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See detailCombining responsiveness and durability in liquid crystal-functionalised electrospun fibres with crosslinked sheath
Vats, Shameek UL; Lagerwall, Jan UL

in Liquid Crystals (2021)

Responsive functional composite fibre mats that are mechanically stable and impervious to water exposure are produced by coaxial electrospinning of thermotropic liquid crystal (LC) core inside a water ... [more ▼]

Responsive functional composite fibre mats that are mechanically stable and impervious to water exposure are produced by coaxial electrospinning of thermotropic liquid crystal (LC) core inside a water-based solution of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) forming the sheath. Because thermotropic LCs usually cannot be spun inside water-based solutions due to excessive interfacial tension γ, a n enabling step is the addition of ethanol or dioxane to the LC as a co-solvent compatible with both core and sheath fluids. This reduces γ sufficiently that coaxial jet spinning is possible. After spinning, thermal cross-linking of the PVA+PAA sheath yields LC-functionalised fibres that can be manipulated by hand and remain intact even upon full immersion in water. The LC core retains its behaviour, nematics showing well-aligned birefringence and transitioning to isotropic upon heating above the clearing point, and cholesterics showing selective reflection which is even enhanced upon water immersion due to the removal of sheath scattering. Our results pave the way to producing LC-functionalised responsive fibre mats using durable polymer sheaths, thereby enabling numerous innovative applications in wearable technology, and they also open new opportunities to study LCs in confinement, without visible impact of the container walls. [less ▲]

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See detailElectrospinning Ethanol–Water Solutions of Poly(Acrylic Acid): Nonlinear Viscosity Variations and Dynamic Taylor Cone Behavior
Vats, Shameek UL; Lagerwall, Jan UL

in Macromolecular Materials and Engineering (2021)

Electrospinning of polymer solutions is a multifaceted process that depends on the careful balancing of many parameters to achieve a desired outcome, in many cases including mixtures of multiple solvents ... [more ▼]

Electrospinning of polymer solutions is a multifaceted process that depends on the careful balancing of many parameters to achieve a desired outcome, in many cases including mixtures of multiple solvents. A systematic study of how the solution viscosity 𝜼—a good probe of solvent–polymer interactions—and the electrospinnability change when poly(acrylic acid) (PAA) is dissolved in ethanol–water mixtures at varying mixing ratio is carried out. A pronounced maximum is found in 𝜼 at a water-to-ethanol molar ratio of about 2:1, where the solvent mixture deviates maximally from ideal mixing behavior and partial deprotonation of carboxyl groups by water coincides synergistically with dissolution of the uncharged protonated PAA fraction by ethanol. The PAA concentration is tuned as a function of water–ethanol ratio to obtain a common value of 𝜼 for all solvent mixtures that is suitable for electrospinning. For high PAA content, the Taylor cone grows in volume over time despite minimum solution flow rate, even experiencing surface gelation for ethanol-rich solutions. This is attributed to the hygroscopic nature of PAA, drawing excess water into the Taylor cone from the air during spinning. [less ▲]

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See detailStable Electrospinning of Core-Functionalized Coaxial Fibers Enabled by the Minimum-Energy Interface Given by Partial Core− Sheath Miscibility
Vats, Shameek UL; Lagerwall, Jan UL; Anyfantakis, Emmanouil UL

in Langmuir (2021), 37(45), 1326513277

Core−sheath electrospinning is a powerful tool for producing composite fibers with one or multiple encapsulated functional materials, but many material combinations are difficult or even impossible to ... [more ▼]

