Short Chain Branches Distribution Characterization of Ethylene/1‐Hexene Copolymers by Using TREF +  13 C‐NMR and TREF + SC Methods

• Published in: Macromolecular symposia. 2012-02, Vol.312 (1), p.63-71
• Main Authors: Zhang, Shiliang, Zhao, Ning, Wu, Yuxiang, Dong, Qi, Wang, Quntao, Tang, Yan, Yu, Yongling, Da, Jianwen, He, Xuelian, Cheng, Ruihua, Liu, Boping
• Format: Article
• Language: English
• ISSN: 1022-1360; 1521-3900
• DOI: 10.1002/masy.201100018

Short chain branches distribution (SCBD) is the key factor for high density polyethylene (HDPE) pipe materials to achieve their excellent performance for long term (50 years) applications. However, the precise SCBD characterization of these HDPE materials with relatively low content of comonomer incorporation still remained as a challenge in this field. In this work, two characterization methods, namely temperature rising elution fractionation (TREF) cross step crystallization (SC) (TREF + SC) and TREF cross 13 C‐NMR (TREF +  13 C‐NMR), have been respectively used to qualitatively and quantitatively investigate the SCBD for two HDPE pipe materials (PE‐1 and PE‐2 with different long term performances) with small amount of 1‐hexene incorporation prepared from SiO 2 ‐supported silyl chromate catalyst system (S‐2 catalyst) during UNIPOL gas phase polymerization. The comparison of SCBD between the two samples showed that: although short chain branches of PE‐2 with good performance were less than those of PE‐1 with bad performance, PE‐2 showed less comonomer incorporation on the low crystallinity and low molecular weight (MW) fractions keeping even higher comonomer incorporation on the high crystallinity and high MW parts compared with PE‐1. This difference on the SCBD for PE‐1 and PE‐2 was thought to be the key factor which is responsible for their great difference on environment slow crack resistance (ESCR). Moreover, TREF + SC method further reflected the intra‐ and inter‐molecular heterogeneities of each fraction from the two HDPE samples through the lamella thickness distribution compared with TREF +  13 C‐NMR.