Enlarged Pore Size in Mesoporous Silica Films Templatedby Pluronic F127: Use of Poloxamer Mixtures and Increased Template/SiO2Ratios in Materials Synthesized by Evaporation-Induced Self-Assembly.

Although evaporation-induced self-assembly(EISA) has proven tobe a convenient method for synthesizing nanoporous silica films (andparticles), accessing material structures with pore sizes larger thanca. 10 nm remains experimentally inconvenient. The use of pore swellingagents (SAs), commonly used during the hydrothermal synthesis of mesoporoussilicas, results in little or no pore size expansion due to evaporationor phase separation. Moreover, diblock copolymer templates can yieldlarge pores but are quite expensive and generally require the additionof strong organic cosolvents. Here, we hypothesized that pores templatedby the Pluronic triblock polymer F127 could be successfully enlarged,without phase separation, by using a chemically similar, nonvolatile,secondary Pluronic polymer (P103) as the SA. We find pore size increasedup to 15 nm for a spherical pore morphology, with a phase transitionto a multilamellar vesicle (MLV)-based nanostructure occurring asthe P103/F127 ratio is further increased. This MLV phase produceseven larger pore sizes due to the collapse of concentric silica shellsupon template removal. Remarkably, F127 alone exhibits expansion ofpore size (up to ca. 16 nm) as the template/silica ratio is increased.We find appearance of the MLV phase is due to geometric packing considerations,with expansion of F127 micelle size being a result of favorable intermolecularinteractions driven by the large poly(ethylene oxide) content of F127.Other Pluronic polymers with this feature also exhibit variable poresize based on the template/silica ratio, enabling the synthesis ofmesoporous films with 3D pore connectivity and truly variable poresize of ca. 4.5 to almost 20 nm. [ABSTRACT FROM AUTHOR]