Malay Kumar Das

The trapezoidal cylinder phase: a new mode of self-assembly in liquid-crystalline soft matter.

A new rectangular columnar liquid crystalline phase with p2gg lattice is reported, which represents a polygonal cylinder array composed of cylinders with trapezoidal cross section. In these polygonal cylinders, one of the sides has a different length and is composed of a different material than the others. This tiling pattern was obtained in two series of T-shaped facial amphiphilic triblock molecules in which a rigid rod-like p-terphenyl core is substituted laterally by a polar and flexible oligoethylene glycol chain, terminated either by a hydrogen-bonding COOH group or by a Li carboxylate group, and having identical or different alkyl groups in the terminal positions. The trapezoidal cylinder phase provides an improved packing for relatively long and rigid alkyl chains at lower temperature and more space inside the polygonal cylinders than triangular cylinders. This combination of conformational and space-filling effects leads to different phase sequences. The trapezoidal cylinder phases pave the way to a new level of complexity in LC engineering and show the huge potential of the general concept of polyphilic tectons for the design of new complex soft matter structures.

New hockey stick compounds with a lateral methyl group showing nematic, synclinic and anticlinic smectic C phases

New hockey stick-shaped mesogens with a lateral methyl group inserted between the m-alkyloxy-chain and the azomethine connecting group, 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl) phenyl-4-n-alkyloxycinnamates have been synthesised. The interesting feature of these hockey stick-shaped mesogens is the appearance of the nematic phase in addition to two polymorphic tilted smectic phases – the synclinic smectic C (SmCs) and the anticlinic SmCa phase. Physical characterisation of these mesogens has been carried out using polarising optical microscopy, differential scanning calorimetry, optical birefringence, X-ray diffraction (XRD), electro–optical and dielectric spectroscopy measurements. The SmCs–SmCa phase transition is accompanied by only a small calorimetric signal but causes a pronounced change in the optical textures. Orientational order parameters determined from the birefringence measurements have been compared with those estimated from XRD patterns. Dielectric measurements point to the formation of small ‘soft’ ferroelectric clusters responsible for a low frequency absorption range.

Structural Investigations of a Non Calamitic Shaped Liquid Crystalline Compound Showing Unusual Phases

X-ray diffraction, birefringence, density and static dielectric permittivity measurements were performed on a calamitic hockey stick shaped mesogen. In addition to SmA phase, this compound exhibits two tilted mesophases – the SmC, synclinic and the SmCa, anticlinic phase. From x-ray diffraction and birefringence data, the orientational order parameters have been determined. The order parameters values drop at the SmC – SmCa phase boundary. Possible cause for the drop in the order parameter values has been discussed. Interestingly, it has been found that the dielectric anisotropy of this compound changes from positive to negative values in the SmCa phase.

Refractive index and orientational order parameter of a polar–polar binary system showing induced smectic Ad and re-entrant nematic phases

Refractive index measurements have been done on a polar–polar binary system, 4-cyanophenyl [4′(4″-n-heptylphenyl)] benzoate (7CPB) + 4-cyanophenyl-4-nonylbenzoate (9.CN), exhibiting nematic–smectic Ad–re-entrant nematic phase sequence using thin prism technique. From refractive index as well as density data, the polarizability anisotropy in the mesomorphic state has been calculated from the standard Vuks model and the orientational order parameter (⟨P 2⟩) values have been determined. By measuring the refractive indices and density in the solid phase, the validity of the Hallers extrapolation method for systems having higher order phases has been verified. The temperature variation of the density, birefringence, and ⟨P 2⟩ values at the nematic–smectic Ad and smectic Ad–re-entrant nematic transitions for all the mixtures are found to be continuous. The experimental ⟨P 2⟩ values have been compared with our previous X-ray data and also those calculated from the Luckhurst and Timimi model.

Optical, dielectric and visco-elastic properties of a few hockey stick-shaped liquid crystals with a lateral methyl group

Measurements of birefringence (Δn), dielectric permittivity (e‖, e⊥), elastic moduli (K11, K33) and rotational viscosity (γ1) have been carried out on the nematic (N) phase of a few hockey stick-shaped compounds 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-alkyloxycinnamates with a lateral methyl group inserted between the m-alkyloxy chain and the azomethine connecting group. Interestingly, a dual characteristic (i.e. partially calamitic like and partially bent-core like) is revealed in the N phase of these compounds. The SmCa–N transition is found to be of first order in nature while the SmCs–N transition is either second order or weakly first order. All the mesogens exhibit a temperature dependent inversion in the static dielectric anisotropy (Δe = e‖ − e⊥) from positive to negative values on entering the SmCa phase. Remarkably in the entire nematic range, the bend elastic modulus (K33) is substantially lower than the corresponding splay modulus (K11). The rotational viscosity coefficient (γ1) as obtained by extracting the relaxation time (τ0) values from two precise, independent probing methods, viz. the capacitive decay technique and the optical phase-decay-time measurement method, are slightly higher than those of many known calamitic systems. Moreover, the activation energy (Ea) calculated from the viscosity data is found to be considerably higher in the nematic phase than those obtained for conventional calamitics. The observed behaviours are accounted for by considering the intriguing shape-determined inter-molecular interactions and molecular associations appearing in the mesophase.

