Formation of Inter linked Shish-Kebabs in Injection-Molded Polyethylene under the Coexistence of Lightly Cross-Linked Chain Network and Oscillation Shear Flow

It has been well established that the entangled molecular network facilitates the formation of shish-kebabs under flow field, however, the entangled network, usually formed by long chains, tends to disentangle due to molecular relaxation. In the present work, a small amount of lightly crosslinked polyethylene (LCPE), which can be considered as stable molecular chain networks, was added to short-chain polyethylene and then injection-molded using a modified injection molding technology-oscillation shear injection molding (OSIM), which can exert a successive shear field on the melt in the mold cavity during packing stage. The hierarchic structure of the OSIM samples was characterized through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). It was found that the oscillation flow field promoted the formation of interlinked shish-kebabs in the intermediate layer of OSIM samples, while there are still typical spherulites in the core layer of OSLM neat polyethylene (PE). The interlinked shish-kebab structure led to remarkable mechanical enhancement from 27.6 and 810.2 MPa of conventional injection molding (CIM) samples to 42.7 and 1091.9 MPa of OSIM samples for tensile strength and modulus, respectively. More importantly, under the same flow condition, the samples containing LCPE networks (termed PEX) exhibit rich shish-kebab structure both in the intermediate and core layers. Moreover, the addition of LCPE also generated stronger interlinked shish-kebabs, in which kebabs and shishes are connected by covalent bonds, rather than topological entanglement points. This special structure leads to further reinforcement from 29.6 and 879.5 MPa of CIM PEX samples to 57.5 and 1311.7 MPa of OSIM PEX samples for tensile strength and modulus, respectively. The results demonstrated that the networks with stable entanglement points are more helpful to induce the formation of shishes under flow than those with topological entanglement points. Our results set up a new method to reinforce polymer parts by tailoring the structure and morphology.