Shish Formation Research Articles

Shish-kebab formation in the micro-injection molding of polyethylene: In-situ synchrotron small angle X-ray scattering study

The structural formation and evolution in high density polyethylene (HDPE) during the micro-injection molding process were investigated as a function of the mold temperature and injection speed utilizing an in situ synchrotron small angle X-ray scattering (SAXS) technique in conjunction with a portable micro-injection molding setup. It was found that shish-kebab structures were formed in all cases. The beginning of occurrence of shish is not much affected by the mold temperature, but the onset of the formation of kebab crystals is strongly dependent on the mold temperature. This mold temperature dependence of the kinetics of shish-kebab formation is attributed to the interplay between the crystallization rate in the outer layer and the formation rate of the shish-like structure, which was further evidenced by the multilayered structure of molded samples via ex situ microfocus scanning SAXS and wide angle X-ray diffraction (WAXD) measurements. On the other hand, the injection speed affects the long period of oriented lamellae and the dimension of the shish-like structure only slightly but influences the kinetics of structural formation evidently. The larger the injection speed, the earlier occurrence of oriented lamellae and shish formation is observed.

The influence of low-temperature shear on crystallization of shish-kebab of unimodal/ bimodal C4 short-chain branching polyethylene

The shish crystal, a crucial structure in the formation of shish-kebab crystals, has drawn widespread attention due to its highly oriented chain arrangement, significantly enhancing the mechanical properties of products. This study utilizes polyethylene samples with longer C4 short branches, employing in-situ synchrotron SAXS/WAXD to investigate the influence of short branch length and content on the formation of shish-kebab crystals in unimodal and bimodal polyethylene under low-temperature shear conditions (with supercooling degrees of 13.5 °C and 10.5 °C). The results reveal that the presence of C4 short-chain branching affects system crystallization, but low-temperature shear induces stronger mechanical action on the melt, resulting in better-oriented lamellar crystals compared to high-temperature shear (superheat degrees of 22.5 °C and 19.5 °C), achieving higher orientation. This achieves the control of shish-kebab crystal generation through the stretched network mechanism. Both types of samples with different C4 short-chain branching contents generate shish-kebab crystals during the shear stage, with relatively higher short-chain branching content inducing more shish crystal formation but with less regular structure. The bimodal polyethylene exhibit higher proportions and absolute amounts of final shish crystals compared to the unimodal polyethylene, indicating that high molecular weight portions are conducive to shish crystal generation under shear. The result demonstrates the feasibility of generating shish-kebab crystals during the low-temperature shear process of PE in the presence of C4 branches using the stretched network mechanism under shear flow conditions. Moreover, controlling the degree of entanglement of molecular chains by adjusting shear temperature, branch content, and branch length can modulate two formation mechanisms of shish crystals—coil-stretch transition mechanism at high temperature and stretched network mechanism at low-temperature.

Mold temperature- and molar mass-dependent structural formation in micro-injection molding of isotactic polypropylene

The structural formation and development of isotactic polypropylene (iPP) upon the micro-injection molding process was investigated at different mold temperatures and molecular weights utilizing a real-time synchrotron radiation small angle X-ray scattering (SAXS) technique combined with a customized micro-injection molding apparatus. Shish-kebab structure and parent-daughter lamellae were found to be formed during micro-injection molding for all iPP samples. In the case of kebab lamellae, a considerable growth in the long period and in the average thickness of lamellar crystallites and amorphous domains is observed at initial stages of crystallization for samples molded at varying temperatures. This effect is caused by the successive formation of thin lamellae in the outer layer and thick lamellae in the inner layer during the manufacturing process as evidenced by the spatial distribution of the crystalline lamellae across the thickness. In addition, the length of the shish formation increases remarkably at the onset of crystallization, the extent of which is dependent on the mold temperature. Despite the large changes of the lamellar stacks and the shish misorientation, the final length of the shish remains essentially unchanged when varying mold temperature. Since there is a critical orientation molecular weight above which the chains are stretched and oriented to form stable shish, the iPP sample with a low molar mass exhibits an overall decrease in the scattering intensity of SAXS patterns compared to the high molecular weight polypropylene. • The structural formation of iPP is investigated utilizing a customized micro-injection molding device. • Shish-kebab and parent-daughter lamellae are formed during micro-injection molding. • A considerable increase in the long period is observed at initial stages of crystallization. • The length of the shish formation increases remarkably at the onset of crystallization. • The final shish length remains essentially unchanged when varying mold temperature.

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 °C, 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 °C 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.

