Access to Nanostructured Hydrogel Networks through Photocured Body-Centered Cubic Block Copolymer Melts

Abstract

Direct access to nanostructured hydrogel networks through high fidelity photocuring of sphere-forming block copolymer melts is demonstrated. Hydrophobic junction points within the hydrogel network are based on an underlying lattice of body-centered cubic spheres (SBCC), produced via melt-state self-assembly of blended AB diblock and ABA triblock copolymer amphiphiles. Integrated thermally stable photocuring chemistry allows for in situ trapping of these spherical domains, independent from the required melt processing necessary to achieve the highly ordered BCC lattice. Swelling of the photocured solids in aqueous (and organic) media afforded highly elastic gels exhibiting excellent mechanical properties (G′ ∼ 103 Pa) and complete preservation of the cured solid shape. The hydrogels fabricated in this study were produced from partially epoxidized (19.6%, relative to diene repeat units) blends of polybutadiene-b-poly(ethylene oxide) diblock (PB–PEO, fPB = 0.13, Mn = 29 500 g mol–1, 88.5 mol %) and PB–PEO–PB triblock (fPB = 0.13, Mn = 59 000 g mol–1, 11.5 mol %) copolymers synthesized via anionic polymerization. Addition of UV-activated cationic photoinitiator (4-iodophenyl)diphenylsulfonium triflate (0.5 mol %) produced composite samples exhibiting a highly ordered SBCC morphology after annealing at moderate temperatures (4 h at 80 °C or 60 s at 140 °C) above the PEO melting transition. Composite films (0.33 mm thickness) were then photocured directly from the melt state, postanneal. Cured samples retained the SBCC structure with extremely high fidelity, effectively prestructuring the network of junction points prior to swelling. The photopatterning potential of these uniquely designed hydrogels is also demonstrated.