High-performance transparent polyesters: synergistic enhancement of optical and thermal properties via fluorene incorporation into PET backbones

Polyethylene terephthalate (PET) finds extensive application in various aspects of human life. To address the limitations of PET's low refractive index (n) and high birefringence (Δn), while enhancing its heat resistance and toughness, a one-pot two-step method incorporates 1,4-cyclohexanedimethanol (CHDM) and 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene (BPEF) into the PET main chain, resulting in the formation of a copolyesters containing EG,CHDM and BPEF (PEBCT). DMA and DSC data indicate that the rigid fluorene ring structure of BPEF reduces the mobility of the polymer chain, thereby enhancing the heat resistance of the PEBCT copolymer. The incorporation of 10 mol% BPEF raises the T_g of the PEBCT copolymer by 19.3%, increasing it from 80.8 to 96.4 °C. Importantly, the introduction of BPEF does not compromise the high transparency of PET. Furthermore, due to the high polarizability of its fluorene ring and unique three-dimensional vertical structure, BPEF simultaneously elevates the refractive index (n) of the copolyesters while reducing the birefringence (Δn). This characteristic facilitates the application of PET in optical materials.