Hierarchical crystalline structures and dynamic mechanical properties of injection‐molded bars of HDPE: attributes of temperature field

The relationship among the processing parameters, crystalline morphologies and mechanical properties of injected‐molded bar becomes much complicated primarily due to the existence of temperature gradient coupled with the shear gradient along the sample thickness. The effect of thermal gradient field on the microstructural evolution, hierarchical structures and dynamic mechanical properties of high‐density polyethylene parts molded via gas‐assisted injection molding (GAIM) were investigated using scanning electron microscope, differential scanning calorimetry, dynamic mechanical analysis and two‐dimensional wide‐angle X‐ray diffraction. The three‐dimensional temperature profiles during the cooling stage under different melt temperatures of GAIM process were obtained by using a transient heat transfer model of the enthalpy transformation approach, and the phase‐change plateaus were clearly observed in the cooling curves. It was found that a variety of melt temperatures could induce considerable variations of the hierarchical structures, orientation behavior and dynamic mechanical properties of the injection‐molded bars. With reduced melt temperature, GAIM samples with higher molecular orientation and improved dynamic mechanical properties were obtained. Copyright © 2013 John Wiley & Sons, Ltd.     

The mechanism of ultrasonic improvement of weld line strength of injection molded polystyrene and polystyrene/polyethylene blend parts

The effect of ultrasonic oscillations and ultrasonic oscillation‐induced modes on weld line strength of polystyrene(PS) and polystyrene/polyethylene(PS/HDPE) blend was investigated. And the mechanism of ultrasonic improvement of weld line strength of PS and PS/HDPE blend was also studied. The presence of ultrasonic oscillations can enhance the weld line strength of PS and PS/HDPE blend. Compared with mode I(ultrasonic oscillations were induced into mold at the whole process of injection molding), the induced ultrasonic oscillations as mode II(ultrasonic oscillations were induced into mold after injection mold filling) is more effective to increase weld line strength of PS and PS/HDPE blend. The mechanism for ultrasonic improvement of weld line strength of PS and PS/HDPE blend is that the ultrasonic oscillations can improve the molecular diffusion across weld line of the melt at the core, and make against the fusion of melt at the skin. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1520–1530, 2006     

Measuring and simulating morphology gradients in injection‐molded talc‐reinforced isotactic polypropylene

Injection molded polymer parts are known to exhibit structural gradients of crystallinity, crystallite phases and crystallite orientations. The structural variations depend on the geometry, the material properties, and the processing conditions, and affect the mechanical properties of the molded part. We explore the use of raster‐scanning small‐ and wide‐angle X‐ray scattering (SAXS, WAXS) for mapping the microstructure in dogbone specimens of an isotactic polypropylene (PP) homopolymer and a talc‐reinforced isotactic PP compound. The specimens were injection molded with different mold temperatures and injection speeds, and the mapping approach revealed systematic structural heterogeneities and asymmetries. Accompanying numerical simulations of the injection molding process yielded predictions of the flow pattern, including the shear rate distribution and the resulting orientation of the flake‐shaped talc particles. We found a clear correspondence between the experimentally observed data and the simulations, in particular regarding the asymmetry of the orientation distributions relative to the center of the dogbone cross section, caused by asymmetric flow through the entrance of the mold. Furthermore, the shear rate distribution correlated with the occurrence of α‐ and β‐phases. Subtle differences in the crystallized structures along the long axis of the dogbones suggest an explanation to the observation that the specimens studied always tended to break at the same position in tensile tests. The results clearly demonstrate the potential of mapping experiments which combine lateral resolution on macroscopic length scales with the molecular‐scale resolution from scattering. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1157–1167     

Shear‐induced change of exfoliation and orientation in polypropylene/montmorillonite nanocomposites

For the improved dispersion of montmorillonite (MMT) in a polypropylene (PP) matrix, PP/MMT nanocomposites prepared via direct melt intercalation were further subjected to oscillating stress achieved by dynamic packing injection molding. The shear‐induced morphological changes were investigated with an Instron machine, wide‐angle X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. The original nanocomposites possessed a partly intercalated and partly exfoliated morphology. A transformation of the intercalated structure into an exfoliated structure occurred after shearing, and a more homogeneous dispersion of MMT in the PP matrix was obtained. However, the increase of the exfoliated structure was accompanied by the scarifying of the orientation of MMT layers along the shear direction. Some bended or curved MMT layers were found for the first time by TEM after shearing. However, the orientation of PP chains in the PP/MMT nanocomposites became very difficult under an external shear force; this indicated that the molecular motion of PP chains intercalated between MMT layers was highly confined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1–10, 2003     

