Formation mechanism of mesostructured silica in confined space: an in situ GISAXS study

The structural evolution of periodic mesoporous material within the channels of anodic alumina membranes (AAMs) by evaporation-induced self-assembly (EISA) is investigated by a combination of in situ grazing-incidence small-angle X-ray scattering (GISAXS) with parallel detection of solvent evaporation and ex situ transmission electron microscopy (TEM). Kinetically controlled and equilibrium-controlled structural evolution can be distinguished for these EISA processes. A new mechanism for formation of mesostructures in the confined environment of AAMs is proposed. Data are presented for samples synthesized with nonionic surfactants at various surfactant:silica ratios and relative humidities. The formation of and transformations between circular or columnar 2D hexagonal and tubular lamellar structures are observed. The circular hexagonal phase is kinetically favored over the columnar hexagonal orientation. The TEM images provide evidence that phase transformations, depending on their type, either start preferentially at the channel wall or in the center of the mesostructured fibers.