Characterization of linear low‐density polyethylene: Cross‐fractionation according to copolymer composition and molecular weight

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Abstract
The structure of ethylene copolymers modified by α‐olefins has become an area of intense investigation since the successful commercialization of so‐called linear low‐density polyethylene (LLDPE) resins. The molecular structure of a series of typical commercial LLDPE copolymers was investigated and compared to LDPE and HDPE. The commercial LLDPE resins studied contained about 7% by weight of butene‐1. The resins were fractionated according to short‐chain branching content by a technique called temperature rising elution fractionation. Size exclusion chromatography, x‐ray diffraction, 13C nuclear magnetic resonance, intrinsic viscosity, and differential scanning calorimetry were used to fully characterize the whole polymers as well as fractions of a selected LLDPE resin. A broad set of data was assembled in this work to investigate the short‐chain branching, long‐chain branching, and the molecular‐weight distribution of these commercial resins. The melting behavior of the LLDPE resins was found to be strikingly different from that of LDPE and HDPE. The broad and multimodal melting envelope of the LLDPE resins was found to be due to a broad and multimodal short‐chain branching distribution. No significant long‐chain branching was found in the LLDPE resins. The short‐chain branching was found to decrease with the increase of molecular weight in a typical commercial LLDPE resin. The unique physical properties of these resins are certainly strongly controlled by the expression of the distinctive heterogeneous comonomer incorporation in the solid‐state morphological structure. The physical and mechanical properties of these materials should be ultimately understandable on the basis of the unique morphology which results from the extremely heterogeneous incorporation of modifying α‐olefin in these commercial LLDPE resins.