Using a pressuring and shearing device (PSD), we explored the simultaneous effects of pressure and flow on β-crystal formation. Interestingly, pressure plays a versatile role in β-crystal formation in isotactic polypropylene (iPP) mixed with β-nucleating agent (β-NA) when a flow field exists simultaneously. At a low pressure (5 MPa), a mass of β-crystals can be obtained over the entire range of applied shear rates (0.0–24.0 s–1). As the pressure increases (50–100 MPa), a weak shear flow (3.2 s–1) can profoundly suppress the β-crystal formation. When elevating the pressure to 150 MPa, β-crystals cannot be generated. The pressure and flow window to produce β-crystals in iPP mixed with β-NA were successfully summarized for the first time. The diversified β-crystal formation behaviors in iPP mixed with β-NA under the simultaneous effects of pressure and shear flow were well elucidated by combining classical nucleation theory and the growth of different crystalline phases. Our current work lays a solid foundation to tailor β-crystals in iPP mixed with β-NA in practical processing and thus to optimize the mechanical properties of iPP products.

Atactic polypropylene (aPP) and isotactic polypropylene (iPP) were incorporated into a new blending material with tailored microstructure. Improved mechanical properties were realized through the application of a shear flow field to injection molding aimed at making use of aPP on a large scale. The hierarchic structure of the oscillation shear injection molding (OSIM) parts was characterized through wide-angle X-ray diffraction, small-angle X-ray scattering, and scanning electron microscopy. It was found that a high and homogeneous orientation inner structure, that is, intense shear flow induced shish-kebabs, is successfully obtained in aPP/iPP blends, which can markedly improve the mechanical performance of blends and offset the mechanical properties decline due to the addition of aPP with poor properties. Owing to the tailored microstructure, as 10 wt % aPP is added, the tensile strength and impact toughness of OSIM samples climb from 30.6 MPa and 4.8 KJ/m2 for normal injection molded samples to 57.8 MPa and 16.8 KJ/m2, respectively. Even when the aPP content reaches 30 wt %, the OSIM samples retain tensile strength of 52.8 MPa and impact strength of 13.1 KJ/m2 compared to only 20.2 MPa and 7.3 kJ/m2 for normal samples. This shows the potential for practical applications because of the satisfactory properties of blends and efficient utilization of aPP. In addition, we found that the practical tensile strength of OSIM samples is significantly higher than the theoretical value calculated through mixing principle. For normal samples, an opposite behavior is observed. Although the addition of aPP usually will lead to a remarkable degradation of mechanical properties, it plays a positive role in modifying the inner structure of injection-molded blend samples when structuring processing is used to provide continuous shear flow during processing. Through OSIM technique, we successfully turn “waste” into wealth, which opens a new field for efficient usage of aPP and oil resources.

In this work, we explored the crystallization of poly(lactic acid) (PLA) blended with poly(ethylene glycol) (PEG) under two inevitable processing fields (i.e., flow and pressure) that coexist in almost all processing for the first time. Here, the PEG was incorporated into PLA as a molecular chain activity promoter to induce PLA crystallization. A homemade pressuring and shearing device was utilized to prepare samples and necessary characterization methods, such as differential scanning calorimetry, scanning electron microscopy, and synchrotron radiation, and were used to investigated the joint effects of PEG, pressure, and shear flow on the crystallization behaviors and morphologies of PLA/PEG samples. The results reveal that adding 3–5 wt % PEG into PLA can significantly increase the PLA crystallinity due to the efficient plasticization effect of PEG, while the PEG content reaches 10 wt %, the PLA crystallinity decreases drastically as the phase separation between PEG and PLA occurs. We also find that applying a higher pressure (∼100 MPa) can facilitate the formation of thicker lamellae with fewer defects as well as higher crystallinity under an equal degree of supercooling compared to normal pressure or a low pressure condition because the slip of molecular chains during crystallization makes the lamellae thicker under higher pressures. The PLA crystalline structure in the PLA/PEG sample is not influenced by the shear flow, yet the crystallinity is largely enhanced by applying a shear flow with an appropriate intensity (0–3.5 s–1). It is worth noting that pressure and shear flow show a synergetic effect to fabricate PLA/PEG samples with high crystallinity. These meaningful results could beyond doubt help comprehend the relationship between crystallization conditions and crystallization behaviors of PLA/PEG samples and thus provide guidance to obtain high-performance PLA/PEG products via controlling crystallization conditions.

