Structural transformation from shish‐kebab crystals to micro‐fibrils through hot stretching process of gel‐spun ultra‐high molecular weight polyethylene fibers with high concentration solution

Structural evolution of gel‐spun ultra‐high molecular weight polyethylene fibers with high concentration solution via hot stretching process was investigated by in situ small‐angle X‐ray scattering, in situ wide‐angle X‐ray diffraction measurements, scanning electron microscopy, and differential scanning calorimetry. With the increase of stretching strain, the long period continuously increases at relative lower stretching temperature, while it first increases and then decreases rapidly at relative higher stretching temperature. The kebab thickness almost keeps constant during the whole hot‐stretching process and the kebab diameter continually decreases for all stretching temperatures. Moreover, the length of shish decreases slightly and the shish quantity increases although there is almost no change in the diameter of shish crystals during the hot stretching process. The degree of crystal orientation at different temperatures is as high as above 0.9 during the whole stretching process. These results indicate that the shish‐kebab crystals in ultra‐high molecular weight polyethylene fibers can transform continuously into the micro‐fibril structure composed mostly of shish crystals through the hot stretching process. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 225–238     

Formation of a large‐scale shish‐kebab structure of polyoxymethylene in the melt spinning and the crystalline morphology evolution after hot stretching

Polyoxymethylene (POM) fiber was produced by melt spinning with a high take‐up speed, which imposed a strong flow field. An unexpected formation of a shish‐kebab morphology with multiple shish of POM fibers was reported for the first time. This morphology is a large‐scale shish kebab with a diameter of 10.5 µm. Further orientation of the POM fiber was obtained by hot stretching twice at 160°C. Two crystalline morphology evolution processes were also observed: (i) the process from the large‐scale shish‐kebab to the deformed small shish‐kebab and (ii) the process from the deformed small shish‐kebab to the perfect whiskers. Compared with the melt spinning fiber, fiber tensile strength with first and second hot stretching increased by 976% and 1705%, respectively. The crystalline melting behavior of fibers significantly changes after the first and second hot stretching. The flow field induces a large number of extended chain crystals. Copyright © 2014 John Wiley & Sons, Ltd.     

Relaxation dynamics of stretched polymer chains traced by shear induced Shish‐Kebab structure

We have developed a morphologic method to investigate the relaxation processing of the stretched polymer chains in melts, in which an atomic force microscope probe was used to shear the surface of an isotactic polypropylene melt to obtain the isolated shish‐kebab structure. We present the results of the time dependence of length of the isolated shish‐kebab structure and the stress dependence of the kebab density along the direction of shish in this paper. Our results demonstrate that the shear‐oriented polymer melts show the relaxation dynamics of worm‐like chain where the length deficit of the isolated shish‐kebab structure is scaled with the relaxation time as a power of 1/3. The melting behavior of shish‐kebab structure was also investigated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 907–914     

The influence of short chain branch on formation of shish‐kebab crystals in bimodal polyethylene under shear at high temperatures

Formation of shish‐kebab crystals due to the coil–stretch transition under shear in the molten state using a bimodal polyethylene system with high molecular weight (HMW) fraction having different branch content was investigated. In specific, in situ small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques were used to study the structure evolution of shish‐kebab crystals at high temperatures under simple shear. The SAXS results revealed that with the increase of branch content, shish‐kebab crystals became more stable at high temperatures (e.g., 139 °C). However, the shish length of the bimodal PE containing 0.11% branch was shorter than that with no branch. The WAXD results showed that the degree of crystallization for bimodal PE with HMW fraction having 0.11% branch increased with time but reached a plateau value of 1%, while that with no branch increased continuously till 11%. Furthermore, the crystal orientation of bimodal PE with HMW fraction having 0.11% branch was above 0.9 and maintained at a constant value, while that with no branch decreased from 0.9 to 0.1 upon relaxation. This study indicates that even though the crystallizability of the HMW fraction with branch content decreased, they could effectively stabilize the shear‐induced crystalline structure with shorter shish‐kebab crystals. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 786–794     

A novel hierarchical crystalline structure of injection-molded bars of linear polymer: co-existence of bending and normal shish–kebab structure

