Photochemical phase and alignment control of a nematic liquid crystal in core-sheath nanofibers

Electrospinning serves as a versatile means of understanding the effects of strong cylindrical confinement on encapsulated liquid crystals (LCs) and is a promising technique for developing functional fabrics and surfaces. In this work, we use electrospinning to create core-sheath nanofibers composed of a polyvinylpyrrolidone (PVP) sheath doped with a photochromic, azobenzene-based surfactant, and a low-molecular weight nematic liquid crystal core. The incorporation of an azobenzene surfactant into the polymer sheath allows for photochemical control over the nematic to isotropic transition temperature of the LC core resulting in ultra violet (UV) irradiation-induced depression of the phase transition temperature. Owing to the photochromic nature of the surfactant, the photoinduced decrease in the transition temperature is reversed with visible light irradiation with no notable fatigue resistance. At high surfactant loading rates, the temperature of the photoinduced phase transition is reduced to below room temperature allowing for the birefringence of the nanofibers "on" and "off" with irradiation and requires no external heating. Based on these oberservations, we propose photoisomerization of the azobenzene-based surfactant at the PVP/LC interface leads to surface induced disorder of the LC core changing the orientation from planar axial to random with UV irradiation and reversing that change with visible irradiation. This is further substantiated by examinig the LC alignment within nanofibers by imaging the fibers with the addition of a quarter-wave retardation plate. This method adds to the growing set of tools available for the control of the phase behavior of encapsulated liquid crystals and expands the functionality of electrospun materials.