The present article reports the nonisothermal crystallization process and morphological evolution of oriented iPP melt with and without in situ poly(ethylene terephthalate) (PET) microfibrils. The bars of neat iPP and PET/iPP microfibrillar blend were fabricated by shear controlled orientation injection molding (SCORIM), which exhibit the oriented crystalline structure (shish-kebab), especially in the skin layer. The skin layer was annealed at just above its melting temperature (175 °C) for a relatively short duration (5 min) to preserve a certain level of oriented iPP molecules. It was found that the existence of ordered clusters (i.e. oriented iPP molecular aggregates) leads to the primary nucleation at higher onset crystallization temperature, and formation of the fibril-like crystalline morphology. However, the overall crystallization rate decreases as a result that the relatively high crystallization temperature restrains the secondary nucleation. With the existence of PET microfibrils, the heterogeneous nucleation distinctly occurs in the unoriented iPP melt and results in the increase of crystallization peak temperature and overall crystallization rate, for the first time, we observed that the onset crystallization temperature has been enhanced further with addition of PET microfibrils in the oriented iPP melt, indicating the synergistic effect of row nucleation and heterogeneous nucleation under quiescent condition. 相似文献
In situ microfibrillar reinforced blend (MRB) based on poly(ethylene terephthalate) (PET) and isotactic polypropylene (iPP) was elaborated by a slit die extrusion, hot stretching, and quenching process. The scanning electronic microscopic images show well-developed PET microfibers in the blends. The on-line small angle X-ray scattering (SAXS) test shows that PET microfibers have high nucleation for iPP crystallization. At the same time, after shear, neat iPP and microfibrillar blend both can faster crystallization rate. Three nucleation origins are proposed in microfibrillar reinforced blends under shear flow field: (a) the classical row nuclei model, (b) fiber nuclei and (c) nuclei induced by fiber assistant alignment. The polarized optical microscopic images indicate that, during the non-isothermal crystallization at a cooling rate of 10 °C/min from 200 °C to room temperature, the neat iPP forms common spherulites, while the diluted microfibrillar blend with 1 wt% of PET has a typical transcrystalline structure. 相似文献
The influence of different shear stresses induced by changing injection molding speeds on molecular chain orientation and lamellar branching of β-nucleated iPP/poly(ethylene terephthalate) (PET) microparts was investigated using two-dimensional wide-angle X-ray diffraction and 2D-small-angle X-ray scattering. Results indicated that the prevailing shear stress can promote the formation of parent–daughter α-crystal structure and twisted shish–kebab structure in subsequent microparts. The diffraction of (300) plane of β-crystals was also observed at varying injection speeds. Increasing injection speeds can significantly enhance the content of β-crystals from 24 to 41% for β-nucleated iPP microparts. Additionally, the content of β-crystals was further enhanced in β-nucleated iPP/PET microparts with in situ formation of PET microfibrils under intensive shearing conditions. The addition of both PET and β-nucleation agents coupling with high shearing conditions exerts a synergetic effect on the development of β-crystals. However, the orientation degree of crystal lattice decreased with increasing injection speeds for both β-nucleated iPP and iPP/PET microparts. 相似文献
In order to better understand the effect of shear stress on the crystal morphology and orientation of polyolefins, dynamic packing injection molding was used to prepare oriented pure polypropylene (PP) and its blends with linear low density polyethylene (LLDPE). The obtained samples were characterized via 2d-SAXS, 2d-WAXD and AFM. Macroscopically, shear induced morphology with surface skin, central core and oriented layer between the skin and the core was observed in the cross-section areas of the samples. For pure PP, a highly oriented structure was seen in the sheared layer but much less oriented structure exists in the core. The orientation in the skin lies in between. The shish-kebab structure, composed of stretched chains (shish) and layered crystalline lamellae (kebabs), was found in the sheared layer. Shish structure exists mainly in the skin layer and oriented spherulits dominates in the core. For PP/LLDPE (50/50) blends, a change of phase morphology from less-phase-separated structure (homogeneous) in the skin, to co-continuous structure in the sheared layer and sea-island structure in the core was observed. PP formed a shish-kebab structure in all the three layers. And on the other hand, a very unique crystal morphology and lamellar orientation of LLDPE were obtained, with the lamellar stack oriented either perpendicularly or 45-50° away from the shear flow direction. 相似文献
Microfibrillar blends were prepared from polypropylene and poly (ethylene terephthalate) by extrusion followed by cold drawing.
The draw ratio employed had a prominent effect on the aspect ratio of the microfibrils produced, as revealed by scanning electron
microscopy. The subsequent isotropization step between the Tm of the polymers created microfibrillar composites with randomly oriented short microfibrils of poly (ethylene terephthalate).
