Monitoring the Chemical Composition of Polyolefins along the Molar Mass Axis with High-Temperature Size Exclusion Chromatography Coupled with an Infrared Detector (HT SEC-IR5)

Series of ethylene/1-alkene and ethylene-propylene-diene copolymers are separated with size exclusion chromatography (SEC) at 150°C using 1,2,4-trichlorobenzene as the mobile phase. Methyl (CH3–) and methylene (–CH2–) groups in the separated polymer samples are monitored at specific wavelengths via a filter-based infrared detector (IR5). It is found that the ratio of CH3/CH2 may increase, be constant, decrease or change irregularly along the molar mass axis, depending on the sample under investigation. Because the CH3/CH2 profiles may be very different even for a given series of copolymers synthesized with the same catalyst, it is supposed that one or more experimental parameters, which are not strictly controlled throughout synthesis, are responsible for these substantial differences in the chemical composition along the molar mass axis. The correlation between the ratio of CH3– to –CH2– groups and the average chemical composition of the investigated series of polymer samples as well as the reproducibility of the measurements and the limit of detection of the SEC-IR5 measurements are evaluated. Coupling of SEC with the advanced IR5-detector enables the convenient and reliable characterization of the chemical composition of many polyolefin materials along their molar mass axis. The monitoring of the chemical composition may help to improve quality of synthesized copolymers.