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Due to the properties of a positive temperature coefficient (PTC) effect and a negative temperature coefficient (NTC) effect, electrically conductive polymer composites (CPCs) have been widely used in polymer thermistors. A dual percolated conductive microstructure was prepared by introducing the polybutylene adipate terephthalate phase (PBAT) into graphene nanoplatelets (GNPs)-filled polylactic acid (PLA) composites, intending to develop a favorable and stable PTC material. To achieve this strategy, GNPs were selectively distributed in the PBAT phase by injection molding. In this study, we investigated the crystallization behavior, electrical conductivity, and temperature response of GNP-filled PLA/PBAT composites. The introduction of GNPs into PLA significantly increased PLA crystallization capacity, where the crystallization onset temperature (To) is raised from 116.7 °C to 134.7 °C, and the crystallization half-time (t1/2) decreases from 35.8 min to 27.3 min. The addition of 5 wt% PBAT increases the electrical conductivity of PLA/PBAT/GNPs composites by almost two orders of magnitude when compared to PLA/GNPs counterparts. The temperature of the heat treatment is also found to play a role in affecting the electrical conductivity of PLA-based composites. Increasing crystallinity is favorable for increasing electrical conductivity. PLA/PBAT/GNPs composites also show a significant positive temperature coefficient, which is reflected in the temperature–electrical resistance cycling tests. [ABSTRACT FROM AUTHOR]

Multiflow injection molding is used to prepare a highly oriented shish‐kebab structure, and the mechanical properties of Polypropylene in the weak direction (WD) and strong direction (SD) are analyzed in this work. The oriented structure shows strong anisotropy. For example, the impact strength in SD is 6 times that of WD. One or three minutes of high‐temperature‐annealing (HTA, 30°C higher than Tm) has been proven to be an effective method to trigger the partial melting and re‐crystallization process, which in turn, can improve the shish‐kebab structure and greatly improve yield and impact strength in both WD and SD. Furthermore, results confirm that the chains in the amorphous region play a key role in the mechanical properties in the WD, while the shish‐kebab crystal structure determines the properties in the SD. Using shish‐kebab structure and HTA to improve mechanical properties in both SD and WD directions will be helpful to avoid generating fiber composites that create recycling difficulties and loss of properties with cycle times. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

ABSTRACT: High oriented isotactic polypropylene prepared by self‐designed two‐stage die drawing apparatus was explored through kinds of methods, including differential scanning calorimetry, two‐dimensional wide‐angle X‐ray diffraction, polarized light microscope, and tensile test. The results showed that there was a great difference between the orientation mechanism and structure evolution of two‐stage solid die drawing process and single‐stage solid die drawing process. All samples would undergo free drawing process after die drawing process. Die drawing and free drawing process were of equal importance to single‐stage die drawing process while die drawing process showed an prominent contribution to the two‐stage die drawing process. Drawing speed showed beneficial influence on die drawing process for both single‐ and two‐stage die drawing process routes. Under the same processing condition, tensile strength and modulus of samples prepared by two‐stage die drawing process were higher than those prepared by single‐stage die drawing process and the maximum value could reach 241.93 MPa and 3.57 GPa, respectively. Moreover, two‐stage die drawing samples showed better dimensional stability than single‐stage die drawing samples especially under low draw rate. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46581. [ABSTRACT FROM AUTHOR]

In this paper, the microstructural evolution of controlled-rheology polypropylene (CRPP) with different melt viscoelasticities was investigated by polarized optical microscopy, scanning electronic microscopy, differential scanning calorimeter, and wide-angle X-ray diffraction. It is found that a typical 'skin-core' structure formed in CRPP microparts and the thickness of oriented layer of CRPP microparts decreases notably with the addition of peroxide. The thickness of oriented layer and the distribution of different layers strongly depend on the melt flow properties and the corresponding relaxation time ( λ). Furthermore, the mechanisms of the suppressed formation of oriented layers during the micro-injection molding process are discussed mainly from the viewpoint of rheology and thermodynamics. It is revealed that the shear-induced orientation is one of the key factors for the formation of oriented molecular structure (row nuclei). The final thickness of the oriented layer is the result of the competition between the orientation behavior and the disorientation behavior. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The morphological structure and crystallization behavior of in situ poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) microparts prepared through micro-injection molding are investigated using a polarized light microscope, differential scanning calorimeter, scanning electron microscope, and two-dimensional wide-angle X-ray. Results indicate that both the shear effect and addition of PET fibers greatly influence the morphologies of the iPP matrix. Typical 'skin-core' and oriented crystalline structures (shish-kebab) may simultaneously be observed in neat iPP and iPP/PET microparts. The presence of PET phases reveals significant nucleation ability for iPP crystallization. High concentrations of PET phases, especially long PET fibers, correspond to rapid crystallization of the iPP matrix. The occurrence of PET microfibrils decreases the content and size of β-crystals; by contrast, the orientation degree of β-crystals increases with increasing PET content in the microparts. This result suggests that the existence of the microfibrillar network can retain the ordered clusters and promote the development of oriented crystalline structures to some extent. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Highlights Melt‐mixed polypropylene (PP) and multi‐walled carbon nanotubes (CNTs) are molded by compression molding, conventional injection molding, and interval injection molding (IntM). IntM can provide high‐shear stress promoting the dispersion of CNTs and forming many shish‐kebab structures, which can improve the strength at the cost of toughness. Meanwhile, high‐temperature annealing (HTA) can be finished in 3 min and can release free molecular chains, enhance orientation, and further promote the dispersion of CNTs, thus improving toughness at the expense of strength. Therefore, in this work, IntM and HTA are combined, which can balance and enhance both the toughness and strength of PP‐CNT nanocomposites. For different CNT contents, the enhancement effect also differs. The combined IntM and HTA do not require replacing existing production equipment, so this research can provide a more economical and practical approach to enhancing PP‐CNT nanocomposites. HTA can be finished in 3 min and promote the dispersion of CNT agglomerates. IntM can enhance the orientation and dispersion of CNTs. Combined IntM and HTA can enhance and balance toughness and strength. [ABSTRACT FROM AUTHOR]

In this work, a series of isotactic polypropylene/poly(ethylene terephthalate) (iPP/PET) samples were prepared by microinjection molding (μIM) and mini-injection molding (IM). The properties of the samples were investigated in detail by differential scanning calorimetry (DSC), Wide-Angle X-ray Diffraction (WAXD), Polarized light microscope (PLM) and scanning electron microscopy (SEM). Results showed that the difference in thermomechanical history between both processing methods leads to the formation of different microstructures in corresponding iPP/PET moldings. For example, the dispersed spherical PET phase deforms and emerges into continuous in-situ microfibrils due to the intensive shearing flow field and temperature field in μIM. Additionally, the incorporation of PET facilitates both the laminar branching and the reservation of oriented molecular chains, thereby leading to forming a typical hybrid structure (i.e., fan-shaped β-crystals and transcrystalline). Furthermore, more compact and higher degrees of oriented structure can be obtained via increasing the content of PET. Such hybrid structure leads to a remarkable enhancement of mechanical property in terms of μIM samples. [ABSTRACT FROM AUTHOR]