In situ synchrotron small angle X-ray scattering investigation of structural formation of polyethylene upon micro-injection molding

A customized micro-injection molding apparatus with a pair of diamond windows in the filling cavity was specifically designed to simulate the extreme processing conditions inherent in the practical micro-molding process. This setup combined with synchrotron radiation allows real-time investigation of multi-scale structural development during the manufacturing process, which enables evaluation of flow-induced crystallization features for a range of polymeric materials. In this study, the structural evolution of high density polyethylene at the nanoscale was investigated by in situ synchrotron small angle X-ray scattering technique under injection molding. The respective structural parameters of kebab lamellae (including the long period, and the average thickness of crystalline lamellae and amorphous phase) and shish formation (such as the radius of gyration, length, and misorientation of the shish) were extensively analyzed from the resulting data. The growth of kebab crystals was found to exhibit a two-step behavior: melting of thin crystallites followed by a recrystallization process occur at the onset of molding whereas an annealing-induced perfectioning of the lamellae is activated at a later stage. In spite of the initial decrease of the shish length and radius, there is an overall increase in the length and misorientation of the shish structure at later times, which can be attributed to a crystallization of partially oriented chains onto the longitudinal surface of the existing shish. The results acquired by the unique combination of this apparatus and synchrotron radiation can provide an effective means to predict the structural formation and modulate the morphology of final products in practical micro-injection molding process of polymers.

The influence of short chain branch on formation of shear-induced crystals in bimodal polyethylene at low shear temperatures

In-situ synchrotron X-ray scattering and diffraction techniques were employed to explore the effect of short chain branch on shear-induced crystallization of bimodal polyethylene, and the shear experiment was conducted at a low shear rate under the temperatures lower than melting point. It has shown that both shish-kebabs and conventional lamellar crystals can form and the crystallinity increases sharply during the shear process. In addition, there are more shish-kebabs but less lamellar crystals generated and the crystallinity reduces markedly with the increase of content of short chain branch. While in the subsequent isothermal crystallization at 121 °C, all samples display a slightly enlarged crystallinity and an approximately unvaried lower crystal orientation. The results illustrate that although the introducing of short chain branch in high molecular weight fraction depresses the crystallizablity of bimodal polyethylene, but it can facilitate the formation of shish-kebabs at low shear temperatures, and the structure formation follows the stretched network mechanism. Combined with our previous papers that the structure formation followed the CST mechanism at the shear temperature higher than melting point, the modulation of shish crystals formation owing to different mechanisms of the CST and the stretched network by changing shear temperature was achieved in the bimodal PE samples.

Flow induced crystallization prevents melt fracture of HDPE in uniaxial extensional flow

This work concerns extension induced crystallization of a commercial high density polyethylene above the equilibrium melting temperature. We compare the nonlinear response during uniaxial elongation to the morphology obtained in the quenched fibers after cessation of the flow at a Hencky strain of 5. At 12 °C above the melting temperature, the samples undergo brittle fracture. Samples stretched at 2 and 6 °C above the melting temperature remain intact throughout the entire course of deformation and exhibit a strain hardening behavior that does not follow time temperature superposition. We propose that stabilization of the filament at lower temperatures, as well as the failure of time temperature superposition, is caused by flow-induced nucleation and growth of shish structures oriented along the flow direction. Further justification is obtained from small-angle X-ray scattering performed on the quenched filament showing an increased formation of shish with an increase in the deformation rate. We find the critical Hencky strain for the onset of the shish formation to be between 0 and 0.6, which is significantly lower than the values reported in the existing literature. We model the influence of shish nucleation on the rheological response in an extension using the hierarchical multimode stress function, which is modified to include the stretched network assumption.This work concerns extension induced crystallization of a commercial high density polyethylene above the equilibrium melting temperature. We compare the nonlinear response during uniaxial elongation to the morphology obtained in the quenched fibers after cessation of the flow at a Hencky strain of 5. At 12 °C above the melting temperature, the samples undergo brittle fracture. Samples stretched at 2 and 6 °C above the melting temperature remain intact throughout the entire course of deformation and exhibit a strain hardening behavior that does not follow time temperature superposition. We propose that stabilization of the filament at lower temperatures, as well as the failure of time temperature superposition, is caused by flow-induced nucleation and growth of shish structures oriented along the flow direction. Further justification is obtained from small-angle X-ray scattering performed on the quenched filament showing an increased formation of shish with an increase in the deformation rate. We find the ...

