Surfactant-Driven Bidirectional Tuning of Viscoplastic Physical Hydrogels and the Underlying Mechanism

Broad-range modulation of the aggregation structure and physical interactions enable high performance and multifunctionality in viscoelastic hydrogels; however, structural regulation mechanisms for viscoplastic hydrogels, particularly those based on polymer aggregation design, remain largely unexplored. Herein, we introduce surfactants with hydrophobic alkyl chains and hydrophilic head groups as modulators of physical interactions, which unexpectedly achieve bidirectional mechanical tuning of viscoplastic poly(vinyl alcohol) (PVA) hydrogels. With low concentrations, surfactants reduce surface tension, lubricate polymer chains, and disrupt intrinsic hydrogen bonds of PVA through competitive bonding, synergistically accelerating polymer chain relaxation and thus enhancing viscoplasticity. Beyond a critical concentration, excess surfactants self-assemble into micelles whose charged headgroups form electrostatic bridges with PVA hydroxyls while simultaneously self-organizing into lamellar stacks (L = 33.1 Å) that establish a secondary network, reinforcing viscoelasticity. Consequently, this approach achieves both broad-range mechanical modulation and a sharp viscoplastic-to-viscoelastic transition. The versatility of this bidirectional control strategy is demonstrated for various surfactants, with the critical concentration dependent on alkyl chain length.