Origin of various lamellar orientations in high-density polyethylene/isotactic polypropylene blends achieved via dynamic packing injection molding: bulk crystallization vs. epitaxy

Epitaxial growth of high-density polyethylene (HDPE) onto lamellae of isotactic polypropylene (iPP), with HDPE chains inclined about 50° to that of iPP, has been achieved for the first time in their blends via dynamic packing injection molding. Even more, the epitaxial growth was found to be dependent on composition of the blends. The sequence of crystallization is not the dominant factor, but the fact that iPP crystallizes before HDPE is prerequisite for epitaxial growth of PE. Various lamellar orientations with composition can be explained by the competition between bulk crystallization and epitaxy at interfaces (i.e. iPP lamellae). In 20PP (20 wt% iPP by weight in blends), HDPE can readily crystallize in the bulk as a result of shear, and no epitaxial growth of PE is observed. For 80PP, however, bulk crystallization of HDPE can be depressed due to lack of nuclei in its bulk, resulting from a much finer droplets dispersed in the iPP matrix, and then epitaxial growth prevails.     

Epitaxy growth and directed crystallization of high-density polyethylene in the oriented blends with isotactic polypropylene

Various lamellar orientations of high-density polyethylene (HDPE), due to competition between bulk nucleation and interfacial nucleation, have been realized in its melt drawn blends with isotactic polypropylene (iPP) upon cooling after subjected to 160 °C for 30 min. Directed crystallization, with heterogeneous nucleation in the bulk (within domains), is defined as lamellar growth along boundary of anisotropic domains and is favored in larger domains at higher temperature (slow cooling), since overgrowth of lamellae can feel the interface rather than impingement with neighbor ones as a result of scare nuclei at higher temperature. Moreover, lamellar growth caused by directed crystallization is dependent of dimension of confinement. Due to 2D confinement of cylindrical domains, lamellae can only grow along the axis of cylinder and thus b-axis orientation is formed. While in the layered domains with 1D confinement, however, lamellae grow with the normal of (110) plane along the melt drawn direction. On the other hand, epitaxial growth of HDPE chains onto iPP lamellae is related to the surface-induced crystallization and dominated by the interfacial nucleation. Only interfacial nucleation is preferred can epitaxial growth occur. Therefore, retarded crystallization, realized by either strong confinement in finer domains or rapid cooling or both, is favorable for it.     

Ultrasonic measurement of orientation in HDPE/iPP blends obtained by dynamic packing injection molding

The orientation of polymer chain has a great effect on its mechanical properties, therefore, it is always an important issue on how to characterize, accurately and quickly, the orientation of polymer chain during processing. In this article, according to the property that ultrasound travels in different velocities in anisotropic media, normal incident shear wave was utilized to explore the orientation structure of HDPE/iPP blends obtained by dynamic packing injection molding. The ultrasonic technique is consistent with the 2D-WAXS in charactering the orientation degree of polymer chains, although ultrasonic technique focuses on the overall orientation of polymer blends while the 2D-WAXS reveals the crystalline orientation of each component. Our work demonstrates that ultrasonic technique might be a reliable, fast and easy way to characterize the orientation structure of crystalline polymer blends. The ultrasonic measurements were performed off-line, but the achievement provides the possibility for on-line detection of orientation structure in injection molding by using ultrasonic technique.     

Shear-induced crystallization in isotactic polypropylene containing ultra-high molecular weight polyethylene oriented precursor domains

Melt blends of short ultra-high molecular weight polyethylene (UHMWPE) fibers and isotactic polypropylene (iPP) were subjected to shear at 145 °C, above the melting point of polyethylene (PE). Structural evolution and final morphology were examined by in situ synchrotron X-ray scattering/diffraction as well as ex situ microbeam X-ray diffraction and high resolution scanning electron microscopy, respectively. Results indicate that the presence of oriented UHMWPE molten domains significantly facilitated the crystallization of iPP and enhanced the initial ‘shish-kebab’ structure leading to the final cylindritic morphology. It is argued that shear flow aligns the fibrillar UHMWPE domains, where the interfacial frictions between PE and iPP effectively retards the relaxation of iPP chains, allowing the aligned iPP chains to create a shish-like structure. Nucleation on the iPP shish initiates the folded chain lamellae (kebabs), which grow perpendicularly to the iPP/PE interface.     

