One-pot construction of hierarchical dynamic polyurethane networks for impact-resistant and shape-stable flexible aramid composites

One-pot HDPU networks were constructed for flexible aramid composites. • Fast/slow boron dynamics couple impact dissipation with shape stability. • Permanent PU skeleton suppresses cold flow and long-term chain slippage. • AF/HDPU achieved 91.20% energy absorption efficiency under 25 J impact. • AF/HDPU retained 76.35% energy absorption efficiency after seven impacts. A hierarchical dynamic polyurethane network (HDPU) combining a permanent polyurethane (PU) skeleton, fast Si-O-B dynamics, and slow aromatic boronate ester dynamics was constructed through a one-pot strategy and applied to flexible aramid composites. The fast-dynamic network promoted rate-sensitive energy dissipation under impact, whereas the slow-dynamic network and permanent PU skeleton suppressed long-term segmental slippage and cold flow. HDPU retained its macroscopic shape after standing for 68 d, whereas the shear-stiffening gel control exhibited pronounced cold flow. After the same storage period, the aramid fabric/HDPU (AF/HDPU) composite retained 98.08% of its initial energy absorption efficiency. Under a 25 J impact, AF/HDPU achieved an energy absorption efficiency of 91.20%. After seven repeated impacts, the energy absorption efficiency remained 80.13% and 76.35% at 15 and 20 J, respectively, without obvious perforation. AF/HDPU also retained 92.5%-97.2% of its initial impact performance after moisture exposure, UV irradiation, and cyclic bending. This work provides a molecular-network strategy for balancing impact-energy dissipation, dimensional stability, and service durability in flexible protective composites.