Small Angle X-Ray Diffraction Studies of an Ester/Biphenyl Mixture (5CB/ME 50.5) Showing an Injected Smectic Phase

Abstract The molecular organisation in a binary mixture of a strongly polar mesogen (5CB) and a weakly-polar mesogen (ME50.5) is investigated using results of X-ray scattering and density studies. Analysis of the intensity distribution of X-ray scattering has yielded results for orientational order parameters in addition to smectic layer spacings and effective molecular lengths. The results are compared with a mean field theory of the smectic phase. Previously unexplained results for mixtures showing a minimum in the anisotropy of physical properties coincident with a maximum in the nematic/smectic A transition entropy are shown to be consistent with the theory. We conclude that favourable packing of molecules in the injected smectic A phase stabilises the layers but increases the orientational free volume, consistent with a lower orientational order parameter.

Study of the physical properties of an ester/biphenyl mixture, exhibiting an injected smectic phase, by small-angle x-ray diffraction

Abstract Small angle x-ray diffraction intensities from binary mixtures of 4-n-pentyl 4′cyanobiphenyl (5CB) and 4-n pentyl phenyl 4-n′ hexyloxy benzoate (ME60.5), which show an injected smectic phase, have been analysed to determine the orientational order parameters (P 2) and (P 4), the translational order parameter, the layer thickness in the smectic phase, and the apparent molecular length in the nematic phase. The order parameters have been fitted satisfactorily to those calculated from McMillans theory by varying the potential parameters α and [sgrave]. The experimental variation of the layer thickness with mole fraction of 5CB has been explained fairly well by assuming the presence of both homo dimers (5CB + 5CB) and hetero dimers (5CB + ME 60.5) in the mixture. Orientational order parameters, layer thickness, and packing fraction all exhibit minima at about 50% molar concentration of 5CB.

Rotational viscosity measurements of bent-core nematogens

Two simple but precise probing methods have been utilized to investigate the influence of structure, properties and molecular association on the director response time (τ0) and hence on the rotational viscosity (γ1) in the nematic phase of three 4-cyanoresorcinol bent-core derivatives (1/7, 1/9 and 1/10) having different terminal chain lengths. The results obtained from the two methods are in good agreement with a small deviation of about 10–15%. The γ1 values are more than an order of magnitude higher compared to the conventional calamitic nematics. Measurements of birefringence, dielectric anisotropy, splay and bend elastic constants and relaxation time of these compounds have also been carried out. For the lower homologues (1/7, 1/9) an inversion in dielectric anisotropy (Δe) is observed in the nematic phase, while the higher homologue (1/10) exhibits negative Δe throughout the entire mesomorphic range. The splay elastic constant (K11) is found to be comparatively greater than the bend elastic constant (K33); however, on approaching the low temperature CybC phase (a phase comprising elongated cybotactic clusters) K33 exhibits a small pretransitional divergence. For all three compounds the magnitude of the rotational viscosity coefficient (γ1) is found to exhibit a strong enhancement on approaching the CybC phase, while in the nematic phase activation energies are found to be nearly an order of magnitude higher than the normal calamitics. The observed behaviours are explained by considering the strong dipolar correlation of the bent-core molecules and the existence of the short range cybotactic clusters, their gradual growth and uniform alignment under the presence of strong surface anchoring.

Comparison of the orientational order parameters determined from X-ray diffraction and 13C NMR studies of a hockey stick-shaped compound

Orientational order parameters (OOPs) have been determined in the Smectic A, Smectic C1 and Smectic C2 phases of a hockey stick-shaped compound from X-ray diffraction measurements and compared with NMR measurements. While the OOP values from NMR measurements abruptly drop at the Smectic C1 to Smectic C2 transition, those determined from X-ray diffraction measurements decreases sharply at the Smectic A to Smectic C1 transition. This drop in the OOP values from X-ray measurements has been explained using a model where the molecular director is randomly oriented in a cone about the smectic layer normal. OOPs from X-ray diffraction and NMR measurements have been successfully fitted with McMillans theory, after necessary correction to the potential, incorporating tilt angles, for three homologues with n = 10, 11 and 12. The temperature dependences of the optical birefringence, dielectric anisotropy and OOP values determined from birefringence measurements of compound 11 are also reported.

Determination of the orientational order parameter of the homologous series of 4-cyanophenyl 4-alkylbenzoate (n.CN) by different methods

The orientational order parameters of a homologous series of 4-cyanophenyl 4-alkylbenzoates have been determined at different temperatures from (13)C-NMR, x-ray diffraction, optical birefringence, high resolution density and diamagnetic susceptibility anisotropy measurements. To determine the temperature dependence of the orientational order parameter from the (13)C chemical shift values, the two-dimensional polarization inversion spin exchange at the magic angle (PISEMA) method was also used for the measurement of (13)C-(1)H dipolar couplings at different sites in the oriented phase. The nematic order parameters determined from each of these methods have been compared. Apart from a slight shift in their values their trends with temperature are very similar. The differences among the results obtained by these five different methods have been discussed. From the high resolution density data, the values of the critical exponents near the T(N-I) transition are found to lie between the Ising model and tricritical behaviour.