Formation Mechanism of Silica/Diblock Mesophases by Solvent Evaporation-Induced Self-Assembly
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Formation Mechanism of Silica/Diblock Mesophases by Solvent Evaporation-Induced Self-Assembly Kui Yu, Celeste A. Drewien, Alan J. Hurd, C. Jeffrey Brinker, Adi Eisenberg1 Sandia National Laboratories, MS 1349, Albuquerque, NM; 1 McGill Univ., Dept. of Chem., Montreal, Canada. ABSTRACT Intermediate structures were trapped during the mesophase transition from lamellae to higher curvature structures in a sol-gel matrix. The target structures included normal hexagonally arranged cylinders and/or normal spheres in a cubic array distributed in a hydrophilic matrix. The present system is believed to be the first to trap these intermediates. Through solvent evaporation-induced self-assembly (EISA), mesostructured silica/diblock films with large characteristic length scales were prepared. The structure-directing agents were polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers with high molecular weight, which are water insoluble and alcohol insoluble. We believe that no micellization took place in the present system; a disorder-to-order transition occurred due to the cooperative selfassembly of the diblock and silicates as the solvent preferentially evaporated from a film cast from a dilute homogeneous solution. During further preferential evaporation, the morphogenic effect of the increase of species concentration facilitates the mesophase development in the direction of a normal cubic to hexagonal to lamellar pathway. However, the morphogenic effects of both the decrease of the PS coil dimension and the siloxane condensation drive the mesophase development in opposite directions. The decrease of the PS coil dimension plays an important role in the present self-assembly process. Trapping of the intermediates and coexisting multiple mesophases are related to the facts that PS has high Tg and high hydrophobicity in particular, as well as to the fact that polymers have relatively low mobility in general.
INTRODUCTION Very recently, we reported the preparation of mesostructured silica films with regular and reverse mesophases through solvent evaporation-induced self-assembly (EISA) [1,2]. The structure-directing agents used were polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers and the prepared mesophases had large characteristic length scales. For the mesostructured silica film with regular mesophases, it is the hydrophobic (PS) domains that are distributed in a hydrophilic (silica and PEO) matrix; subsequent calcination produced mesoporosity. The pore sizes and wall thicknesses were rationally adjusted by the diblock copolymer block lengths. The pore diameters of the mesostructured porous silica prepared were in the range of 5 nm –25 nm, and the wall thicknesses were in the range of 5 nm – 15 nm. For the mesostructured silica film with reverse mesophases, it is the hydrophilic (silica and PEO) domains that are distributed in a hydrophobic (PS) matrix. After calcination, only silica spheres or silica rods were left [3]. It is the relatively hydrophilic to hydrophobic volume ratio that determ
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