Transition from transparent aerogels to hierarchically porous monoliths in polymethylsilsesquioxane sol-gel system.

A transition from hierarchical pore structures (macro- and meso-pores) to uniform mesopores in monolithic polymethylsilsesquioxane (PMSQ, CH(3)SiO(1.5)) gels has been investigated using a sol-gel system containing surfactant Pluronic F127. The precursor methyltrimethoxysilane (MTMS) undergoes an acid/base two-step reaction, in which hydrolysis and polycondensation proceed in acidic and basic aqueous media, respectively, as a one-pot reaction. Porous morphology is controlled by changing the concentration of F127. Sufficient concentrations of F127 inhibit the occurrence of micrometer-scale phase separation (spinodal decomposition) of hydrophobic PMSQ condensates and lead to well-defined mesoporous transparent aerogels with high specific pore volume as a result of the colloidal network formation in a large amount of solvent. Phase separation regulates well-defined macropores in the micrometer range on decreasing concentrations of F127. In the PMSQ-rich gelling domain formed by phase separation, the PMSQ colloidal network formation forms mesopores, leading to monolithic PMSQ gels with hierarchical macro- and meso-pore structures. Mesopores in these gels do not collapse on evaporative drying owing to the flexible networks and repulsive interactions of methyl groups in PMSQ.

In situ SAXS observation on metal-salt-derived alumina sol-gel system accompanied by phase separation.

The structure formation process of hierarchically porous alumina gels has been investigated by in situ small angle X-ray scattering (SAXS). The measurement was performed on the sol-gel solution containing aluminum chloride hexahydrate (AlCl(3)·6H(2)O), poly(ethylene oxide) (PEO), and propylene oxide (PO). The temporal divergence of scattering intensity in the low q regime was observed in the early stage of reaction, indicating that the occurrence of spinodal-decomposition-type phase separation. Detailed analysis of the SAXS profiles revealed that phase separation occurs between weakly branched polymerizing aluminum hydroxide (AH) and PEO. Further progress of the condensation reaction forms phase-separated two phases, that is, AH-rich phase and PEO-rich phase with the micrometer-range heterogeneity. The growth and aggregation of primary particles occurs in the phase-separated AH-rich domain, and therefore, the addition of PEO influences on the structure in nanometer regime as well as micrometer regime. The moderate stability of oligomeric species allows homogeneous condensation reaction parallel to phase separation and successful formation of hierarchically porous alumina gel.

Hierarchically porous carbon monoliths with high surface area from bridged polysilsesquioxanes without thermal activation process.

Hierarchically porous carbon monoliths with high specific surface areas have been fabricated by removing nano-sized silica phase from carbon/silica composites pyrolyzed from bridged polysilsesquioxane. This activation method improves the homogeneity between inner and outer parts of the monoliths compared to the conventional thermal activation methods.

Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths.

Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol-gel reaction. Starting from AlCl3.-6H2O and poly(ethylene oxide) (PEO) dissolved in a H2O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H2O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H2O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO.

Rigid crosslinked polyacrylamide monoliths with well-defined macropores synthesized by living polymerization.

Rigid crosslinked polyacrylamide monoliths with well-defined macropores have been successfully fabricated by organotellurium-mediated living radical polymerization (TERP) accompanied by spinodal decomposition. The TERP forms homogeneous networks derived from N,N-methylenebis(acrylamide) (BIS), in which spinodal decomposition is induced to form macropores. Macropore diameter can be controlled from submicrons to a few microns, and also the obtained networks contain mesopores in the macroporous skeletons, which are collapsed by evaporative drying. They are promising materials with hydrophilic polyacrylamide surfaces and have enough strength to preserve the macropores from the surface tension arising in the repetitive swelling and drying that may occur in many applications.

Thick silica gel coatings on methylsilsesquioxane monoliths using anisotropic phase separation.

Silica gel coatings on methyltrimethoxysilane (MTMS)-derived monoliths have been studied using wetting transition. Wetting transition is observed in a small confined space, where a coating solution phase-separates into a well-coarsened dimension, making all the phase-separating polymerizing silica phase dynamically flow onto the existing surface of a mold. Bulk coating experiments have shown reductions of both macropore volume and diameter due to the coated layer. Comparing HPLC efficiencies of the coated monolith with those of the non-coated MTMS monolith revealed that the retention factors drastically increased in both normal- and reversed-phase modes. This is attributed to the existence of considerable amounts of accessible micropores left inside the coated layer, where analyte molecules travel and adsorb for a considerable period of time.