Processing–Nanostructure–Property Relationships of All‐Polyethylene Composites Reinforced by Flow‐Induced Oriented Crystallization of UHMWPE

AbstractAll‐polyethylene composites exhibiting substantially improved toughness/stiffness balance are readily produced during conventional injection molding of high density polyethylene (HDPE) in the presence of bimodal polyethylene reactor blends (RB40) containing 40 wt% ultrahigh molar mass polyethylene (UHMWPE) dispersed in HDPE wax. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses shows that flow‐induced crystallization affords extended‐chain UHMWPE nanofibers forming shish which nucleates HDPE crystallization producing shish‐kebab structures as reinforcing phases. This is unparalleled by melt compounding micron‐sized UHMWPE. Injection molding of HDPE with 30 wt% RB40 at 165 °C affords thermoplastic all‐PE composites (12 wt% UHMWPE), improved Young's modulus of 3400 MPa, tensile strength of 140 MPa, and impact resistance of 22.0 kJ/m2. According to fracture surface analysis, the formation of skin‐intermediate‐core structures accounts for significantly improved impact resistance. At constant RB40 content both morphology and mechanical properties strongly depend upon processing temperature. Upon increasing processing temperature from 165 °C to 250 °C the average shish‐kebab diameter increases from the nanometer to micron range, paralleled by massive loss of self‐reinforcement above 200 °C. The absence of shish‐kebab structure at 250 °C is attributed to relaxation of polymer chains and stretch‐coil transition impairing shish formation.

Full Characterization of Multiphase, Multimorphological Kinetics in Flow-Induced Crystallization of IPP at Elevated Pressure.

Understanding the complex crystallization behavior of isotactic polypropylene (iPP) in conditions comparable to those found in polymer processing, where the polymer melt experiences a combination of high shear rates and elevated pressures, is key for modeling and therefore predicting the final structure and properties of iPP products. Coupling a unique experimental setup, capable to apply wall shear rates similar to those experienced during processing and carefully control the pressure before and after flow is imposed, with in situ X-ray scattering and diffraction techniques (SAXS and WAXD) at fast acquisition rates (up to 30 Hz), a well-defined series of short-term flow experiments are carried out using 16 different combinations of wall shear rates (ranging from 110 to 440 s–1) and pressures (100–400 bar). A complete overview on the kinetics of structure development during and after flow is presented. Information about shish formation and growth of α-phase parents lamellae from the shish backbones is extracted from SAXS; the overall apparent crystallinity evolution, amounts of different phases (α, β, and γ), and morphologies developing in the shear layer (parent and daughter lamellae both in α and γ phase) are fully quantified from the analysis of WAXD data. Both flow rate and pressure were found to have a significant influence on the nucleation and the growth process of oriented and isotropic structures. Flow affects shish formation and the growth of α-parents; pressure acts on relaxation times, enhancing the effect of flow, and (mainly) on the growth rate of γ-phase. The remarkably high amount of γ-lamellae found in the oriented layer strongly indicates the nucleation of γ directly from the shish backbone. All the observations were conceptually in agreement with the flow-induced crystallization model framework developed in our group and represent a unique and valuable data set that will be used to further validate and implement our numerical modeling, filling the gap for quantitatively modeling crystallization during complicated processing operations like injection molding.

Chain Deformation on the Formation of Shish Nuclei under Extension Flow: An in Situ SANS and SAXS Study

By combining extensional rheological and in situ small-angle neutron and synchrotron X-ray scattering (SANS, SR-SAXS) techniques, the correlation between chain deformation and morphology of nucleus in a lightly cross-linked deuterated PE (D-PE)/hydrogenated PE (H-PE) blend has been studied at different draw ratios. The deformation of molecular chain obtained by SANS shows that shish forms at a rather small chain deformation of about 1.3, which does not support either coil–stretch transition or the simple stretch network model. Combining these data with the SAXS results, we speculate that the conformation ordering couples with density, instead of single chain conformation, is the key factor for shish formation. Meanwhile, due to the different alignment of the center of mass of the stretched network in samples with different draw ratios, a periodic concentration modulation of D-PE appears after crystallization of the samples stretched to the hardening zone, which, however, does not occur under the small dra...

