Determination of Isophthalic Acid Comonomer Content in Poly(Ethylene Terephthalate) (PET) Using Raman Spectroscopy

Poly(ethylene terephthalate) (PET) is one of the leading polymers in the packaging industry. It is often copolymerized with isophthalic acid (IPA) to tune its properties, yet its comonomer content is not always known. In this work, we develop simple Raman spectroscopy methods to quantify the IPA content of PET samples in the range most used for bottle production without any pretreatment. The calibration curves allow precise quantification of IPA content for amorphous samples (R 2 = 0.997), and good estimates for semicrystalline samples (R 2 = 0.952) and commercial products exhibiting common spectroscopic challenges. This work leverages the speed and accessibility of Raman spectroscopy for solid-state IPA quantification, making it a practical alternative to established techniques.