Dispersed microfibril‐dominated deformation and fracture behaviors of linear low density polyethylene/isotactic polypropylene blends

Linear low density polyethylene/isotactic polypropylene (LLDPE/iPP) blends, with oriented microfibrils of iPP dispersed in the nearly isotropic LLDPE matrix, has been prepared via melt extrusion drawing and subsequent thermal treatment at 160°C to melt LLDPE matrix. The presence of oriented microfibrils of iPP in the LLDPE/iPP blends not only promotes the homogenous deformation, with no drop of nominal stress around yield point, but also enhances the fracture toughness significantly. The specific Essential Work of Fracture w_e, which is a pure crack resistance parameter per ligament area unit, is 24.7 and 33.6 N/mm for the blends with 15 and 30 wt % microfibrils of iPP, respectively. Moreover, with the deduced deformation parameters, such as true yield stress and strain hardening modulus, the relationship between deformation parameters and fracture toughness is explored. It is demonstrated that the fracture toughness can be well correlated with the ratio of true yield stress to strain hardening modulus σ_ty/G, and either a decrease in yield stress or an increase in strain hardening can improve fracture toughness. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1291–1298, 2007     

Morphology and thermal properties in the binary blends of poly(propylene-co-ethylene) copolymer and isotactic polypropylene with polyethylene

The morphology and thermal properties of isothermal crystallized binary blends of poly(propylene-co-ethylene) copolymer (PP-co-PE) and isotactic polypropylene (iPP) with low molecular weight polyethylene (PE) were studied with differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). In PP-co-PE/PE binary blends, however, the connected PE acted as a phase separating agent to promote phase separation for PP-co-PE/PE binary blends during crystallization. Therefore, the thermal properties of PP-co-PE/PE presented double melting peaks of PE and a single melting temperature of PP during melting trace; on the other hand, at cooling trace, the connected PE promoted crystallization rate because of enhanced segmental mobility of PP-co-PE during crystallization. At isothermal crystallization temperature between the melting points of iPP and PE, the binary blend was a crystalline/amorphous system resulting in persistent remarkable molten PE separated domains in the broken iPP spherulite. And then, when temperature was quenched to room temperature, the melted PE separated domains were crystallized that presented a crystalline/crystalline system and formed the intra-spherulite segregation morphology: these PE separated domains/droplet crystals contained mixed diluent PE with connected PE components. On the other hand, in the iPP/PE binary blends, the thermal properties showed only single melting peaks for both PE and iPP. Moreover, the glass transition temperature of iPP shifted to lower temperature with increasing PE content, implying that the diluent PE molecules were miscible with iPP to form two interfibrillar segregation morphologies: iPP-rich and PE-rich spherulites. In this work, therefore, we considered that the connected PE in PP-co-PE functioned as an effective phase separating agent for PP and diluent PE may be due to the miscibility between connected PE and diluent PE larger than that between PP and dispersed PE.     

