Interactions of thermochromic liquid crystals with surface-modified PET substrates for thermal sensing applications

Liquid crystals have gained significant attention worldwide for use in digital displays, sensing, smart textures, etc. Thermochromic liquid crystals, which respond to temperature, find applications in medical diagnostics and industrial monitoring through temperature sensing and mapping. Here, we deposited a thermochromic cholesteric liquid crystal (CLC) ink on surface-modified polyethylene terephthalate (PET) substrates using doctor blade coating and direct ink writing (DIW) methods, enabling its use for monitoring temperature changes in various applications. We investigated the thermochromic responses of the CLC ink films doctor blade-coated at different temperatures and analyzed how the processing temperature influences the surface characteristics, alignment of the liquid crystals, and their thermochromic behavior. Based on these observations, we identified the optimal printing temperature for the DIW process. We elucidated the interaction of CLCs with surface treatment, such as hydrophobic and hydrophilic coatings, and how they affect the spreading behavior and thermochromic response of CLCs printed on the coated PET substrates. The results demonstrate that the hydrophobic coating intensifies the temperature response of the CLC by drastically reducing ink spreading. This result is supported by the wetting studies of the CLC ink on various substrates and the wetting envelopes, which are developed to predict the contact angles of the ink on these surfaces. Direct ink writing of liquid crystals at elevated temperatures together with the surface treatment of substrates provides a facile way to pattern the liquid crystals with desirable alignment and robust thermochromic performance for temperature sensing applications.