The effect of shear on mechanical properties and orientation of HDPE/mica composites obtained via dynamic packing injection molding (DPIM)

The interfacial interaction and orientation of filler play important roles in the enhancement of mechanical performances for polymer/inorganic filler composites. Shear has been found to be a very effective way for the enhancement of interfacial interaction and orientation. In this work, we will report our recent efforts on exploring the development of microstructure of high density polyethylene (HDPE)/mica composites in the injection‐molded bars obtained by so‐called dynamic packing injection molding (DPIM), which imposed oscillatory shear on the melt during the solidification stage. The mechanical properties were evaluated by tensile testing and dynamic mechanical analysis (DMA), and the crystal morphology, orientation, and the dispersion of mica were characterized by scanning electron microscopy and two‐dimensional wide‐angle X‐ray scattering. Compared with conventional injection molding, DPIM caused an obvious increase in orientation for both HDPE and mica. More importantly, better dispersion and epitaxial crystallization of HDPE was observed on the edge of the mica in the injection‐molded bar. As a result, increased tensile strength and modulus were obtained, accompanied with a decrease of elongation at break. The obtained data were treated by Halpin–Tsai model, and it turned out that this model could be also used to predict the stiffness of oriented polymer/filler composites. Copyright © 2009 John Wiley & Sons, Ltd.     

Development of morphology and properties of injection‐molded bars of HDPE/PA6 blends along flow direction

The structure and mechanical properties of injection‐molded bars of high‐density polyethylene (HDPE)/PA6 blends were studied in this article. The experimental results showed that the morphologies of injection‐molded bars change gradually along the flow direction, which is tightly related to the melt viscosity and processing conditions. The higher melt viscosity, lower mold temperature, and shorter packing time, restricting the macromolecular relaxation, enhance the difference in morphologies and properties at near and far parts of a mold. An injection‐molded bar (namely H2C5), consisting of 75 wt % of HDPE, 20 wt % of PA6, and 5 wt % of compatibilizer (HDPE‐g‐MAH), showed a greater difference in mechanical properties at near and far parts because of its higher melt viscosity. A clear interface between the skin and core layers of near part in it leads to a much higher impact strength than that of far part. And tensile tests show that its tensile strength of near part is higher than that of far part due to the higher orientation degrees of HDPE matrix and PA6 dispersed phase in near part. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 184–195, 2007     

Morphology evolution and crystalline structure of controlled‐rheology polypropylene in micro‐injection molding

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.     

Study of the structure development during the melt spinning of nylon 6 fiber by on‐line wide‐angle synchrotron X‐ray scattering techniques

The melt spinning of nylon 6 has been studied with on‐line wide‐angle synchrotron X‐ray scattering techniques. The apparatus consisted of a single screw extruder and a metering pump mounted on a horizontal platform that could be translated in the vertical direction allowing a range of distances to be sampled with the X‐ray beam. The structure development, equatorial crystallinity index, and crystalline orientation were studied as a function of take‐up speed and position along the spinline. For low‐speed (50 mpm) situations, the nylon chains crystallize into independent hydrogen bonded sheets that start to interact with each other as their concentration starts to increase. For higher speed situations, the nylon chains crystallize directly into the interacting hydrogen‐bonded sheet structure. Upon conditioning at room temperature for 24 h, this interacting hydrogen‐bonded sheet structure transforms into the well‐known three‐dimensional alpha and gamma phases of nylon 6, probably existing in a shish‐kabob structure. The equatorial crystallinity index increases as distance from the spinneret increases and as take‐up speed decreases. The crystalline orientation function is constant along the spinline for a constant take‐up speed, and increases as take‐up speed is increased. Conditioning further increases both the crystallinity and crystalline orientation of the fibers. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1277–1287, 1999     

Crystallization and melting of isotactic polypropylene crystallized from quiescent melt and stress‐induced localized melt

Temperature dependency of crystalline lamellar thickness during crystallization and subsequent melting in isotactic polypropylene crystallized from both quiescent molten state and stress‐induced localized melt was investigated using small angle X‐ray scattering technique. Both cases yield well‐defined crystallization lines where inverse lamellar thickness is linearly dependent on crystallization temperature with the stretching‐induced crystallization line shifted slightly to smaller thickness direction than the isothermal crystallization one indicating both crystallization processes being mediated a mesomorphic phase. However, crystallites obtained via different routes (quiescent melt or stress‐induced localized melt) show different melting behaviors. The one from isothermal crystallization melted directly without significant changing in lamellar thickness yielding well‐defined melting line whereas stress‐induced crystallites followed a recrystallization line. Such results can be associated with the different extent of stabilization of crystallites obtained through different crystallization routes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 957–963     

