Bound Water as a Reinforcing Element for Ultra-Strong Polyacrylamide.

Water is generally considered a plasticizer that weakens polymer mechanical properties. Here, we show that in polyacrylamide (PAM), water plays distinct roles depending on its state in the polymer matrix: free water plasticizes PAM into a soft gel, whereas bound water forms strong PAM-water-PAM hydrogen-bond networks that surpass direct PAM-PAM interactions, enabling ultra-strong PAM. Confined bound water reinforces the polymer matrix, increasing the tensile modulus from ∼0.6 to 9.2 GPa and the tensile strength from 2.7 to 140 MPa, while also yielding record-high flexural (197 MPa) and compressive (178 MPa) strengths. Molecular dynamics simulations indicate that bound water organizes into linear nanoclusters that dynamically couple polymer chains, maximizing interchain cohesion and enabling efficient energy dissipation under stress. The resulting material surpasses many high-performance polymers, including polyamide-imide (PAI), polyether ether ketone (PEEK), and polyimide (PI), and exhibits exceptional flame resistance (limiting oxygen index ∼46). These findings establish bound water as an intrinsic structural enhancer, redefining its role in polymer reinforcement and providing a new design strategy for high-performance synthetic polymers.