Microstructure Characterization of Low Density Polyethylene by Temperature Rising Elution Fractionation

Two low density polyethylene resins (sample A and sample B) were fractionated through temperature rising elution fractionation (TREF), and GPC,C-13-NMR, DSC and SSA are exploited to study the chain structure of original resins and their fractions. The two resins with unimodal distribution have different properties. Compared with sample B, sample A possesses higher tensile break strength and film puncture resistance. By(13) C-NMR, the total branch contents of samples A and B are 3. 93 mol% and 4. 82 mol%, respectively. Both resins have short chain branch and long chain branch structures. It is also found that the branching content of the fractions decreases with the elution temperature, which is in accordance with TREF mechanism. By the TREF-GPC cross-fractionation results, the fractions of two resins focus on higher temperature region. The dominant fraction of sample A is eluted at 80 C, and its molecular weight is concentrated at 32 x 10(4). The dominant fraction of sample B is eluted at 75 CC, and its molecular weight is concentrated at 18 x 104. Sample A has a slightly higher molecular weight, more content on high molecular weight, broader molecular weight distribution. In DSC results, the melting temperatures of samples A and B are 110. 5 degrees C and 110. 2 degrees C, respectively. Both samples have broader melting range corresponding to 50 similar to 120 degrees C. In the SSA thermal fractionation, each fraction of the two samples shows a broad range endotherm with multiple melting peaks (more than eight peaks). The arithmetic mean methylene sequence length of fractions for the two samples gradually increases from 34 to 86 and from 32 to 84 with the increase in the elution temperature, respectively. In addition, the characteristics of chain microstructure and the relationship of structure and properties were discussed.