Structure–property relationships of ionic poly(ethylene terephthalate) (PET): effect of ion content and species

The functionalization of poly(ethylene terephthalate) (PET) can potentially expand the applications of PET, particularly in high-value-added fields. To address the limitations of the low ionic content of traditional anionic PET, our study employs sodium dimethyl isophthalate-5-sulfonate (SIPM) as the ionic monomer. The copolymer was synthesized by transesterification and polycondensation. Ethylene glycol (EG) or butanediol (BDO) was applied to react with SIPM to obtain different types of ionic SIPE or SIPB. The ionic monomers are introduced into the PET macromolecular chains by reacting dimethyl terephthalate (DMT) and EG with SIPE or SIPB; for the first time, the effect of ion types and contents on PET properties was systematically studied. The incorporation of ionic groups leads to a notable enhancement in the mechanical, thermal, and hydrophilic properties of PET ionomers. The introduction of SIPB effectively improves the elongation at break and impact strength of PET, while the incorporation of SIPE substantially enhances the tensile and flexural strength of PET. Furthermore, an increase in the ion content evidently improves the hydrophilicity of the PET ionomer, with maximum water absorption of 30% and lowest water contact angle of 67°. With the introduction of the ion group, the macromolecular chain's regularity was disrupted, and the crystallinity of the PET ionomers was reduced. These changes promise an increase in transparency. Consequently, PET exhibits great potential to be applied in packaging materials, the textile industry, and the optical field.