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Polypropylene (PP) was compounded with β-nucleating agent and injection-molded at 180 °C or 220 °C. The samples were subsequently treated by supercritical carbon dioxide (scCO 2 ) at different temperatures. Results show that processing temperature and scCO 2 treatment could strongly influence the tensile and impact properties of β-nucleated PP. In particular, the sample processed at 220 °C and treated at 120 °C exhibit much enhanced impact strength (4.8 times that of its untreated counterpart). FTIR, WAXD, SEM and DMA were performed to explore the effects of processing temperature and scCO 2 treatment on structure of the samples. Deformation-induced plastic flow and micro-voids were also evaluated to construct structure-property relations. It was found that the influence of processing temperature on mechanical properties is mainly associated with the β-form content and β-crystalline morphology, while the structural changes in the crystalline lamellar scale may be responsible for the toughening effect of scCO 2 treatment. [ABSTRACT FROM AUTHOR]

Abstract: Large amount of work has been reported on the annealing of polypropylene (PP) and the related changes in mechanical properties. However, the structure–property correlations and the physical origin of annealing induced microstructural evolution are still not very clear. In this work, taking β-form PP (β-PP) as example, the microstructural changes induced by annealing were investigated from macromolecular to crystalline lamellae level with Fourier transform infrared (FTIR) spectroscopy, conventional differential scanning calorimetry (DSC), temperature-modulated DSC (TMDSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA). Besides mobile amorphous fraction (MAF), the role of rigid amorphous fraction (RAF) in toughening PP is particularly taken into consideration. It is shown that annealing increases the chain mobility in MAF and decreases it in RAF. Such an effect is believed to be mainly associated with the formation of looser MAF and more RAF by the microstructural re-arrangement involving conformational ordering of partial amorphous chain segments and a significant interlamellae thickening. A thorough analysis of structure–property relationship through observing plastic deformation behaviors by scanning electron microscope (SEM) and estimating stress transmission between crystalline and amorphous phases, suggests that both MAF and RAF play important role on toughening β-PP. They can promote the initiation of microvoids effectively upon deformation by reducing the stress transmission. As a result, large-scale plastic deformation is triggered. This work is important and provides a new insight into the mechanisms of microstructural evolution and subsequent improvement in impact toughness during annealing. [Copyright &y& Elsevier]

Abstract: Poly(propylene)/elastomer blends with β-form nucleating agent (β-NA) aryl amides compound (TMB-5) were prepared. The effects of β-NA on crystallization, melting behaviors and elastomer morphologies of PP/elastomer blends were studied through polarization optical microscope (POM), differential scanning calorimetry (DSC) and scanning electronic microscope (SEM). The fracture behaviors, including notched Izod impact fracture and single-edge notched tensile (SENT) fracture, were comparatively studied to establish the role of NA in improving the fracture toughness of PP/elastomer blends. Our results showed that the presence of β-NA leads to determinable β-PP formation in the blends, and as a consequence the fracture toughness of the blend is improved dramatically. Compared with notched Izod impact testing, which can efficiently characterize the fracture toughness of the blends only at lower elastomer content, SENT testing provides more detail of fracture behavior in all the compositions. Furthermore, SENT test shows that the significant improvement in fracture toughness of PP/elastomer/β-NA is contributed to the simultaneous enhancement of crack initiation energy and crack propagation energy, but largely dominated by crack propagation stage. [Copyright &y& Elsevier]

Highlights: [•] ScCO2-assisted annealing was adopted to toughen polypropylene for the first time. [•] Polypropylene with well-balanced mechanical properties was prepared. [•] Polypropylene random copolymer with superior toughness was prepared. [•] Toughening mechanism of scCO2-assisted annealing was proposed. [ABSTRACT FROM AUTHOR]

Abstract: The synergistic effect of β-modification and annealing on the impact toughness of a commercially available polypropylene random copolymer (PPR) was investigated. Interestingly, the impact toughness of β-nucleated PPR after annealing at low temperature (0°C) was almost five times as high as that of the virgin PPR without annealing. The crystalline structure, supermolecular structure, phase morphology, the relaxation of chain segments and fracture behavior of matrix were investigated to explore the toughening mechanism related to the β-modification and annealing. It was found that annealing improved the mobility of chain segments in amorphous phase as well as the strength of ligament of PPR matrix with profuse β crystals having the intrinsic low plastic deformation resistance, responsible for the superior toughness achieved. This work provides a possible method to toughen semi-crystalline polymers at low temperature by combination of β-modification and suitable annealing. [Copyright &y& Elsevier]