Molecular orientation,crystallinity, and flexural modulus correlations in injection molded polypropylene/talc composites

In order to promote better understanding of the structure‐mechanical properties relationships of filled thermoplastic compounds, the molecular orientation and the degree of crystallinity of injection molded talc‐filled isotactic polypropylene (PP) composites were investigated by X‐ray pole figures and wide‐angle X‐ray diffraction (WAXD). The usual orientation of the filler particles, where the plate planes of talc particles are oriented parallel to the surface of injection molding and influence the orientation of the α‐PP crystallites was observed. The PP crystallites show bimodal orientation in which the c‐ and a*‐axes are mixed oriented to the longitudinal direction (LD) and the b‐axis is oriented to the normal direction (ND). It was found that the preferential b‐axis orientation of PP crystallites increases significantly in the presence of talc particles up to 20 wt% in the composites and then levels‐off at higher filler content. WAXD measurements of the degree of crystallinity through the thickness of injection molded PP/talc composites indicated an increasing gradient of PP matrix crystallinity content from the core to the skin layers of the molded plaques. Also, the bulk PP crystallinity content of the composites, as determined by DSC measurements, increased with talc filler concentration. The bulk crystallinity content of PP matrix and the orientation behavior of the matrix PP crystallites and that of the talc particles in composites are influenced by the presence of the filler content and these three composite's microstructure modification factors influence significantly the flexural moduli and the mechanical stiffness anisotropy data (E_LD/E_TD) of the analyzed PP/talc composites. Copyright © 2009 John Wiley & Sons, Ltd.     

Dispersion and distribution of organically modified montmorillonite in nylon‐66 matrix

Nylon‐66 nanocomposites were prepared by melt‐compounding nylon‐66 with an alkyl ammonium surfactant pretreated montmorillonite (MMT). The thermal stability of the organic MMT powders was measured by thermogravimetric analysis. The decomposition of the surfactant on the MMT occurred from 200 to 500 °C. The low onset decomposition temperature of the organic MMT is one shortcoming when it is used to prepare polymer nanocomposites at high melt‐compounding temperatures. To provide greater property enhancement and better thermal stability of the polymer/MMT nanocomposites, it is necessary to develop MMT modified with more thermally stable surfactants. The dispersion and spatial distribution of the organic MMT layers in the nylon‐66 matrix were characterized by X‐ray diffraction. The organic MMT layers were exfoliated but not randomly dispersed in the nylon‐66 matrix. A model was proposed to describe the spatial distribution of the organic MMT layers in an injection‐molded rectangular bar of nylon‐66/organic MMT nanocomposites. Most organic MMT layers were oriented in the injection‐molding direction. Layers near the four surfaces of the bar were parallel to their corresponding surfaces; whereas those in the bulk differed from the near‐surface layers and rotated themselves about the injection‐molding direction. The influence of the spatial distribution of the organic MMT on crystallization of nylon‐66 was also investigated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1234–1243, 2003     

Interlocking shish‐kebab morphology in polybutene‐1

The aim of this research was to explore the effect of shear‐controlled orientation injection molding (SCORIM) on polybutene‐1 (PB‐1). This article describes the methods and processing conditions used for injection molding and discusses the properties of the moldings. Both conventional and SCORIM have been used for the production of moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt that facilitates enhanced molecular alignment. The effect of the process was investigated by performing mechanical tests, X‐ray studies, differential scanning calorimetric studies, polarized light microscopy, and atomic force microscopy (AFM). Moldings exhibited an improved mechanical performance as compared with conventional moldings. Young's modulus was increased over twofold, and the impact energy was enhanced by 60%. The improvement in mechanical performance was combined with an increase in crystallinity and enhanced molecular orientation. The application of SCORIM also favored the formation of the stable Form I′ in PB‐1. The formation of interlocking shish‐kebab morphology following the application of SCORIM was observed in the AFM studies. Relationships between the mechanical properties of PB‐1 and the micromorphologies formed during processing are demonstrated. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1828–1834, 2002     

Structure Evolution upon Uniaxial Drawing Skin‐ and Core‐Layers of Injection‐Molded Isotactic Polypropylene by In Situ Synchrotron X‐ray Scattering

