From Brittle to Ultraductile: Precise Ionic PEG with Dynamic Networks and Reconfigurable Crystallites

Achieving high ductility and toughness in semicrystalline polymers without sacrificing stiffness remains a longstanding challenge. Here we show that sparse precise ionic functionalization can fundamentally alter the mechanical behavior of a semicrystalline polymer. Ionic poly-(ethylene glycol) (iPEGs) containing precisely spaced imidazolium groups (∼1.5 mol %, every 62 repeat units) undergo a striking transition from brittle to ultraductile behavior, which exhibits stable neck propagation and apparent strain hardening at moderate molecular weight of 41 kDa. Tensile behavior, melt rheology, and X-ray scattering spectra reveal that this highly ductile behavior arises from a coupled ionic-semicrystalline architecture. Dynamic ionic associations introduce long-lived transient constraints that enhance the amorphous phase resistance to deformation and sustain load-bearing connectivity between crystallites, while ionic incorporation suppresses lamellar thickening and produces thin PEG crystallites capable of deformation-induced reorganization. The resulting interplay between crystallite connectivity, tie-chain load transfer, and reconfigurable crystallites enable stable neck propagation rather than brittle fracture. These results establish sparse ionic "stickers" as a molecular design principle for strengthening crystal-mobile semicrystalline polymers without introducing additional phases or permanent covalent cross-links.