Mechanism of Mesoporous Silica Formation. A Time-Resolved NMR and TEM Study of Silica−Block Copolymer Aggregation

Abstract

The dynamics of the synthesis of a mesoporous silica material SBA-15 is followed using time-resolved in situ 1H NMR and transmission electron microscopy (TEM). Block copolymer-silica particles of two-dimensional hexagonal symmetry evolve from an initially micellar solution. The synthesis was carried out with the block copolymer Pluronic P123 (EO20-PO70-EO20) at 35 degrees C and using tetramethyl orthosilicate as the silica precursor. By using TEM, we can image different stages during the evolution of the synthesis. Flocs of spherical micelles held together by the polymerizing silica are observed prior to precipitation. With time, the structure of these flocs evolves and the transition from spherical to cylindrical hexagonally packed micelles can be monitored. The signal from the methyl protons of the PO part was recorded with 1H NMR. One observes a continuous increase in the signal width but with distinct changes in the spectral characteristics occurring in narrow time intervals. The spectral changes can be attributed to structural changes of the self-assembled aggregates. The 1H NMR and TEM studies reveal the same mechanism of formation. It is concluded that the aggregation is caused by a micelle-micelle attraction induced by oligomeric/polymeric silica that adsorbs to the EO palisade layer of the micelles and has the ability to bridge to another micelle. This adsorption also favors the formation of cylindrical aggregates relative to spherical micelles. The sequence of NMR and TEM observations can then be interpreted as the following sequence of events: (i) silicate adsorption on globular micelles possibly accompanied with some aggregate growth, (ii) the association of these globular micelles into flocs, (iii) the precipitation of these flocs, and (iv) micelle-micelle coalescence generating (semi)infinite cylinders that form the two-dimensional hexagonal packing.

Nucleation and growth of zeolites and inorganic mesoporous solids: Molecular insights from magnetic resonance spectroscopy

“…In the second stage, which lasts approximately 1 h, silica polymerization occurs at the core/corona interface of the polymer aggregates and then extends progressively outward to the corona region in the third stage (∼ 1 h). In situ solution-state 1 H NMR measurements during the formation of SBA-15 suggest that block copolymer micelles initially increase in size after which they aggregate and subsequently precipitate [143], coalescing into cylinders that adopt, from in situ SAXS measurements, two-dimensional hexagonal order [144]. The stages of silica polymerization at the corona/core interface and in the corona as indicated by EPR appear to play a role in the increased sizes of the polymer aggregates and their aggregation.…”

“…The principal requirements are that conditions must exist in which the structuredirecting agents self-assemble into ordered systems, and under which the inorganic species interact with or otherwise associate selectively with certain of the self-assembling components. One system that has been extensively studied mechanistically has used amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer species under acidic solution conditions to incorporate and cross-link hydrolyzed silica precursor species selectively in the hydrophilic regions of hexagonal and cubic composite mesophases [139][140][141][142][143][144][145], While the underlying physical basis by which self-assembly occurs is similar for non-ionic blockcopolymer species and the charged-surfactant structure-directing agents discussed above, several mechanistic differences can be identified. Among them, is that the two systems often occur in different regimes where attractive versus repulsive interactions dominate among the self-assembling agents.…”

Expanding mesoporosity of triblock-copolymer-templated silica under weak synthesis acidity

“…In addition, the hydronium ions can also strengthen the hydrogen bond between the silica species and the PEO chains. Both electrostatic interaction and hydrogen bond were taken as assembly forces involved in the copolymer-templated synthesis of mesoporous silica under acidic conditions [1,2,42], which cause the accumulation of the silica species around the template phase and thus promote kinetically their polymerization to form silica-capped micelles as well as their energetically favorable bundling into repeated structure due to entropic effect [43][44][45].…”

Transmission electron microscopy in formation and growth of ordered mesoporous materials

“…The formation of 2D hexagonal [13] and two cubic structures [14 •• ] were studied by TEM in combination with insitu studies by 1 H NMR and SAXS/XRD. A small volume of the original solution was diluted 10 times at specific times after the onset of the syntheses.…”