Session E50: Chirality in Polymers and Soft Matter II: Liquid Crystals and Liquid Crystalline Polymers

Programming Emergent Chiral and Polar Symmetries with Saddle-Splay Elasticity

Emergent chiral symmetries in liquid crystals (LCs) are usually achieved by symmetry breaking in close boundary confinements. Gaining precise control over the location, creation and manipulation of broken symmetries remains a formidable challenge. In this work, we achieve pre-programming of broken chiral symmetries in the nematic phase of two achiral LCs (5CB and 8CB) through a ubiquitous but oft-neglected property of LCs: the “saddle-splay” elasticity. Using combinations of lithographic patterning and selective surface functionalization, we create surface patterns with spatially defined geometry and precisely controlled surface chemistry. Through this, we characterize broken symmetry regimes on a variety of surface patterns, including arrays of circular, trefoil, and annulus posts, in which saddle splay-driven effects gain physical expression. In turn, we unlock the resultant director field patterns, and identify spontaneously broken symmetries within domains exhibiting chiral and polar regimes. By fine-tuning the patterning geometry, we then program the location, energy landscape, and means of manipulation of the symmetry breaking processes. As a result, we demonstrate a multi-state stable LC display device that can be switched at an extremely low voltage density (~0.5 V/µm).
  

Twist-bend-like phases and elastic response of model bent-core liquid crystals

Bent-core liquid crystals have recently attracted significant attention because of their novel mesostructure and the intriguing behavior of their elastic constants, which are strongly anisotropic and have an unusual temperature dependence. For instance, experiments report an abnormally low bend elastic constant, which dips near the nematic-twist-bend transition, and increases again as the transition is crossed; the molecular mechanisms responsible for these behaviors are unclear. Here, we utilize Density of States algorithms in Monte Carlo simulation applied to a bent-core variant of the Lebwohl-Lasher model to analyze the mechanism behind elastic response in this novel mesostructure and understand the temperature dependence of its Frank-Oseen elastic constants.   

Chiral helical nanofilament and nanocylinder phases and a new type of polymorphism in liquid crystals

Helical nanofilaments, consisting of bundles of twisted smectic layers with a helical pitch of 200 nm, are formed by achiral bent-core liquid crystal (BC-LC) molecules due to an intralayer mismatch between top and bottom molecular halves relieved by local saddle-splay. Here, by introducing a chiral center to one of the sides of asymmetric BC-LCs (shorter side, called meta-side), we observed the first example of the polymorphism in liquid crystalline materials. They form a not heretofore helical microfilament (HF) phase upon rapid cooling and an oblique columnar upon slow cooling.¹Interestingly, another not reported morphology observed when the chiral center migrates to the longer para-side). In this case, the BC-LC molecules form layers that are rolled up into coaxial cylinders resulting in the formation of heliconical-layered nanocylinders (HLNCs).²HLNCs form within 80-100 nm width and micrometer lengths. This optically active cylinders form feather-like structures, braid, and assemble into hollow structures totaling six levels of hierarchical self-assembly.
[1] Li, Lin, et al. Nature communications 9.1 (2018): 714.
[2] S. Shadpour, et al. (HLNCs) – hierarchical self-assembly in a unique B4 phase liquid crystal polymorph, submitted (2018).   

Indication of a Twist-Grain-Boundary-Twist-Bend Phase of flexible bent-shape chiral dimers.

Flexible bent-core oligomers with odd-numbered methylene spacers exhibit a “twist-bend” nematic (N_TB) phase characterized by a nanoscale heliconical pitch. We designed mixtures of achiral dimers, which exhibit the N_TB phase, with a chiral additive. By differential scanning calorimetry, resonant soft X-ray scattering, polarized optical microscopy, and induced circular dichroism studies on condensed phases of these mixtures, we find, while in the nematic phase the micron-scale pitch of the additive-induced helical structure decreases with increasing additive concentration, in the N phase both the micron-scale and the nanoscale pitch of the ambidextrous spontaneous heliconical structure increase. At chiral additive concentrations above ~2% by weight, a new phase appears between the N* and N_TB* phases, which we propose to be analogous to the twist-grain-boundary (TGB) phase of chiral smectics; we designate this new phase by TGB_TB.   

Nano-confinement of chiral liquid crystals gives rise to exotic blue phases

Blue phases (BPs) arise spontaneously in chiral liquid crystals (ChLCs) as a means to minimize the global free energy, by forming networks of defects with specific cubic symmetries. By confining BPs we introduce an approach to manipulate their structure through geometrical frustration. In this work, we present a new family of BPs by modeling ChLCs in toroidal and cylindrical cavities. The configurations are obtained following a theoretically-informed Monte Carlo relaxation of the free energy functional, which is described in the framework of the Landau-de Gennes formalism. We vary temperature and chirality to build phase diagrams and summarize the portfolio of chiral morphologies, which can be classified into twisted cholesterics, and BPs with helical and cubic symmetries. We also study the effects of surface anchoring and curvature to highlight the stability of these new phases. The formation of these new kind of morphologies offers interesting opportunities to direct the assembly of macromolecules and colloids at the nanoscale.   

Temperature Dependence of the Pitch in Chiral Lyotropic Chromonic Liquid Crystals

Molecular chirality is a subtle symmetry-breaking operation, yet its presence can produce profound macroscopic effects. One example is the nematic phase of liquid crystals, in which the presence of molecular chirality causes the preferred direction of molecular orientation to rotate in helical fashion resulting in striking optical effects. The pitch of the helix is sensitive to many parameters, including the type of molecules, concentration of chiral molecules, and temperature. Most liquid crystals studied to date consist of molecules or strongly bonded molecular assemblies that are not affected by experimental conditions. One exception is a lyotropic chromonic liquid crystal (LCLC), in which the size of the assemblies depends strongly on concentration and temperature. Investigation of the pitch of the helix in such systems reveals a highly unusual temperature dependence in which the pitch diverges as the temperature increases and the assemblies decrease in size. Theoretical considerations both explain this effect and allow the basic chiral interactions to be explored.   

