Mechanical Reinforcement of Lamellar Bilayer Hydrogels by Small Amounts of Co-surfactants

Anisotropic photonic hydrogels with alternatively stacked poly(dodecyl glyceryl itaconate) (PDGI) bilayers and polyacrylamide (PAAm) gel layers are unique soft materials with various functions. It is known that to form the lamellar phase of bilayers, a small amount of co-surfactant sodium dodecyl sulfate (SDS) should be present in the precursor monomer solutions of the gels. However, little is known about the influence of the co-surfactant on the structure of bilayers and on the mechanical properties of such photonic hydrogels. Herein, we chose several co-surfactants and studied the effect of the co-surfactants on the self-assembly behavior of the bilayers and on the mechanical properties of the resulting photonic hydrogels. A macroscopically aligned lamellar phase could be induced for all the co-surfactants. Interestingly, the mechanical response of the photonic hydrogels sensitively depends on the chemical structure of the co-surfactant, especially at large deformation. We hypothesize that doping by small amounts of co-surfactants dramatically changes the anchoring strength and density of PAAm strands onto the bilayer surface, thereby influencing the load transfer efficiency from the bilayer to the PAAm gel layer at large deformation and the rupture of the bilayer. This work provides new understanding in the molecular mechanisms of deformation and strengthening in this soft and anisotropic nanocomposite, helping to design more robust photonic hydrogels.