Hierarchical structure and unique impact behavior of polypropylene/ethylene-octene copolymer blends as obtained via dynamic packing injection molding

Controlling the hierarchical structure of melt-processed polymers is vital to “structuring” processing and tailoring properties of the product. In this work, polypropylene (PP)/octene-ethylene copolymer (POE) blends were injection-molded using so-called dynamic packing injection technique, which imposed oscillatory shear on the gradually cooled melt during the packing solidification stage. In this way, samples with highly oriented PP matrix and elongated POE particles were obtained. Most interestingly, it was found for the first time that the elongated POE particles could not improve any impact toughness of oriented PP, which is completely different from that for the isotropic ones. Polarized optical microscope, scanning electron microscope, micro-Fourier transform infrared spectroscopy and differential scanning calorimetry were used to characterize the microstructures along sample thickness. The crack-initiation term, impact fractured surface and cross-section of the impact surface were inspected to understand the difference in impact behavior between the oriented PP/POE blends and their isotropic counterparts. The results show that massive crazing or plastic flow of the matrix could not be effectively initiated in the oriented blends. Our work provides a good example for better understanding structure–property relationship of polymers via well controlling their internal hierarchical structure.     

Alternating multilayer structure of polyethylene/polypropylene blends obtained through injection molding

The formation of multilayer structures in the high‐speed thin wall injection‐molded samples of high‐density polyethylene/isotactic polypropylene blends is reported. Based on the morphology development in injection runner and mold, a possible formation mechanism of multilayer structure was proposed in this study. Injection molding could be used as a simple and an effective method for the fabrication of multifunctional multilayer structure. This work is interesting and important for scientific research as well as several potential applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Super polyolefin blends achieved via dynamic packing injection molding: Tensile strength

The tensile strength of some polyolefin blends, HDPE/PP, HDPE/LDPE, HDPE/ LLDPE, and PP/LLDPE, achieved by dynamic packing injection molding have been investigated as a function of composition and melt temperature. Molecular architecture and phase behavior play an important role in chain orientation, hence the tensile strength. For HDPE, which has a linear structure, the highest enhancement of tensile strength is obtained. LDPE, which has a highly branched structure, the smallest enhancement is seen. PP and LLDPE lie in between. Super polyolefin blends with high tensile strength and high elongation have been obtained by this method. The shear‐induced morphologies with core in the center, oriented zone surrounding the core and skin layer were observed in the cross‐section areas of the samples. The tensile strength was found to be directly proportional to the area of the oriented zone. When the area of oriented zone is less than 35%, the tensile strength is not only the orientation dependency but the blending components dependency as well. When the area of oriented zone is more than 35%, however, our new finding is that the orientation will be the dominating parameter to determine the tensile strength of the blends, independent of the components, the composition, molecular architecture, phase behavior, and crystal morphology. The maximum tensile strength for all the polyolefin blends is extrapolated as to 230MPa, as the area of oriented zone reaches to 100%. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 236–243, 2002     

Super polyolefin blends achieved via dynamic packing injection molding: the morphology and mechanical properties of HDPE/EVA blends

As a part of long-term project aimed at super polyolefin blends, in this work, we report the mechanical reinforcement and phase morphology of the blends of high-density polyethylene (HDPE) and ethylene vinyl acetate (EVA) achieved by dynamic packing injection molding. The shear stress (achieved by dynamic packing injection molding) and interfacial interaction (obtained by using EVA with different VA content) have a great effect on phase morphology and thus mechanical properties. The super HDPE/EVA blends having high modulus (1.9–2.2 GPa), high tensile strength (100–120 MPa) and high impact strength (six times as that of pure HDPE) have been prepared by controlling the phase separation, molecular orientation and crystal morphology of the blends. The phase inversion was also found to shift towards lower EVA content under shear stress. The enhancement of tensile strength and modulus originates from the formation of oriented layer, while the high impact strength is related to shear induced phase morphology. DSC studies indicated that the shish kebab crystal structure that also contributes to the enhancement of tensile strength is formed in the oriented layer. The dramatic improvement of impact strength may result from the formation of microfibers and elongated EVA particles along the flow direction. Wu's toughening theory was found non-applicable for the elongated and oriented rubber particles, and a brittle–ductile–brittle transition was observed with increasing EVA content.     

