Studies on the Application of Filter-Based IR Detector for Polyolefin Characterization with HT-SEC

Even though polyolefins are simple polymers from a chemical structure point of view, their full characterization in practice is still an intriguing task. Basic macromolecular characteristics of polyethylene or polypropylene like molar mass moments and their distributions (MMD) but also chain conformation and thus information on long-chain branching, can be addressed with high-temperature size-exclusion chromatography (HT-SEC). Hyphenation of infrared detection to size-exclusion chromatography expands possibilities of SEC even more and allows to reveal comonomer incorporation across molecular weight and thus generate a fingerprint of a given catalytic system used in polyolefin synthesis. Multiband filter-based infrared detector gives an easy and fast access to so-called short chain branching distribution (SCB) vs MMD by coupling to HT-SEC. In this work, we summarize recent findings on application of a filter-based IR detector (IR5-MCT) towards characterization of polyolefins synthesized with different catalytic systems and varying comonomer types. It is found that for copolymers of polyethylene with 1-butene, 1-hexene or 1-octene (non-C3), one linear calibration line can be used in the range up to 70 CH3/1000TC, thus covering the range necessary for common applications like for instance HDPE or LLDPE. For ethylene-propylene copolymers (C3), over the broad range up to 333 CH3/1000TC, the calibration line is best fitted to a second order polynomial. C3 copolymers show a different behavior compared to non-C3 copolymers, irrespective the amount of comonomer incorporated in the polymer. We show that mixtures of PP and PE homopolymers result in equivalent response as the copolymers of the same average composition and thus can also be used to set up calibration lines. Based on this evaluation practical aspects of IR5-MCT calibration are discussed.