Dependence of the Color-Changing Properties of Mechanochromic Liquid Crystal Composites on Polymer Matrix Mechanical Properties

Cholesteric liquid crystals (CLCs) are a unique class of materials that exhibit mechanochromic properties upon applied external stress. To improve their utility in potential optical and sensing applications, CLCs need to be contained in a polymer matrix to be processed into films or fibers. This entrapment presents an interesting situation where the mechanical properties of the polymer matrix may affect the overall color-changing properties of the CLC. Here, a CLC blend is mixed into different grades of thermoplastic polyurethane (TPU) with varying Shore hardness. These CLC–polymer composites are subjected to mechanical stress while collecting visible reflection spectra. By increasing the stiffness of TPU used to entrap the CLC blend from Shore hardness 70A to 93A, we can blue shift the selective reflection band (SRB) by >30 nm causing a color change from red to orange/yellow. Through stress relaxation experiments, the extent of color change in the CLC is correlated to peak stress of the TPU, providing an approach to tuning the color change. Increasing the stretching speed from 1 to 10 mm/s results in a >35 nm blue shift of the SRB, causing a color change from red to orange/yellow. A combination of the hardest TPU (93A) and the highest stretching speed (10 mm/s) at the largest strain explored (30%) results in the largest overall SRB blue shift of >70 nm. The reflected color returns to within 20 nm of the original SRB position, highlighting the reversibility of the system. Finally, a time-dependent generalized Maxwell model is derived that captures the shifts in reflected color upon stretching and stress relaxation in CLC–polymer composites. A greater understanding of the mechanical factors affecting the color-changing behavior of polymer-dispersed mechanochromic liquid crystals will increase applicability in areas such as strain/damage sensing, inkless writing, and anticounterfeiting.