Nanoporous Silicas by Casting the Aggregates of Amphiphilic Block Copolymers: The Transition from Cylinders to Lamellae and Vesicles

Abstract

The synthesis of mesoporous ceramic oxides with pore sizes between 2 and 50 nm is a recent trend in materials science. Most mesoporous silicas are prepared using ionic low molecular weight surfactants as structure-directing agents and sometimes inert oils as swelling additives.[1] This process, involving the precipitation of a surfactant-rich gel phase, is restricted to the synthesis of materials with pore sizes smaller than 8 nm, as the silica walls are too thin to support a larger-pore network. The use of bulk lyotropic liquid crystal phases of low molecular weight non-ionic surfactants [2±4] as templates partially solves the problem of mechanical stability. The wall thickness depends on the amount of inorganic precursor present in the preparation mixture, because the nanostructure is a result of directly casting the lyotropic bulk phase. However, surfactants are only available with restricted lengths of the hydrophobic chain, so that again the pore diameter is limited to approximately 4.5 nm.Casting the lyotropic phases of amphiphilic block copolymers (ABCs) is the method of choice for the synthesis of mechanically stable large-pore systems.[5] Here, the solidification of a siliceous precursor takes place in the aqueous domains of the microphase-separated medium, hence producing a monolithic cast of the original supramolecular aggregate. In addition, the ABC liquid crystal approach affords coherent porous coatings and macroscopic objects with continuous pore systems. [6] This approach was subsequently adopted by Chmelka et al., [7] who employed commercially available Pluronic-type triblock copolymers and obtained materials with pore diameters between 4.7 and 30 nm, and wall thicknesses between 3 and 5 nm. Their observations confirm the superiority of polymeric templates as porogens. [6] In this contribution, we present new non-ionic polymer templates with improved water-solubilities and a broader range of accessible molecular weights, which allows pore diameters of ceramic nanostructures to be extended beyond the known limits of this casting procedure. Templating the lyotropic aggregate structure of poly(butadiene-bethylene oxide) (PB-PEO) was studied over a wide range of block lengths, block length ratios and polymer concentration. The resulting silicas were characterized by transmission electron microscopy (TEM), porosimetry, and small-angle X-ray scattering (SAXS). The results demonstrate the great potential inherent in ABC templating as the method of choice for pore design in ceramic oxides. The polymers used in the experiments and their analytical data are summarized in Table 1. Table 1. Physical data of the amphiphilic block copolymers.[a] GPC measurement using CHCl 3 for the precursor PB (PB standard), the PEO block length is calculated from 1 H-NMR spectra (M n = number average molecular weight´10 3 kg/mol).[b] Repeat units.[c] Polydispersity, GPC in CHCl 3 , RI.The lyotropic phase behavior of the amphiphilic block copolymers in water was studied by polarized-light optical microscopy, as SAXS experime...

Porous materials via nanocasting procedures: innovative materials and learning about soft-matter organization

“…Because of these advantages, a whole variety of amphiphilic block copolymers has been used. Beside the first used non-ionic structures with PEO chains to mediate silica compatibility, 19,[59][60][61] cationic and anionic polyelectrolyte blocks have also been employed. 31 The first reported systems have been poly(styrene)-PEO block-copolymers of low molecular weight.…”

“…[62][63][64] Fig. 8 depicts two unconventional phases and their nanocasting replicas, the bicontinuous sponge-like phase (Fig.…”

Preparation of Porous Silica Materials via Sol−Gel Nanocasting of Nonionic Surfactants: A Mechanistic Study on the Self-Aggregation of Amphiphiles for the Precise Prediction of the Mesopore Size

“…This approach was introduced and extended by Göltner et al [14][15][16][17][18][19] She termed the whole procedure "nanocasting", because a liquid-crystalline phase formed by the amphiphilic block copolymers in water is fixed (usually under preservation of the phase structure) by the solgel formation of silica in the hydrophilic domains, thus resulting in a solid 1:1 replica. Recently, also Stucky, Melosh, Chmelka et al applied the nanocasting concept to prepare hexagonal mesoporous silica with Pluronics (PEO-PPO-PEO block copolymers) as templates.…”

Evaporation‐Induced Self‐Assembly for the Preparation of Porous Metal Oxide Films

“…Their hydrophilic block is in most cases poly(ethyleneoxide) (PE) or, more seldom, poly(vinylpyridine). 15 The variety of available hydrophobic blocks is broader and is represented by poly(propylene oxide) (PP), 16 polystyrene (PS), 17 polybutadiene (PB), 18 and poly(ethylene -cobutylene) (KL). 19 The most often used block of copolymer templates are a commercially available family of Pluronic triblock -copolymers with a general formula PE x PP y PE x and Brij diblock -copolymers with a formula of CH 3 (CH 2 ) x PE y OH.…”

“…This was demonstrated by introduction of two families of block copolymer templates: KLE (KL -PEO) 19 and PIB -b -PEO, 18 which substantially extend the applicability of EISA toward preparation of different classes of crystalline metal oxides with developed mesoporous architechture. Both families of polymers feature large hydrophobic/hydrophilic contrast because of a large size and the different polarities of hydrophilic and hydrophobic blocks.…”