The X ray diffraction patterns of the microfibrillar blends were different from those of corresponding composites although
the polypropylene phase in both exhibited predominantly the presence of α crystallites. The crystallization of the polypropylene
phase was affected by the orientation and diameter of the poly (ethylene terephthalate) microfibrils. The short microfibrils
in the microfibrillar composites were not effectual in hastening the crystallization of polypropylene. The thermal decomposition
studies revealed the capability of microfibrillar blends to delay the degradation better than the microfibrillar composites. 相似文献
In this work, normal blends, microfibrillar blends and composites were prepared from low density polyethylene (LDPE) and poly(ethylene terephthalate) (PET) in 85/15 and 75/25 w/w% ratio in the presence and absence of a compatibilizer polyethylene grafted with maleic anhydride (PE-g-MA). The microfibrillar composites (MFCs) were prepared using extrusion – drawing – isotropization technique. The morphology development of the microfibrillar blends and composites was studied using scanning electron microscopy (SEM). The presence of 5 wt% PE-g-MA compatibilizer affected the continuity of the fibrils differently in 75/25 and 85/15 w/w% microfibrillar blends. In the case of normal blends the addition of compatibiliser reduced the size of the dispersed PET phase. The presence of PET microfibrils improved the tensile properties of the microfibrillar composites. The normal blends exhibited a relatively ductile failure during tensile loading in comparison with the microfibrillar composites. The microfibrillar nature of the dispersed phase was found to improve the stiffness of the composite rather than their impact strength. 相似文献
We prepared uniaxially oriented thin films of poly(butylene terephthalate) (PBT) by applying shear strain to the melt and studied their resulting morphology by transmission electron microscopy (TEM), and could show visually that stacked-lamellar structures are formed in aromatic polyesters. On the basis of crystallographic consideration, we assigned each of the recognized stacked-lamellar structures to a shish-kebab structure or a part of it. In addition, we successfully demonstrated that in one shish-kebab structure all or almost all kebabs (namely, lamellae) have a same crystallographic orientation. 相似文献
In this study, isotactic polypropylene (iPP) samples were prepared by conventional injection molding (CIM) and pressure vibration injection molding (PVIM), in which a periodical shear field was imposed on the iPP melt during the cooling solidification. The distribution of supermolecular structures of samples was investigated by Polarized Light Microscopy (PLM) and Scanning Electron Microscopy (SEM). Results show that the through-the thickness-morphology of sample prepared by CIM features a typical skin-core structure, as a result of general shear-induced crystallization. This structure can be divided into three layers, including a skin layer in which the shish-kebab structure was found, a transition region with deformed spherulite structure and a core layer with spherulitic structure. However, the morphology of the sample prepared by PVIM, as a result of periodical shear-induced crystallization, features a richer and fascinating supermolecular structure and can not be roughly divided into three layers. A region full of shish-kebab-like cylindrulite structures was found between the transition region and the core layer, which is rare to be seen in conventional injection molding. Based on their various core structures, two kinds of shish-kebab-like cylindrulites were defined: one is multi-fibril-core cylindrulite of which core is an assembly of multiple fibrils, and the other is single-fibril-core cylindrulite of which the core just contains a single fibril. Based on the investigated results, a schematic illustration is proposed to depict the through-the thickness-distribution of supermolecular structure of iPP sample prepared by PVIM. The mechanism of the formation of the two kinds of shish-kebab-like cylindrulite structures is also depicted by a schematic illustration, and it was discussed in terms of periodical shear-induced crystallization. 相似文献
A strong shear flow was imposed on the melt of polycarbonate (PC) microfibril reinforced high density Polyethylene (HDPE) during a secondary melt flow process, i.e. gas assisted injection molding (GAIM). Classic shish-kebabs and typical transcrystallinity were simultaneously observed in the entire thickness of the GAIM HDPE/PC microfibril composites, which were closely related to the strong shear flow that was further amplified and distributed by incorporated PC microfibrils. Interestingly, some nano-sized ultrafine PC microfibril inclined to absorb extended chain bundles to form shish nuclei on its surface first, which subsequently evolved into hybrid shish-kebab superstructures. It was deemed that the induced formation of hybrid shish-kebab superstructures on nano-sized ultrafine PC microfibril was due to the absorbing of extended chain bundles for hybrid shish nuclei with the strong shear flow serving as the driving force. Importantly, large scale formation of these highly oriented crystalline superstructures can bring significant mechanical reinforcement in GAIM HDPE/PC microfibril composite. For GAIM HDPE/PC microfibril composites, its yield strength is increased by 68% and 66%, compared to the GAIM HDPE parts and the common injection molded (CIM) HDPE/PC composites, respectively; meanwhile, the Young's modulus is enhanced by 253% and 17%, compared to the GAIM HDPE parts and the CIM HDPE/PC composites, respectively. 相似文献
Repetitive processing was employed to assess the recyclability of in situ microfibrillar poly(ethylene terephthalate) (PET)/high‐density polyethylene (HDPE) blends which were fabricated through a “rectangular slit die extrusion–hot stretching–quenching” process. For comparison, the conventional PET/HDPE blends were also obtained using the same processing operation but without hot stretching. The morphological observation indicated that slit die extrusion and hot stretching successfully made the dispersed PET phase deform in situ into well‐defined microfibrils. The average diameter of the microfibrils increased with the processing cycles. The rheological properties obtained from the parallel‐plate dynamic rheometer suggested that the microfibrillar blends have higher viscosity and viscoelastic moduli (storage and loss moduli) as well as better flow stability than the conventional PET/HDPE blend. More importantly, with the increase in the processing cycles, an increase in yield strength and unchanged tensile modulus were observed for in situ microfibrillar blends, while a decrease in these properties for conventional blend, indicating that the in situ microfibrillar PET/HDPE blends have promising recycling potential.