Shear and pressure fields unavoidably coexist in practical polymer processing operations, but their combined influence on the crystalline structure of poly(l-lactic acid) (PLLA) has never been studied due to the limit of experiment device. In the current work, we utilized a homemade pressuring and shearing device to study the crystalline morphology and structure of PLLA under the coexistence of shear and pressure. Interestingly, we obtained almost exclusive β-form directly from PLLA melt crystallization at our experimental condition (shear 13.6 s–1, pressure 100 MPa, and crystallization temperature 160 °C). Undoubtedly, abundant β-form is helpful to tackle the major shortcoming of PLLA performance, i.e., poor toughness. This meaningful result is different from the common viewpoints that PLLA β-form can usually be obtained by hot-drawing or solid coextrusion under a high tensile ratio, suggesting that PLLA β-form can be obtained through shear-induced crystallization. In addition, the fraction of β-PLLA strongly depends on supercooling and shear intensity. A higher supercooling (pressure 150 MPa and crystallization temperature 160 °C) could also induce predominant β-form even under a very low shear rate of 1.0 s–1. While, under a lower supercooling (pressure 50 MPa and crystallization temperature 160 °C), we did not observe any trace of β-form. In the heating experiment to investigate crystal form transformation, we also found that partial β-form transformed into α-form through melting–crystallization, and meanwhile some β-form crystals melted directly without transformation. These results could beyond doubt help to comprehend the relationship between crystallization condition and inner crystal structure and thus afford guidance in practical processing to toughen final PLLA products via controlling crystalline structure.

Flow and pressure frequently coexist in practical polymer processing operations, but their combined influence on the microstructure of polymer parts has received very limited attention in the academic community. In the current work, we utilized a home-made pressuring and shearing device with a reliable dynamic sealing design to study the formation and microstructure of γ-form isotactic polypropylene (iPP) obtained under the coexistence of flow and pressure. We observed a strong shear dependence of pressure-induced γ-form iPP. There are three regions depending on shear flow intensity, i.e., facilitation (<3.7 s−1), suppression (3.7–9.1 s−1) and inexistence (>9.1 s−1) regions of the γ-form. As the shear rate is below 3.7 s−1, the pressure-induced γ-form dominates and the shear flow slightly facilitates formation of γ-form. Unexpectedly, above 3.7 s−1, the shear flow is unfavorable for γ-form growth. Even under a pressure of 100 MP, a flow field with a shear rate above 9.1 s−1 could entirely suppress the γ-form. Moreover, we did not observe any trace of the β-form in the obtained iPP that is generally generated under shear flow alone. These interesting results have never been reported, which undoubtedly help manipulate the inner structure and thus enhance the performance of final iPP products.

An ultrahigh molecular weight polyethylene (UHMWPE) composite containing carbon nanotube–carbon black (CNT–CB) hybrid was fabricated via a facile method, i.e., mechanical mixing plus hot compaction in order to obtain low-cost conductive polymer composites with balanced electrical properties. Optical microscope and scanning electron microscope observations indicate the formation of a typical segregated structure in the CNT–CB/UHMWPE composite, with the CNT–CB hybrid selectively located at the interfaces of UHMWPE granules. Compared to the single CNT loaded UHMWPE composite, the CNT–CB/UHMWPE segregated composite with a quarter replacement of CNT with CB shows only 8% decline in electrical conductivity, with the same filler content of 4 wt%, realizing a significant reduction in the material cost. More interestingly, the CNT–CB/UHMWPE composite presents 273% higher positive temperature resistivity intensity than that of CNT/UHMWPE composites, exhibiting strong sensitivity to ambient temperature. Our work demonstrates a novel strategy to fabricate low-cost and high-performance conductive polymer composites by the combination of hybrid fillers and a segregated structure.

The physically entangled chain networks, formed by a long-chain polymer component, have been demonstrated to be very effective for shish-kebab formation in a flow field. However, an open question still remains whether there is an optimal content of entangled chain networks. The answer is crucial in the academic and industrial fields but faces a challenge since physically entangled chain networks are not stable, which tend to disentangle during flow. In the current work, we utilized lightly cross-linked chain networks, which can be considered as stably entangled chain networks (SECN), to study the role of SECN density in shish-kebab formation under an intense flow field. The SECN density was adjusted by adding various contents of lightly cross-linked polyethylene (PEX) in short-chain polyethylene, and the intense flow field was applied by a modified injection molding machine, so-called oscillation shear injection molding (OSIM). The results show that the structure in the core layer of the OSIM samples could well clarify the role of SECN density. There is a threshold of SECN density for shish-kebab formation under the given flow field, below which (PEX content <30 wt %) only the isotropic lamellae were formed in the core layer. However, when the SECN density was high enough (PEX content >30 wt %), a large amount of shish-kebabs appeared throughout the whole sample. Unexpectedly, excessive SECN (PEX content >70 wt %) caused imperfect shish-kebab, possibly due to contact and even entanglement between neighboring SECNs. A major superiority of our experiments performed on the scalable processing equipment is that the mechanical performance of sample can be evaluated. Thanks to the significant promoting effect of SECNs and intense flow field on shish-kebab formation, the tensile strength of OSIM sample gained a remarkable enhancement from 29.9 MPa for conventional injection-molded neat PE to 69.7 MPa for OSIM sample with 70 wt % PEX. For OSIM sample with 100 wt % PEX, the tensile strength shows a slight decrease to 66.2 MPa owing to the imperfect shish-kebabs resulting from the excessive SECN. Our results demonstrate that SECN is favorable for flow-induced shish-kebab formation, however excessive SECN (i.e., highly dense entanglement) could hinder the formation of shish-kebab.

