Network-Formation Sequence Controls Entanglement Topology and Antifreezing Mechanics in Double-Network Hydrogels

Double-network (DN) hydrogels are typically synthesized by a fixed, sequential polymerization order, yet the role of network-formation order on topological architecture and mechanical performance remains largely unexplored. Here, we report that the polymerization sequence is a decisive topology-control variable in DN hydrogels via a two-step polymerization. By reversing the network-formation order of the same polymer pair of poly( N -hydroxyethyl acrylamide) (pHEAA) and polyacrylamide (pAAm), the pHEAA-first (pHEAA/pAAm) DN hydrogels form a “tightly entangled” architecture, while the reversed pAAm-first (pAAm/pHEAA) DN hydrogels yield a “restricted entanglement” network. This topological contrast governs mechanical properties at both ambient and subzero temperatures. At room temperature, pHEAA-first hydrogels achieve superior tensile strength (∼1.2 MPa) compared to their pAAm-first counterparts (∼0.45 MPa) despite comparable compositions. The sequence effect becomes more pronounced in their antifreezing resilience at −10 °C. pHEAA-first hydrogels retain near-invariant tensile behavior, robust fatigue resistance (>50 cycles), and high flaw tolerance (fractocohesive length ≈ 1.91 mm), while pAAm-first counterparts suffer from early fatigue failure and high notch sensitivity. This superior antifreezing performance arises from stronger sequence-defined confinement and entanglement-mediated stress delocalization, which suppress freezing-induced mechanical heterogeneity and delay crack propagation. These findings establish network-formation sequence as a practical, additive-free design strategy for tuning topological confinement and enabling durable hydrogel mechanics in subzero environments.