Cholesteryl ester liquid crystals exhibit thermochromic properties related to the existence of a twisted nematic phase. When used in applications such as thermal mapping, a color change is often monitored by video cameras. Thus, quantitative methods to evaluate thermochromic behavior (e.g., blue-start, red-start, red-end, color play and bandwidth) from video analysis are desirable. However, obtaining quantitative color measurements from digital images remains a significant technical challenge, especially for highly reflective samples such as liquid crystals (for which ultraviolet−visible (UV−vis) reflectance spectroscopy is typically used). We developed a method to determine thermochromic properties from videos of liquid crystal cooling under polarized light microscopy. We relate observed color transitions to quantifiable changes in the cumulative color difference in the International Commission on Illumination (CIE) L*a*b* color space and validate this method with UV−vis reflectance spectroscopy. The measured thermochromic behavior and associated measurement uncertainties (coefficient of variations) were comparable to UV−vis reflectance measurements.

Cholesteryl ester liquid crystals exhibit thermochromic properties related to the existence of a twisted nematic phase. We formulate ternary mixtures of cholesteryl benzoate (CB), cholesteryl pelargonate (CP), and cholesteryl oleyl carbonate (COC) to achieve thermochromic behavior. We aim to achieve thermochromic fibers by incorporating the liquid crystal formulations into electrospun fibers. Two methods of incorporating the liquid crystal (LC) are compared: (1) blend electrospinning and (2) coaxial electrospinning using the same solvent system for the liquid crystal. For blend electrospinning, intermolecular interactions seem to be important in facilitating fiber formation since addition of LC can suppress bead formation. Coaxial electrospinning produces fibers with higher nominal fiber production rates (g/hr) and with higher nominal LC content in the fiber (wt. LC/wt. polymer assuming all of the solvent evaporates) but larger fiber size distributions as quantified by the coefficient of variation in fiber diameter than blend electrospinning with a single nozzle. Importantly, our proof-of-concept experiments demonstrate that coaxially electrospinning with LC and solvent in the core preserves the thermochromic properties of the LC so that thermochromic fibers are achieved.

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Achieving fibers that change color with temperature may be promising for applications such as sensors and smart wearable textiles (woven and nonwoven). In this study, temperature-responsive cholesteryl ester liquid crystal formulations were blended with polycaprolactone or polystyrene using chloroform as a solvent for electrospinning to achieve thermochromic nonwoven products. Using polystyrene, beaded fibers were achieved and the thermochromic behavior was only observed under polarized light microscopy. To achieve fibers with visible thermochromic behavior observed by a video camera or smart phone, polycaprolactone was used as a carrier polymer. The comparison between polystyrene and polycaprolactone provides insight into polymer/solvent selection achieving responsive materials via blend processing. High loadings of liquid crystal were achieved with polycaprolactone, blends of 10 wt % polycaprolactone with 15 wt % liquid crystal formed fibers (fiber contained 60 wt % liquid crystal). Colored fibers could be achieved by varying the formulation of the liquid crystal. For example, liquid crystal formulations that were green at ambient conditions resulted in fibers that were green at ambient conditions (22 °C). Nonwoven fibers with dynamic color with temperature were also demonstrated. For example, liquid crystal formulations were colorless at ambient conditions and underwent a reversible color change from red to blue when heated and cooled between 32 and 37 °C. When incorporated into fibers, the fiber mats changed from white at ambient conditions to red at 32 °C to blue at 37 °C. The color change was reversible over multiple cycles.