Unraveling the internal structure of 4D-printed stimuli-responsive materials using a molecular probe

Stimuli-responsive 4D-printing offers unparalleled potential in various research fields, enabling the combination of complicated mechanical design with programmable functionalities. The switchable polymer network structures, e.g., crystalline domains, free volume, and phase separation, are the key to achieving macroscopic responsiveness. However, despite a growing repertoire of new materials, most studies rely on rudimentary imaging techniques to visualize the materials' shape change under external stimuli. It remained challenging to correlate macroscopic behavior with the nanoscopic structures and dynamics of polymers. Here, leveraging the aggregation-induced emission (AIE) phenomena, we introduce a novel method that can offer direct insights into the network structures and the chain mobility of the printed polymers. We developed a new photo-polymerizable polyurethane with multiple responsive characteristics, including temperature, mechanical strain, and pH, as an example of responsive 4D-printing materials. By embedding aggregate-induced emission luminogen (AIEgen) in the polymer matrix, we demonstrated that the emission intensity and wavelength can serve as reporters and correlate the intramolecular motions of the AIEgen with the stimuli-responsive properties of the polymers. These observations were confirmed by small-angle X-ray scattering revealing the underpinning structural evolution. With potential applications for real-time structural monitoring, this study provides a new tool for the characterization of 4D-printed materials.