Rui-Yan Zhang

Hierarchically oriented crystalline structures of HDPE induced by strong second melt penetration

Recently, a melt-penetrating process in which the first melt suffered from only one direction penetrating action was achieved by our home-made multi-melt multi-injection molding (MMMIM). In this work, a high-density polyethylene (HDPE) melt was penetrated by a high-speed second HDPE melt via an MMMIM instrument. It was found that hierarchically oriented crystalline structures were generated in the melt-penetrating sample along the thickness, investigated by SEM, synchrotron 2D-WAXD and 2D-SAXS; however, only isotropic spherulites were formed in non-melt-penetrating samples. 2D-WAXD/2D-SAXS results demonstrated that in the melt-penetrating sample, the degree of orientation in the subskin layer was larger than that in other layers, and confirmed the existence of the shish–kebab structures.

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.

Formation of double skin-core orientated structure in injection-molded Polyethylene parts: Effects of ultra-high molecular weight Polyethylene and temperature field

In the current work, the effect of ultra-high molecular weight polyethylene (UHMWPE) and temperature field on the unique double skin-core orientated structure and mechanical properties of high-density polyethylene (HDPE) parts molded by multi-melt multi-injection molding (MMMIM) were investigated using a variety of characterization techniques including rheological experiments, scanning electron microscopy (SEM), synchrotron small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and tensile testing. The SEM results revealed that a distinct double skin-core orientated structure was formed in samples molded via MMMIM. That is, compact lamellar together with typical shish-kebab structures was formed from the skin to the sub-skin, and large area of oriented lamellar was formed again near the core layer due to the significantly improved relaxation time of the UHMWPE/HDPE blend and intensive shear flow resulted from the secondary melt penetration process. Additionally, with increase temperature of the second melt, the oriented lamellar near the core layer tended to develop into irregularly-arranged lamellar and the double skin-core orientated structure weakened gradually. These results were further authenticated by SAXS. Results of tensile testing indicated that with reduced temperature of second melt, samples with higher tensile strength and modulus were obtained.

Effect of Viscosity Ratio on the Crystalline Morphologies and Mechanical Property of Multi-Melt Multi-Injection Molded Parts

The effect of viscosity ratio (ηcore/ηskin) on the crystalline morphologies of the bars molded by multi-melt multi-injection molding (MMMIM) was investigated by fixing a kind of high density polyethylene (HDPE) with a high molecular weight as the skin material and tuning the viscosity ratio by the viscosities of different core melts. The hierarchical morphologies and mechanical properties were investigated by PLM, SEM, 2D-WAXS and tensile testing. It was found that higher viscosity ratio facilitated the interaction between the skin and core melts ascribed to greater chain entanglement, and induced a higher degree of molecular orientation accompanied by improved mechanical properties.

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.