Rhodamine-containing double-network hydrogels for smart window materials with tunable light transmittance, low-temperature warning, and deformation sensing

Abstract

Smart window materials have attracted much attention because of their energy savings ability, light passage control in response to external stimuli, but most studies focus on the tunability of a single property, specifically, optical transmittance. Here, we report a rhodamine (Rh) mechanophore-functionalized PNIPAAm-based double-network (DN) hydrogel with the potential to act as multifunctional smart window materials that have the combination of a transmittance regulation ability, freezing−/compression-induced mechanochromic and mechanofluorescent properties. Benefiting from naturally reversible hydrophilic/hydrophobic phase transitions of PNIPAAm chains, DN hydrogels exhibit an excellent temperature-induced optical transmittance change between transparency and opaqueness. Because of the covalent incorporation of Rh mechanophores into the first network within DN hydrogels, the gels can display visualizable mechanochromism or mechanofluorescence upon freezing or compression. This is attributed to the generation of the colored and fluorescent ring-opened Rh spirolactams via mechanoactivation. This work is the first example of thermochromic smart window materials with a visualized low-temperature warning and damage sensing abilities and provides a new strategy to fabricate multifunctional smart window materials.