Isothermal crystallization of isotactic polypropylene blended with low molecular weight atactic polypropylene. Part I. Thermal properties and morphology development

The thermal properties and morphology development of isotactic polypropylene (iPP) homopolymer and blended with low molecules weigh atactic polypropylene (aPP) at different isothermal crystallization temperature were studied with differential scanning calorimeter and wide-angle X-ray scattering. The results of DSC show that aPP is local miscible with iPP in the amorphous region and presented a phase transition temperature at T_c=120 °C. However, below this transition temperature, imperfect α-form crystal were obtained and leading to two endotherms. While, above this transition temperature, more perfect α- and γ-form crystals were formed which only a single endotherm was observed. In addition, the results of WAXD indicate that the contents of the γ-form of iPP remarkably depend both on the aPP content and isothermal crystallization temperature. Pure iPP crystallized was characterized by the appearance of α- and γ-forms coexisting. Moreover, the highest intensity of second peak, i.e. the (0 0 8) of γ-form coexisting with (0 4 0) of α-form, and crystallinity were obtained for blended with 20% of aPP, the γ-form content almost disappeared for iPP/aPP blended with 50% aPP content. Therefore, detailed analysis of the WAXD patterns indicates that at small amount aPP lead to increasing the crystallinity of iPP blend, at larger amount aPP, while decreases crystallinity of iPP blends with increasing aPP content. On the other hand, the normalized crystallinity of iPP molecules increases with increasing aPP content. These results describe that the diluent aPP molecular promotes growth rate of iPP because the diluent aPP molecular increases the mobility of iPP and reduces the entanglement between iPP molecules during crystallization.     

Phase morphology and toughening mechanism of polyamide 6/EPDM-g-MA blends obtained via dynamic packing injection molding

One of the most important findings in polymer-toughening is known as the critical matrix ligament thickness (τ_c) theory, which is directly related to both rubber concentration and average size of particles. All these studies assume that rubber particles are spherical and randomly distributed in the matrix. Rubber particles may be stretched and oriented along the shear flow direction in real processing. In this paper the effect of stretched and oriented rubber particles on the impact strength of PA6/EPDM-g-MA blends have been studied via dynamic packing injection molding (DPIM). The impact strength of specimens obtained by DPIM was found substantially increase at all the blends investigated, compared with the one obtained via conventional injection molding. Particularly, more than 30 kJ m⁻² increase of the impact strength was observed for specimens with a higher rubber content (more than 15 wt%). SEM results showed a remarkably decrease of rubber particle size and more uniform dispersion of the dynamic molded specimens. This can be attributed to the shear induced reaction at the interface between polyamide 6 and EPDM-g-MA during the packing stage. The rubber particles were found stretched along the melt shear flow direction when it is content above 15 wt%. A master curve can be also constructed by plotting the impact strength versus the inter-particle distance, indicating that Wu's criterion still works for blends with stretched and oriented rubber particles when the crack propagation direction is perpendicular to the orientation direction of rubber particles. The observed higher impact strength in dynamic specimens could be due to, in part, the enhanced flexural stiffness, which will absorb more energy during impact process when the fracture of IZOD bars is incomplete, but more importantly due to the existence of the stretched and oriented rubber particles, which are more efficient in slowing the velocity of crack propagation and thus cause higher impact resistance when the fracture propagation direction is perpendicular to the rubber oriented direction.     

Morphology and mechanical properties of injection‐molded ultrahigh molecular weight polyethylene/polypropylene blends and comparison with compression molding

The mechanical properties and morphology of UHMWPE/PP(80/20) blend molded by injection and compression‐molding were investigated comparatively. The results showed that the injection‐molded part had obviously higher Young's modulus and yield strength, and much lower elongation at break and impact strength, than compression‐molded one. A skin‐core structure was formed during injection molding in which UHMWPE particles elongated highly in the skin and the orientation was much weakened in the core. In the compression‐molded part, the phase morphology was isotropic from the skin to the core section. The difference in consolidation degree between two molded parts that the compression molded part consolidated better than the injection one was also clearly shown. In addition, compositional analysis revealed that there was more PP in the skin than core for the injection‐molded part, whereas opposite case occurred to the compression‐molded one. All these factors together accounted for the different behavior in mechanical properties for two molded parts. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Shear-induced crystallization of isotactic polypropylene within the oriented scaffold of noncrystalline ultrahigh molecular weight polyethylene