In practical processing, polymer melts generally experience flow and pressure fields simultaneously, but their flow-induced crystallization behavior under pressure was barely investigated. For this reason, we provided an insight into the crystallization behavior and crystalline morphology and structure of isotactic polypropylene (iPP) obtained under the combination of flow (2.5–22.5 s–1) and high pressure (200 MPa) by using self-designed pressurizing and shearing device. Unprecedented iPP spherulites were observed, which are composed of oriented thick lamellae (18 nm) with ultrahigh melting temperature (179.5 °C). Such spherulitic crystals with lamellae aligning perpendicular to flow have never been reported. All samples have a double melting peak behavior including an additional low temperature peak (165–169 °C) apart from the ultrahigh melting peak. The in situ synchrotron X-ray measurements show that the parents of cross-hatched structure are responsible for the ultrahigh melting temperature and the daughters give rise to the low temperature melting peak because the parents crystallized much earlier and had a higher growth rate than the daughters. Moreover, the parents are all in α-form while the daughters consist of α- and γ-forms. Our results afford a new method to obtain thick lamellae in a relatively short time.

The crystallization behavior of poly(butylene succinate) (PBS) nanocomposites with a wide range of contents of clays was revealed. It was of interest to find that the crystallization rate of PBS was accelerated obviously at relatively low contents of clays; while a retarded crystallization kinetics and a decreased crystallinity of PBS were found in the nanocomposites with higher clay contents. Two interplaying effects existed in the nanocomposites, i.e., a suppression effect of clays on nucleation and a templating effect of clays on crystal growth, were clarified to contribute to this intriguing crystallization behavior.

A new homemade apparatus, i.e. vibration assisted extrusion equipment, is employed to extrude polypropylene. Vibration assisted extrusion is based on the application of a specific macroscopic shear vibration field. Reduction of apparent melt viscosity as a function of vibration frequency is measured at different screw speeds and die temperatures. The effect of the process is investigated by performing mechanical tests, differential scanning calorimetry studies, polarized light microscopy and wide-angle X-ray diffraction. It is found that, compared with conventional extrusion, vibration assisted extrusion could effectively improve the rheological properties of PP melt by incorporating an extra shear vibration field. Both the tensile strength and elongation at break increased under the shear vibration field. For vibration assisted extrusion samples, both the melting temperature and crystallinity increased, accompanied by remarkable grain refinement. Vibration assisted extrusion induced a significantly enhanced bimodal orientation with a high fraction of a*-oriented α-crystallites, while only a limited improvement in the flow direction orientation. A structural model, i.e. bimodal c-axis and a*-axis orientation of PP macromolecular chains, was adopted to explain the experimental results.

•aPP/iPP binary blends were prepared to make a large amount usage of aPP.•Blends with desired mechanical performance were fabricated via the OSIM technique.•Tensile strength of the aPP/30 wt% iPP blend is comparable to HDPE.•Invariable increment of the tensile strength of OSIM samples is observed.To make large scale, effective use of atactic polypropylene (aPP), normally regarded as industry waste, isotactic polypropylene (iPP) was blended with aPP with a guiding ideology of “structuring processing”. Herein, the aPP/iPP blends were melt processed through modified injection molding, i.e., oscillation shear injection molding (OSIM), in which an oscillation shear flow field was applied to induce self-reinforcing oriented iPP crystals. With addition of only 30 wt% iPP, the tensile strength of the blend could increase from 1.6 MPa for neat aPP to 26.6 MPa, which is comparable to that of conventionally injection molded high density polyethylene. Further increasing iPP content to 50 wt%, the tensile strength of OSIM aPP/iPP sample rose up to 41.6 MPa, already higher than those of industrial-scale extruded and injection-molded iPP. The results of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) testified that the increased enhancement of mechanical performance of OSIM blend with the increase of iPP content can be ascribed to the progressive formation of iPP shish-kebab networks. It is a mechanism to reinforce amorphous polymer under shear flow by adding crystalline component. Meanwhile, adding iPP into aPP could effectively enhance the viscosity of aPP and hence the processability of aPP could be significantly improved. This technique opens a gate to manufacture aPP into practical products through the most common processing methods like extrusion and injection-molding.

