Morphological distribution of polymeric nucleating agents in injection‐molded isotactic polypropylene plates and its influence on nucleating efficiency

The morphological development of a special polymeric nucleating agent [acrylonitrile–styrene copolymer (SAN)] in the isotactic polypropylene (iPP) matrix in the process of injection molding has been investigated by means of wide‐angle X‐ray diffraction and scanning electron microscope. The current experimental results indicate that the shear field, in combination with the temperature gradient, has great influence on the morphological distribution of SAN in the process of injection molding. For injection‐molded SAN/iPP specimens with higher SAN concentration (≥4%), SAN assembles to many microspheres and disperses uniformly in the isotropic core region; while from isotropic core region to oriented skin region, these SAN microspheres are gradually stretched into fibrils as a result of shear effect. On the contrary, for the specimens with lower SAN concentration (<4%), only microspheres can be observed in the core region and the skin region. At the same time, SAN has been proved to be a kind of special β‐nucleating agent. The addition of SAN into iPP helps enhances the crystallinity and the content of β crystal form of injection‐molded specimen. The morphology and the distribution of SAN in iPP matrix have great influence on the SAN's nucleating activity, which will ultimately affect the final crystalline structures of injection‐molded specimens. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Hierarchical structure of injection-molded bars of HDPE/MWCNTs composites with novel nanohybrid shish-kebab

Injection-molded products usually show hierarchical structure from skin to core due to the existence of shear gradient and temperature gradient. Investigating the hierarchical structure is helpful to better understand the structure-property relationship of injection-molded sample, which is important for design and preparation of polymer products with high performance. In this work, the hierarchical structures of injection-molded bars of high-density polyethylene (HDPE)/multi-walled carbon nanotubes (MWCNTs) composite were explored by examining the microstructure and crystal morphology, layer by layer, along the sample thickness, using SEM, DSC and 2D-WAXS. To enhance the shear effect, a so-called dynamic packing injection molding (DPIM) technique was used to prepare the molded bar with high orientation level. Interestingly, SEM revealed that in the skin and core zones, the lamellae of PE anchored randomly on the surface of MWCNTs, while well-defined nanohybrid shish-kebab (NHSK) entities, in which fibrillous carbon nanotubes (CNTs) act as shish while HDPE lamellae act as kebab, exist in the oriented zone. The changed NHSK crystal structure along the thickness direction of molded bar is considered as due to the shear gradient and thermal gradient in injection molding. And the underlying origin of in situ formation of NHSK under shear effects is discussed based on experimental observations.     

Effect of Melt‐Memory on the Crystal Polymorphism in Molded Isotactic Polypropylene

The influence of γ‐quinacridone as a β‐crystal nucleating agent in injection molded isotactic polypropylene (iPP) is discussed. Samples are injection molded and characterized via polarized‐light optical microscopy and X‐ray diffraction. Mold‐filling simulation is used to understand the shear and cooling processes during sample preparation. The cooling rate associated with the quench near the mold wall is estimated to be greater than 600 K s^?1 using simulation, confirming previous studies that β‐crystal growth is not supported at that cooling rate. The non‐nucleated samples form β‐crystals at a distance of 100–300 µm from the skin and in the core of the sample, which is not expected based on quiescent cooling data. Since the mold‐filling simulation does not predict shear in the core, the formation of the β‐crystals formed in this region is attributed to shear‐induced crystallization effects in the injection unit of the molding machine that are not modeled in flow simulation, as they are typically excluded from any molding simulation analysis. This “melt‐memory” effect has shown to be significant, and it is suggested that the prediction of final properties of injection moldings requires understanding and knowledge of the entire shear history of the material including that of the injection unit.     

The interplay of thermodynamics and shear on the dispersion of polymer nanocomposite