Applications of shear-induced polarized light imaging (SIPLI) technique for mechano-optical rheology of polymers and soft matter materials

A new experimental method for studying the mechano-optical rheology of polymeric liquids and soft matter materials is presented. The method is based on a combination of rotational rheology and a recently developed optical technique—shear-induced polarized light imaging (SIPLI). The method provides a unique opportunity to monitor a complete sample view during rheological measurements in plate–plate and cone-and-plate geometry. Applications of the method are presented including simultaneous SIPLI and the rheology of the oriented lamellar phase of block copolymers and liquid crystals as well as a study of the thermally induced reversible transformation of worm-like micelles to spherical micelles. In addition, a direct relation between the shish formation and the polymer melt viscosity upturn during flow-induced crystallization of semi-crystalline polymers is demonstrated. An application of SIPLI for quantitative birefringence measurements is also shown. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2151–2170

Dissolution and Re-emergence of Flow-Induced Shish in Polyethylene with a Broad Molecular Weight Distribution

Flow-induced shish formation in semicrystalline polymers is an important phenomenon since shish can strongly influence final material properties. The formation, dissolution, and re-emergence of flow-induced shish were studied for a polyethylene system with a broad molecular weight distribution using time-resolved synchrotron small-angle X-ray scattering (SAXS). The results show that shish are generated for a critical strain of 100 at various shear rates, ranging from 25 to 200 s–1. The shish formed at 140 °C are not stable and decay during a subsequent isothermal process. The specific shish dissolution dynamics is in contrast with the shish formation, affected by the shear rate applied. Unexpectedly, when the polymer melt with dissolved shish was cooled down, shish re-emerges. It demonstrates that the shish created did not dissolve completely into relaxed random coils but relaxed only partially into some ordered precursors that are below the limit of SAXS detection. Moreover, shish dissolution dynamics wa...

Flow-Induced Crystallization of Polymers: Molecular and Thermodynamic Considerations

Flow-induced crystallization (FIC) of polymers is a long-standing, industrial relevant, nonequilibrium thermodynamic challenge. Thanks to the development of in situ time and spatial resolved techniques like rheology and synchrotron radiation X-ray scattering, substantial progress on the understanding of FIC has been achieved in past 20 years. In this Perspective, we first discuss some recent modifications and improvements on early coarse-grained approaches like the coil–stretch transition model for shish formation and the entropic reduction model for the enhancement of nucleation rate. Then breaking out the two-phase model of classical nucleation theory, flow-induced coil–helix transition, density fluctuation or phase separation, and isotropic–nematic transition are considered as intermediate orders in FIC. Establishing flow morphology and phase diagrams is essential for revealing the nonequilibrium nature of FIC, which will serve as structural roadmap for the processing of high performance polymer products.

Kinetic Process of Shish Formation: From Stretched Network to Stabilized Nuclei

On the basis of the duality of the shish-kebab superstructure, coil–stretch transition (CST) is well recognized as the molecular mechanism for shish-kebab formation in polymer melts, which, however, is challenged by recent results in flow-induced crystallization (FIC). In this work, we perform a real time investigation on FIC of polyethylene bimodal blends by combing a unique homemade extensional rheometer and synchrotron radiation small-angle X-ray scattering. The results show that the critical strain for shish formation decreases with increasing long chain concentration, which contradicts the role of CST but agrees well with stretched network model (SNM). Quantitative analyses indicate that the formation of shish is determined by the degree of network deformation rather than solely by strain or long chain concentration at a specific temperature. In addition, three types of shish with different stability are observed sequentially by increasing strain. On the basis of our results, strong support is given ...

Reexamination of shish kebab formation in poly(ethylene oxide) melts

Although recent experiments concerning shear effects on crystallization in polymer melts are inclined to support the oriented network model, two important questions still remain open: the effect of polydispersity on shish formation and the growth mechanism of shish. In this work, a narrowdisperse poly(ethylene oxide) (PEO) is chosen as a model system to clarify the role of polydispersity for shish kebab formation after a pulse of flow. The intact structure of shish kebab observed via scanning electron microscopy (SEM), with a 150 nm wide shish core and perpendicular epitaxially grown kebabs with a long period of about 35 nm, reveals that polydispersity is not a necessary condition for shish kebab formation. On the other hand, two-step shear experiment is utilized to elucidate the growth mechanism of shish. It has been proved in a latest study that shish growth is related to the size of the growing nuclei (interval time) and second step shear rate. In this manuscript, a third factor, shear time in the second step shear, is found to be an effective factor to promote the growth of shish, which might provide a convenient processing method.

