Synthetic Reversible Fibrous Network Hydrogels Based on a Double-Helical Polyelectrolyte

Abstract

The unique mechanical properties of fibrous networks in biological tissues have inspired the development of synthetic fibrous network hydrogels, yet few polymers can reversibly form such structures. Here, we report the first reversible fibrous network hydrogel composed of synthetic polyelectrolytes with extremely rigid conformation (persistence length is ∼1 µm), made up of double-helical poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT) and tetrabutylphosphonium bromide ([P₄₄₄₄]Br). The hydrogel exhibits a unique sol–gel transition, triggered by the hydrophobicity increase of [P₄₄₄₄]Br above lower critical solution temperature (LCST). This drives PBDT aggregation into fibrous bundles through electrostatic interactions. These bundles grow and branch into a continuous network, with the molecular rigidity of PBDT's double-helix conformation being key to gel formation. The hydrogel displays strain-stiffening mechanical responses akin to biological systems and shows a significant hysteresis (21 °C) between heating and cooling cycles. Uniquely, the effects of salts on the transition temperature deviate from the Hofmeister series, highlighting coordination with sulfonate groups as the dominant factor. Leveraging its modulus change during gelation, the hydrogel was successfully applied as a spray coating on superhydrophobic vertical Teflon surfaces. This study broadens the scope of thermoreversible hydrogels introducing gelation mechanisms for rigid polyelectrolytes and demonstrates their potential in advanced coatings.