Electrospun Microfibres With Temperature Sensitive Iridescence From Encapsulated Cholesteric Liquid Crystal

We apply coaxial electrospinning to produce core-sheath polymer composite fibres with encapsulated short-pitch cholesteric liquid crystal, giving the fibres iridescent colours due to selective reflection within a narrow band of the visible wavelength spectrum. By modifying the feed rate of the liquid crystal during spinning we can tune the fibre diameter from the sub-micron range to about 7 mm, other ranges being accessible via further modifications of the spinning parameters. We demonstrate that the thinnest fibres display quantised colours, determined primarily by the core diameter, whereas the thicker fibres allow a quasi-continuous change in colour if the cholesteric helix pitch changes. Because of the strong response function of liquid crystals, phases as well as structures changing in response to small changes in the environment, the resulting non-woven fibre mats have potential for smart textiles, in particular in sensing applications

Coaxial electrospinning of liquid crystal-containing poly(vinylpyrrolidone) microfibres

With the relatively new technique of coaxial electrospinning, composite fibres of poly(vinylpyrrolidone) with the liquid crystal 4-cyano-4′-octylbiphenyl in its smectic phase as core material could be produced. The encapsulation leads to remarkable confinement effects on the liquid crystal, inducing changes in its phase sequence. We conducted a series of experiments to determine the effect of varying the relative flow rates of inner and outer fluid as well as of the applied voltage during electrospinning on these composite fibres. From X-ray diffraction patterns of oriented fibres we could also establish the orientation of the liquid crystal molecules to be parallel to the fibre axis, a result unexpected when considering the viscosity anisotropy of the liquid crystal kept in its smectic phase during electrospinning

Liquid Crystals in Novel Geometries prepared by Microfluidics and Electrospinning

We describe two new techniques of preparing liquid crystal samples and discuss their potential for novel research and applications. Very thin polymer composite fibers func- tionalized by a liquid crystalline core are realized by coaxial electrospinning of a polymer solution surrounding the liquid crystal during the spinning process. The re- sulting fiber mats exhibit the special properties and responsiveness of the liquid crystal core, e.g. temperature dependent selective reflection when a short-pitch cholesteric is encapsulated. In the second approach an axisymmetric nested capillary microfluidics set-up is used to prepare liquid crystalline shells suspended in an aqueous continuous phase. The spherical geometry of the shell imposes specific defect configurations, the exact result depending on the prevailing liquid crystal phase, the director anchoring conditions at the inner and outer surfaces, and the homogeneity of the shell thickness. With planar director anchoring a variety of defect configurations are possible but for topological reasons the defects must always sum up to a total defect strength of s = +2. Homeotropic anchoring instead gives a defect-free shell, in contrast to a droplet with homeotropic boundary conditions, which must have a defect at its core. By varying the inner and outer fluids as well as the liquid crystal material and temperature, the defect configuration can be tuned in a way that makes the shells interesting e.g. as a versatile colloid crystal building block

Confinement-Sensitive Optical Response of Cholesteric Liquid Crystals in Electrospun Fibers

Soft self-assembling photonic materials such as cholesteric liquid crystals are attractive due to their multiple unique and useful properties, in particular, an optical band gap that can be continuously and dynamically tuned in response to weak external influences, easy device integration, compatibility with flexible architectures, and, as shown here, potential for submicrometer optical applications. We study such a system formed by a short-pitch cholesteric confined in the core of polymer fibers produced by coaxial electrospinning, showing that the selective reflection arising from the helical photonic structure of the liquid crystal is present even when its confining cavity is well below a micrometer in thickness, allowing as little as just half a turn of the helix to develop. At this scale, small height variations result in a dramatic change in the reflected color, in striking difference to the bulk behavior. These conclusions are made possible by combining focused ion beam (FIB) dissection and imaging of the internal fiber morphology with optical microscopy. The FIB dissection further reveals that the cross section of the cavity within the fiber can have a shape that is quite different from that of the outside fiber. This is critical for the photonic behavior of the composite fiber because different optical textures are generated not only by change in thickness but also by the shape of the cavity. Our results provide insights into the behavior of cholesterics in submicrometer cavities and demonstrate their potential at such dimensions

Macroscopic-scale carbon nanotube alignment via self-assembly in lyotropic liquid crystals

By dispersing carbon nanotubes (CNTs) in a lyotropic liquid crystalline matrix, uniaxial alignment of the nanotubes can easily be achieved over macroscopic areas. We briefly describe the principles behind the technique and then show that it can be applied to multiwall as well as single-wall nanotubes and that a variety of different dispersing materials can be used, from industrial surfactants to DNA. We also present a new microfluidics-based method for transferring the liquid crystal-dispersed CNTs to a substrate, maintaining a fair control of tube direction. (C) 2009 Elsevier B.V. All rights reserved

On the balance between syn- and anticlinicity in smectic phases formed by achiral hockey-stick mesogens with and without chiral dopants

A series of achiral hockey-stick-shaped mesogens forming tilted smectic liquid crystal phases of synclinic SmC- as well as anticlinic SmCa-type was prepared and characterized. While all homologues exhibit both phases, the balance shifts from anticlinic to synclinic order upon elongation of the terminal chain at the meta-position, defining the hockey-stick shape. The elongation also leads to an increased kinetic hindrance of the transition between syn- and anticlinic phases and a decreased transition enthalpy. These observations indicate that a well-defined kink (short meta-substituted chain) promotes the anticlinic structure while a higher flexibility between kinked and rod-shape (long meta-substituted chain) promotes synclinic order. An intermediate chain-length homologue was selected as host material for doping with syn- and anticlinic rod-shaped chiral dopants, respectively, at varying concentrations. Opposite of what might be expected the balance between syn- and anticlinic order was not simply dictated by the choice of dopant. Instead, both types of tilting order prevailed with roughly the same strength as in the achiral host regardless of which chiral material was added, up to concentrations well beyond normal doping conditions. Thus, at least with hockey-stick-shaped achiral hosts, syn- as well as anticlinic chiral compounds can be used effectively as chiral dopants without necessarily having an important impact on the clinicity of the resulting mixture. The hockey-stick design concept should be useful in producing achiral anticlinic-forming mesogens for low-polarization, long-pitch antiferroelectric liquid crystal mixtures. Finally, we point out that a mixture study like the one carried out here yields a conclusive means of establishing the clinicity of achiral tilted smectics, an endeavour that can sometimes be far from trivial

Morphology and Core Continuity of Liquid-crystal-functionalized, Coaxially Electrospun Fiber Mats Tuned Via the Polymer Sheath Solution

By electrospinning liquid crystals coaxially inside a polymer sheath, responsive fibers with application potential, e.g., in wearable sensors can be produced. We conduct a combined scanning electron/polarizing microscopy study of such fibers, concluding that a match between the properties of the sheath solution and that of the core fluid is vital for achieving well-formed and well-filled fibers. Problems that may otherwise arise are fibers that are continuously filled, but partially collapsed; or fibers in which the core breaks up into droplets due to a mismatch in elongational viscosity between inner and outer fluids