The hierarchy structures and orientation behavior of high-density polyethylene (HDPE) molded by conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were intensively examined by using scanning electronic microscopy (SEM) and 2D wide-angle X-ray diffraction (2D-WAXD). Results show that the spatial variation of crystals across the thickness of sample molded by CIM was characterized by a typical skin–core structure as a result of general shear-induced crystallization. Unusually, the crystalline morphologies of the parts prepared by GAIM, primarily due to the penetration of secondary high-compressed gas that was exerted on the polymer melt during gas injection, featured a richer and fascinating supermolecular structure. Besides, the oriented lamellar structure, general shish–kebab structure, and common spherulites existed in the skin, sub-skin, and gas channel region, respectively; a novel morphology of shish–kebab structure was seen in the sub-skin layer of the GAIM parts of HDPE. This special shish–kebab structure (recognized as “bending shish–kebab”) was neither parallel nor perpendicular to the flow direction but at an angle. Furthermore, there was a clear interface between the bending and the normal shish–kebab structures, which may be very significant for our understanding of the melt flow or polymer rheology under the coupling effect of multi-fluid flow and complex temperature profiles in the GAIM process. Based on experimental observations, a schematic illustration was proposed to interpret the formation mechanism of the bending shish–kebab structure during GAIM process.     

Shish-kebab structures of nylon-6 obtained from quiescent solutions by application of self-seeding

A study has been made of the morphology and structure of nylon-6 crystals grown from diluted 1,4-butanediol solution. Isothermal crystallization from homogeneous solution resulted in smooth ribbons or lath-shaped crystals aggregated into sheaves. Shish-kebab structures of nylon-6 could be grown from the quiescent solution by self-seeding techniques. Electron microscopic investigations and small-angle x-ray measurements showed that the molecules in the lath-shaped backbones of the shish kebab are folded and oriented perpendicular to the long axis of the crystals. The polyamide laths have the α-monoclinic crystal structure with the hydrogen bonds parallel to the long axis. It is suggested that due to the anisotropic type of bonding in the crystal lattice the crystals fragment laterally during the heating stage in the self-seeding technique. Structural defects, e.g., twinning sites introduced during the dissolution and subsequent crystallization may cause the growth of shish-kebab structures from quiescent solution.     

Effects of different ultrahigh molecular weight polyethylene contents on the formation and evolution of hierarchical crystal structure of high-density polyethylene/ultrahigh molecular weight polyethylene blend fibers

In this paper, the blend fibers of ultrahigh molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) were prepared by solution blending and gel spinning process. The uniformity of the blend fibers has been confirmed by rheological data and thermodynamic unimodal curve. They were further characterized by single fiber strength test, scanning electron microscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, and so forth, to explore the structural evolution mechanism with the change of UHMWPE content. The results showed that when the molar content of UHMWPE was only 2.9 mol%, entanglement appeared in the structure of shish-kebab, and when the proportion reached 20 mol%, an interlocking structure could be observed. With the increase of UHMWPE content, kebab began to be networked, and when the content reached 33 mol%, kebab's orientation reached its peak. After that, the interlocking network structure gradually improved. When the content reached 50 mol%, the shish's orientation reached saturation, and the shish-kebab network became perfect. In addition, with the increase of UHMWPE content, stress-induced recrystallization occurred on the wafer, some kebab would be converted into shish crystals, and when the content exceeded 50 mol%, the microfibers began to merge, and the wafer became denser, but still had entanglements. Our work has proposed a quantitative explanation for the evolution of hierarchical crystal structure of HDPE/UHMWPE blend fibers.     

Mechanism of formation of shish kebab structures

To describe the characteristic crystalline structure of polyolefins, Pennings first proposed a model consisting of a combination of an extended chain crystal (a “shish”) and folded chain crystals (“kebabs”). In Pennings' model the “shish” forms first during a crystallization process under stress and then later the “kebabs” overgrow this “shish” structure epitaxially. Because we had some doubts about such a mechanism, we undertook a series of experimental studies on linear polyethylene, particularly in regard to the crystallization process from a solution under shear. Our conclusion is that the crystals grow first by a screw dislocation mechanism, like whiskers, and then later these are deformed by the shear stress to form the shish kebab structures.     

Molecular weight dependence of hybrid shish kebab structure in injection molded bar of polyethylene/inorganic whisker composites