The structure evolution of the oriented layer (skin) and unoriented layer (core) from injection‐molded isotactic polypropylene samples upon uniaxial drawing is probed by in situ synchrotron X‐ray scattering. The X‐ray data analysis approach, called “halo method”, is used to semiquantitatively identify the transformation process of crystal phase upon uniaxial drawing. The results verify the validation of the stress‐induced crystal fragmentation and recrystallization process in the deformation of the injection‐molded samples under different temperatures. Furthermore, the end of strain softening region in the engineering stress‐strain curves explicitly corresponds to the transition point from the stress‐induced crystal fragmentation to recrystallization process. Basically, the skin and core layers of the injection‐molded parts share the similar deformation mechanism as aforementioned. The stretching temperature which dramatically affects the relative strength between the entanglement‐induced tie chains and the adjacent crystalline lamellae determines the crystal structural evolution upon drawing. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1618–1631     

A study on the micro‐injection molding of multi‐cavity ultra‐thin parts

In this study, we report the micro‐injection molding of ultra‐thin parts (100, 250, and 500 µm). The results show that the flow resistance increases as the cavity becomes thinner. The melt front is not symmetric when filling a four‐cavity ultra‐thin part and filling the eight‐cavity mold under a low temperature. If we increase the mold temperature or cavity thickness, the melt front becomes symmetric. Finally, we construct the operation windows of molding for three kinds of plastics (PS, PMMA, PC) and provide a molding range based on mold temperature and injections speed. Meanwhile, the relationship between the thickness and the operation windows are also investigated. The thinner the cavity is, the smaller the operation window is. We need to increase the injection speed significantly for molding the ultra‐thin parts with micro‐features on both surfaces which are 60 µm in thickness. Furthermore, we succeed in molding 30 µm ultra‐thin parts in this experiment. Copyright © 2009 John Wiley & Sons, Ltd.     

Hierarchical crystalline structures induced by temperature profile in HDPE bars during melt penetration process

The hierarchical crystalline morphologies and orientation structures across the thickness direction in high-density polyethylene(HDPE) molded bars were investigated via a novel melt-penetrating processing method named multi-melt multi-injection molding(M3IM). The samples with various mold temperatures(20, 40 and 60 °C) were prepared, and the effects of the external temperature profile on the evolution of crystalline microstructures were studied. With scanning electron microscopy(SEM), the transition of crystalline morphology from ring-banded structure to oriented lamellae was observed with decreasing mold temperature, and the oriented lamellae were formed at the sub-skin layer of the samples at the lowest mold temperature, which was further testified by differential scanning calorimetry(DSC). With the decline of mold temperature, the degree of orientation, obtained from two-dimensional small angle X-ray scattering(2D-SAXS), was increased and long periods rose a little. Thus, decreasing mold temperature was beneficial to the formation of orientation structures because the relaxation of chains was weakened.     

Effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene

The effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene (UHMW‐PE) are discussed, based on a combination of in situ X‐ray measurement and stress–strain behavior. The sample films of metallocene‐ and Ziegler‐catalyzed UHMW‐PEs with a similar viscosity average MW of ～10⁷ were prepared by compression molding at 180 °C. Stress profiles recorded at 160 °C above the melting temperature of 135 °C exhibited a plateau stress region for both films. The relative change in the intensities of the amorphous scattering recorded on the equator and on the meridian indicated the orientation of amorphous chains along the draw axis with increasing strain. However, there was a substantial difference in the subsequent crystallization into the hexagonal phase, reflecting the molecular characteristics, that is, MW distribution of each sample film. Rapid crystallization into the hexagonal phase occurred at the beginning point of the plateau stress region in melt‐drawing for metallocene‐catalyzed UHMW‐PE film. In contrast, gradual crystallization into the hexagonal phase occurred at the middle point of the plateau stress region for the Ziegler‐catalyzed film, suggesting an ease of chain slippage during drawing. These results demonstrate that the difference in the MW distribution due to the polymerization catalyst system dominates the phase development mechanism during melt‐drawing. The effect of the processing conditions, that is, the including strain rate and drawing temperature, on the melt‐drawing behavior is also discussed. The obtained results indicate that the traditional temperature–strain rate relationship is effective for transient crystallization in to the hexagonal phase during melt‐drawing, as well as for typically oriented crystallization during ultradrawing in the solid state. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2455–2467, 2006     

Influence of scale effect and injection speed on morphology and structure of the microinjection molded isotactic polypropylene parts

