Non-isocyanate polyurethane (NIPU) Films for Dual-Function Humidity Sensing and Hydroplasticization Monitoring by harnessing Clusteroluminescence

This work presents non-isocyanate polyurethane (NIPU) films of the polyhydroxyurethane (PHU)-type, characterized by pendent vicinal diol groups, as highly effective materials for humidity sensing and monitoring of matrix hydroplasticization. These films eliminate the need for fluorophores, as they exhibit inherent, unconventional fluorescence based on clusteroluminescence. This fluorescence originates from extensive hydrogen bonding and electron-rich heteroatomic sub-luminophores. The fluorescence properties of these films are highly sensitive to environmental humidity. As relative humidity increases, both fluorescence intensity and lifetime decrease linearly. This phenomenon is attributed to the disruption of hydrogen bonding by absorbed water molecules, which alters the clusteroluminescence mechanism. Furthermore, the fluorescence response also reflects the hydroplasticization of the NIPU matrix. A linear correlation was observed between relative fluorescence intensity and the glass transition temperature (Tg), enabling the first-ever use of nonconventional fluorescence to monitor Tg variations as a function of humidity. These findings demonstrate the dual functionality of non-isocyanate PHU-type films as accessible, fluorophore-free materials for real-time humidity sensing and monitoring of softening behavior in these NIPU matrices.