Silica-based mesoporous hybrid materials

1.4 Silica-based mesoporous hybrid materials

Hybrid organic-inorganic materials can be defined as nanocomposites made of organic and inorganic components combined over length scales ranging from a few Ångstroms to a few tens of nanometres [59]. In case of silica-based mesoporous organic-inorganic hybrid materials, the positioning of organic and inorganic building blocks on a molecular level allows one to profit from the functional variation of organic chemistry, with the advantage of both thermally stable and robust inorganic compounds. There are different pathways reported in the literature to obtain silica-based mesoporous organic-inorganic hybrid materials on a molecular level: (1) The co-condensation of condensable inorganic silica species and silylated organic compounds; (2) The post-synthetic functionalization of the pore surface of a preformed, silica-based network (grafting), and (3) the usage of bissilylated single-source organosilica precursors such as organo-brigded trialkoxysilane precursors, leading to periodic mesoporous organosilicas, PMOs [28]. One advantage of the sol-gel processing is the possibility to combine inorganic and organic species on a very small level. The distribution and interplay between soft and hard matter can be controlled with a high accuracy by variation of the synthesis conditions.

(1) Co-condensation

The co-condensation of tetraalkoxysilanes, (RO)4Si, with terminal trialkoxy-organosilanes of the type (R´O)3SiR in the presence of structure-directing agents leads to materials with organic residues anchored covalently to the pore walls. Problems encountered with this method are the sometimes inhomogeneously distributed residues, the loss of mesoscopic order with increasing concentration of (R´O)3SiR in the synthesis and problems to remove the template without destroying the organic functionality.

(2) Surface modification through postsynthetic functionalization of silica (“grafting”) In this method, inner surfaces of preformed mesostructured silica gels are modified with organic components, primarily by reaction of organosilanes of the type (R´O)3SiR, chlorosilanes ClSiR3 or silazanes HN(SiR3)2 with the silanol groups on the pore surfaces.

Such modification influences the optical, electronic, separation, chemical or biochemical properties and leads to a wide range of applications [60]. Disadvantages are the possible blocking of the pores, e.g., if the organosilanes react preferentially at the pore opening.

(3) Periodic mesoporous organosilicas (PMOs)

The sol-gel synthesis of organic-inorganic hybrid materials by reaction of bridged organosilica precursors of the type (R´O)3Si-R-Si(OR´)3 has been known for a long time.

1.4 SILICA-BASED MESOPOROUS HYBRID MATERIALS

The combination with liquid crystal templating allows the fabrication of PMOs. Here, the organic moieties are an inherent part of the network. One advantage of this method is that the organic moiety is homogeneously distributed in the hybrid network. The covalently bonded organic spacer allows varying properties of the final gel framework, such as flexibility, hydrophobicity, tunable refractive index or catalytic activity. In the meantime the fabrication of PMO´s has been widely investigated [61-72] and PMOs with crystal-like pore walls have been reported [45, 73, 74]. The latter ones exhibit a long-range order of the organic bridges within the pore walls of the mesoporous material. An in-situ SAXS investigation with synchrotron radiation published by Morell et al [45] shows that mesostructure formation and organization of the molecular precursor species take place simultaneously. The employment of non-ionic triblock copolymers such as Pluronic P123 permits the synthesis of large-pore PMOs and was first published in 2001 [66].

Nanocomposites

Besides the combination of organic-inorganic building blocks on a molecular level several attempts have been reported to take advantage of sol-gel-derived materials in the fabrication of composites, e.g., as reinforcing particles [75]. The combination of soft and hard matter to form nanocomposites is a sophisticated way of tuning the mechanical properties of a material, as is done by many biological systems. Artificial nanocomposites consisting of silica and polymer can be derived by different approaches. The first involves mixing polymer with silica particles or silica precursors. In the second synthesis approach polymerizable monomers are mixed with silica particles followed by polymerization. The third method involves mixing both silica precursors and polymer monomers. Both species are simultaneously or consecutively polymerized. Since the interface of the organic and inorganic building blocks plays a crucial role in the final material properties, mesoporous silica materials with their high surface area provide interesting scaffolds for the fabrication of hybrid organic-inorganic nanocomposites [76-78]. Improvements of the material properties are found in thermal stability, tensile strength, modulus and toughness [77].

The use of polymerizable surfactants (surfmers) [79] in the LC-templating process represents a special case. In 1958, Freedman et al. [80] reported the first synthesis of a monomer which was also an emulsifying agent. Since this time a wide variety of surfmers have been studied [81-88]. The combination of monomer self-assembly and condensation of inorganic species was, e.g., employed by Gray et al. [89] They used an inverse hexagonal phase around a hydrophilic solution containing an inorganic precursor. They postulated three criteria for success: The amphiphile must (1) contain a readily polymerizable group, (2) be compatible with the inorganic precursor and form the desired LC phase over a definable range of compositions and (3) the polymerization of the

1.4 SILICA-BASED MESOPOROUS HYBRID MATERIALS

monomer must proceed with retention of overall phase architecture. The use of surfmers as structure directing agents in the templating process of periodic mesoporous materials leads to a nanocomposite material with a geometrically confined, permanent organic phase inside the mesopores or vice versa [89, 90].

Another possibility to combine inorganic and organic building blocks is the addition of polymerizable species to the synthesis mixture (precursor, surfactant, solvent + monomer) [91-94]. Sellinger et al [93] report the fabrication of nanocomposite films mimicking nacre by a simple dip-coating process (evaporation-induced self-assembly, EISA) of a silica/surfactant/monomer mixture. Subsequent polymerization of the monomeric species, induced by light or heat, completes the nanocomposite assembly process. A coupling agent such as an organofunctional silane, e.g., methacryloxy propyltrimethoxy silane, covalently binds the organic phase to the silica framework.