Silica-polymer hybrids manufactured by the sol-gel process are frequently nanocomposites, with phase sizes on the nanometre scale. These types of products are becoming increasingly well established.

A relatively new type of hybrid is based on the silsesquioxane cage structure, which can be regarded as one of the smallest forms of silica or even as ‘molecular silica’. This approach has been developed by Hybrid Plastics (USA) and is called polyhedral oligomeric silsesquioxane (POSS) nanotechnology. These products are now close to commercial exploitation. The particle diameter of the POSS molecules is in the range 0.7-30 Å, on average much smaller that the diameter of, for example, colloidal silica particles. Polymers containing the POSS molecules are therefore true nanocomposites. Typical structures of POSS molecules are represented by (figure 1). The physical form of the materials varies from liquids through waxes to crystalline solids. The cost of the POSS chemicals is projected to fall between that of fumed silica and the silicones. The POSS structures can be functionalised with a wide variety of groups, leading to a range of POSS monomers. The nature of the functional group determines compatibility with the polymer matrix.

Figure 1. Representation of a polyhedral oligomeric silsesquioxane or POSS.

The R group can be simple alkyl, cycloalkyl or aryl, or reactive/ polymerisable groups such as acrylic, α-olefin, styrene, epoxide, carboxylic acid, isocyanate, amine, alcohol and silane. Using such functionalisation, the POSS structures can be either copolymerised with a range of monomers, or grafted onto polymer chains. A variety of phase morphologies is possible, ranging from phase separated to phase inverted to single phase, truly molecular level reinforcement. The POSS compounds can be readily incorporated into polymers using standard processing methods. Interestingly, and in contrast to clay nanocomposites, incorporation of POSS can lead to a reduction in the polymer melt viscosity, which should represent a distinct advantage for many applications. A major effect of POSS incorporation, e.g. in polypropylene, is retention of modulus above the glass transition temperature (Tg) of the unreinforced polymer. Increasing the concentration of the POSS structures in copolymers can have a dramatic effect in increasing the Tg of the polymer. POSS nanocomposites occupy the performance region indicated as ‘Hybrids’ in Figure 2. Flammability behaviour of the polymer is also claimed to be improved by incorporating POSS.

Figure 2. Relative property performance of polymers, ceramics and hybrids.

POSS silanols can be used as coupling agents for fillers and offer a number of advantages over conventional silane coupling agents, including moisture stability, low volatility, no requirement for water addition and no formation of volatile by-products. Surface coverage by POSS cages is also very high. Use of POSS surface treatment can enhance the reinforcing effect of the filler.

POSS products are attracting interest in a wide range of materials markets, including aerospace, sporting goods, medical applications and electronics.