Oriented "shish-kebab" structure can enable remarkable mechanical enhancement in polymers. Therefore, the formation mechanism and practical application of this structure have been extensively studied. However, the effect of shish-kebab content on mechanical properties is still uncertain. Knowledge of this effect is crucial in the academic and industrial fields but remains elusive because shish-kebab content is difficult to control. In this work, a self-developed multiflow vibrateinjection molding was used to produce samples with different shear layer thicknesses. The content of shish-kebab was represented by R, i.e., the thickness ratio of shear layer (composed by shish-kebab) to the whole sample. Results showed that with increased R impact/tensile strength exponentially increased, whereas elongation at break exponentially decreased. Based on the results, a modified model was proposed to interpret the strengthening and toughening mechanism. This study established a new method of predicting and controlling the mechanical properties of samples with shish-kebab and spherulite structures.

Abstract:The main goal of this research is to study the development of crystalline morphology and compare it to various mechanical properties of microfibrillar composites (MFCs) based on polypropylene (PP) and poly(ethylene terephthalate) (PET), by adding a functional compatibilizer and a non-functional rubber in two different steps in the processing sequence. The MFCs were prepared at a weight ratio of 80/20 PP/PET by twin screw extrusion followed by cold drawing and injection moulding. The non-functionalized polyolefin-based elastomer (POE) and the functional compatibilizer (i.e., POE grafted with maleic anhydride (POE-g-MA)) were added in a fixed weight percentage at two stages: during extrusion or during injection moulding. The morphology observations showed differences in crystalline structure, and the PP spherulite size was reduced in all MFCs due to the presence of PET fibrils. Their relationship with the mechanical performances of the composite was studied by tensile and impact tests. Adding the functional compatibilizer during extrusions showed better mechanical properties compared to MFCs. Overall, a clear relationship was identified between processing, structure and properties.

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In situ microfibrillation and multiflow vibrate injection molding (MFVIM) technologies were combined to control the phase morphology of blended polypropylene (PP) and poly(ethylene terephthalate) (PET), wherein PP is the majority phase. Four kinds of phase structures were formed using different processing methods. As the PET content changes, the best choice of phase structure also changes. When the PP matrix is unoriented, oriented microfibrillar PET can increase the mechanical properties at an appropriate PET content. However, if the PP matrix is an oriented structure (shish-kebab), only the use of unoriented spherical PET can significantly improve the impact strength. Besides this, the compatibilizer polyolefin grafted maleic anhydride (POE-g-MA) can cover the PET in either spherical or microfibrillar shape to form a core–shell structure, which tends to improve both the yield and impact strength. We focused on the influence of all composing aspects—fibrillation of the dispersed PET, PP matrix crystalline morphology, and compatibilized interface—on the mechanical properties of PP/PET blends as well as potential synergies between these components. Overall, we provided a theoretical basis for the mechanical recycling of immiscible blends.

To achieve the goal of preparing an in situ microfibril composite (MFC), multiple strong shear flows were imposed on the melt of high-density polyethylene (HDPE)/polystyrene (PS). During vibration injection molding (VIM), both phase morphology and crystalline structure show big differences from the common injection molding (CIM) samples. The PS phase would deform into ultrafine microfibrils and then absorb the HDPE matrix to form a shishkebab super crystalline structure. The morphology analysis shows that when PS content increases, the size and morphology would change correspondingly. When PS content reaches a certain level, it would impair the mechanical performance. This work also analyzes the relationship between blending ratio and crystalline structure and explains the difference. These results provide valuable insight into immiscible polymer systems under a shearing field and have future industrial prospects.

In this work, isotactic polypropylene and high density polyethylene blends with tailored crystalline structures were prepared through an accessible injection-molding method. Two hierarchic structures, i.e., shish-kebab structures and epitaxy structures, were both successfully obtained among the whole range of samples, which were carefully characterized through polarized light microscopy, scanning electron microscopy, differential scanning calorimetry and small-angle X-ray scattering. It was found that the special epitaxy crystalline structure showed better mechanical properties than the shish-kebab one. The inclined polyethylene lamellae among the oriented polypropylene matrix not only enhances the interfacial adhesion but also facilitates the transmission of external force within matrix. Consequently, the tensile strength of the sample with epitaxy structures is around 46.58 MPa and its impact strength reaches up to 63.33 kJ/m2. These results provide a new method to industrially manufacture samples with tailored crystalline structures, making it possible for preparing general polymer materials with advanced properties.

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Blends of isotactic polypropylene (iPP)/b nucleation agent (b-NA)/polyolefin elastomer (POE) were prepared by injection molding. The microstructure and mechanical properties of these blends before and after being annealed at various temperatures and times were studied. It was found that annealing simultaneously increased the tensile strength and impact strength. As known, the degree of orientation decreased from the skin layer to the core layer. The orientation of all layers decreased with the increase of annealing temperature and time. The results showed that annealing gave rise to chain rearrangement in both the crystalline and POE phases which, we suggest, played a crucial role in determining the mechanical properties of the blends.