Thermochromic Nonwoven Fabrics Via Electrospinning

Cholesteryl ester liquid crystals (LC) are a class of unique soft materials with thermochromic properties arising from their helical structure with a temperature-dependent pitch length. Near the mesophase transition temperature, the liquid crystal first reflects blue light (λ = 450 nm) at relatively short pitch lengths. As the temperature decreases, the wavelength reflected shifts to the red end of the spectrum (λ = 760 nm) due to the increase in pitch length. Such materials have been used in thermal mapping and analysis in medical, industrial, and engineering applications. The LC in this study are composed of cholesteryl oleyl carbonate, cholesteryl pelargonate, and cholesteryl benzoate (30:60:10 by weight).. Electrospinning is a useful approach to incorporate such functional additives into nonwoven fibers. In this work, we aim to achieve thermochromic nonwoven fabrics in a single-step, single nozzle electrospinning process using solvent induced phase separation. Specifically, we blend the LC with spinnable polymer solutions (one-phase). During the fiber spinning process, as the solvent evaporates and the polymer concentration increases, phase separation of the liquid crystal will occur. Polycaprolactone (PCL) (10 wt.% which formed uniform fibers alone) was blended with the LC and electrospun using chloroform as the solvent. The LC concentration was varied between 2.5 and 27.5 wt.%. The fibers were analyzed using polarized light microscopy (PLM) on a temperature-controlled stage. At low LC concentrations (below 15 wt%), no thermochromic behavior in the fiber was observed. Increasing the LC concentration above 15 wt.% resulted in non-uniform fiber formation and nonwoven fabrics with an oily feel. Thus, the intermediate LC concentration of 10 wt% PCL 15 wt% LC-3 ideal for achieving uniform fibers and robust fabrics. The resulting fibers and fabrics were thermochromic and demonstrated reversible color change with no significant change in color transition temperatures over at least 4 cycles. Overall, we demonstrate that electrospinning is a promising approach to achieving thermochromic nonwoven fabrics.