Sol-Gel

By Prof. Dr Clement Sanchez
Director of the Laboratory of Condensed Matter Chemistry (LCMC) 

Sol-Gel chemistry is based on the polymerization of molecular precursors such as metal alkoxides M(OR)n . Hydrolysis and condensation of these alkoxides lead to the formation of metal oxopolymers. The mild characteristics offered by the sol-gel process allows the introduction of organic molecules inside an inorganic network. Inorganic and organic components can then be mixed at the nanometric scale, in virtually any ratio leading to so-called hybrid organic-inorganic nanocomposites. These hybrids are extremely versatile in their composition, processing and optical and mechanical properties.

The progress in the field of hybrids materials largely depends on the core competencies of chemists and illustrate the central role of chemistry in the development of advanced materials with unprecedented performances. An incredible amount of research investigations have appeared the last 10 years in the field of hybrids materials indicating the growing interest of chemists, physicists and materials researchers to fully exploit this technical opportunity for creating materials and device with benefits of the best of the two worlds namely inorganic and organic. This land of research initially worked out by the sol-gel community is at present exploding with the appearance of a new class of mesoscopic hybrid structures engineered at the molecular scale to satisfy requirements for a variety of applications from biological sensing and catalysis to optical communications.

Classification of hybrids

Before exploring this new world of advanced materials it is justified in order to have a unified representations of what they represent to define and humbly try to classify them. We call hybrid material any organic-inorganic or bio-mineral system in which at least one of the components, organic or inorganic, is present with a size scaling from tenths to tens of nanometers. Components making up the hybrids could be molecules, oligomers or polymers, aggregates and even particles. Therefore they are considered as nanocomposites or even composites at the molecular scale.

Hybrids materials can be classified in many ways depending of the relative composition of the constituents components, the nature of chemical interactions between them or the chemicals bonds involved.
Chemical composition is one of the most important parameter since its variation leads to hybrid materials with distinctive physico-chemical behaviors and profoundly different properties.

Two kind of hybrids materials can be synthesized :

- Organics or inorganic doped systems

They are usually based on one major phase which contains a second one in relatively low amounts (generally less than 1%).

- Organic-inorganic systems or hybrids

Here the fraction of each component in the system is of the same order of magnitude. Systems were one of the components is present at levels higher than 10 % belongs to this category.

Chemical composition is not by itself a pertinent criterion for classification. We have proposed in the past a criterion which is now well adopted by the scientific community and relates to the type of interaction or the nature of chemical bonding between the organic and inorganic species. Following this criteria the different organic-inorganic hybrids can be classified in two broad families:

Class I: Includes hybrids systems where one of the component (organic, biologic or inorganic), which can be molecules, oligomers or polymers is entrapped within a network of the other component.
In that case we are in presence of weak-type interactions between the hosting network and the entrapped species. The systems of this kind are essentially based on Van der Walls, Hydrogen bonding or electrostatic interactions.

Class II: Gathers the hybrids materials where the inorganic and organic parts are chemically bonded by a covalent or iono-covalent bond. The frontier between both class is not always simple and we can eventually have hybrids systems with class I and class II characteristics. A typical example of such case are hybrids materials for optical applications made by encapsulation of organic chromophores within an hybrid matrix which belongs to class II. Although the dye interacts with the hybrid host via Van der Waals or Hydrogen bonding forces, the strong chemical bonds between organic and inorganic parts which make the host material has a significant impact on the overall properties of the system and therefore this kind of hybrids will be also classified as class II.

Hybrids synthesis strategies

Although they initially have been worked out by chemists from the sol-gel scientific community today hybrids are elaborated by researchers coming from a variety of disciplines, polymers chemists, solid state chemists, catalysis, materials researchers etc. Each of these communities elaborate hybrids using their own tools, specific disciplinary methods and more important their own raw materials. It is not seldom to see that a polymerist will work out an hybrid system having an emphasis on the polymer side of the hybrid, using even pre-formed polymers, capped oligomers etc. Sol-gel and inorganic chemists will preferably use as precursors, silicon or comprar cialis sublingual or metal alkoxides or even inorganic building units such as clusters or nanoparticles. They can also use lamellar inorganic compounds as host for organic components. Many names have been given to these materials: Ceramers, Polycerams, Ormosils or Ormocers. These name were labelled by materials science researchers coming from different sides (polymers scientists, glass or ceramic scientists, organometallic scientists). Nanomers from materials were the colloidal side of the hybrids chemistry is preponderant and so on.
However it is now commonly accepted that a molecular approach for the synthesis of hybrids reflects better the wide opportunities offered by this compounded chemistry.

Synthesis Approaches used by polymer chemists

Starting from oligomers or polymers, the polymer chemist objective is to improve or modulate mechanical, thermal or adhesion properties by the adjunction of minerals charges while still preserving a number of advantages due to the organic polymeric nature of the system (high flexibility, low density,…). Interface by its nature and extent in such systems has a primordial role in those properties. Therefore a number of precaution are usually taken for the mineral charges adjunction. Used charges could be independently pre formed and non aggregated or generated in situ by hydrolysis and polycondensation of metallic alkoxides precursors. The organic network linked with these charges through Van der Waals, electrostatic or hydrogen bonding interactions.

