Vat photopolymerization of stretchable foam with highly entangled and crosslinked structures

Vat photopolymerization 3D printed flexible polymer foams, characterized by their porosity and lightweight nature, are in high demand for applications in thermal insulation, sound absorption, noise reduction, and biomedicine. The vat photopolymerization used UV-curable resin composed of oligomer can provide excellent fabrication accuracy, however the high cross-linking density after curing prevents the green part from expanding and further being processed into foam parts. In this work, a facile additive manufacturing polyurethane foam preparation method that successfully balances fabrication accuracy and expansion ratio is presented. The oligomer containing difunctional dynamic polyurea bonds in the resin system ensures the printing accuracy. Additionally, the dynamic urea bonds disassociate under a heating condition, reducing the cross-linking density and providing free space for expansion. Moreover, heat stimulated chain extension and crosslinking enhance the stretchability of the foams, demonstrating a strain of up to 650% at a density of 0.25 g/cm3. This work addresses the challenge associated with fabricating free foaming parts via vat photopolymerization by delivering favourable surface quality and high expansion ratio without compromising mechanical properties.