Stabilization of the liquid crystalline blue phase by the addition of short-chain polystyrene

Kasch, Nicholas; Dierking, Ingo; Turner, Michael L.

doi:10.1039/c3sm00065f

Cited by 29 publications

(25 citation statements)

References 25 publications

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“…Polycrystalline structures are generally riddled with grain boundaries that can severely limit electro-optical performance. Recent efforts have sought to develop strategies to significantly increase the thermal stability of BPs (14,(16)(17)(18)(19)(20)(21)(22)(23). Control of the lattice orientations of BPs, however, has been difficult to achieve; unlike nematic LCs, the molecular alignment of BPs is not uniform, and multiple nucleation seeds occur simultaneously during the course of a BP phase transition (24-28).…”

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confidence: 99%

Mesoscale martensitic transformation in single crystals of topological defects

Martínez‐González

Hernández-Ortíz

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Liquid-crystal blue phases (BPs) are highly ordered at two levels. Molecules exhibit orientational order at nanometer length scales, while chirality leads to ordered arrays of double-twisted cylinders over micrometer scales. Past studies of polycrystalline BPs were challenged by the existence of grain boundaries between randomly oriented crystalline nanodomains. Here, the nucleation of BPs is controlled with precision by relying on chemically nanopatterned surfaces, leading to macroscopic single-crystal BP specimens where the dynamics of mesocrystal formation can be directly observed. Theory and experiments show that transitions between two BPs having a different network structure proceed through local reorganization of the crystalline array, without diffusion of the doubletwisted cylinders. In solid crystals, martensitic transformations between crystal structures involve the concerted motion of a few atoms, without diffusion. The transformation between BPs, where crystal features arise in the submicron regime, is found to be martensitic in nature when one considers the collective behavior of the double-twist cylinders. Single-crystal BPs are shown to offer fertile grounds for the study of directed crystal nucleation and the controlled growth of soft matter.T here is considerable interest in understanding at a fundamental level the processes through which crystals are nucleated, and how they grow and transform between different lattice symmetries. Among crystal-crystal transformations, martensitic transitions are of particular interest, as they involve a collective and diffusionless atomic rearrangement that occurs at velocities comparable to the speed of sound in the material (1). Here, we use liquid crystals (LCs) that exhibit long-range order and crystalline symmetries with lattice constants in the submicron regime, the socalled blue phases (BPs), to study a liquid analog of a crystalcrystal transformation. BPs are thermodynamically stable liquid crystalline states that occur in a narrow range of temperatures between the cholesteric (Chol) and isotropic (I) phases of chiral LCs. In BPs, the molecules are locally oriented and organized in such a way as to form structures known as double-twist cylinders. These cylinders, which arise from a balance of long-range elastic distortions and short-range enthalpic contributions to the free energy, adopt a cubic lattice structure, which is interdispersed by an ordered network of topological defects (disclination lines). Depending on the thermodynamic conditions, the double-twist cylinders adopt a body-centered cubic structure (BCC), which produces the so-called blue phase I (or BPI), or they adopt a simple cubic structure (SC), which corresponds to the so-called blue phase II (or BPII). In both cases, the lattice parameters are on the order of a few hundred nanometers (2-4). BPs exhibit selective light reflections; they have fast electro-optical switching characteristics, with submillisecond response times, and their viscosity is relatively large (3,(5)(6)(7)(8). This c...

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confidence: 99%

Mesoscale martensitic transformation in single crystals of topological defects

Martínez‐González

Hernández-Ortíz

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…In the bulk, however, BPs are only found in a narrow range of temperature, between the cholesteric and the isotropic phases (1-6, 10-18), thereby placing limits on their practical utility. Recent efforts have therefore focused on increasing their stability over wider ranges of temperature and chirality, for example through addition of nanoparticles (19,20,(21)(22)(23), polymers (24)(25)(26), or by manipulating their flexoelectricity (14).…”

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Blue-phase liquid crystal droplets

