High-refractive-index polymer

A high-refractive-index polymer (HRIP) is a polymer that has a refractive index greater than 1.50. A high-refractive-index polymer (HRIP) is a polymer that has a refractive index greater than 1.50. Such materials are required for anti-reflective coating and photonic devices such as light emitting diodes (LEDs) and image sensors. The refractive index of a polymer is based on several factors which include polarizability, chain flexibility, molecular geometry and the polymer backbone orientation. As of 2004, the highest refractive index for a polymer was 1.76. Substituents with high molar fractions or high-n nanoparticles in a polymer matrix have been introduced to increase the refractive index in polymers. A typical polymer has a refractive index of 1.30–1.70, but a higher refractive index is often required for specific applications. The refractive index is related to the molar refractivity, structure and weight of the monomer. In general, high molar refractivity and low molar volumes increase the refractive index of the polymer. Optical dispersion is an important property of an HRIP. It is characterized by the Abbe number. A high refractive index material will generally have a small Abbe number, or a high optical dispersion. A low birefringence has been required along with a high refractive index for many applications. It can be achieved by using different functional groups in the initial monomer to make the HRIP. Aromatic monomers both increase refractive index and decrease the optical anisotropy and thus the birefringence. A high clarity (optical transparency) is also desired in a high refractive index polymer. The clarity is dependent on the refractive indexes of the polymer and of the initial monomer. When looking at thermal stability, the typical variables measured include glass transition, initial decomposition temperature, degradation temperature and the melting temperature range. The thermal stability can be measured by thermogravimetric analysis and differential scanning calorimetry. Polyesters are considered thermally stable with a degradation temperature of 410 °C. The decomposition temperature changes depending on the substituent that is attached to the monomer used in the polymerization of the high refractive index polymer. Thus, longer alkyl substituents results in lower thermal stability. Most applications favor polymers which are soluble in as many solvents as possible. Highly refractive polyesters and polyimides are soluble in common organic solvents such as dichloromethane, methanol, hexanes, acetone and toluene. The synthesis route depends on the HRIP type. The Michael polyaddition is used for a polyimide because it can be carried out at room temperature and can used for step-growth polymerization. This synthesis was first succeeded with polyimidothiethers, resulting in optically transparent polymers with high refractive index. Polycondensation reactions are also common to make high refractive index polymers, such as polyesters and polyphosphonates.