In our previous work, a hybrid shish kebab structure, with polyethylene (PE) crystal lamellae periodically decorated on the surface of an inorganic whisker (SMCW) and aligned approximately perpendicular to the long axis of the whisker, has been observed in the injection molded bar of PE/SMCW composites. To investigate the effect of the molecular weight of the PE matrix on the formation of the hybrid shish kebab structure and the corresponding physical properties of HDPE/SMCW composites, in this work, three types of PE with different molecular weights were used to prepare the composites. They were first melt blended and then subjected to dynamic packing injection molding (DPIM), in which the prolonged shear was exerted on the melt during the solidification stage. An obvious hybrid shish kebab (HSK) structure, with PE crystal lamellae closely packed on the surface of the SMCW, was found in the samples with a low molecular weight PE (LMW-PE) matrix and a medium molecular weight PE (MMW-PE) matrix. However, in samples with a high molecular weight PE (HMW-PE) matrix, an incomplete HSK structure with PE crystal lamellae loosely decorated on the surface of the SMCW was observed. Furthermore, DSC results indicated that SMCW served as a good nucleating agent only for the composite with a LMW-PE matrix and the nucleation efficiency decreased with increasing PE molecular weight. Correspondingly, the tensile strength of the PE/SMCW composites was significantly improved by adding SMCW for the samples with a LMW-PE or MMW-PE matrix. Especially for samples with a LMW-PE matrix, the tensile strength was remarkably enhanced by the presence of only 1 wt % SMCW. For the composites with a HMW-PE matrix, the addition of SMCW had almost no reinforcing effect on the composites. The molecular weight dependence of the formation of HSK and property enhancement was discussed on the basis of the chain mobility and crystallization capability of the PE matrix.     

The Influence of Ethyl Branch on Formation of Shish-Kebab Crystals in Bimodal Polyethylene under Shear at Low Temperature

Formation of shish-kebab crystals using a bimodal polyethylene system containing high molecular weight(HMW)component with different ethyl branch contents was investigated.In situ small-angle X-ray scattering(SAXS)and wide-angle X-ray diffraction(WAXD)techniques were used to monitor the formation and evolution of shish-kebab structure sheared at low temperature in simple shear mode and low rate.Only the bimodal PE with no branch formed shish-kebab crystals at the shear temperature of 129℃,and the shish length increased with the crystallization time,while bimodal PE with branch has no observable shish under the same conditions.The degree of crystallization for bimodal PE with no branch increased with time up to above 7％,while those with ethyl branch increased continually up to above 23％.Furthermore,bimodal PE's Hermans orientation factor with no branch increased to 0.60,while those with ethyl branch only increased to a value below 0.15.This study indicated that the shish-kebab crystal formed at the low temperature of 129℃is due to the stretch of entangled chains under shear for the bimodal PE with no branch.Only partly oriented lamellar crystals were formed for the bimodal PE with ethyl branch.All the results at the shear temperatures higher,closed to,and lower than the melting point,the modulation of shish crystals formation owing to different mechanisms of the coil-stretch transition and the stretched network by changing shear temperature was achieved in the bimodal PE samples.     

Morphological evolution and orientation development of stretched iPP films: Influence of draw ratio

Varying the processing conditions of semicrystalline polymers can produce different morphologies of crystallization, which leads to different properties. There have been extensive studies of flow‐induced crystallization on isotactic polypropylene (iPP) using predominantly shear flow. A stretching method, deduced from extrusion, was introduced to study the morphological evolution of elongation‐induced shish‐kebab crystallization. Different morphologies of the resultant samples with different draw ratios (DRs) were carefully investigated and characterized via differential scanning calorimetry, polarizing light microscopy, scanning electron microscopy, atomic force microscopy, and 2D small‐angle X‐ray scattering. In addition, the degree of orientation of the samples with different DRs was also investigated using the 2D wide‐angle X‐ray scattering technique. The results indicate that the elongation‐induced morphology is strongly dependent on the effective stretching flow expressed in terms of the DR, which is defined as the ratio of rates between take‐up and the extrusion. The spherulite is dominant at low DRs, but it starts to deform along the stretching direction with increasing DR. The shish‐kebab structure in the stretched film, composed of stretched chains (shish) and layered crystalline lamellae (kebabs), increases gradually with an increase in the DR, whereas the spherulites gradually decreased. Furthermore, the overall orientation of α‐phase crystals, expressed by the Hermans orientation parameter, is also found to increase dramatically with the DR, and the rate of increase strongly depends on the DR. The different crystal morphologies are attributed to crystallization induced by different elongations of the stretched iPP films. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1223–1234, 2010     

Combined Effects of Stretching and Nanofillers on theCrystalline Structure and Mechanical Properties ofPolypropylene and Single-walled Carbon NanotubeComposite Fibers简

The combined effects of stretching and single-walled carbon nanotubes （SWCNTs） on crystalline structure and mechanical properties were systematically investigated in melt-spun polypropylene （PP） fibers prepared at two different draw ratios. The dispersion, alignment of the SWCNT bundles and interfacial crystalline structure in the composite fibers are significantly influenced by the stretching force during the melt spinning. The nanohybrid shish kebab （NHSK） superstructure where extended PP chains and aligned SWCNT bundle as hybrid shish and PP lamellae as kebab has been successfully obtained in the composite fibers prepared at the high draw ratio and the related formation mechanism is discussed based on the results of morphological observations and 2d-SAXS patterns. Large improvement in tensile strength and modulus has been realized at the high draw ratio due to the enhanced orientation and dispersion of SWCNT bundles as well as the formation of NHSK.     

