High refractive index polymers are being considered good alternatives to inorganic glass owing to their impact resistance, ease of handling, light weight and low cost. These polymers find an array of applications in optoelectronics such as optical lens, cameras, sensors, and light emitting diode sealer. Unfortunately, common optical polymers, for example, polycarbonate, polystyrene, and poly(methyl methacrylate) have a low refractive index limiting their application in the field of optoelectronics. Therefore, developing polymeric materials with high refractive index is invaluable.
Structures with high molar refraction have been identified to contribute to the refractive index of the final polymers. For this reason, introducing structures with high molar refraction, such as halogens, Sulphur atoms, and aromatic ring can be helpful in improving the refractive index of these polymers. Selenium, which is located in the same group in the periodic table as Sulphur, exhibits physical and chemical properties similar to Sulphur. Selenium molar refraction is however higher than that of Sulphur.
Researchers led by Professor Jian Zhu at Soochow University in China investigated the possibility of synthesizing high refractive index polymers with selenium-containing structures. They implemented a post-modification approach. Poly(glycidyl methacrylate) was fabricated via mediated radical polymerization then modified by a reaction between selenium-functional agent and pendent epoxy group. Their work is published in Reactive and Functional Polymers.
The authors prepared Poly(glycidyl methacrylate) through Fe(0)-mediated living radical polymerization. A mixture of glycidyl methacrylate, Fe(0), and 2-Cyanoprop-2-yl-1-dithionaphthalate were added to an ampoule and the solution deoxygenated. After the desired polymerization time, the contents were diluted and filtered.
Pendent epoxy group and selenium functional agent were used to post-modify the obtained poly(glycidyl methacrylate).
Living radical polymerization proved to be an efficient method for the fabrication of poly(glycidyl methacrylate) with predetermined molecular weights as well as narrow molecular weight distribution. Polymerization at 25°C minimized the possibility of unwanted side reactions. Poly(glycidyl methacrylate) with narrow molecular weight distribution was synthesized. However, its molecular weight could be adjusted by altering the molar ratio of the monomer to 2-Cyanoprop-2-yl-1-dithionaphthalate.
While selenide structures exhibited high molar refractions, which was necessary for the synthesis of high refractive index polymers, little attention had to be paid to such an excellent material. Poly(glycidyl methacrylate) indicated a refractive index of 1.511. The refractive index significantly improved with the addition of the selenide functional groups. The refractive index improved gradually from 1.511 to about 1.719 after modification with the selenide functional groups.
A linear relationship was observed between the concentration of selenide and the refractive index. A higher amount of selenide led to a higher refractive index, which was because of the resulting high molar refraction of the selenide functional group. However, the Abbe’s number was more than 50 with low selenium content. This was necessary for feasible applications. When the selenium content was raised to 10% the Abbe’s number dropped sharply to 10-15.
Therefore, selenide post-modification offered an effective approach to control both the Abbe’s number and refractive index of the polymer by selectively introducing selenide functional groups. However, introduction of selenium did not affect light transmittance of the original poly(glycidyl methacrylate). With these unique attributes, this polymeric material would be an essential building block in optics.
Huinan Jiang, Xiangqiang Pan, Na Li, Zhengbiao Zhang, Jian Zhu, Xiulin Zhu. Selenide-containing high refractive index polymer material with adjustable refractive index and Abbe’s number. Reactive and Functional Polymers, volume 111 (2017), pages 1–6.Go To Reactive and Functional Polymers