Coaxial electrospinning of microfibres with liquid crystal in the core

Abstract

Liquid crystal containing composite fibres were produced via coaxial electrospinning, demonstrating that this technique can be used for producing new functional fibres and/or to study the impact of extreme confinement on liquid crystal phases.Electrospinning offers a simple yet versatile means of producing nano-and microfibres of various materials and with properties tailored for use in diverse applications, ranging from photovoltaics 1 and photocatalysis 2 to energy storage 3 or tissue engineering. 4 By applying a DC voltage of several kV between a spinneret and a collector, a thin jet is ejected from a polymer melt or solution that is pumped through the spinneret. The jet is further stretched and thinned on the way to the collector due to electrostatic self repulsion. During the past few years a number of important advances in the technique were reported, such as fibre alignment, 5,6 tuning of fibre diameter from the nanometre to the micrometre range, 6 combination with sol-gel processes for the production of inorganic tubes, 7,8 and coaxial electrospinning of different polymers 9 or even of non-polymeric liquids in a polymer-inorganic composite sheath. 3,8 While the latter has so far been demonstrated using simple alkanes 3 or mineral oil, 8 such coaxial electrospinning opens the door to incorporation of a wide range of materials, including complex fluids, allowing for the introduction of new functionality to the fibre. As an example of such development we here report coaxial electrospinning of composite fibres consisting of a poly(vinylpyrrolidone) (PVP)/TiO 2 sheath and a nematic liquid crystal core.Liquid crystals constitute a unique class of liquids in that they combine fluidity with long-range order, orientational and in some cases also 1D or 2D translational. 10 This gives them a strong response to external influences, which is the basis for their very successful application in devices such as displays, modulators and sensors. The nematic phase exhibits only orientational order, described by the director n, a signless unit vector indicating the average direction of the principal symmetry axis of the molecules or molecule aggregates (rodor disclike). When incorporated into an electrospun fibre, the various types of self-assembled nano-and microstructures and the strong response function could provide the fibre with novel functionality, with potential application fields ranging from cheap temperature sensors to clothing with switchable textures. On the other hand, electrospinning such a composite fibre also constitutes a simple method of studying the effects of extreme confinement on various liquid crystal phases and phase transitions. Such effects can be very strong. 11,12 Our coaxial electrospinning set-up is schematically shown in Fig. 1. A small hole was made in the side of a syringe containing the outer fluid, allowing a flexible polyimide-coated silica capillary tube (outer diameter 320 mm) to be inserted and led through the needle of the syringe (inner diameter 500 mm). The hole was sealed with epo...

A new era for liquid crystal research: Applications of liquid crystals in soft matter nano-, bio- and microtechnology

“…Lagerwall and co-workers demonstrated this so far on regular nonchiral nematics [256], non-chiral smectic-A [267] as well as shortpitch chiral nematics [268]. In particular the latter is interesting from an applicational point of view, since the selective reflection of the liquid crystal is visible through the sheath for sufficiently thick fibers, thus giving the whole mat the color of the liquid crystal, easily visible by the naked eye.…”

“…A different approach to producing liquid crystal-core e polymer sheath fibers, with considerably smaller diameter, was recently introduced by Lagerwall and co-workers [256]. Using coaxial electrospinning they could introduce a variety of liquid crystals into the core of the spinning jet, where the sheath fluid was a solution of water soluble polymers like polyvinylpyrrolidone (PVP) or PVA, sometimes with inorganic sol-gel precursor for producing a composite sheath, e.g.…”

“…This may be a way forward also for the fiber mats functionalized by liquid crystal. Alternatively one might see the liquid crystal electrospinning activity primarily as a useful exploratory phase technique, its versatility being of great value as long as the questions to be answered deal with the spinnability of certain phases, their response to the strong cylindrical encapsulation within the fibers (quite strong effects on liquid crystal phase sequence were found in some of the mats studied so far [256,268]), the response to changes in the environmental conditions or to external fields etc. Once a concept identified as useful should be brought to a stage where true textiles can be produced, this could then be transferred to conventional spinning techniques, a task that should be feasible provided that the very thin diameter of electrospun fibers is not required.…”

Non-electronic gas sensors from electrospun mats of liquid crystal core fibres for detecting volatile organic compounds at room temperature

“…A potential solution to this conundrum recently appeared with experimental demonstrations of fibre production with liquid crystal contained within the fibre core [11,[49][50][51][52][53][54]. The concept is highly attractive, because the LC-functionalised fibres can be produced at low cost, following a simple one-step standard fibre production procedure that yields flexible non-woven responsive fibre mats that can be used as sensors.…”

“…The concept is highly attractive, because the LC-functionalised fibres can be produced at low cost, following a simple one-step standard fibre production procedure that yields flexible non-woven responsive fibre mats that can be used as sensors. The fibres have been produced through electrospinning, the core-sheath geometry ensured using a coaxial spinneret [11,49,51,53,54] or via in-situ phase separation [52], as well as through airbrushing [50]. In the following, we will focus on the electrospinning technique [55][56][57], in which a high voltage applied between spinneret and a grounded counter electrode leads to ejection of a thin charged jet from a droplet of spinning liquid that protrudes from the spinneret.…”