Analysis of comonomer content and cyclic oligomers of poly(ethylene terephthalate)

The concentrations of diethylene glycol (DEG) and isophthalate (IPA) units present in two commercial polyesters were measured using Fourier transform infrared (FTIR) spectroscopy and by a lowering of the melting point as measured by differential scanning calorimetric (DSC) method. To carry out the FTIR spectroscopic analysis, it was necessary to synthesise poly(diethylene glycol terephthalate) and poly(ethylene isophthalate). With FTIR spectroscopy, it was possible to measure with reasonable accuracy the DEG content of the two commercial polyesters, whereas by DSC, the presence of IPA in one material affected the results. Cyclic oligomers of the two commercial polyesters were extracted using chloroform and analysed by preparative high performance liquid chromatography and electrospray mass spectrometry. It was found that polymer containing more DEG units promoted the formation of oligomers less than trimer in size, whilst the polymer containing more IPA units promoted the formation of oligomers greater than trimer in size.