o-Chao Xia

The coupling relation between chain architectures and secondary flow field determined by an unusual dependence of shish-kebabs on molecular weight of high-density polyethylene

Selecting high molecular weight has become a quite popular approach for effective tuning of more shish-kebabs in semi-crystalline polymers. However, here, an unusual dependence of shish-kebabs on molecular weight of high-density polyethylene (HDPE) is found under the injection molding with a secondary flow. Characterization with electron microscope and X-ray scattering of the crystal structures reveals that the richest shish-kebabs develop in the HDPE with medium high molecular weight (HMW) chains rather than in the HDPE with more HMW chains. Both macroscopic scales (fluid behaviors related to the intensity of flow) and molecular scales (rheological properties related to orientation and relaxation) need to be overall considered and a physical model has been proposed to explain how the coupling between the chain architectures and the secondary flow field contributes to the unusual phenomenon. The factors, such as molecular parameters, interfaces between dispersed phase, and matrix as well as the properties of filler, profoundly influence both the two scales, which can be employed to tune the morphology related to physical properties. These significant results provide a simple but effective morphology control technique under a secondary flow field.

An unusual transition from point-like to fibrillar crystals in injection-molded polyethylene articles induced by lightly cross-linking and melt penetration

Recently, a melt penetrating process in which a first polymer melt is rapidly penetrated by a second polymer melt has been realized on our home-made multi-melt multi-injection molding (MMMIM) machine. Although great deformation can be provided by the rapid melt penetration process, it has been found that hardly any oriented crystalline structures can be kept and formed due to the quick chain relaxation at high temperatures. In the present work, lightly cross-linked structures were introduced to prolong the relaxation time of linear high density polyethylene (HDPE) molecular chains. The hierarchical structures of MMMIM samples were characterized by scanning electron microscopy (SEM), polarized light microscopy (PLM) and two-dimensional small angle X-ray scattering (2D-SAXS). It was found that the melt penetrating process promoted the formation of cylindritic crystalline structures in the subskin layer, whereas only isotropic spherulites were formed in the subskin layer of the corresponding conventional injection molding (CIM) sample. From linear to lightly cross-linked macromolecular chain structures, a transition from cylindritic structures composed of banded-spherulites along the flow direction towards shish–kebab-like structures was observed in the subskin layer of the MMMIM samples, and also the distances between two nuclei decreased as well as the orientation degree increased gradually in the transition layer. These results indicate that lightly cross-linked HDPE structures with longer relaxation times are beneficial to keep the point-like nuclei along the flow direction and are helpful for the transition to shish–kebab-like structures with thread-like nuclei. Modified models are proposed to interpret the mechanism of the formation of shish–kebab-like structures under the melt penetrating of samples with lightly cross-linked structures.

Hierarchical crystalline morphologies induced by a distinctly different melt penetrating process

Recently, a melt penetrating process which allows one kind of polymer melt to penetrate another polymer melt has been achieved on our home-made multi-melt multi-injection molding (MMMIM) instrument. It is the first time that hierarchically crystalline morphologies induced by melt penetration of the melt with different interactions are reported. In this work, high density polyethylene (HDPE) melt was penetrated by a distinctly different polypropylene (PP) melt and also by the same HDPE melt for comparison. The crystalline morphologies were observed using SEM and PLM, and the lamellar structures were characterized using a synchrotron 2D-SAXS/WAXD. The results showed that β-form transcrystallization occurred along the interface of the PP penetrating sample, and only α-iPP spherulites were observed in the core layer. Interestingly, shish–kebabs with flat lamellae were found in the subskin layer of the PP penetrating sample, while spherulites and cylindrites consisting of a banded-structure were observed in the sample at the penetration of the HDPE melt.