Hybrid material
[1] Mixing at the microscopic scale leads to a more homogeneous material that either show characteristics in between the two original phases or even new properties.
Class II hybrid materials are those that show strong chemical interactions between the components such as covalent bonds.
The latter type of system is known as network builder Blends are formed if no strong chemical interactions exist between the inorganic and organic building blocks.
One example for such a material is the combination of inorganic clusters or particles with organic polymers lacking a strong (e.g. covalent) interaction between the components.
The term nanocomposite is used if the combination of organic and inorganic structural units yield a material with composite properties.
That is to say that the original properties of the separate organic and inorganic components are still present in the composite and are unchanged by mixing these materials.
[4] Two different approaches can be used for the formation of hybrid materials: Either well-defined preformed building blocks are applied that react with each other to form the final hybrid material in which the precursors still at least partially keep their original integrity or one or both structural units are formed from the precursors that are transformed into a new (network) structure.
Representative examples of such well-defined building blocks are modified inorganic clusters or nanoparticles with attached reactive organic groups.
The building block approach has one large advantage compared with the in situ formation of the inorganic or organic entities: because at least one structural unit (the building block) is well-defined and usually does not undergo significant structural changes during the matrix formation, better structure–property predictions are possible.
Chemists can design these compounds on a molecular scale with highly sophisticated methods and the resulting systems are used for the formation of functional hybrid materials.
Many future applications, in particular in nanotechnology, focus on a bottom-up approach in which complex structures are hierarchically formed by these small building blocks.
In these cases well-defined discrete molecules are transformed to multidimensional structures, which often show totally different properties from the original precursors.
Generally simple, commercially available molecules are applied and the internal structure of the final material is determined by the composition of these precursors but also by the reaction conditions.
For example, low temperature derived inorganic materials are often amorphous or crystallinity is only observed on a very small length scale, i.e. the nanometer range.
Depending on these prerequisites the material can be pretreated, for example a pure inorganic surface can be treated with surfactants or silane coupling agents to make it compatible with the organic monomers, or functional monomers can be added that react with the surface of the inorganic material.
One problem that also arises from the simultaneous formation of both networks is the sensitivity of many organic polymerization processes for sol–gel conditions or the composition of the materials formed.
