Insight into the effect of pre-polymerization with propylene on the evolution of crystalline texture and mechanical response during high-density polyethylene production

This paper aims to evaluate the effect of propylene pre-polymerization on the crystalline structure and mechanical response of HDPE resin. It was deduced from the PLM and DSC outcomes that the pre-polymerization with isotactic homo-polypropylene resulted in the formation of an in-reactor blend of the HDPE/iPP, in which the concentration and molecular weight of the iPP as the minor phase were heterogeneous within the HDPE powder particles of various sizes as the major phase. On the other hand, the presence of the iPP pre-polymer in the HDPE matrix led to significant phase separation. The results highlighted that the ultra-high molecular weight chains in the iPP acted as the nuclei for the formation of the shish-like structure, resulting in enhanced oriented α-type spherulites during the melt stretching fields. Moreover, in the molding processes involving the elongational fields, the similarity of the crystallization temperature of HDPE resin and the ultra-high molecular weight of the iPP led to the replication of stretch-induced molecular orientation caused by the iPP to the HDPE matrix. The crystalline structure consisted of highly ordered lamellae in the flow direction, causing reinforcement of the mechanical properties in the flow direction and weakening the mechanical properties in the counter-flow direction. Nevertheless, in the compression molding process, the ultra-high molecular weight iPP only acted as the nucleation agent for polyethylene, which caused the fine crystalline texture. Besides, more crystalline boundaries, a low degree of bonding between the whole crystalline domains, and inadequate bonding between the iPP and HDPE crystalline domains diminished the mechanical properties. Furthermore, a comparison between ambient and low-temperature impact strength exhibited that the pre-polymerized sample had a higher low-temperature impact strength, which may be attributed to the resin's finer crystalline texture.