The influence of chitin nanocrystals on structural evolution of ultra-high molecular weight polyethylene/chitin nanocrystal fibers in hot-drawing process

Ultra-high molecular weight polyethylene (UHMWPE)/chitin nanocrystal (CNC) fibers were prepared. Compared with the pure UHMWPE fibers, the ultimate tensile strength and Young’s modulus of UHMWPE/CNC fibers are improved by 15.7% and 49.6%, respectively, with the addition of chitin nanocrystals (CNCs) of 1 wt%. The melting temperature (T _m) of UHMWPE/CNC fibers was higher than that of pure UHMWPE fibers. Pure UHMWPE fibers and UHMWPE/CNC fibers were characterized with respect to crystallinity, orientation and kebab structure by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM). It is found that the CNCs act as the shish structure in UHMWPE/CNC fibers and the kebab crystals are grown around the CNCs. There was almost no difference between pure UHMWPE fibers and UHMWPE/CNC fibers in orientation. But the degree of crystallinity of various stages of UHMWPE/CNC fibers was respectively higher than the corresponding stage of pure UHMWPE fibers. Moreover, the addition of 1 wt% CNCs improved the thickness of kebab crystals and accelerated the transformation of kebab to shish.     

Structural evolution of melt-drawn transparent high-density polyethylene during heating and annealing: Synchrotron small-angle X-ray scattering study

The morphological development of melt-drawn transparent high-density polyethylene during heating was investigated employing in-situ synchrotron small-angle X-ray scattering (SAXS) technique. The results confirm that at lower temperatures only meridional scattering peaks aligned perpendicular to the extensional flow direction can be observed, indicating a highly oriented lamellar crystallite structure; whereas at higher temperatures an equatorial streak additional to the layer-like meridional scattering pattern develops, reflecting the presence of shish-kebab-like objects in the specimen under investigation. Upon heating, the average thickness of the kebab crystals remains essentially unaffected below 110 °C, and subsequently the selective melting of the less stable kebabs proceeds yielding thicker layered lamellar crystals. When the temperature is raised to 131 °C, the shish-like formation and the thermally stable kebab crystals melt simultaneously. In addition, the microstructure of the melt-drawn specimen subjected to annealing at elevated temperatures was probed at room temperature. As opposed to the SAXS patterns registered at high temperatures, the SAXS diagram measured after annealing shows no equatorial streak, suggesting that the cylindrical structures could be re-formed. This observation can be explained by assuming that the plate-like kebab crystals with their normal parallel to the stretching direction grow and impinge during cooling to room temperature due to secondary crystallization, which can be verified by in-situ SAXS experiments during annealing and subsequent cooling.     

MORPHOLOGY OF POLYBUTADIENE “SHISH KEBABS”

Shish kebab structure of cis-1 , 4-polybutadiene has been obtained by quiescent solution crystallization at suitable temperature. The morphology and growth mechanism of formation of shish kebab structure have been studied in detail.The higher the molecular weight the faster the sample crystallizes.     

Polymer decoration on carbon nanotubes via physical vapor deposition

The polymer decoration technique has been widely used to study the chain folding behavior of polymer single crystals. In this article, we demonstrate that this method can be successfully adopted to pattern a variety of polymers on carbon nanotubes (CNTs). The resulting structure is a two-dimensional nanohybrid shish kebab (2D NHSK), wherein the CNT forms the shish and the polymer crystals form the kebabs. 2D NHSKs consisting of CNTs and polymers such as polyethylene, nylon 66, polyvinylidene fluoride and poly(L-lysine) have been achieved. Transmission electron microscopy and atomic force microscopy were used to study the nanoscale morphology of these hybrid materials. Relatively periodic decoration of polymers on both single-walled and multi-walled CNTs was observed. It is envisaged that this unique method offers a facile means to achieve patterned CNTs for nanodevice applications.     

