Hierarchical ABA triblock hydrogels: Midblock entanglement governs mechanics

Synthetic hydrogels that mimic the extracellular matrix must combine toughness, elasticity, and porosity. Non-equilibrium processing of hydrogels by rapidly injecting an amphiphilic ABA triblock copolymer dissolved in a water-miscible organic solvent into water induces nano and microscale ordering, resulting in porous materials. Previously published results using poly(ethylene oxide) (PEO) as the midblock yields highly extensible and tough gels. Moving beyond PEO seemed trivial, but most hydrogels prepared with hydrophilic midblocks of poly(N-isopropylacrylamide) (PNIPAM), poly(2-hydroxyethyl acrylate) (PHEA), and poly(acrylic acid) (PAA) are brittle. Confocal microscopy confirms that all gels are porous, indicating structure is not the reason for the pronounced mechanical differences. Viscosity measurements of midblock polymers PNIPAM, PHEA, and PAA in water indicate that hydrogels are brittle when the midblock concentration in the hydrated pore walls is below the polymer’s respective entanglement concentration threshold. Interestingly, PAA as the midblock results in highly extensible hydrogels when swollen in salt solutions due to the screening of electrostatic charges. These results establish design rules for toughening porous, hierarchically ordered ABA triblock hydrogels.