for Soft Matter Research Jalahalli | And 3 more authors.
Journal of Materials Chemistry C

We report here X-ray, electrical switching and dielectric measurements on a binary system comprising achiral bent-core and chiral rod-like components. While the pure bent-core compound exhibits a single mesophase, namely the B2 phase, the rod-like compound shows smectic A and smectic C* mesophases. A particular mixture, studied in detail here, presents a novel sequence of three columnar type B1 phases, smectic A-B1OPAF1-B1OPAF2-B1 TPAF-smectic C* phase, as the sample is cooled, where the subscripts O, T, P, F and AF indicate orthogonal, tilted, polar, ferroelectric and antiferroelectric properties, respectively, with the B1 phases being of the B1rev type. In the electrical switching measurements a twin peak profile-characteristic of an antiferroelectric structure-is seen for the B1 OPAF phases, but with no textural change except on a transient scale. On the other hand the B1TPAF phase, which also shows antiferroelectric-type switching, exhibits clear changes between the field-off and field-on states, as well as for the two signs of the field. This phase also possesses a three-fold higher value of polarization than the smectic C* phase, indicating the stronger influence of the polar ordering. Dielectric studies show the presence of a soft mode of relaxation in the vicinity of the B1OPAF2-B1TPAF transition, with the relaxation frequency of the mode exhibiting behavior similar to that seen for the smectic A-smectic C* phase transition. Mean field coefficients determining the soft mode behavior as well as the thermal variation of the tilt angle have been determined. © 2013 'The Royal Society of Chemistry. Source

Krishna Prasad S.,Center for Soft Matter Research Jalahalli | Vijay Kumar M.,Center for Soft Matter Research Jalahalli | Shilpa T.,Center for Soft Matter Research Jalahalli | Yelamaggad C.V.,Center for Soft Matter Research Jalahalli
RSC Advances

We report complex permittivity characteristics in composites of gold nanoparticles (GNP) and a weakly polar nematic liquid crystal possessing a low frequency director relaxation. Differential calorimetric measurements show that the inclusion of GNP has a strong influence on the isotropic-nematic (fluid-orientational fluid) transition temperature as well its first order character in terms of the transition entropy. The absolute value of conductivity increases by two to three orders of magnitude with respect to that for the host liquid crystal and its concentration dependence is demonstrated to be described by the percolation scaling law generally observed in composites of metal particles and polymers. However, the obtained exponent is much smaller, possibly owing to thermal fluctuations present in the fluid-like nematic medium. The activation energy governing the temperature dependence of conductivity is much higher in the nematic than in the isotropic phase. The frequency dependence of the ac conductivity exhibits a critical frequency that is concentration- dependent, but the exponents obtained defy Jonscher's Universal Response principle. A surprising feature is the observation of a substantial increase of not only the principal permittivity values, but their anisotropy as well. These studies also constitute the first report on the influence of GNP on the director relaxation mode of nematics. In contrast to the behaviour of the static permittivity, the dynamics of the system as measured using the director relaxation is seen to become faster with the presence of GNP. We provide an explanation for this antagonistic behaviour in terms of the alignment of the liquid crystal molecules in the vicinity of GNP, and the importance of the weak polarity of the liquid crystals used. © 2014 The Royal Society of Chemistry. Source

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