Shear-induced crystallization of isotactic polypropylene (iPP) within the oriented scaffolds of noncrystalline ultrahigh molecular weight polyethylene (UHMWPE) was investigated by means of in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The study was carried out using iPP/UHMWPE blends under isothermal crystallization at 145 °C (i.e., above the melting point of polyethylene) and step shear (shear rate=60 s⁻¹, duration=5 s) conditions. The oriented and isotropic iPP crystalline phases were extracted from the 2D WAXD pattern, and their kinetics data were evaluated with the Avrami equation. The dominant component in the oriented iPP phase was a kebab structure, whose nanostructure dimensions were determined by a novel SAXS analysis scheme. The minor non-crystalline but oriented UHMWPE component played a key role in the nucleation of iPP, which could be explained in terms of mutual diffusion at the interface, resulting in a significant increase in the relaxation time of iPP chains. As a result, after shear, the interfacial iPP chains could also retain their orientation and formed oriented nuclei to initiate the kebab growth.     

The morphology and mechanical properties of dynamic packing injection molded PP/PS blends

As a part of long-term project aimed at super polyolefin blends, in this work, we report the mechanical reinforcement and phase morphology of the immiscible blends of polypropylene (PP) and polystyrene (PS) achieved by dynamic packing injection molding (DPIM). The shear stress (achieved by DPIM) and interfacial interaction (obtained by using styrene-butadiene-styrene (SBS) as a compatibilizer) have a great effect on phase morphology thus mechanical properties. The shear-induced morphology with core in the center and oriented zone surrounding the core was observed in the cross-section areas of the samples. The phase inversion was also found to shift towards lower PS content under shear stress, at 70 wt% in the core and 30 wt% in the oriented zone, compared with 80 wt% for static samples (without shear). The tensile strength, tensile modules and impact strength were found largely increase by means of either shear stress or compatibilizer. The PS particle size is greatly reduced with adding of SBS, and the reduced particle size results in greater resistance to deformation, which causes the co-continuous structure at oriented zone change into droplet morphology. The morphology resulting from blending and processing was discussed based on effect of interfacial tension, shear rate, phase viscosity ratio and composition. The observed change of mechanical properties was explained based on the combined effect of phase morphology (droplet-matrix or co-continuous phase) and molecular orientation under shear stress.     

Tensile properties and interfacial adhesion of silicone rubber/polyethylene blends by reactive blending

Two‐phase blends of silicone rubber (SR) and linear low density polyethylene (LLDPE) were prepared by reactive blending using peroxide crosslinking agent of SR. The tensile strength and elastic modulus of the SR were found to be increased by reactive blending with LLDPE without sacrifice of the elongation. The improvement of the tensile properties is attributed to the strong adhesion at the interface between SR matrix and LLDPE domain due to the chemical reaction by peroxide. The observation by polarized optical microscopies revealed that the debonding did not occur at the interface, but the LLDPE domains were elongated to longer one. Despite the elongation of the LLDPE domain, the blends exhibited good deformation recoverability. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46192.     

