Separation of amorphous and semicrystalline parts of LLDPE with high-temperature interactive liquid chromatography

Linear low-density polyethylene (LLDPE) materials may contain components, which are soluble at room temperature. These may influence physical properties or raise concerns for end-users and regulatory bodies because LLDPE is widely used for food packaging. Due to the nonpolar and amorphous nature of the room temperature soluble part of LLDPE, its molecular characterization with common methods for polyolefin analysis, such as Temperature Rising Elution Fractionation (TREF), Crystallization Fractionation (CRYSTAF), or Differential Scanning Calorimetry (DSC), has proven to be very challenging. Separation of the amorphous part of LLDPE, as well as its semicrystalline part, is, however, realizable with high-temperature liquid chromatography (HT-LC), which is demonstrated for the first time in this work. This technique enables to evaluate the chemical composition distribution of the room-temperature solubles and non-solubles in LLDPE samples. It was found that LLDPE synthesized with metallocene catalysts contained only a very small amount of amorphous material. In contrast, LLDPE prepared with Ziegler-Natta catalysts contained up to 14 wt.% of the amorphous copolymer. The latter was soluble at room temperature, had a weight-average molar mass ranging from 60 to 200 kg/mol, and contained about 10 mol% more 1-olefin than the non-soluble part of LLDPE.