Hydrochromic Nanocapsule with Real Time Visual In Situ Water Level Sensing Function and Toughening Effect for Plastic Materials

Abstract Despite the recent developments in hydrochromic materials, visual detection of water level in plastic materials without sacrificing mechanical properties remains a meaningful yet challenging topic. Currently, to optimize the sensitivity, hydrochromic agents are usually incorporated into hygroscopic matrixes to form bulk solids, which have restricted compatibility for the direct application inside other polymer materials. In this study, this challenge is tackled by nanoscale incorporation of hydrochromic agent into a hygroscopic polymer by miniemulsion polymerization. The obtained hydrochromic nanocapsule has an average diameter ≈110 nm, shows reversible hydrochromic property after 10 cycles, and can offer the first directly eye‐readable signal about the real time in situ water level in plastic materials with a water level below 4 wt.%, while achieving a 214% enhancement of the toughness at the same time. Comprehensive researches by investigating the composition and viscoelastic transition of the nanocapsules has shed light on the mechanisms concerning hydrochromic sensitivity and toughening effect. This simple methodology not only offers a new route for the nanoscale incorporation and application of different hydrochromic materials, but also inspires the development of smart nanoparticles with other stimuli‐responsive functionalities. All these aspects should render this work interesting for both scientific researches and engineering practices.