Molecular and mesoscopic structures of transparent block copolymer-silica monoliths

Mesoscopically ordered, transparent silica-surfactant monoliths have been p repared using amphiphilic triblock poly(ethylene oxide)-poly(propylene oxid e)-poly(ethylene oxide) (PEO - PPO - PEO) copolymer species to organize pol ymerizing silica networks. The block copolymer acts as a structure-directin g agent, as the aqueous silica cations partition within the hydrophilic reg ions of the self-assembled system and associate preferentially with the PEO blocks. Subsequent polymerization of the silica precursor species under st rongly acidic conditions (pH similar to 1) produces a densely cross-linked silica network that may be mesoscopically organized by the block copolymer species into composites with characteristic ordering length scales of >10 n m. When this is accompanied by slow evaporation of the aqueous solvent, suc h composite mesostructures can be formed into transparent and crack-free mo noliths (e.g., 2.5 cm diameter x 3 mm thick). Distributions and dynamics of the PEO and PPO copolymer blocks within the silica matrix were investigate d in situ using Si-29{H-1} and C-13{H-1} two-dimensional solid-state hetero nuclear correlation NMR techniques and H-1 NMR relaxation measurements, Mes ostructural ordering was determined by X-ray diffraction and transmission e lectron microscopy. The degree of microphase separation and the resulting m esostructure of bulk samples were found to depend strongly upon the concent ration of block copolymer, with higher concentrations producing higher degr ees of order.