Design and synthesis of gradient-refractive index isosorbide-based polycarbonates for optical uses

The synthesis of bio-based polymers using renewable bio-monomers have received extensive research attention to meet the concept of environmental sustainability. Isosorbide (ISB) derived from biomass is commonly used in the polycarbonate industry as an alternative to bisphenol A (BPA) because it is green, non-toxic, and more widely available. Compared to BPA-based polycarbonate, isosorbide-based polycarbonate has excellent properties such as high transparency, easy coloration, and outstanding rigidity. However, the naturally low refractive index of isosorbide triggers the latter to fall outside the threshold of optical applications. In order to further improve the refractive index of isosorbide-based polycarbonate without affecting its transparency and rigidity, a copolymerization scheme of bisphenol monomers (BPs) with isosorbide is proposed in this work. A series of isosorbide–bisphenol copolycarbonates were synthesized by a melt polycondensation process using ionic liquid 1,4-(1,4-diazabicyclo[2.2.2]octane)butyl dibromide ([C4(DABCO)2][Br]2) as the catalyst. As expected, the monomer structure had significant effect on the optical property and molecular weight of the copolycarbonates, the monomer bis(p-hydroxyphenyl) ether (BPO) had better reactivity among the screened seven monomers resulting in relatively higher molecular weight. Moreover, poly(BPO-co-ISB carbonate) (POIC) using BPO as the monomer exhibited a high Abbe number (vd = 39.7), low yellowness index (YI = 0.93), and a higher refractive index (nd = 1.536), which was much higher than poly(isosorbide carbonate) (1.496). Therefore, a series of POICs were further prepared to investigate the effect of BPO content on the optical, thermal, mechanical, and hydrophobic properties of the materials. The results showed that the copolymers POICs had higher refractive indices (1.511–1.573), better thermal stability, flexibility, hydrophobicity and processing-friendly glass transition temperature (Tg) and pencil hardness, and thus are expected to be sustainable materials for optical lenses.