The Sol-Gel Route to Nanocomposites

Nanocomposites are multiphase materials where one of the structural units, either organic or inorganic, is in a defined size range 1-100 nm. In order to prepare organic-inorganic nanocomposites either the particles or the polymers or both can be synthesized “in situ” or used in the final state. When dispersing the nanoparticles, the need of lowering the interface free energy would lead to their agglomeration. Alternatively interactions between inorganic particles and matrix polymer or between particles and an ultrathin layer of surrounding organic molecules must be set up. This is commonly indicated by saying that the inorganic nanoparticles must be compatibilized (Kickelbick 2007). When organic reactive groups are present at the surface of the nanoparticle allowing attachement of matrix polymer molecules, a firm polymer–particle interface can also be obtained. The proper nanoparticle surface engineering can give: 1. good dispersion of NP into polymer at high filler content 2. adjustment of rheology at high filler content 3. covalent bonds between filler and polymer and higher network density 4. Materials with improved mechanical properties (fatigue strength, toughness, scratch resistance...) and flame retardancy and coatings with barrier properties Recently numerous synthesis methods were explored and successfully used to produce organic-inorganic hybrid nanoparticles with controlled defined shapes (core-shell, multinuclear, hairy-like raspberry ...), nanoscale sizes, structure and composition. They have potential applications in a variety of domains, starting from their use as components of advanced both functional than structural nanocomposite materials, where, as reminded above, the engineering of the interface is critical to have good dispersion and for the tailoring of the final properties. The paper will show that the Sol-Gel method is an outstanding route to the synthesis of hybrid nanoparticles and nanocomposites. Generally speaking it gives inorganic high purity materials at mild synthesis conditions (temperature and pH). An outstanding variation is the so called Stober method allowing colloidal particles (with well-defined size and shape and with narrow size distribution) be produced through hydrolysis and polycondensation of silicon alcoxides in water /alcohol /ammonia mixtures. The sol gel method allows, also, easy functionalization of silica nanoparticles with specific organic groups. Many of the hybrid nanoparticles synthesis methods take advantage of all this. The sol gel method finds, however, applications in many other research fields. The aim of this paper is to give also, shortly, a contribution to the knowledge of the basic principles of the method, with