The morphology and microstructure as well as their forming mechanism of the parts in microinjection molding process are critical. In this work, the coupling effect of scale factor and injection speed on the morphology of the microparts was systematically investigated. Neat isotactic polypropylene parts with thicknesses of 1 mm, 200 μm, and 100 μm were molded at different injection speeds. Polarized light microscope and wide‐angle X‐ray diffraction were used to inspect the microstructures along the sample thickness. In this way, three kinds of typical morphology were observed in the parts, including typical skin‐core structure for the parts with the thickness of 1 mm, noncore shear layer structure for the parts with the thickness of 200 μm, and special skin‐core structure with large fraction of columnar crystal for the parts with the thickness of 100 μm. Most interestingly, it was intuitively and straightforward found that the wall slip occurs when the injection speed exceeds a certain value. Specifically, opposite morphological change trend can be obtained when the parts were molded at different levels of injection speeds. Based on these experimental observations, the formation mechanism was proposed to interpret the morphological evolution. Our work provides a new insight for better understanding the morphology evolution mechanism for microinjection molding parts.     

Morphological comparison of isotactic polypropylene parts prepared by micro‐injection molding and conventional injection molding

The morphological feature of microparts evolved during micro‐injection molding may differ from that of the macroparts prepared by conventional injection molding, resulting in specific physical properties. In this study, isotactic polypropylene (iPP) microparts with 200 µm thickness and macroparts with 2000 µm thickness were prepared, and their morphological comparison was investigated by means of polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimeter (DSC), and wide‐angle X‐ray diffraction (WAXD). The results presented some similarities and differences. PLM observations showed that the through‐the thickness‐morphology of micropart exhibited a similar “skin–core” structure as macropart, but presented a large fraction of shear layer in comparison to the macropart which presented a large fraction of core layer. The SEM observation of shear layer of micropart featured highly oriented shish‐kebab structure. The micropart had a more homogeneous distribution of lamellae thickness. The degree of crystallinity of the micropart was found to be higher than that of the macropart. High content of β‐crystal was found in micropart. The 2D WAXD pattern of the core layer of macropart showed full Debye rings indicating a random orientation, while the arcing of the shear layer indicates a pronounced orientation. The most pronounced arcing of the micropart indicates the most pronounced orientation of iPP chains within lamellae. Copyright © 2011 John Wiley & Sons, Ltd.     

Relationship between fine structure of polyoxymethylene copolymer plates and methanol transport properties

An experimental study was made of diffusion behavior of methanol through three kinds of injection‐molded plates of a polyoxymethylene (POM) copolymer with different molecular weights M at 60 °C. Fine structure of the three sample plates was also examined by wide‐angle X‐ray diffraction and small‐angle X‐ray scattering, and moreover, their dynamic properties were investigated by the dynamic mechanical analysis (DMA). It is shown that the diffusion behavior may be well explained by the one‐dimensional Fick diffusion equation with a constant diffusion coefficient, and that the steady‐state transport rate increases with increasing M. As for fine structure, the crystallinity decreases slightly, and the preferential orientation and the long period increase, with increasing M. The long period of the lamellar stacking structure increases with increasing M, and it also increases with methanol transport. In DMA, the loss tangent tan δ becomes higher after the methanol transport in the wide range of temperature around the glass transition one. These results indicate that amorphous regions serve as channels for methanol molecules in the lamellar stacking structure, leading to the conclusion that the dependence of the steady‐state transport rate on M arises from the factors of crystallinity and long period. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1234–1242, 2007     

Layer structure estimation of three‐dimensional crystal and two‐dimensional molecular film for fluorinated comb copolymers

Comb copolymers containing both hydrogenated and fluorinated side‐chains were prepared by copolymerization using acrylic or methacrylic monomers in several ratios. The crystal structures of these copolymers and layer structures of their organized molecular films were investigated by wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering (SAXS), and out‐of plane X‐ray diffraction. Further, to selectively estimate the regularity of shorter fluorocarbon side‐chains, organized molecular films of copolymers were investigated by polarized near‐edge X‐ray adsorption fine structure (NEXAFS) spectroscopy. From the results of these measurements, it was inferred that these copolymers formed highly ordered layer structures, and a long spacing was predominantly determined by the arrangement of hydrogenated side‐chains, except in copolymers having extremely high fluorocarbon contents. In the case of the organized molecular films, the fluorinated side‐chains of methacrylate copolymers cannot form a highly ordered arrangement, whereas those of acrylate copolymers were oriented on monolayers. However, in both cases, the hydrogenated side‐chains predominantly formed layer structures in the organized films, and the fluorinated side‐chains did not contribute to the formation of the layer structures. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 534–546, 2008