Preparation and performance of reversible thermochromic phase change microcapsules based on negative photochromic spiropyran

Thermochromic phase change materials have been widely studied for their importance in energy-efficient buildings, thermal control clothing, and low-temperature energy storage. Herein, we successfully designed and synthesized a spiropyran compound with negative photochromic properties in a solid matrix, 3-(3′,3′-dimethyl-6,8-dinitrospiro[chromene-2,2′-indolin]−1′-yl) propyl methacrylate (SPMA), as a thermochromic dye. The synthesized spiropyran was copolymerized with methyl methacrylate (MMA) using photoinitiation, and then n-octadecane was encapsulated by the copolymer to prepare reversible thermochromic phase change microcapsules (TC-microPCMs) through the solvent evaporation technique. The encapsulation efficiency of n-octadecane by copolymers prepared with different photoinitiator concentrations was also investigated. According to the results of SEM, particle size analysis (DLS), and DSC, the copolymer prepared at 3% concentration possessed the best encapsulation effect on n-octadecane, and the melting enthalpy of the microcapsules reached 168.7 J/g and the encapsulation ratio was 89.5%, which was not significantly different from that of microcapsules without the addition of thermochromic dyes. This di-functionalization process effectively avoids the "impurity effect" of dye addition. In addition, the homogeneous spherical structure and stable chemical composition of the microcapsules are the key factors to achieve a fully reversible thermochromic behavior. What's more, the functionalized film based on TC-microPCMs, TC-microPCMs@PVA, was used to explore the potential for applications in thermal storage and temperature sensing. The results demonstrated that the composite film has excellent reversible thermochromic performance, high latent heat storage performance, and good cycling durability, which make the bifunctional microcapsules highly promising candidates for thermal sensors, solar energy storage, and smart textiles.