Silica-type mesostructures from block copolymer phases: Formation mechanism and generalization to the dense nanoparticle regime

The present study elucidates the local structure and formation mechanism of a hybrid system based on poly(isoprene-block-ethylene oxide) diblock copolymers (PI-b-PEO) as structure directing agents for organically modified ceramic (ormocer) precursors, (3-glycidyloxypropyl)trimethoxysilane (GLYMO), and aluminum sec-butoxide (Al((OBu)-Bu-s)(3)). Employing results of pH measurements, solid-state NMR, scanning force microscopy, and mechanical properties analysis, it is shown that silica nanoparticles are an intermediate in the condensation process and that the final composites show structural heterogeneities within the PEO/inorganic containing domains of the hybrids. On the basis of the experimental results, a two-step model for the polymer-inorganic hybrid formation is developed. After sol-gel nanoparticle formation and subsequent mixing with the block copolymer, further condensation provides a locally heterogeneous PEO/morganic domain conceptually similar to what is observed in alcogels, but replacing the solvent by PEO. Finally, implications of working in the high nanoparticle density regime with block copolymers or other amphiphiles for the generation of novel nanostructured, functional polymer-inorganic hybrid materials are discussed.