Elucidating Molecular-Scale Principles Governing the Anchoring of Liquid Crystal Mixtures on Solid Surfaces

Computational chemistry calculations are broadly useful for guiding the atom-scale design of hard-soft material interfaces including how molecular interactions of single-component liquid crystals (LCs) at inorganic surfaces lead to preferred orientations of the LC far from the surface. The majority of LCs, however, are not single-component phases but comprise of mixtures, such as a mixture of mesogens, added to provide additional functions such as responsiveness to the presence of targeted organic compounds (for chemical sensing). In such LC mixtures, little is understood about the near-surface composition and organization of molecules and how that organization propagates into the far-field LC orientation. Here, we address this broad question by using a multiscale computational approach that combines density functional theory (DFT)-based calculations and classical molecular dynamics (MD) simulations to predict the interfacial composition and organization of a binary LC mixture of 4'-cyano-4-biphenylcarbolxylic acid (CBCA) and 4'-n-pentyl-4-biphenylcarbonitrile (5CB) supported on anatase (101) titania surfaces. DFT calculations determine the surface composition and atomic-scale organization of CBCA and 5CB at the titania surface, and classical MD simulations build upon the DFT description to describe the evolution of the near-surface order into the bulk LC. A surprising finding is that the 5CB and CBCA molecules adopt orthogonal orientations at the anatase surface and that, above a threshold concentration of CBCA, this mixture of orientations evolves away from the surface to define a uniform far-field homeotropic orientation. These results demonstrate that molecular-level knowledge achieved through a combination of computational techniques permits the design and understanding of functional LC mixtures at interfaces.

Exploring Quantum Dots Size Impact at Phase Diagram and Electrooptical Properties in 8CB Liquid Crystal Soft-Nanocomposites

We explore the influence of functionalized core-shell CdSe/ZnS quantum dots on the properties of the host liquid crystal compound 4-cyano-4'-octylbiphenyl (8CB) through electrooptical measurements. Two different diameters of quantum dots are used to investigate the size effects. We assess both the dispersion quality of the nanoparticles within the mixtures and the phase stability of the resulting anisotropic soft nanocomposites using polarizing optical microscopy. The temperature-mass fraction phase diagrams of the nanocomposites reveal deviations from the linear behavior in the phase stability lines. We measure the birefringence, the threshold voltage of the Fréedericksz transition, and the electrooptic switching times of the nanocomposite systems in planar cell geometry as functions of temperature, mass fraction, and diameter of the quantum dots. Beyond a critical mass fraction of the dopant nanoparticles, the nematic order is strongly reduced. Furthermore, we investigate the impact of the nanoparticle size and mass fraction on the viscoelastic coefficient. The anchoring energy at the interfaces of the liquid crystal with the cell and the quantum dots is estimated.

Application of Bombyx mori Silk Fibroin Films for Liquid-Crystal Devices

Biocompatible and biodegradable silk fibroin films show promise as an eco-friendly biomaterial with excellent mechanical, thermal, and optical transparency properties. In contrast, polyimide (PI) films adopted in the liquid-crystal display (LCD) industry for aligning LC molecules are synthesized using toxic chemicals, which are nonrecyclable and nonbiodegradable. In this work, Bombyx mori silk fibroin films are fabricated from the aqueous solution and applied as alignment films for LCDs. The thermal properties of the prepared regenerated silk fibroin materials under different heat treatment temperatures are investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The crystallinity of different heat-treated silk fibroin materials is determined by infrared spectroscopy. The silk fibroin film treated at a higher temperature exhibits better thermal stability due to the higher crystallinity of the β-form structure. The LCDs using silk fibroin alignment films show a low pretilt angle of 0.5° and an anchoring energy of ∼10^-3 J/m² similar to those of the conventional polyimide films.

Electro-optical performance of nematic liquid crystals doped with gold nanoparticles

The effect of gold nanoparticles on the dielectric, electro-optical, and rheological properties of the ZhK-1289 liquid-crystal mixture that define the response time of liquid-crystal devices with a concentration range of 0.06-5 wt% was investigated in this study. A phase diagram of the obtained composites was formed demonstrating an increase in the clearing temperature and a broadening of the mesophase existence range in the case of doping nanoparticles. It was found that in the obtained dispersions there are structural rearrangements in the low concentration range leading to an increase in the lateral bending stiffness of the liquid-crystal matrix, a decrease in the response time and threshold voltage of the Freedericksz transition, and also an increase in the anisotropy of the dielectric permittivity and the refraction index. The improvement of the electro-optical performance of the liquid crystal can be caused by the nanoparticle adsorption of impurity ions, which reduces the field-screening effect in the liquid crystal. According to the results obtained in this study, the optimal values of the physical parameters of liquid-crystal composites doped with gold nanoparticles for their application in practice are achieved in a concentration range of 0.5-1 wt%.

Improving liquid-crystal-based biosensing in aqueous phases

Liquid crystal (LC)-based biological sensors permit the study of aqueous biological samples without the need for the labeling of biological species with fluorescent dyes (which can be laborious and change the properties of the biological sample under study). To date, studies of LC-based biosensors have explored only a narrow range of the liquid crystal/alignment layer combinations essential to their operation. Here, we report a study of the role of LC elastic constants and the surface anchoring energy in determining the sensitivity of LC-based biosensors. By investigating a mixture of rod-shape and bent-shape mesogens, and three different alignment layers, we were able to widen the useful detection range of a LC-based sensor by providing an almost-linear mapping of effective birefringence with anionic surfactant concentrations between 0.05 mM and 1 mM (model target analyte). These studies pave the way for optimization of LC-based biosensors and reveal the importance of the choice of both the LC material and the alignment layer in determining sensor properties.