Transparent cubic Fd3m mesoporous silica monoliths with highly controllable pore architectures

Abstract

Ordered cubic Fd3m mesoporous silica monoliths (HOM-11) were fabricated by using lyotropic and microemulsion phases of block copolymers (EO_m–PO_n–EO_m) as templates. Aromatic and aliphatic hydrocarbon molecules were used in the formation of true microemulsion liquid crystal phases, in the expansion of pore sizes, and in the changes in the phase geometrical shape. Large cylinder-like pores in the range of 6–11 nm with uniform constrictions and bimodal mesopore sizes can be easily produced (within minutes) by adopting this simple and reproducible strategy. The degree of solubilization of the hydrocarbons crucially influenced the generation of the more open-pore systems with cylindrical cubic Fd3m channels. Our results also show that enlargement of pore sizes of cubic Fd3m monoliths was achieved with the use of a high concentration of copolymers in the composition phase domains, a high degree of swelling and a large size of PO–EO blocks (core–corona) of the copolymer templates. In addition, this synthetic approach is also efficient in designing cubic Fd3m silica monoliths with large-sized glass, thick-walled frameworks up to 30 nm thick and high mesopore/micropore volumes. Although XRD patterns show well-defined Bragg diffraction peaks indicative of highly ordered cubic Fd3m structures, TEM micrographs reveal that worm-like mesopore channels in large domains were observed with samples synthesized from copolymers with small EO–PO block ratios. This finding indicates that the molecular nature (i.e. the flexibility of the corona- and core-blocks) of the copolymer templates not only led to disordered pore channels but also reduced the ability to design more mesostructured phases.