Structure transition from hexagonal mesostructured rodlike silica to multilamellar vesicles

We studied the synthesis of siliceous structures by using a nonionic block copolymer (Pluronic P123) and perfluorooctanoic acid (PFOA) as cotemplates in an acid-catalyzed sol-gel process. Different siliceous structures were obtained through systematically varying the molar ratio (R) of PFOA/P123, and the resultant materials were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen sorption analysis, and Fourier-transform infrared spectroscopy. The results are consistent and reveal a structure transition from a highly ordered 2D hexagonal (HEX) mesostructure with a rodlike morphology to multilamellar vesicles (MLVs) with sharp edges when R is increased. The fact that the MLVs are initiated from the end of hexagonally mesostructured rods provides key evidence in such a novel structure transition. Our finding indicates that, at least in our observations, the MLVs are developed gradually from HEX structures, rather than by a direct cooperative self-assembly mechanism. It is suggested that PFOA molecules with rigid fluorocarbon chains closely interact with PEO. This interaction model may well explain (1) the "wall-thicken" effect in HEX mesostructures by enlarging the hydrophilic PEO moiety (R = 0-1.4), (2) the subsequent HEX to multilamellar structure transition by modifying the hydrophilic/hydrophobic volume ratio (R = 1.4-2.8), and (3) the formation of MLVs with sharp edges by increasing the bending energy. This model provides insight into the fabrication of novel porous materials by the use of block copolymers and fluorinated surfactant mixed templates.