Poly(Z-lysine)-Based Organogels: Effect of Interfacial Frustration on Gel Strength

A series of poly(Z-lysine)-based block copolymers were synthesized with poly(propylene oxide), poly(ethylene oxide-stat-propylene oxide), and polyhedral oligomeric silsesquioxane. These copolymers form thermoreversible gels in tetrahydrofuran with critical gel concentrations as low as 0.5 wt %. Infrared spectroscopy indicates that the peptide block adopts an antiparallel beta-sheet conformation whereby the assembly is facilitated by intermolecular H-bonding. The modulus of the gels was determined as a function of poly(Z-lysine) molecular weight and solution concentration (1-4 wt %). It was found that increasing the poly(Z-lysine) molecular weight or concentration increases gel strength and the range of linear viscoelastic response with stress. Remarkably, gel strengths as high as 220 and 348 Pa in THF and chloroform, respectively, were observed. In addition, the gel strength is highly dependent on the solvent interactions with the non-peptide block of the copolymer. We propose a model where interfacial frustration destabilizes beta-sheet assembly of peptide block, leading to weaker gel.