The sphere-to-rod transition kinetics of tri-block copolymer micelles are especially important for the synthesis of mesoporous silica materials, such as SBA-15, where these surfactants are used as templating agents [1]. Such a synthesis typically starts with spherical micelles, which, during silica hydrolysis and condensation, are converted into long cylindrical micelles. When the materials are calcined, the micelles are removed, resulting in a silica matrix with hexagonally ordered mesopores. It is highly probable that the kinetics of the sphere-to-rod transition versus the kinetics of silica hydrolysis and condensation will determine the final properties of the synthesized inorganic materials. Therefore, we have studied the kinetics of the sphere-to-rod transition of the tri-block copolymer P123 (EO₂₀PO₇₀EO₂₀) in aqueous solutions, using dynamic light scattering (DLS) and cryogenic electron transmission microscopy (Cryo-EM).

Sphere-to-rod transition is induced by a solvent jump, initiated by adding KCl and ethanol to an aqueous micellar solution. The growth process of the worm-like micelles depends on the experimental conditions and has two distinct regions that can be described as a slow initiation period, and actual growth to equilibrium. All growth curves exhibit a single relaxation time (τ_r ) that represents the lifetime of the micelles. The growth curves collapse into a master curve, when shifted by the relaxation time, indicating that the actual growth process of the micelles in all samples occurs through the same mechanism. The lifetime decreases with increasing surfactant and ethanol concentration. Additionally, as evidenced by Cryo-EM images, some of the formed micelles exhibit a caterpillar-like shape, in which several of the original spherical species can still be detected. These facts suggest that the micelles grow longer predominantly by random coagulation/fragmentation reactions involving species of different sizes [2].

Conclusions from this study allow us to enhance our understanding of the role played by salts and ethanol in the formation of micelle-templated mesoporous materials, such as SBA-15.

References:

[1] Zhao D.; Huo Q.; Feng J.; Chmelka B. F.; Stucky G. D. J. Am. Chem. Soc. 1998, 120, 6024

[2] Denkova A.G., Mendes E.,Coppens M.-O., submitted for publication.