Phase behavior of mesoporous silicas templated by the amphiphilic diblock copolymer poly(ethylene-b-ethylene oxide)

In this study we used an amphiphilic diblock copolymer, poly(ethylene-b-ethylene oxide) (PE-b-PEO), as an organic template for the synthesis of mesoporous silica through evaporation-induced self-assembly (EISA). Using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and nitrogen adsorption/desorption isotherms, we investigated the effects of the tetraethyl orthosilicate (TEOS) and hydrochloric acid (HCl) contents on the EISA process. At a constant HCl(aq)-to-PE-b-PEO ratio (0.12:1), we observed an interesting mesophase transition—from lamellae to distorted lamellae to cylinders mixed with spheres to spherical structures—upon increasing the TEOS-to-PE-b-PEO ratio. Meanwhile, at a constant TEOS-to-PE-b-PEO ratio (10:1), a mesophase transition from distorted lamellae to cylinders mixed with spheres to single spherical morphology occurred upon decreasing the HCl(aq)-to-PE-b-PEO ratio. Because the TEOS-to-PE-b-PEO ratio changed the volume fraction of the microphase separation system and the HCl(aq)-to-PE-b-PEO ratio affected the rate of hydrolysis of TEOS during the EISA process, we attribute the various mesophase phenomena to competition between thermodynamic and kinetic control, respectively. In addition, we also observed a metastable structure, so-called “buckled cylinders,” in some of our systems.

The formation of each organized hybrid mesostructure resulted from a balance of two competitive processes: microphase separation and organization of the template and inorganic polymerization.Figure optionsDownload as PowerPoint slideHighlights
► Poly(ethylene-b-ethylene oxide) copolymer as a template for mesoporous silica.
► A lamellae to spherical structures upon increasing the silica ratio.
► A distorted lamellae to single spherical morphology upon decreasing the HCl ratio.
► Various mesophase phenomena competition between thermodynamic and kinetic control.