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

This study presents a DN hydrogel reinforced with HMW PPz jeff bottlebrush polymers, achieving enhanced mechanical strength, toughness, and thermal stability. • Introducing polyphosphazenes with Jeffamine side chains (PPz jeff BBPs) to form unique double network (DN) hydrogels. • Cryo-SEM confirms PPz jeff is embedded in DN hydrogels, resulting in reduced pore size and swelling capacity. • PPz jeff significantly enhances the DN hydrogels' tensile/compression strength, modulus, and toughness. • Novel dense, brush-like architectures reinforce the network, boosting mechanical properties while maintaining high water content. 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-PPz jeff ) monomers, along with N,N′ -methylene bisacrylamide (NMBA) crosslinker and 2-oxoglutaric acid (OGA) photo-initiator, producing PPz jeff @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, M n ≈ 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 PPz jeff BBPs. Mechanical testing revealed significant enhancements, including tensile strength up to ∼ 4 MPa, toughness ∼ 540 kJ/m 3 , 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.