Low-Temperature Visual Mechanical Sensing via Uniaxial Compression of Blue Phase Liquid Crystal Elastomer.

Low-temperature visual mechanical sensing materials hold great promise for applications in cryogenic manufacturing, polar exploration, etc. Blue phase liquid crystals (BPLCs) are a promising candidate for low-temperature visual sensing materials based on their excellent optical properties and unique chiral helical structure. However, there remain challenges for low-temperature (<0°C) visual mechanical sensing BPLCs due to difficulties in fabrication of low-temperature flexible films and inadequate understanding of their thermal-optical-mechanical-structural relationships. Herein, a high-quality BPLC elastomer (BPLCE) is fabricated via "full chain extension followed by cooling-assisted assembly" strategy, and it exhibits visual mechanical sensing capabilities across -30 to 60°C based on the synergistic effect of the polymer cross-linked network (mechanical responsiveness) and BPLC structure (optical signal). There appear high- (20°C∼60°C) and low-sensitivity (-30°C∼20°C) mechanochromic regimes for the as-prepared BPLCE originating from stress-induced phase transition of unpolymerized liquid crystals upon cooling, which is validated by temperature-dependent dramatic change in spectra evolution, stress for equivalent stopband shift, recovery time, etc. Additionally, BPLCE undergoes distinct chiral optical transitions under compression. Potential applications including polarization recognition, critical pressure alert, and in situ low-temperature full-field stress monitoring are demonstrated, which may pave a way for advanced visual mechanical sensing in cryogenic engineering.