Core−sheath electrospinning is a powerful tool for producing composite fibers with one or multiple encapsulated functional materials, but many material combinations are difficult or even impossible to spin together. We show that the key to success is to ensure a well-defined core−sheath interface while also maintaining a constant and minimal interfacial energy across this interface. Using a thermotropic liquid crystal as a model functional core and polyacrylic acid or styrene-butadiene-styrene block copolymer as a sheath polymer, we study the effects of using water, ethanol, or tetrahydrofuran as polymer solvent. We find that the ideal core and sheath materials are partially miscible, with their phase diagram exhibiting an inner miscibility gap. Complete immiscibility yields a relatively high interfacial tension that causes core breakup, even preventing the core from entering the fiber- producing jet, whereas the lack of a well-defined interface in the case of complete miscibility eliminates the core−sheath morphology, and it turns the core into a coagulation bath for the sheath solution, causing premature gelation in the Taylor cone. Moreover, to minimize Marangoni flows in the Taylor cone due to local interfacial tension variations, a small amount of the sheath solvent should be added to the core prior to spinning. Our findings resolve a long-standing confusion regarding guidelines for selecting core and sheath fluids in core−sheath electrospinning. These discoveries can be applied to many other material combinations than those studied here, enabling new functional composites of large interest and application potential. [less ▲]

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See detailElastic sheath–liquid crystal core fibres achieved by microfluidic wet spinning
Honaker, Lawrence William UL; Vats, Shameek UL; Anyfantakis, Emmanouil UL et al

in Journal of Materials Chemistry C (2019)

While coaxial polymer sheath–liquid crystal core fibres attract interest for fundamental research as well as applied reasons, the main method for achieving them so far, electrospinning, is complex and has ... [more ▼]

While coaxial polymer sheath–liquid crystal core fibres attract interest for fundamental research as well as applied reasons, the main method for achieving them so far, electrospinning, is complex and has significant limitations. It has proven particularly challenging to spin fibres with an elastic sheath. As an alternative approach, we present a microfluidic wet spinning process that allows us to produce liquid crystal core–polyisoprene rubber sheath fibres on a laboratory scale. The fibres can be stretched by up to 300% with intact core–sheath geometry. We spin fibres with nematic as well as with cholesteric liquid crystal in the core, the latter turning the composite fibre into an elastic cylindrical photonic crystal. Iridescent colours are easily observable by the naked eye. As this coaxial wet spinning should be amenable to upscaling, this could allow large-scale production of innovative functional fibres, attractive through the various responsive characteristics of different liquid crystal phases being incorporated into an elastic textile fiber form factor. [less ▲]

Detailed reference viewed: 173 (27 UL)
See detailMicrofluidic Wet Spinning of Core-Sheath Elastomer-Liquid Crystal Fibers
Honaker, Lawrence William UL; Vats, Shameek UL; Anyfantakis, Emmanouil UL et al

Scientific Conference (2019, March 29)

Liquid crystals encapsulated in fibers have a wide variety of applications in sensing. In order to produce these, several methods have been explored. Electrospinning is among the better-known techniques ... [more ▼]

Liquid crystals encapsulated in fibers have a wide variety of applications in sensing. In order to produce these, several methods have been explored. Electrospinning is among the better-known techniques with considerable successes. Only a limited range of polymers, though, has been used for electrospinning with liquid crystal cores, and the process of electrospinning has many obstacles to its utility at an industrial scale. On the other hand, wet-spinning techniques are better suited for industrial applications and are widely used in textile manufacturing, but are not commonly used for coaxial fiber production, especially with the large experimental scales that are difficult to replicate in a standard liquid crystal research laboratory. We therefore propose a method for wet-spinning coaxial core-sheath liquid crystal-filled elastomer fibers using a microfluidic set-up. Based on the flow-focusing method used for the production of liquid crystal shells and emulsions, this technique generates coaxial filaments by pumping a core-sheath flow of a liquid crystal surrounded by a rubbery polymer solution into a co-flowing coagulation bath. The coagulation bath is tuned to quickly extract the elastic polymer solution solvent, leaving behind a dry, continuous fiber. We have employed this method to produce fibers of polybutadiene and polyisoprene containing a core of a liquid crystal, such as 4-cyano-4'-pentylbiphenyl (5CB). Investigations into the choice of polymer solution, i.e. both the polymer and solvents used, will be presented in addition to discussion on parameters affecting the contiguity of the core. [less ▲]

Detailed reference viewed: 323 (9 UL)