Molecular and mesoscopic structures of transparent block copolymer-silica monoliths

Mesoscopically ordered, transparent silica-surfactant monoliths have been prepared using amphiphilic triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymer species to organize polymerizing silica networks. The block copolymer acts as a structure-directing agent, as the aqueous silica cations partition within the hydrophilic regions of the self-assembled system and associate preferentially with the PEO blocks. Subsequent polymerization of the silica precursor species under strongly acidic conditions (pH approx. 1) produces a densely cross-linked silica network that may be mesoscopically organized by the block copolymer species into composites with characteristics ordering length scales of > 10 nm. When this is accompanied by slow evaporation of the aqueous solvent, such composite mesostructures can be formed into transparent and crack-free monoliths (e.g., 2.5 cm diameter × 3 mm thick). Distributions and dynamics of the PEO and PPO copolymer blocks within the silica matrix were investigated in situ using ²⁹Si{¹H} and ¹³C{¹H} two-dimensional solid-state heteronuclear correlation NMR techniques and ¹H NMR relaxation measurements. Mesostructural ordering was determined by X-ray diffraction and transmission electron microscopy. The degree of microphase separation and the resulting mesostructure of bulk samples were found to depend strongly upon the concentration of block copolymer, with higher concentrations producing higher degrees of order.