The presence of hydrogen bonding allows generally to obtain a relatively good homogeneity of the hybrid. Inorganic-organic hybrids of class I have been synthesized by generating silica nanoparticles upon hydrolysis and condensation of silicon tetra alkoxides Si(OR)4 (OR = EtO, MeO,…) in presence of polyoxazolines in ethanol.
A good mutual dispersion of the two phases is ensured by the presence of hydrogen bonds between silanols groups (Si-OH) of the silica network and the carbonyl and amides functions present in the polymer. This homogeneity can be further improved by functionalizing the organic polymer by Si(OR)3 groups which after hydrolysis increase the chemical affinity between organic and inorganic components through covalent or partly covalent bonds. The resulting hybrids belongs to class II

Synthesis Approaches used by the inorganic and sol-gel chemists

- Intercalation and/or grafting of organic or polymeric molecules within a mineral lamelar network: (clays, phosphates, phosphonates, oxydes,..).

The lamelar compounds are used as network host preserving the bi dimensional pre-established order. Organic molecules are then inserted or grafted into this pre-structured mineral network. Recently this community has started experimenting synthetic routes to hybrids directly from molecular precursors in solution using especially metallic phosphonates (B. Bujoli Nantes, A. Clearfield (USA) , G. Alberti (Italie), A. Vioux Montpellier, etc…).

- Synthesis by electrocristallisation of hybrid molecular assemblies ;

This synthesis method allows the elaboration of well organized mineral networks with long range order favorable to control and adjust electronic intermolecular transfer.  (P. Batail Nantes)

- Impregnation of preformed inorganic gels:

A typical example of that is impregnation of a silica xerogel formed by hydrolysis and polycondensation of silicon alkoxides with organic monomers susceptible to be polymerized within the porous gel structure. Methylmethacrylate (MMA) is the most usual case and the inorganic-organic hybrid obtained after polymerization of the MMA has optical and mechanical properties better than the individual components.

- Synthesis from heterofunctional metalic alkoxides or silsesquioxannes:

Precursors of this kind have the formula RxM(OR')n-x or 3(R'O)Si-R-Si(OR')3. The hydrolysis of alkoxy groups (OR') followed by a condensation reaction will form the mineral network and the R groups will imprint in the network the organic function.

- Synthesis of hybrid through the connection of well defined functionnal nanobuilding Blocks.

The pre-formatted species or building blocks could be in this case oxo-metalic clusters, nanoparticles (CdS, CdSe,…), metalic or oxides colloids, organic molecules or oligomers. These blocks are functionalized during or after their synthesis with complementary species for tailoring the interface between organic and inorganic domains.
A review of this strategy has been presented in Comments in Inorganic Chemistry 20(4-6), 327-371 (1999).

- Templated growth of inorganic or hybrid networks by using organic molecules and macromolecules as structure directing agents.

Some molecules like amines, alkyl ammonium ions, amphiphilic molecules or surfactants can be used as templates to build a structured mineral network. Materials of the zeolites families are among the most intensively investigated systems. Although the fundamental understanding of mineral networks formation in presence of organic templates is by itself of a great importance, the aim of this research is to synthesize new micro or mesoporous hybrid or inorganic materials with tailored porosity in both size, shape and function for applications in selective separation, sensors, catalysis and low dielectric constant materials.

Nowdays, the basic understanding of the role of molecular and supramolecular interactions between template molecules (surfactants, amphiphilic block copolymers, organogelators, etc…) and the growing hybrid or metal-oxo based network is allowing the construction of complex hybrid hierarchical architectures. These strategies, try, in some naive way to mimic the growth processes occurring in biomineralization.

Groups includes : G. Stucky, UC SantaBarbara, A. Stein (USA), G. Ozin (Canada) J. Brinker ( USA), S. Mann Bristol, M. Antonietti and  F. Schueth (Max Planck , Germany), T. Bein (University of Munich, Germany), C. Sanchez LCMC Paris)

The combination at the nanosize level of inorganic and organic or even bio-active components in a single material makes accessible an immense new area of materials science that has extraordinary implications for developing novel multi-functional materials exhibiting a wide range of properties. This fascinating new field of research brings together scientists working in many different domains. Among soft chemistry processes, sol-gel chemistry offers versatile access to the chemical design of new hybrids organic-inorganic materials. Many new combinations between inorganic and organic or even biological components will probably appear in the future. 

All these synthetic routes make hybrids extremely versatile and open the window for the creation of new materials with unexpected properties

Editorial links to hybrids

 

Properties comparison between glass and polymers: The window for hybrids

Optical waveguides

Refractive index modulation

More links about hybrids..

 

 

 

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