Martínez‐González

Zhou

Rahimi

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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Blue phases of liquid crystals represent unique ordered states of matter in which arrays of defects are organized into striking patterns. Most studies of blue phases to date have focused on bulk properties. In this work, we present a systematic study of blue phases confined into spherical droplets. It is found that, in addition to the so-called blue phases I and II, several new morphologies arise under confinement, with a complexity that increases with the chirality of the medium and with a nature that can be altered by surface anchoring. Through a combination of simulations and experiments, it is also found that one can control the wavelength at which blue-phase droplets absorb light by manipulating either their size or the strength of the anchoring, thereby providing a liquid-state analog of nanoparticles, where dimensions are used to control absorbance or emission. The results presented in this work also suggest that there are conditions where confinement increases the range of stability of blue phases, thereby providing intriguing prospects for applications.blue phases | chiral liquid crystals | droplets | confinement B lue phases of liquid crystals (LCs) are characterized by a local director field that forms double-twisted cylinders, arranged into defect regions that are periodically repeated in space (1, 2). The symmetry properties of blue phases (BPs) have attracted considerable attention, as have their potential applications in a wide range of technologies (3-7). Theoretical and computational studies of the bulk structure and dynamics of BPs have helped elucidate their nature (1, 8, 9-15) in considerable detail. In the bulk, however, BPs are only found in a narrow range of temperature, between the cholesteric and the isotropic phases (1-6, 10-18), thereby placing limits on their practical utility. Recent efforts have therefore focused on increasing their stability over wider ranges of temperature and chirality, for example through addition of nanoparticles (19, 20, 21-23), polymers (24-26), or by manipulating their flexoelectricity (14).Confined chiral liquid crystals are of interest for applications in optical devices (27)(28)(29)(30)(31). Simulations of chiral LCs in channels have shown that their defect structure can be manipulated through confinement, thereby raising intriguing prospects for technology (32)(33)(34)(35). Chiral LC droplets have also been studied both numerically (36-38) and experimentally (39,40). More recent, intriguing simulations of planar chiral droplets with strong anchoring by Seč et al. (41) have examined their phase behavior as a function of chirality. In particular, it was reported that the twist bipolar structure (BS) is stable in the low chirality regime, whereas the radial spherical structure (RSS) is preferred at high chirality. The BS state has a cylindrical symmetry, where the director field is uniform along the symmetry axis of the droplet and rotates in the perpendicular direction forming bent cholesteric layers. In the RSS state, the director field also forms curved ...

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“…This theory was applied previously to the case of a blue phase stabilized by oligomeric polystyrene, with some success in replicating experiments. 17 Here, it is also applied to a stabilization of the twist grain boundary phase that might occur along similar lines by a mechanism of microscopic phase separation. It has been proposed that the stability of twist grain boundary phases observed in various mixtures of compounds 18 and in mixtures of liquid crystalline materials with nanoparticles 19 is enhanced by such an effect.…”

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confidence: 99%

Phase transitions and separations in a distorted liquid crystalline mixture

Kasch

Dierking

2015

The Journal of Chemical Physics

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A theoretical method is proposed for modelling phase transitions and phase ranges in a multi-component liquid crystalline mixture where the liquid crystal structure is distorted and defects are formed. This method employs the Maier-Saupe and Kobayashi-McMillan theories of liquid crystalline ordering and the Flory-Huggins theory of mixtures. It builds on previous work on mixed systems that can form smectic-A and nematic phases by incorporating "distortion factors" into the expression for the local free energy of the mixture, which account for the effects of a deviation of the liquid crystal structure from the uniform nematic and smectic-A states. The method allows a simple description of chiral defect phases such as the blue phase and the twist grain boundary phase. In a previous work, it was shown that a model of the blue phase along these lines could effectively explain the observed effect whereby an added guest compound can stabilize the phase by separating into the high energy defect regions of the structure. It is shown here that with the correct choice of guest material a similar effect could be observed for the twist grain boundary phase.

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Stabilization of the liquid crystalline blue phase by the addition of short-chain polystyrene

Cited by 29 publications

References 25 publications

Mesoscale martensitic transformation in single crystals of topological defects

Mesoscale martensitic transformation in single crystals of topological defects

Blue-phase liquid crystal droplets

Phase transitions and separations in a distorted liquid crystalline mixture

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