Strengthening of double network hydrogels by high-molecular-weight polyphosphazene-based bottlebrush polymers

Typical hydrogels generally display poor mechanical properties due to their high water content, limiting their practical applications. To address this, we designed a double-network hydrogel (DNH) with superior strength while retaining high water content. The first network was constructed using 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and single styryl-terminated polyphosphazene with jeffamine sidechains (St-PPzjeff) monomers, along with N,N′-methylene bisacrylamide (NMBA) crosslinker and 2-oxoglutaric acid (OGA) photo-initiator, producing PPzjeff@SN hydrogels. These SN samples underwent further internal polymerization to build the second network from acrylamide (AAm), OGA, and NMBA. To reinforce the system, high-molecular-weight PPz-based bottlebrush polymers (BBPs, Mn ≈ 200 and 400 kDa) with varied side chains were incorporated into the first network of the DN hydrogels. This unique brush-like architecture, containing PPz grafted with hydrophilic jeffamine chains (ethylene oxide (EO) and propylene oxide (PO) units), synergistically stabilized the matrix via a hydrophilic/hydropobic balance. Characterization confirmed a dense, uniform network and chemical integration to the network through the styryl end of PPzjeff BBPs. Mechanical testing revealed significant enhancements, including tensile strength up to ∼ 4 MPa, toughness ∼ 540 kJ/m3, and markedly higher compression resistance. These improvements arise from BBP architecture and network embedding. This strategy demonstrates that HMW PPz BBPs enable the fabrication of mechanically robust and tough hydrogels.