UV photocatalyzed polyurethane formulations for controlled foam generation

Polymeric foams are used across a wide range of applications from insulation to tissue engineering. However, finely tuning their morphologies for property control remains challenging due to the difficulty of decoupling the foaming and the solidification. To achieve this decoupling, we use here an external stimulus to trigger the solidification of polyurethane foams. Our original strategy relies on the use of a photobase generator (OXTA-DBU), which, via the release of a cyclic amidine upon irradiation at the appropriate wavelengths, acts as a photolatent catalyst for the hydroxyl-isocyanate addition reaction. The response of the resulting bulk formulations to UV irradiation is characterized via the use of photorheology. Then, using a purpose-designed millifluidic circuit, these initially liquid formulations are continuously mixed and foamed. The obtained foam templates are solidified through UV irradiation. The structure of the resulting solid polyurethane foams is studied via X-ray microtomography to analyze the impact of irradiation conditions on the repartition of the polymeric matrix across the foam, providing a qualitative insight into the solidification kinetics of the foam. We thus demonstrate that the use of OXTA-DBU allows for the generation of well-controlled polyurethane foams whose final morphology can be adjusted through irradiation conditions. We also show that formulations mixing the photobase generator with a more classical catalyst provide access to an even wider range of solidification kinetics which, in turn, enables a finer tuning of the solid foam morphology.