Most of work on polymer/layered silicate nanocomposites (PLSN) focused on the importance of the chemistry used to modify the surface of the clay, usually montmorillonite (MMT) and characterization of the nano-scale structure obtained in the last decade. The role and importance of processing has been paid attention only recently. Our purpose of this study is to determine the effect of the chemistry and shear on the dispersion of clay in polymer matrix via dynamic packing injection molding (DPIM), in which the melt is firstly injected into the mold then forced to move repeatedly in a chamber by two pistons that moved reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part. A special orientation region between the skin region and the core region is produced via imposing the reversible shear effect during the cooling of the melt. The initial MMT and organic modified MMT (abbreviated as OMMT) were directly injection molded with powdered iPP in the DPIM without pre-extrusion. Without shearing, only an intercalated structure was obtained through organic modification and the diameter of OMMT particles has a significant reduction compared to the initial MMT. Under the effect of shear, however, an intercalated morphology in the oriented region and an exfoliated morphology in the core region can be achieved. Our result suggests that both the chemistry and shear are important to determine the dispersion of clay in polymer matrix. PP/clay nanocomposites with excellent impact strength can be prepared via simple one step, direct injection molding without a conventional pre-extrusion, by dynamic packing injection molding. The observed change of impact strength could be related to the local hierarchical structure developed during the processing.     

Study of the molecular orientation heterogeneity in polypropylene injection‐molded parts by Raman spectroscopy

Polarized Raman microspectroscopy has been used to study oriented‐skin layers induced in injection‐molded isotactic polypropylene (iPP) parts. A method based on the intensity sensitivity of several Raman bands to laser light polarization was employed to estimate the degree of molecular orientation in iPP. The skin‐core molecular orientation heterogeneity in injection‐molded iPP is then evaluated via two different experimental methods. Results show that an in‐depth profile using micro‐Raman confocal technique is as valuable as an edge profile performed on a sample cross‐section because both are correlated with optical microscopy measurements. Both Raman measurements are in good agreement with optical microscopy measurements. The skin development was found to be narrowly related to the shear strain rate at the mold walls. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Comparison of structure development in injection molding of isotactic and syndiotactic polypropylenes

A comparative study of the crystallization and orientation development in injection molding isotactic and syndiotactic polypropylenes was made. The injection molded samples were characterized using wide angle X‐ray diffraction (WAXD) techniques and birefringence. The injection molded isotactic polypropylene samples formed well‐defined sublayers (skin, shear and core zones) and exhibited polymorphic crystal structures of the monoclinic α‐form and the hexagonal β‐form. Considerable amounts of β‐form crystal were formed in the shear and core zones, depending on the injection pressure or on the packing pressure. The isotactic polypropylene samples had relatively high frozen‐in orientations in the skin layer and the shear zone. The injection molded syndiotactic polypropylene exhibited the disordered Form I structure, but it did not appear to crystallize during the mold‐filling stage because of its slow crystallization rate and to develop a distinct shear zone. The core zone orientation was greatly increased by application of high packing pressure. The isotactic polypropylene samples exhibited much higher birefringence than the syndiotactic polypropylene samples at the skin and shear layers, whereas both materials exhibited similar levels of crystalline orientation in these layers.     

The hierarchy structure and orientation of high density polyethylene obtained via dynamic packing injection molding

The evaluation of microstructure and crystal morphology in injected-molded bar becomes much complicated because of the existence of a shear gradient and a temperature gradient from the skin to the core of the samples. To understand the relationship between shear rate-molecular weight and oriented structure of injection molded bar, in this work, the hierarchy structure and the effect of molecular weight on the formation of shish-kebab structure were investigated by examining the lamellar structure of injection molded samples of high density polyethylene (HDPE) with different melt flow index (MFI), layer by layer, along the sample thickness. To enhance the shear effect, so-called dynamic packing injection molding (DPIM), in which the melt is firstly injected into the mold and then forced to move repeatedly in a chamber by two pistons that move reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part, was used to obtain the molded bar. Furthermore, a small amount of ultra-high molecular weight polyethylene (UHMWPE) was added into HDPE to explore the effect of UHMWPE on the crystal morphology and orientation. Our results indicated (1) that the overall orientation in the molded bar increased with decreased MFI, and a small amount of UHMWPE could enhance substantially HDPE orientation; (2) at the skin, there existed intertwined lamellae constituting an interlocked lamellar assembly, a typical shish-kebab structure gradually developed from the subskin-layer to the core, with increased shish content toward the center, but in the core was a spherulite-like superstructure with randomly distributed lamellae; (3) UHMWPE played an important role not only in the formation of shish, but also in the transformation from spherulite to shish-kebab oriented structure for HDPE with a low molecular weight (high MFI).     

