Role of Stably Entangled Chain Network Density in Shish-Kebab Formation in Polyethylene under an Intense Flow Field

The physically entangled chain networks, formed by a long-chain polymer component, have been demonstrated to be very effective for shish-kebab formation in a flow field. However, an open question still remains whether there is an optimal content of entangled chain networks. The answer is crucial in the academic and industrial fields but faces a challenge since physically entangled chain networks are not stable, which tend to disentangle during flow. In the current work, we utilized lightly cross-linked chain networks, which can be considered as stably entangled chain networks (SECN), to study the role of SECN density in shish-kebab formation under an intense flow field. The SECN density was adjusted by adding various contents of lightly cross-linked polyethylene (PEX) in short-chain polyethylene, and the intense flow field was applied by a modified injection molding machine, so-called oscillation shear injection molding (OSIM). The results show that the structure in the core layer of the OSIM samples could well clarify the role of SECN density. There is a threshold of SECN density for shish-kebab formation under the given flow field, below which (PEX content <30 wt %) only the isotropic lamellae were formed in the core layer. However, when the SECN density was high enough (PEX content >30 wt %), a large amount of shish-kebabs appeared throughout the whole sample. Unexpectedly, excessive SECN (PEX content >70 wt %) caused imperfect shish-kebab, possibly due to contact and even entanglement between neighboring SECNs. A major superiority of our experiments performed on the scalable processing equipment is that the mechanical performance of sample can be evaluated. Thanks to the significant promoting effect of SECNs and intense flow field on shish-kebab formation, the tensile strength of OSIM sample gained a remarkable enhancement from 29.9 MPa for conventional injection-molded neat PE to 69.7 MPa for OSIM sample with 70 wt % PEX. For OSIM sample with 100 wt % PEX, the tensile strength shows a slight decrease to 66.2 MPa owing to the imperfect shish-kebabs resulting from the excessive SECN. Our results demonstrate that SECN is favorable for flow-induced shish-kebab formation, however excessive SECN (i.e., highly dense entanglement) could hinder the formation of shish-kebab.