Enlarged Pore Size in Mesoporous Silica Films Templated by Pluronic F127: Use of Poloxamer Mixtures and Increased Template/SiO₂ Ratios in Materials Synthesized by Evaporation-Induced Self-Assembly

Abstract

Although evaporation-induced self-assembly (EISA) has proven to be a convenient method for synthesizing nanoporous silica films (and particles), accessing material structures with pore sizes larger than ca. 10 nm remains experimentally inconvenient. The use of pore swelling agents (SAs), commonly used during the hydrothermal synthesis of mesoporous silicas, results in little or no pore size expansion due to evaporation or phase separation. Moreover, diblock copolymer templates can yield large pores, but are quite expensive and generally require the addition of strong organic cosolvents. Here, we hypothesized that pores templated by the Pluronic triblock polymer F127 could be successfully enlarged, without phase separation, by using a chemically similar, non-volatile, secondary Pluronic (P103) as the SA. We find pore size increased up to 15 nm for a spherical pore morphology, with a phase transition to a multilamellar vesicle (MLV) based nanostructure occurring as the P103/F127 ratio is further increased. This MLV phase produces even larger pore sizes due to collapse of concentric silica shells upon template removal.Remarkably, F127 alone exhibits expansion of pore size (up to ca. 16 nm) as the template/silica ratio is increased. We find appearance of the MLV phase is due to geometric packing considerations, with expansion of F127 micelle size a result of favorable intermolecular interactions driven by the large polyethylene oxide content of F127. Other Pluronic polymers with this feature also exhibit variable pore size based on the template/silica ratio, enabling the synthesis of mesoporous films with 3D pore connectivity and truly variable pore size of ca. 4.5 to almost 20 nm. Particles and films of mesoporous silica (MPS) templated by condensation of solution-phase sol-gel precursors around self-assembled surfactant mesophases, either through solution processing 1,2 or evaporation-induced self-assembly (EISA), 3,4 have become an important class of nanostructured materials, with potential applications ranging from catalytic supports 5 and adsorbents for environmental remediation 6 to drug delivery 7,8 and nanoporous molecular separation membranes. 9,10 A prominent feature of these materials is the ease in which pore size can be engineered through the identity of commonly available off-the-shelf surfactant templates, starting at ca. 2 nm, 11,12 though the use of alkylammonium surfactants such as cetyltrimetylammonium bromide (CTAB), 1 up to approximately 10 nm with amphiphilic block co-polymer templates such as the poloxamer series of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblocks (PEO-PPO-PEO, exemplified by the SBA series of mesoporous silicas). 13-15 However, a need for greater pore size (pushed in part by the use of MPS to deliver large biomolecular cargos such as DNA or functional proteins) 16-19 has driven research into methods for expanding the upper limit of this range. One strategy that has been successful in synthesizing porous materials with pore sizes of even greater than 40...

Ordered meso-/macroporous silica and titania films by breath figure templating in combination with non-hydrolytic sol–gel processing

“…6, bottom). A maximum for all samples at a pore diameter of 6 nm is observed which is comparable to the values obtained for the siliceous materials and corresponds to the size that can be expected by using Pluronic F127 as structure directing agent (SDA) [22,23]. However, no narrow pore size distribution is found for the titania materials.…”

Synthesis of new porous clay heterostructures: The influence of co-surfactant type

“…The nonionic polymers (Pluronic F127) based on polypropylene oxide and polyethylene oxide units were used as primary organic template for synthesis of the silica with pore sizes larger than 10 nm [44].…”

Growth and Functionalization of Particle-Based Mesoporous Silica Films and Their Usage in Catalysis

“…The most common method for synthesizing SBA-15 films is evaporation-induced self-assembly (EISA) [18], where an ethanol-containing solution is deposited onto a substrate using spin- or dip-coating. The method can be used on large substrates with various compositions, but is limited, as the dipping angle is crucial for the film thickness [19], resulting in non-homogenous coatings on non-flat substrates, and pores larger than 10 nm are rare [20]. Recently, methods have been developed for depositing thin layers of pre-synthesized mesoporous silica particles on substrates, e.g., by spinning them onto a substrate [21], or using Langmuir-Blodgett deposition [22], making also cylindrical pores easily accessible.…”