3D Printing of Thermally Activated Polymer Foams with Controlled Porosity and Tunable Mechanical Properties via Digital Light Processing*

Many natural lightweight materials display a distinctive combination of low density, high strength and high toughness. These unique features are achieved through complex hierarchical architectures that introduce porosity and have characteristic dimensions spanning from the nano- to macroscale. This level of control, however, has been proven difficult to mimic synthetically in polymeric materials, limiting the use of soft porous composites in load-bearing applications. To address this challenge, we developed a two-step process involving 3D photopolymerization followed by a thermal processing step to fabricate thermally-activated polymer foams with tunable porosity and material properties. The material is processed by incorporating stimuli-responsive microspheres that act as a foaming agent into a photocurable resin leading to microporosity throughout the resulting polymer network. During thermal treatment, 3D printed objects undergo irreversible volume expansion while maintaining their geometry. Structural analysis and mechanical characterization on the polymer foams showed that a broad range of porosities up to 75% and tunable specific moduli can be achieved. Implementing this approach allows the development of lightweight materials with superior bulk properties for applications in aerospace and biomedicine.