Self‐Seeded Nucleation of PET in a Benign Solvent Yields a High Modulus Aerogel With Ultra‐Low Thermal Conductivity

Poly(ethylene terephthalate) (PET) is the most recycled plastic; however, upcycling of this commodity polymer to value‐added products has remained limited. Recent attempts to produce PET aerogels have relied on toxic solvents, chemical conversions and cross‐linking, and reinforcement to achieve useful mechanical performance. Now, using a new benign solvent (1,3‐diphenylacetone), we report a simple, safe, and sustainable dissolution and gelation procedure to convert waste PET into low density, monolithic aerogels with high mechanical strength (E = 20 ± 2 MPa) and remarkably low thermal conductivity (k = 21.9 to 28.9 mW m −1 K −1 ). The key to this process is controlled crystal nucleation by a self‐seeding phenomenon during gelation that establishes a mesoscale fibrillar framework. Our strategy offers a benign sol–gel process to upcycle plastic waste into a new form of pure PET for advanced thermal insulation.