Gas‐assisted injection molded polypropylene: The skin‐core structure

In this article, gas penetration‐induced skin‐core structure of isotactic polypropylene(iPP), which is molded by gas‐assisted injection molding at different gas pressures, was investigated. For comparison, the counterpart was also molded by conventional injection molding (CIM) using the same processing parameters but without gas penetration. They were characterized via PLM, DSC, and SEM. And the crystal morphology at different gas pressures was principally concerned. For the GAIM parts, highly oriented structure is formed in the skin zone, and much less oriented structure in the inner zone (near the gas channel surface). Furthermore, it is suggested that the naked shish structure can be developed in the skin zone of GAIM part, which is molded at higher gas pressures, and shish‐kebab structure is mainly formed in the skin zone of that, which is molded at lower gas pressure. However, for the CIM part, from the skin to the core zone, the dominant morphological feature is spherulite. In a word, the presence of gas penetration notably enhances the oriented structure formation and gives rise to the skin‐core structure. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Oriented re-crystallization of polypropylene through partial melting and its dramatic influence on mechanical properties

A promising method for toughening isotactic polypropylene (iPP) without loss of stiffness is to enhance molecular orientation during processing. However, conventional injection molding cannot impart remarkable molecular orientation to the molded bars as a result of retarded lamellar growth upon rapid solidification and thus has little contribution to the toughness enhancement. Herein, we demonstrated that molecular orientation in the conventional injection molded iPP bars has been notably enhanced through oriented re-crystallization after partial melting at 175 °C. As a result, about 3.5 times increase in the toughness, in parallel with apparent strength enhancement, has been gained in these re-crystallized bars. Our findings offer a simple and economic approach to produce tough iPP and maybe other semi-crystalline polymers in practice.     

Unique clay orientation in the injection-molded bar of isotactic polypropylene/clay nanocomposite

In this article, the injection-molded bars of isotactic polypropylene/organoclay nanocomposite with different clay contents have been obtained via dynamic packing injection molding (DPIM). The oriented microstructure including layered nanoparticles and PP lamellae has been inspected through 2D-WAXS analyses along the sample thickness of the molded bars. Depending on the clay content and sample thickness, various oriented clay structures with nanoparticles uniplanar-axially oriented parallel to the surface of molded bar, or partially tumbled around the flow axis of the molded bar, or even a random orientation, could be observed. The observed orientation behavior of nanoparticles could be temporarily elucidated as the results of the sensitive response of layered nanoparticles to shear deformation and the structural recovery of clay network assisted by the electrostatic attraction existing between adjacent nanoplatelets.     

Enhanced β‐crystal formation of isotactic polypropylene under the combined effects of acid‐corroded glass fiber and preshear

The acid‐corroded glass fiber (GF)/isotactic polypropylene (iPP) composite was injection molded by mixing–injection molding (MIM). Through this method, preshear can be imposed on melt during mix–plasticization process. The crystalline structure across the thickness direction of the injection‐molded bars was investigated by wide‐angle X‐ray diffraction and differential scanning calorimetry (DSC). It was unexpectedly found that, in core region, the acid‐corroded GF/iPP sample has the highest content of β‐form crystals, followed by uncorroded GF/iPP and neat iPP. Additionally, the crystalline morphology was investigated by polarized optical microscopy (POM) and scanning electron microscopy, and the results showed that β‐transcrystallization is preferably present in the acid‐corroded GF/iPP system. Confirmed by POM and DSC, the acid‐corroded GF shows strong β‐nucleation ability to iPP under static condition. Combined with the main features of MIM, three β‐nucleation origins in the acid‐corroded GF/iPP system under injection molding condition are proposed: (1) precursors induced by preshear in the barrel, (2) row‐nuclei induced by local shear, and (3) the acid‐corroded GF nuclei. POLYM. COMPOS. 34:1250–1260, 2013. © 2013 Society of Plastics Engineers     

Crystalline morphology of isotactic polypropylene (iPP) in injection molded poly(ethylene terephthalate) (PET)/iPP microfibrillar blends

The poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) in situ microfibrillar blends have been prepared through a “slit die extrusion-hot stretch-quenching” process, in which PET assumes microfibrils with 0.5-15 μm in diameter depending on the hot stretching ratios (HSR, the area of the transverse section of the die to the area of the transverse section of the extrudate). The injection molded specimens of virgin iPP and the PET/iPP blends were prepared by conventional injection molding (CIM) and by shear controlled orientation injection molding (SCORIM), respectively. The effect of shear stress and PET phase with different shape on superstructures and their distribution of injection molded microfibrillar samples were investigated by means of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). The shear (or elongational) flow during CIM and SCORIM can induce oriented lamellae (i.e. kebabs induced by shish). The shish-kebab structure appears not only in the skin and intermediated layers of CIM samples, but also in the whole region of SCORIM samples. For the neat iPP samples, a more “stretched” shish-kebab structure with higher orientation degree can be obtained in the interior region (intermediate and core layers) by the SCORIM method; moreover, the SCORIM can result in the growth of β-form crystal both in intermediate layer and in core layer, which only appears in intermediate layer of the neat iPP samples obtained by CIM. For the PET/iPP blends, interestingly, the addition of microfibrils as well as their aspect ratios can affect the orientation degree of kebabs only in the intermediate layers, and the addition of microfibrils with a low aspect ratio can bring out a considerable increase in the orientation degree of kebabs along the flow direction. However, for the SCORIM, the addition of microfibrils seems to be a minor effect on the orientation degree of kebabs, and it tends to hamper the formation of a more “stretched” shish-kebab structure and suppresses the growth of β-form crystal distinctly. Furthermore, It appears from experiment that γ-form crystals can grow successfully in this oriented iPP melt with the synergistic effect of shear and pressure only when the growth of β crystals can be restrained by some factors, such as the PET dispersed phase and thermal conditions (cooling rate).     

Injection molding‐induced morphology of thermoplastic polymer blends

Various morphologies induced by injection molding are reviewed here. The structural hierarchy in the direction perpendicular to flow direction always appears in the injection‐molded blends. The hierarchy involves three aspects: phase behavior hierarchy for the dispersed phase, crystalline or orientated structural hierarchy for the matrix, as well as hierarchy structure of co‐continuous phase morphology. There are usually three layers in the injection‐molded bars, i.e., the skin layer and the core region as well as the shear zone between them. The morphology of the dispersed phase is usually different between the skin zone (usually including shear zone) and the core region: deformed particles may exist in the skin layer, whereas spherical droplets in the core region. Moreover, the size of crystals usually increases with the increase of the distance to the surface due to the increase of crystallization time, and the orientation is often severe near the surface due to the high shear stress. Defects of the injection‐molded parts, such as weldline and flow marks, are also reviewed. The dispersed phase morphology in the weldline region is significantly different from the region out of the weldline and the morphology of the flow marks is also different from the out‐of‐flow marks. POLYM. ENG. SCI., 45:1655–1665, 2005. © 2005 Society of Plastics Engineers     

Hierarchic structure and mechanical property of glass fiber reinforced isotactic polypropylene composites molded by multiflow vibration injection molding

Maleic anhydride‐grafted polypropylene (Ma‐PP) and β nucleation agents (β‐NA) were used to modify the glass fiber (GF)/isotactic polypropylene (iPP) composite. The interface adhesion, degree of orientation, and crystalline morphologies of the PP/GF composites molded by multiflow vibrate‐injection molding (MFVIM) and conventional injection molding (CIM) were studied by polarized light microscopy (PLM), scanning electronic microscopy (SEM), and X‐ray measurements. Results prove that the interface adhesion was improved by the Ma‐PP; γ crystal was generated by the MFVIM due to the instant high pressure and shear during the multiflow; and a hierarchical structure which has a strengthened skin and a toughened core was formed. As a result, the final PP/GF/β‐NA composite has a 60% increase in tensile strength and 80% improvement in impact strength compare with the CIM pure PP/GF composite. Based on the observations, a modified model is proposed to interpret the strengthening and toughening mechanism. Our work paves the way to obtain high‐performance GF/iPP composites. POLYM. COMPOS., 38:2707–2717, 2017. © 2015 Society of Plastics Engineers     

An Alternating Skin–Core Structure in Melt Multi‐Injection‐Molded Polyethylene

Shish‐kebab, which is endowed with superior strength and modulus, provides the potential to fabricate self‐reinforced polymer products. However, the injection‐molded product usually exhibits a typical skin–core structure, and the shish‐kebab is only located in an extremely thin shear layer. Therefore, the controlling and tailoring of crystal structures in complex flow field to improve the mechanical properties of the injection‐molded sample are still a great challenge. Herein, for the first time, high‐density polyethylene sample with a novel macroscopic alternating skin–core structure is achieved using a melt multi‐injection molding technique. Results show that, with increasing the amount of melt injection, the layers of skin–core structure increase in the form of arithmetic progression, and therefore the tensile strength of the samples progressively increases due to an increase of shish‐kebab content. This study demonstrates a new approach to achieve multilayer homogeneous materials with excellent tensile strength via macroscopic structural design during the practical molding process.     

