3D printable polymer foams with tunable expansion and mechanical properties enabled by catalyst-free dynamic covalent chemistry†

Thermoset foams are some of the most common polymer materials in our lives. Despite their prevalence, they are notoriously difficult to form into complex shapes and finding a balance between mechanical strength, pore size and crosslinker density poses a significant challenge in optimizing their performance for specialized applications. 3D printing offers a solution by enabling the production of complex structures that can be foamed on demand using closed cell foaming microspheres, where a post-processing thermal treatment triggers expansion. However, foam expansion is typically constrained by its crosslinking density. This work introduces dynamic phosphodiester bonds into 3D printed polymers embedded with foaming agent microspheres to facilitate dynamic bond exchange during the thermal foaming process. With the inclusion of dynamic bonds, the foaming rate can be increased while also maintaining higher levels of crosslinking. These printed materials exhibit versatility, functioning effectively both before and after foaming, and offer potential for a diverse range of applications. Overall, this dynamic bond approach yields stronger, more expandable foams with improved energy dissipation and allows for the use of the printed foams in multiple lifecycles.