Polypropylene/polystyrene blends prepared by diffusion and subsequent polymerization in polypropylene pellets after injection molding

PP/PS quasi‐nanoblend pellets were synthesized by diffusion and subsequent polymerization of styrene in iPP pellets via a two‐step procedure and then processed by injection molding. The PS distributions along the thickness direction of the molded bars were investigated by Micro‐FTIR, showing almost homogeneous distribution no matter whether the PS distribution in the blend pellets is homogeneous. The morphology of the molded bars was investigated by FESEM, revealing two types of particles (small spherical and bigger irregular‐shaped complex aggregates) and good interfacial adhesion between particles and matrix. The particles are mainly in nano and submicron sizes, and only few particles approach 1 μm. The mechanical properties of the molded bars were evaluated by uniaxial tensile testing, showing a significant reinforcing effect without significantly loosing ductility. The yield strength of all the blends increase 20–27% compared to neat PP and the elongations at break are all over 300%. The remarkable mechanical properties of the molded bars were correlated with their morphology. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43983.     

Morphology and mechanical behavior of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends and fibers

The crystallization morphologies and mechanical behaviors of iPP/sPP blends and the corresponding fibers were investigated in the present work. For all the investigated iPP/sPP blends, the starting crystallization temperature of sPP during cooling process was significantly increased with increasing iPP content. The iPP/sPP blends are strongly immiscible at the conventional melt processing temperatures, in consistence with the literature results. As isothermally crystallized at 130 °C, sPP still keeps melt state, while iPP component is able to crystallize and the spherulites become imperfect accompanied by decreasing of the crystallite size as sPP content increases. The addition of sPP decreases the crystallinity of iPP/sPP blends and fibers. The storage modulus, E′, of the iPP/sPP blends is higher than that of sPP homopolymer in the temperature range from −90 to 100 °C. The iPP/sPP fibers can be prepared favorably by melt-spinning. As sPP content exceeds 70%, the elastic recovery of the iPP/sPP fibers is approximately equal to that of sPP homopolymer fiber. The drawability of the as-spun fiber of iPP/sPP (50/50) is better than that of sPP fiber, which improves the fiber processing performance and enhances the mechanical properties of the final product. The drawn fiber of sPP presents good elastic behavior within the range of 50% deformation, whereas the elastic property of the iPP/sPP (50/50) fiber slightly decreases, but still much better than that of iPP fiber.     

Dispersion and mechanical properties of polypropylene/multiwall carbon nanotubes composites obtained via dynamic packing injection molding

In this article, polypropylene (PP)/multiwall carbon nanotubes (MWNTs) composites were prepared through dynamic packing injection molding, in which the oscillatory shear was exerted on the molten composite during packing and solidification stage of injection‐molding. A simultaneous increase of tensile strength and impact strength has been achieved for PP/MWNTs composites containing only 0.6 wt % MWNTs. Particularly, the impact strength was found increased by almost 50% at such low MWNTs content. These improvements in properties were attributed to uniform dispersion and possible orientation of nanotube induced by shear stress. It was suggested that the dynamic packing injection molding could provide much strong shear force for better dispersion of MWNTs in PP matrix, on one hand, but breakdown the aspect ratio of MWNTs, on the other. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1880–1886, 2007     

Self‐interference flow of an isotactic polypropylene melt in a cavity during injection molding. I. Effect of the self‐interference flow on the properties

The self‐interference flow (SIF) of a melt in a cavity during injection molding is introduced. It comes from two streams of the melt being split by a patented mold gate called a twin gate. The effects of this flow on the static and dynamic mechanical properties, thickness distribution, and shrinkage in the transverse direction (TD) of injection‐molded isotactic polypropylene parts are discussed. SIF has an influence on the static mechanical properties, especially the impact strength. There are slight increases in the tensile strength and Young's modulus and an increase of approximately 70–90% in the impact strength in comparison with the properties of samples obtained by a conventional flow process with a common pin gate. Dynamic mechanical thermal analysis studies show an increase in the storage modulus for SIF samples. Results obtained from research into the effect of the mold temperature and injection pressure on the impact strength show that the impact strength of SIF specimens has a weaker dependence on the mold temperature and injection pressure. In addition, the flow brings a more uniform thickness distribution and a smaller shrinkage in the TD to SIF samples. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2784–2790, 2003