Preparation and properties of high refractive index polythioetherimide film based on thiol-ene click chemistry

Three kinds of biscitraconimide (BCI) monomers, 4,4′-(diaminodiphenylsulfone) biscitraconimide (DDS-BCI), 1,2-(ethylenediamine) biscitraconimide (EDA-BCI) and m-(xylylenediamine) biscitraconimide (MXDA-BCI), were respectively prepared from the reaction of citraconic anhydride with three different diamines by reflux dehydration in glacial acetic acid. As-prepared products further reacted with 4,4′-thiodibenzenethiol (TDBT) and 1,6-hexanedithiol (HDT) to synthesize a series of new transparent thermoplastic polythioetherimides (PTEI) films with excellent solubility, high refractive index and low birefringence. All PTEI films had the 5% weight loss temperature between 332 °C and 417 °C and the thermal expansion coefficient ranged from 39.4–94.8 ppm/°C, showing good thermal stability. Besides, the transmittance of PTEI film at 450 nm exceeded 85 %, the average refractive index (nav) measured at 589 nm reached up to 1.6236 ∼1.6635 and in-plane/out of-plane birefringence (Δn) values were between 0.0014 and 0.0036. Especially DDS-BCI/TDBT sample exhibited the highest refractive index, relatively small birefringence and excellent thermal stability among all PTEI films, which is expected to be an ideal advanced optical material.