Formation and Destruction of Polyelectrolyte/Surfactant Complexes for the Toughening of Hydrogels

Tough hydrogels usually have heterogeneous structures. Revealing microstructure formation and variations under specific conditions is important for the structure-property relationship and toughening mechanism of these gels. In previous work, we reported plastic-like supramolecular hydrogels with polyelectrolyte/surfactant complexes (PESCs) as physical cross-links. The gels are synthesized by polymerizing an aqueous solution of methacrylic acid in the presence of N-hexadecyltrimethylammonium chloride micelles. The as-prepared gels are soft and stretchable, while the equilibrated gels with expanded volumes are stiff and tough. What happens and how PESCs form during the swelling process remain unclear. In this article, we investigate PESC formation during the swelling of the gel by small- and wide-angle X-ray scattering measurements. The results indicate that the surfactant micelles in the as-prepared gel are broken at the beginning of the swelling process, accompanied by severe volume expansion of the gel due to the high osmotic pressure. Then, the gel contracts and PESCs are gradually formed with electrostatic interactions between oppositely charged polyelectrolytes and surfactants as well as hydrophobic interactions between alkyl chains of surfactants. The increased amount and strength of PESCs lead to vitrification of the gel with excellent mechanical properties. The swelling and mechanical performances of the gel can be tailored by swelling conditions and the alkyl chain length of surfactants. Under stretching of the gel, the PESC aggregates are destructed into small clusters and even disappear, dissipating vast energy. This strategy of forming robust PESCs is validated by developing other tough gels with various oppositely charged monomers and surfactants. This work should be inspirational for understanding the microstructure formation and its contribution to the mechanical excellence of hydrogels.