Measuring gel content of aromatic polyesters using FTIR spectrophotometry and DSC

Aromatic polyesters, such as poly(butylene adipate-co-terephthalate) or PBAT, are susceptible to photodegradation, which causes crosslinking in their molecular structure and results in a final reduction of their mechanical properties. Gel content is one of the methods to quantify the degree of crosslinking, but the current ASTM method has some serious drawbacks, including a time-consuming procedure and the use of dangerous solvent. Therefore, the objective of this work was to use and to compare two new methods in measuring the gel content of aromatic polyester films using DSC and FTIR spectrophotometry. PBAT samples were exposed to UV radiation and their gel contents were measured according to ASTM D 2765 and ISO 10147. Two new models to measuring and predicting gel contents were developed and established; (1) an empirical model using changes in absorbance intensities at wave number of 796 cm−1 corresponding to 1, 2, 4 trisubstitute benzene and (2) a DSC Avrami equation using melting peak onset time differences between non-crosslinked and crosslinked samples. Both models accurately predicted high gel contents, but the DSC Avrami model seems to better predict gel content in the whole range since the empirical FTIR values were likely to mispredict at low gel contents.