Synthesis and Performance Characterization of Optical Copolyesters Based on Isomannite and Isophthalic Acid

Poly(ethylene terephthalate-co-isomannide terephthalate-co-ethylene isophthalate-co-isomannide isophthalate) (PEⅡT) was synthesized. The crystallization, optical and thermal performances of PEⅡT were investigated by using differential scanning calorimetry (DSC), ultraviolet-visible-near-infrared spectroscopy (UV-Vis-NIR), X-ray diffraction (XRD) and polarized light microscopy (POM). The results demonstrate that the crystallinity and crystallization rate of PEⅡT can be reduced effectively with the increase of the content of isophthalic acid units in copolyester PEⅡT. The increase of the content of isophthalic acid units made the steric hindrance of PEⅡT molecular chains increase, resulting in the decrease of crystallization ability, which is mainly because its cooling crystallization temperature gradually rise and the melt crystallization temperature gradually decrease until no generation of thermal crystallization phenomenon. Due to the decrease of the crystalline part, the haze of PEⅡT can be completely reduced to zero, and the light transmittance can reach 90.7%. At the same time, it was found that the decrease of PEⅡT light transmittance stems from the increase of yellowing degree, which makes the blue violet light of 330~550 nm wavelength absorbed by PEⅡT, leading to the low light transmittance. The melting point of PEⅡT is greatly affected by the content of isophthalic acid, but it still maintains good thermal decomposition performance.