Articles written in Bulletin of Materials Science

• Influence of zinc oxide nanorods on an orientationally ordered fluid comprising soft-bent dimers

Bent-core liquid crystals (LCs) introduced a whole new dimension to the science of LCs. Besides re-emphasizing the importance of the shape of the molecule, they brought in phases with symmetry, not known earlier.Another sub-class of systems that is emerging is that of soft-bent molecules. In contrast with the bent-core molecules, here the bend is achieved through the parity of the aliphatic linker that connects two monomers. They hold the unique advantage that a simple variation of temperature can favour different conformer states and thus govern the self-assembled structure. A highlight of the power of this route is seen in terms of the discovery of a new type of nematic, viz., twist–bend nematic. Investigations on not only this phase but also the regular nematic that often precedes it have received significant attention in the last few years. Here we present results on the regular nematic phase of a binary mixture comprising such a soft-ben dimer, known in the literature as CB7CB, by incorporating zinc oxide nanorods (NRs) into the system. The NRs with anaspect ratio of $\sim$7.2, not very different from that of LCs, further accentuate the importance of shape and shape anisotropy of the entities. Specifically, we observe that the nematic–isotropic transition temperature increases by $\sim$1.9K even for a low concentration of 4% NRs.While the dielectric anisotropy decreases, birefringence shows a substantial increase, adding to thecomplexity of the influence. Upon addition of minute amount of NRs (1%), while the splay elastic constant gets enhanced, its bend counterpart not only gets reduced but retains the convex-shaped thermal profile seen for the parent mixture.

• Wide thermal range, exclusive occurrence of technically significant chiral nematic phase: synthesis and mesomorphism of cholesterol-based non-symmetric dimers

Fifteen newnon-symmetric chiral dimers belonging to three different series have been synthesized and evaluated for their mesomorphic properties. They are formed by interlinking cholesterolwith salicylaldimine (SAN) cores (with reverse imine groups) via an $\omega$-oxyalkanoyloxy spacer. Within a series, the length of the terminal $n$-alkoxy tails has been varied for a fixed even-parity spacer. Three even-parity spacers such as 4-oxybutanoyloxy, 6-oxyhexanoyloxy and 8-oxyoctanoyloxy have been used to join two cores, whereas the terminal tails such as $n$-butyloxy, $n$-hexyloxy, $n$-octyloxy, $n$-decyloxy and $n$-dodecyloxy chains have been attached to the SAN core. Microscopic and calorimetric experimental results show that all the dimers behave identically exhibiting the chiral nematic (${\rm N}^*$) phase solely, which was authenticated by powder X-ray diffraction studies carried out on some selected samples. In the vast majority of the cases, this phase is thermodynamicallystable, and while cooling, it exists over a wide thermal range covering room temperature (RT) due to supercooling. This finding is notable given the fact that the ${\rm N}^*$ phase possesses technologically significant optical properties. At RT, the ${\rm N}^*$ phase displayed one of the iridescent colours characteristically caused by interference and diffraction of the reflected and scattered light. A comparative study reveals that the lengths of both the terminal chain and central spacer influence the clearing temperature of the dimers, and also the temperature range of the ${\rm N}^*$ phase. The selective reflection measurements revealed that the pitch of the ${\rm N}^*$ phase is either temperature sensitive or temperature insensitive. Temperature-dependentcircular dichroism (CD) spectra were recorded for the planar texture of the ${\rm N}^*$ phase formed by a dimer, as a representative case. The presence of an intense negative CD band suggests the left-handed screw sense of the ${\rm N}^*$ phase helix.

• # Bulletin of Materials Science

Volume 44, 2021
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Continuous Article Publishing mode

• # Dr Shanti Swarup Bhatnagar for Science and Technology

Posted on October 12, 2020

Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
Chemical Sciences 2020