Abstract: In this work, we investigated the epitaxial crystallization and its determinant factor in the blends of high density polyethylene (HDPE) and a low crystallizable propylene-ethylene random copolymer (PPR) during injection molding. For PPR dominated blend (blended with 30 wt.% HDPE), epitaxial growth of HDPE on the PPR crystals can be achieved, as demonstrated by two-dimensional wide-angle X-ray diffraction and small-angle X-ray scattering measurements, though the crystallization of PPR was far behind that of HDPE component under quiescent conditions. In-situ shear infrared experiments indicated that PPR crystallized in advance mainly due to larger crystallization window and flow-induced crystallization acceleration. As for the blend with 70 wt.% HDPE, however, one could not observe epitaxy. Shear-induced crystallization of HDPE hindered its epitaxial growth. Once the formation of shish-kebab of HDPE was activated, the epitaxial growth would be suppressed to a large extent. [Copyright &y& Elsevier]

In recent years, rubber toughened polypropylene (PP) blends have been extensively investigated with special regard to the importance of PP matrix properties (e.g., crystal morphology and structure) in tailoring the toughening efficiency. Unfortunately, although high melt flowability is indispensable to the processing of polymers into complex thin-wall products, it remains a huge challenge to achieve a marvelous balance between stiffness and toughness in the blends with relatively low matrix molecular weight. Herein, taking PP/EPR (ethylene-propylene copolymer) blends as an example, the combined effects of matrix molecular weight and crystallinity on the mechanical properties of PP/EPR blends have been examined in both experiment and theory. The experimental results show that decreasing molecular weight leads to significant deterioration in notched impact toughness, tensile strength and Young's modulus. Impressively, the toughness loss caused by the decrease of molecular weight can be well-compensated by a slight reduction in the matrix crystallinity, without evidently sacrificing the strength and modulus, demonstrating an improved stiffness-toughness balance. Theoretical analysis indicates that the impact toughness of PP/EPR blends is heavily dependent on the chain entanglement density (v e) of PP matrix, and a linear relationship between critical interparticle distance (ID c) and the v e 1/3 has been verified from both experiment and theory. Moreover, it is interesting to find that decreasing matrix crystallinity can increase the v e of low-molecular-weight PP matrix, which enables the effective toughening at lower concentration of EPR and thus gives rise to less loss in the strength and modulus. We believe this work not only gives a new insight into the role of chain entanglement in the toughening but also provides a promising guidance for the design of high-performance PP with high melt flowability. [Display omitted] • Combined effects of M n and crystallinity on the toughening efficiency of PP/EPR blends were studded. • The relationship between the ID c and v e of PP matrix i.e. ID c ∼ v e 1/3 , was given for the first time. • Matrix with higher M n or lower crystallinity has higher v e , exhibiting larger ID c. • Toughness loss caused by decreasing M n can be compensated by reducing crystallinity. • Low-viscosity PP/EPR blends with good toughness-stiffness balance were obtained. [ABSTRACT FROM AUTHOR]

There are principally two mechanisms to improve the impact resistance of polymer-based composites, intrinsic toughening and extrinsic toughening. But the interplay between them is far from being well understood. Here, glass fiber was incorporated into polypropylene to promote extrinsic toughening mechanism, while addition of elastomer and annealing were adopted for intrinsic toughening. In this way, the interaction among glass fiber, elastomer and annealing could be discussed based on various characterizations, and their combined effect on mechanical properties of the composites could be determined. The results show that the intrinsic toughening mechanism of elastomer will be suppressed by glass fiber irrespective of the support of annealing, though annealing could work synergistically with glass fiber to toughen polypropylene. The possible structure-property relations are discussed. This work will provide deeper insight into the toughening behavior of polymer composites and practical guidance for the design of composites with excellent stiffness-toughness balance. [ABSTRACT FROM AUTHOR]

Abstract: As a part of serial work about the toughening effect of elastomer and nucleating agent on polypropylene (PP), this work is focused on the microstructure changes of PP matrix in PP/elastomer blends with β-phase nucleating agent (β-NA) during the uniaxial tensile deformation process. The microstructure changes have been investigated through differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) measurements. The results show that there is a transformation of β-PP to α-PP, which is dependent on the local strain of the tensile-deformed specimen during the deformation process. The bigger the local strain, the high the degree of the β→α transformation is. At the later stage, β-PP is completely changed into α-PP. The presence of elastomer, especially at high load, prevents such transformation, possibly leading to more β-PP participating in the later deformation process of the fracture, which most likely results in the great improvement of fracture toughness of PP/elastomer/β-NA. Further results show that the β→α transformation occurs mainly in the necking region of the specimen during the deformation process. [Copyright &y& Elsevier]