Reports: DNI549093-DNI5: New Opportunities in the Synthesis of Ultra-Large-Pore Ordered Mesoporous Materials

Michal Kruk, PhD, City University of New York (Staten Island)

The current project explores our recently proposed approach to the judicious selection of micelle expanders (swelling agents) in the surfactant-templated synthesis of ultra-large-pore ordered mesoporous materials (OMMs). The swelling agent candidates are selected on the basis of a hypothesis that substances that solubilize in micelles of a particular surfactant to a moderate extent promise to be micelle expanders leading to well-ordered OMMs with significantly enlarged mesopores, perhaps of size beyond that hitherto achieved. On the other hand, substances that solubilize too strongly are likely to afford non-uniform very-large-pore products (perhaps foams), while substances solubilizing to a small extent may lead to no appreciable pore size enlargement. The suggested way to identify swelling agents is to examine data on the extent of solubilization of organic compounds in surfactant solutions, with a particular attention to families of compounds for which the extent of solubilization varies in a wide range. For instance, in the case of alkyl-substituted benzenes, the extent of solubilization in Pluronic triblock copolymers (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO), decreases as the number and/or size of alkyl substituents increases. If there are experimental data available on the micelle expander performance of one or more of the members of a particular swelling agent family in conjunction with a particular surfactant (or surfactant family), one can consider whether the experimentally observed swelling action was excessive or too weak. On this basis, one can attempt to identify a member of the swelling agent family that would solubilize in the considered surfactant in an optimal way. In particular, in the synthesis of ordered mesoporous silicas (known as SBA-15) with cylindrical mesopores using EO20PO70EO20 surfactant Pluronic P123 (with relatively large fraction of the hydrophobic block), 1,3,5-triisopropylbenzene (TIPB) was identified earlier as a promising swelling agent. TIPB indeed allowed us to synthesize large-pore and ultra-large-pore 2-D hexagonal SBA-15 silica with cylindrical mesopores of diameter from 10 nm up to 26 nm, some of which had never been documented before. On the other hand, it was noticed that EO106PO70EO106 triblock copolymer Pluronic F127 with a low fraction of the hydrophobic block was swollen to a limited extent by 1,3,5-trimethylbenzene (TMB; the most commonly used swelling agent) in the synthesis of LP-FDU-12 silica with face-centered cubic, fcc, structure (Fm3m symmetry) of spherical mesopores. Therefore, more strongly solubilized compounds (xylene and toluene) were selected as swelling agents, indeed leading to significant unit-cell size and pore-diameter increases.

Over the last year, the work carried out and/or published on the project was focused on: (i) better understanding of the selection principles for swelling agents, (ii) application of the micelle expander selection principles to periodic mesoporous organosilicas (PMOs), (iii) application of the micelle expander selection principles to hollow single-micelle-templated silica and organosilica nanoparticles, (iv) improvement in understanding of nanoscale structures of surfactant-templated materials, and (v) the use of silicas prepared in the presence of judiciously selected swelling agents as supports for well-defined polymer brushes in the mesopores.

A significant effort was devoted to the improvement of the understanding of the principles of the selection of swelling agents in the micelle-templated synthesis. For instance, the literature data on solubilization of alkylbenzenes in micelles of Pluronics surfactants suggested that xylene and ethylbenzene, which have the same number (two) of carbon atoms in alkyl substituents, solubilize to a similar extent. We investigated whether this result implies that the suitability of alkylbenzenes as swelling agents in conjunction with Pluronics block copolymers can be predicted on the basis of the total number of carbon atoms in alkyl substituents, without considering the number, position and size of the alkyl substituents. For example, FDU-12 silicas synthesized under the same conditions using Pluronic F127 and either individual xylene isomers, or xylene isomers mixture or ethylbenzene as swelling agents were compared and it was found that all these swelling agents perform in a similar way. These and other results suggest that for alkyl-substituted benzenes, the total number of carbon atoms in the alkyl substituents can be used to predict the suitability of these compounds as swelling agents in combination with Pluronic block copolymers.

We continued to investigate the application of toluene as a swelling agent in the synthesis of well-defined Pluronic-templated materials. We also explored the opportunities in the synthesis of large-pore silicas at room temperature instead of the previously used temperatures from 7 to 20 °C range to make the large-pore materials even more readily available. The preliminary results were encouraging and the work in this direction is continued.

We have also extended the use of judiciously selected micelle swelling agents over two new compositions of periodic mesoporous organosilicas, namely ethenylene-bridged and phenylene-bridged organosilicas with unsaturated and aromatic bridging groups, respectively. In both cases, well-defined materials with face-centered cubic structures of large (~11-14 nm) spherical mesopores have been obtained.

Pluronic block copolymers and judiciously selected swelling agents also afforded hollow silica and organosilica nanoparticles (typically nanospheres) templated by single surfactant micelles. The proper selection of the swelling agent allows one to synthesize the nanospheres of unusually large (~20 nm) pore diameters.

The inspection of transmission electron microscopy images of large-pore materials prepared using micelle expanders suggested that such materials with cylindrical mesopores are likely to form from building blocks composed of cylindrical micelles surrounded by silica (or organosilica) envelopes. This opens interesting opportunities in adjusting the accessibility of the cylindrical mesopores.

The large-pore silicas investigated in the project were used as supports for the grafting of well-defined polymer brushes by surface-initiated atom-transfer radical polymerization.

PI has prepared and published a review discussing the most fundamental aspects of the work on the grant that were published so far.

The PRF award facilitated PI’s advancement as an associate professor. It provided funds to support two Ph.D. students (including one US citizen), as well as two undergraduate students (one US citizen and one minority student). The PRF award also provided much needed funding for chemicals and supplies used by four graduate students (one of which recently graduated), and three undergraduate students (one graduated) working on research projects within the scope of the PRF award.