Engineering photo-responsive recyclability into polymer networks for sustainable 3D printing

The 3D printing industry is growing quickly, with projections that over 10% of manufactured products will be 3D printed within this decade. A consequence of this rapid growth is rapid waste production: over 30% of 3D-printed material is discarded immediately after printing. To enable recycling of scrap materials, we report photo-reversible polymer networks that are crosslinked and un-crosslinked using different wavelengths of light. Light-driven un-crosslinking has potential as a low-cost, low-energy, on-demand recycling technology. We designed photo-reversible polymer networks comprising multi-arm star polyethylene glycol functionalized with anthracene (PEG-anthracene). PEG-anthracene undergoes crosslinking upon irradiation with ultraviolet light (UV, 365 nm) and un-crosslinking upon irradiation with deeper UV light (265 nm). The un-crosslinking of PEG-anthracene with 3, 4, 6, or 8 arms (5 kg/mol per arm) was compared using in situ dynamic rheology. During network un-crosslinking, polymers with fewer arms exhibited “recycling windows,” where the sample transitioned from solid-like to liquid-like, during which re-crosslinking could occur. These findings demonstrate an opportunity for on-demand recycling of photo-reversible polymers to increase 3D printing sustainability.