Three properties of polymers are directly related to their refractive index: focusing power, light reflection and dispersive effect due to the diversity of the wavelengths of the light.
The higher the refractive index, the more the material bends the light, increasing the focusing power. Consequently, high refractive index plastics are extremely useful for optical applications where focusing is required. For a given optical design, less polymer weight is needed, increasing the integration density, saving raw material cost and environmental concerns.
Plastics are becoming increasingly attractive for a variety of optical applications such as, for example, lenses, optical circuits, optical fibres, anti-reflective films and coatings, optical adhesives, LCD displays, waveguides, UV-reactive inks and varnishes...
Presently the most popularly known optical plastics include polymers such as polymethacrylates, polyurethanes, polycarbonates, polystyrene and urethane-acrylates.
Intrinsic high refractive index polymers combine chemical groups and structures such as aromatic, heterocyclic and other rings, aromatic etherketones, sulphur-containing groups, halogen derivatives except fluorine, phosphorus-containing groups (phosphonates and phosphazenes), some organometallic species. Though the refractive indices are often limited below 1.80, it is interesting because of the large possibilities of action on formulae and thus a broad range of the other characteristics can be obtained. Moreover heat or radiation (UV, EB, laser...) treatments such as annealing, crosslinking and high polymerization contribute to a higher refractive index.
Extrinsic high refractive index polymers incorporate high refractive index nanoparticles into conventional polymers or intrinsic high refractive index polymers. It is a promising strategy to achieve refractive indices higher than 1.80. For example, gelatine can reach a 3.2 RI by addition of Si-nanoparticles. However, the obtained organic-inorganic hybrid materials sometimes suffer from poor storage stability, higher optical loss and poor processability.
Some producers or compounders market ready-to-use high refractive index polymers aiming several industrial sectors such as lenses, optical fibres, anti-reflective films and coatings, optical adhesives, UV-reactive inks and varnishes...