THE EFFECT OF EC ON ORIENTATION AND RELAXATION OF PP OBTAINED VIA DYNAMIC PACKING INJECTION MOLDING

As a long term project aiming at engineering plastics based on polypropylene (PP), in this work, we report the effect of ethylene-cellulose (EC) on the orientation and relaxation of PP obtained via dynamic packing injection molding (DPIM). 2d-WAXD results showed that PP with a highly oriented structure and a shish-kebab structure were achieved by DIPM, leading to an increase of tensile strength from 35.0MPa to 48.6MPa. The degree of orientation of PP was increased by adding only 1% of EC, resulting in a further increase of tensile strength from 48.6MPa to 53.8MPa. Shish was found to exist not only in the oriented layer but also in the core of the sample after adding EC, as evidenced by DSC result.This was understood as due to the overall decrease of viscosity by the addition of EC, thus an increasing of shear rate. Higher shear rate can favor the orientation of molecules and continuous growth of shish structure, resulting in a change of shish distribution along the sample thickness. On the other hand, the relaxation mode of shish in the melt recrystallization of PP is also greatly affected by the presence of EC. A retarded relaxation of PP macromolecules was seen.Even more, SEM results showed that EC could form the short fibers in PP matrix along the shear flow direction. Compared with the morphology obtained by conventional injection molding, a much better dispersion and easy break-up of EC in PP matrix were observed for samples obtained by dynamic packing injection molding.     

Crystal and phase morphology of dynamic-packing-injection-molded high-density polyethylene/ethylene vinyl acetate blends

The effect of shear stress, provided by so-called dynamic-packing injection molding, on crystal morphology and phase behavior was investigated for high-density polyethylene (HDPE) in blends with ethylene vinyl acetate (EVA) of various viscosities and vinyl acetate (VA) contents, with the aid of differential scanning calorimetry, two-dimensional small-angle X-ray scattering (2D SAXS), and scanning electron microscopy (SEM). A shish-kebab pattern was found in the oriented zones of dynamic samples, and the ratio of shish to kebab increased as a function of the EVA content in the blends up to 20 wt %, regardless of the VA content. This showed that molecules of HDPE could easily be stretched to form a shish structure in the presence of EVA. Moreover, a large increase in the long spacing, characterized by 2D SAXS measurements, was achieved because of the presence of EVA. The SEM results showed an obvious decrease in the domain size of the EVA phase under the effect of shear stress. All these results suggested shear-induced mixing between HDPE and EVA, in that ethylene segments of EVA molecules could be forged in the shish structure during shear and the other fractions of EVA were located in the amorphous regions between the adjacent lamellae of HDPE. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1831–1840, 2004     

Conformation dependence of DNA electrophoretic mobility in a converging channel

The electrophoresis of λ‐DNA is observed in a microscale converging channel where the center‐of‐masses trajectories of DNA molecules are tracked to measure instantaneous electrophoretic (EP) mobilities of DNA molecules of various stretch lengths and conformations. Contrary to the usual assumption that DNA mobility is a constant, independent of field and DNA length in free solution, we find DNA EP mobility varies along the axis in the contracting geometry. We correlate this mobility variation with the local stretch and conformational changes of the DNA, which are induced by the electric field gradient produced by the contraction. A “shish‐kebab” model of a rigid polymer segment is developed, which consists of aligned spheres acting as charge and drag centers. The EP mobility of the shish‐kebab is obtained by determining the electrohydrodynamic interactions of aligned spheres driven by the electric field. Multiple shish‐kebabs are then connected end‐to‐end to form a freely jointed chain model for a flexible DNA chain. DNA EP mobility is finally obtained as an ensemble average over the shish‐kebab orientations that are biased to match the overall stretch of the DNA chain. Using physically reasonable parameters, the model agrees well with experimental results for the dependence of EP mobility on stretch and conformation. We find that the magnitude of the EP mobility increases with DNA stretch, and that this increase is more pronounced for folded conformations.     

Crystallization of polylactic acid under in situ deformation during nonsolvent‐induced phase separation

In this study, we investigate polylactic acid (PLA) crystallization under in situ biaxial extension in a nonsolvent‐induced phase separation foaming process. Our ternary system consists of PLA, dichloromethane (DCM) as solvent and hexane as nonsolvent. For the first time, the formation of a shish‐kebab crystalline morphology is observed in such a solution‐based foaming process in certain solid–liquid phase separated systems. The formation of shish‐kebabs is described based on the coil‐stretch transition concept. The rapid biaxial deformation caused by macropore growth uniaxially stretches the long chains that are tied with at least two single crystals which eventually leads to the formation of shish structures throughout the polymer‐rich phase. The kebab lamellae then form perpendicularly on the shish cores. The scanning electron microscopy (SEM) observations and our interpretation of the crystallization phenomena are confirmed by differential scanning calorimetry (DSC) analysis. The observation of various crystalline morphologies, particularly shish‐kebabs, and the elucidation of their formation mechanisms contribute to the understanding of phase separation and pore growth as well as crystallization in such polymer–solvent–nonsolvent systems. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1055–1062