Fabrication and characterization of photochromic microencapsulated phase change materials with highly cross-linked polyurethane shell coated by nano-TiO₂

Abstract

A new method of incorporating nano-sized titanium dioxide (nano-TiO₂) particles onto the shell of photochromic microencapsulated phase change materials was introduced, in order to address issues of easy degradation of photochromic dyes’ core components caused by ultraviolet (UV) irradiation and residual organic emulsifiers. Using nano-TiO₂ as the Pickering emulsion stabilizer and cross-linked polyurethane as the shell material, a composite protective structure was constructed to encapsulate core materials with phase-change and photochromic properties, thereby forming photochromic phase change microcapsules (TPT-MPCMs) with UV protection and thermal insulation. Characterization results show that the core‒shell structured TPT-MPCMs possessed high light transmittance, with a particle size of 5‒15 μm and a latent heat of 116.2 J·g⁻¹. The highly cross-linked shell formed by xylitol and isophorone diisocyanate effectively protected the core from thermal degradation up to 180 °C, while the nano-TiO₂ shell surface allowed maintaining the UV responsiveness of microcapsules after exposure to intense UV irradiation for 5 h. This strategy significantly improves the long-term stability and service life of photochromic microcapsules under harsh environments, opening up broad prospects for their applications in fields such as outdoor anti-counterfeiting labels, intelligent temperature-controlled coatings, and multifunctional smart textiles.

Graphical abstract

Keywords

Pickering emulsion / microcapsule / phase change material / cross-linked polyurethane / reversible photochromism

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