Inducing chirality in homeotropic nematics via confinement geometry

The configuration of liquid crystalline phases, in particular nematics, are controlled by both the microscopic properties of their constituents and the macroscopic boundary conditions. Past work has shown that microscopically achiral chromonic liquid crystals, which have a small twisting modulus in comparison to splay and bend moduli, form chiral textures when embedded in cylindrical geometries with homeotropic boundary conditions. We show that when the cylinder is bent into a torus, these chiral configurations form at higher relative twisting moduli than it would in a straight cylinder. We use a boundary preserving Möbius transformation to shift the escaped core region toward the inner walls of the torus, yielding energetically favorable configurations. In order to match the homeotropic boundary conditions, these configurations are necessarily twisted independent of the elastic constants of the achiral mesogens. We experimentally verify the existence of such twisted textures with 5CB in toroidal droplets.   

Mesogen-free Liquid Crystalline Poly(ethylene oxide) with Sulfonyl Side Chains: Effects of Tacticity and Alkyl Side Chain Length

Atactic and isotactic poly(ethylene oxide)s possessing alkylsulfonylmethyl side chains were synthesized. Because of the strong dipolar interactions among sulfonyl groups in the side chains, they have a tendency of forming self-assembled mesophases even without the presence of aromatic mesogens. These polymers were characterized by size-exclusion chromatography (SEC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) at various temperatures. These PEO-based comb-like polymers are capable of forming liquid crystals with a double layer structure, which has been determined by temperature dependent XRD. In this presentation, effects of the tacticity and alkyl chain length on the mesophase structure and dielectric properties will be discussed.
  

Ionic Liquid Crystalline Elastomers Actuated by Low Electric Field

Over the past two decades Liquid Crystalline Elastomers (LCEs) with anisotropic properties attracted immense interest for their superior responsibility to various external stimuli. Hybrid aligned LCE films have anisotropy of thermal extension coefficient and dielectric properties in top and bottom sides, leading to large thermally and/or optically induced bend. Electric field induced bending via Maxwell stress from dielectric coupling of insulating LCEs requires electric fields close to electric breakdown, thus limiting their applications. In ionic electroactive polymers (IEAPs), ion flow causes asymmetric volume change and generates large bending even at low (~1V) voltages. In this work ionic liquids (ILs) are introduced into hybrid aligned LCE films to combine spatial anisotropic properties of hybrid alignment in LCEs with charge separation dynamics of ionic EAPs. We demonstrate that charge separation in LCE-IL system is capable for low electric field induced flexing or flexing-induced electricity effect. Additionally, ion motion induced electro-thermomechanical effects that enable spatial deformations by varying directors of LCEs are also presented.   

Backfolding Transitions in a Liquid Crystalline Polymer Brush

Liquid crystalline (LC) polymer brushes offer a convenient way of modifying surface properties for LC materials in, for example, LC displays. With this motivation in mind, we begin by studying LC polymer brushes in a simple solvent using self-consistent field theory. The polymers are modeled as worm-like chains with Maier-Saupe interactions. For good solvent conditions, the isotropic interactions favor a stretched brush while the anisotropic LC interactions favor folding into a high-density nematically collapsed brush. The brush undergoes first-order transitions as the number of folds increases. The folding transitions can be qualitatively understood through a simple analytic model balancing the energetic benefit from increased LC alignment and the cost associated with the bending energy of hairpin folds.   

Tension-induced nematic phase separation in homopolymer melts

The nematic coupling parameter α, which quantifies the interactions between backbone tangents and nematic fields, governs the nematic phase behaviors and polymer alignment in bulk and at interfaces. Together with external tension, the nematic interactions can also drive phase separation of long chains from short ones in bidisperse homopolymer melts. Combining molecular dynamics (MD) simulations and analytical theory, we extract α for polyethylene (PE) oligomers under applied tension, and construct a mean-field free energy to predict the phase boundary for bidisperse melts in which the longer chains are stretched by uniaxial tension. Phase separation occurs when sufficient tension is applied, consistent with a previous prediction by Olmsted and Milner. By directly observing phase separation in MD simulations, we validate the phase diagram. Using non-equilibrium MD (NEMD) simulations, we also show that extensional and shear flow may lead to nematic phase separation in molten PE oligomers, because the flow can impose a stronger tension on the longer chains than the short ones. We expect the tension-induced nematic phase separation may affect chain configurations for polydisperse polymer melts under flow, and in turn affect flow-induced crystallization.   

Entanglement in the isotropic-nematic crossover regime: does Edwards’ primitive path picture still apply?

Edwards defined entanglements in polymer melts in terms of contacts between individual chains’ primitive paths (PPs). This picture breaks down in the rigid-rod-like limit: while there are no inter-PP contacts, systems remain entangled in the sense that every chain’s transverse motion remains highly constrained by the other chains. How it breaks down as chains approach this limit is largely unknown. Using molecular dynamics simulations and topological analyses, we characterize how entanglement in polymer melts varies and the Edwards picture breaks down as chain stiffness and nematic order increase. We find that the diffusivity and the entanglement length N_e measured by topological analyses are minimized at two different chain stiffnesses k₁ and k₂ (with k₁ < k₂). Both of these are below the stiffness k₃ at which the isotropic-nematic transition occurs. We relate these phenomena to both the gradual onset of local nematic order and the inherent limitations of Edwards’ PP picture for quantifying entanglement in semiflexible polymer melts.