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     

Enhanced orientation of the water‐assisted injection‐molded ipp in the presence of nucleating agent

The nucleated isotactic polypropylene (iPP) was molded by water‐assisted injection molding. The crystalline morphology and orientation distribution were studied. The results show that shear brought by melt filling and pressurized water penetration can separately induce the formation of oriented structures in skin region (i.e., the region near mold cavity wall) and the water channel region. For virgin iPP, slightly oriented lamellae appear exclusively in the above aforementioned regions. However, shish‐kebab structure occurs not only in skin and water channel region of the iPP containing moderate content of nucleator (0.2 wt%) but also in the whole region of the iPP containing a higher content of nucleator (1 wt%). It is well known that nucleator cannot directly induce the development of shish‐kebab in the absence of shear, thus the results indicate: shear flow actually distributes over a much broader range than expected; in shear field, nucleator is significantly helpful for the shear which is not sufficient to solely induce oriented structure to promote the formation of the oriented structure. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Shear‐Induced Skin‐Core Structure of Molten Isotactic Polypropylene and the Formation of β‐Crystal

The study of crystallization behavior and crystalline morphology of polymer melt under shear flow is of great interest due to the strong effect of flow field on the final properties of polymer products in the practical processing. In this respect, the shearing hot stage provides a unique tool which monitors sensitively the changes in crystalline structure induced by precise experimental conditions. Herein, the impacts of both melting temperature and shear rate on the crystallization behavior of isotactic polypropylene (iPP) melt are investigated. Under static conditions, there are only random spherulite structures. Once shear is involved, the cylindrite‐layers appear near both surfaces of the sample, which is consistent with the skin‐core structure in the injection molded parts. Meanwhile, the β‐crystals can be developed and are related to the molecular orientation, depending on the applied melting temperatures and shear rates. More interestingly, the crystallinity of β‐crystal in the pure iPP can reach 15%. The above results indicate that the melting temperature and shear rate are important factors in determining the β‐form crystal development of iPP matrix.     

The effect of talc on the crystallization of isotactic polypropylene

Isotactic polypropylene (iPP) has been crystallized in the presence of talc under the quiescent state and shear flow of injection molding. The resulting morphology has been investigated by means of polarizing microscopy, transmission electron microscopy, and wide angle X‐ray diffraction. In the quiescent state, the iPP lamellae grew from the surface of talc and the transcrystalline region was formed at the interface between iPP melt and the talc. The nucleation of iPP was very frequent on the cleavage plane of talc. The X‐ray diffraction pattern of the transcrystal showed a*‐axis orientation to the crystal growing direction. In injection‐molded samples of the talc‐filled iPP, the morphology of lamella growing from talc appeared as same as that of the transcrystal. However, the crystalline orientation of injection‐molded talc‐filled iPP, in which the b axis was oriented to the thickness direction and the a* and the c axis was oriented to the flow direction, was quite different from that of the transcrystal. This b‐axis orientation results from the orientation of the plate plane of talc, which induces the nucleation and the crystallization under shear flow. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1693–1703, 2001     

Annealing‐induced structural rearrangement and its toughening effect in injection‐molded isotactic polypropylene

To improve the toughness of isotactic polypropylene (iPP), by blending modification or special processing techniques, is an ongoing pursuit in the polymer community. In this study, the toughness of injection‐molded iPP bars has been enhanced by about four times upon annealing at 140°C, in parallel with slight increasing of stiffness. Through various structural characterizations it has been confirmed that the remarkable toughening effect in the annealed iPP bars mostly results from the enhanced molecular mobility in the amorphous phase due to lamellar perfection, rather than from the change of molecular orientation, crystal modification, crystallinity, and so on. Moreover, it is deduced that the gain of mobile amorphous fraction in the hierarchical iPP bars upon annealing follows the order of skin > intermediate > core, same to that contributing to the toughness enhancement. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers