Measuring the Microphase Separation Scale of Polyurethanes with a Vibration-Induced Emission-Based Ratiometric "Fluorescent Ruler".

Polyurethanes (PUs) are a very attractive type of segmented polymer with unique mechanical properties derived from thermodynamic incompatibility between flexible soft segments and hard segments. The performance of PUs is closely related to their microphase separation structures, in which the hard domains serve as physical cross-linking points in the soft matrix. Studying the microphase separation of PUs in a facile manner is thus of great significance but challenging due to the complexity of the internal structures of PUs. N,N'-disubstituted-dihydrophenazine (DPAC) derivatives, the typical molecules featured with vibration-induced emission (VIE) attribute, can emit fluorescence varying with surrounding environment. In this proof-of-concept work, a series of DPAC derivatives were employed as built-in ratiometric "fluorescent rulers" to measure the degree of microphase separation in PUs modulated by temperature variation. The fluorescence of selected DPAC-doped PU films is dependent on the temperature, providing a theoretical basis for this concept. The feasibility of these fluorescent rulers was further validated by the small-angle X-ray scattering (SAXS) analysis. The SAXS curves show significant change in q range of 0.02 to 0.15 Å-1 with the variation in temperature, showing the changes in the internal microstructure. The polydisperse hard sphere model analysis of the scattering data revealed that the volume fraction of hard spheres has a defined relationship with the fluorescence intensity ratio of orange-red light and blue light, thereby demonstrating a novel fluorimetry method for measuring and monitoring the microphase separation of polyurethanes.