In the present work, atactic polypropylene (aPP)/isotactic polypropylene (iPP) with different aPP content was prepared through an injection-molding process to improve the toughness of iPP and make large scale use of aPP. The hierarchic structure of the injection-molded parts was characterized through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and scanning electron microscopy (SEM). It was found that the network of iPP crystals was still integral with shish-kebab structures in the skin layer and the relatively high crystallinity of iPP in injection-molded parts with low aPP content utilizing suitable process conditions. Therefore, the toughness of iPP was enhanced from 31.8 to 42.6 MJ/m3 due to the addition of a 20 wt % aPP component, meanwhile the tensile strength only decreased from 45.9 to 40.5 MPa. Furthermore, when aPP content reached 50 wt %, the toughness of the aPP/iPP blends increased to 52.0 MJ/m3 with the tensile strength staying at the level of 20 MPa, indicating that the A50 sample also has good toughness with reasonable strength. The results demonstrated that the aPP/iPP blends with high mechanical properties owing to the optimized inner structure can be obtained through the suitable processing method. Our results set up a new method to make large scale use of aPP. Moreover, with an increase in aPP content, the mechanical properties of the injection-molded part can be divided into three evolution stages with distinct differences. At high and low aPP content, the mechanical properties were not sensitive to aPP content. However, when aPP content fell between 20 and 50 wt %, the mechanical properties of the injection-molded part, especially the elongation at the break, changed dramatically acting like percolation phenomenon in electro-conductive polymer composites, which may be the result of phase inversion of aPP/iPP blends. The percolation in mechanical properties is meaningful to prepare the blend of crystallizable/noncrystallizable blends with different properties.Keywords: Atactic polypropylene; Injection molding; Isotactic polypropylene; Mechanical properties; Percolation phenomenon

It has been well established that the entangled molecular network facilitates the formation of shish-kebabs under flow field, however, the entangled network, usually formed by long chains, tends to disentangle due to molecular relaxation. In the present work, a small amount of lightly cross-linked polyethylene (LCPE), which can be considered as stable molecular chain networks, was added to short-chain polyethylene and then injection-molded using a modified injection molding technology-oscillation shear injection molding (OSIM), which can exert a successive shear field on the melt in the mold cavity during packing stage. The hierarchic structure of the OSIM samples was characterized through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). It was found that the oscillation flow field promoted the formation of interlinked shish-kebabs in the intermediate layer of OSIM samples, while there are still typical spherulites in the core layer of OSIM neat polyethylene (PE). The interlinked shish-kebab structure led to remarkable mechanical enhancement from 27.6 and 810.2 MPa of conventional injection molding (CIM) samples to 42.7 and 1091.9 MPa of OSIM samples for tensile strength and modulus, respectively. More importantly, under the same flow condition, the samples containing LCPE networks (termed PEX) exhibit rich shish-kebab structure both in the intermediate and core layers. Moreover, the addition of LCPE also generated stronger interlinked shish-kebabs, in which kebabs and shishes are connected by covalent bonds, rather than topological entanglement points. This special structure leads to further reinforcement from 29.6 and 879.5 MPa of CIM PEX samples to 57.5 and 1311.7 MPa of OSIM PEX samples for tensile strength and modulus, respectively. The results demonstrated that the networks with stable entanglement points are more helpful to induce the formation of shishes under flow than those with topological entanglement points. Our results set up a new method to reinforce polymer parts by tailoring the structure and morphology.

Isothermal and nonisothermal crystallization of isotactic polypropylene (iPP)/graphene oxide nanosheet (GONS) nanocomposites were investigated by differential scanning calorimetry, polarizing optical microscopy and synchrotron wide-angle X-ray diffraction. Nonisothermal crystallization measurement revealed that GONSs significantly elevated crystallization peak temperature of iPP. Nucleating activity of the nanocomposites containing 0.05 wt% and 0.1 wt% GONSs was 0.61 and 0.56, respectively, indicating strong nucleation ability of GONSs. Induction period and half crystallization time of nanocomposites was greatly reduced during isothermal crystallization process. Moreover, only 0.05 wt% and 0.1 wt% caused less temperature dependence of crystallization kinetics in a wide temperature range of 132–140 °C. A very low GONS loading provided sufficient surface area for crystallite nucleation. Polarized optical microscopy showed that nucleation density of nanocomposites was much larger than that of neat iPP, accelerating overall spherulite growth rate. Crystal structure of iPP was not affected by GONSs, confirming the α-nucleating ability of GONSs.