Mechanism for Shish Formation under Shear Flow: An Interpretation from an in Situ Morphological Study

Isotactic polypropylene has been systematically studied with in situ optical microscopy to obtain a real space view of the morphology evolution of shear induced crystallization as a function of shear rates and shear time to elucidate the mechanism of shish formation. The critical shear time for inception of shish formation on the shear rate have provided an important understanding of the molecular and entangled network relaxation in relation to the shish formation. Also the observation of a typical shear time dependent comet like shish-kebab structure formation on the interface and the morphological growth of the shish from a fiber inside our sample have led to a new hypothesis that the shish is formed through a multiple discrete steps instead of forming directly to the final most stable state, this means that a transition state may be existed before shish growth. Two steps shear experiments with various time intervals between each step were designed to verify our proposed transition state mechanism, which can be observed much directly and obviously through real-space morphology, especially at low shear rate with long shear time. Another time dependent relaxation time is introduced base on shear rate dependent experiment after the first step shear and has close relationship with the existence of the critical shear time. A general framework for the shish formation has been established which can capture all the observed morphological features well, including the existence of the critical shear time at a given shear rate.

Self-Acceleration of Nucleation and Formation of Shish in Extension-Induced Crystallization with Strain Beyond Fracture

Extension flow induced crystallization of isotatic polypropylene (iPP) has been studied with a combination of extension rheological and in situ small-angle X-ray scattering (SAXS) measurements at 140 °C. Rheological data of step extension on iPP melt are divided into before and beyond fracture strain zones in strain–strain rate space, where intermediate strains between them lead to fracture of samples. Coincidently, weak and strong accelerations of nucleation are observed in the before and beyond fracture strain zones respectively, where distinctly different features of crystallization kinetics and nucleation form occur in these two zones. The microrheological model explains the acceleration of nucleation in the “before fracture strain zone” well, while a “ghost nucleation” mechanism is proposed to interpret the strong acceleration of nucleation in the “beyond fracture strain zone”. The “ghost nucleation” is due to the displacement of initial parent point nuclei, where daughter nuclei are induced along the trails. This new mechanism explains well the acceleration of nucleation in orders of magnitude and the formation of shish in iPP melt.

Shish Formation and Relaxation in Sheared Isotactic Polypropylene Containing Nucleating Particles

The formation and relaxation of shish in molten isotactic polypropylene (iPP) and two samples containing different commercial nucleating particles was studied during and after continuous shear flow. In-situ measurements of the shish were made by simultaneous wide- and small-angle X-ray scattering (rheo-SAXS/WAXS) using synchrotron radiation. In the SAXS patterns, sharp meridional reflections were observed in all three samples during the initial stages of shear flow. Their presence indicates that shish form in discrete steps, by the aggregation of pointlike nuclei into oriented threads. Compared to pure iPP, the addition of both nucleants increased the amount and the dimensions of shish formed. However, the shish also relaxed faster in the presence of both nucleated materials. This behavior was attributed to the particles inducing lower molecular weight chains to participate in shish formation. The particle aspect ratio was observed to greatly influence shish formation. This is explained by modification of...

A Study on the Chain−Particle Interaction and Aspect Ratio of Nanoparticles on Structure Development of a Linear Polymer

The stability of metastable flow-induced precursor (FIPs) in the polymer melts in presence of nanoparticles, viz. single-walled carbon nanotube (SWCNT) and zirconia nanoparticles, is studied at, 142 °C, close to the equilibrium melting point of unconstrained extended chain crystals of linear polyethylene (PE). The results conclusively demonstrate the influence of chain−particle interactions, between PE and the nanoparticles, on the stretch of the long chains. With the applied flow, SWCNTs together with PE chains are observed to align along the flow direction, giving rise a strong streak like pattern along the equator. At the initial stages, intensity of the observed streak in the presence of SWCNTs is stronger than that for the neat polyethylene. The streak intensity stabilizes with time, where the time required for the stabilization depends on the amount of the dispersed nanotubes in the polymer matrix. On the contrary, in the presence of zirconia nanoparticles, where the chain−particle interactions between PE and the nanoparticles are weak the initially observed streak tends to disappear with time, where the time required is strongly dependent on the concentration of the nanoparticles in the polymer matrix. Thus, compared to the neat polymer, the presence of zirconia nanoparticles destabilizes the shish formation. The chain orientation along the flow direction is determined using Herman’s orientation function and the length of the oriented chains (shish) by Ruland’s streak analysis. On cooling, with the crystallization of the polymer, scattering develops along the meridian, indicating the development of folded chain crystals, where the oriented chains present along the flow direction provide the epitaxy matching thus suppressing the nucleation barrier. The meridional intensity (arising with the formation of crystals, called kebabs) at room temperature, shows strong dependence on the stable streak intensity (chain orientation along the flow direction, called shish) along the equator prior to cooling.

Critical Strain for Shish-Kebab Formation

A combination of extensional rheological and in situ synchrotron radiation small-angle X-ray scattering (SR-SAXS) measurements was introduced to investigate critical strain ε* for shish formation and validate whether coil−stretch transition or stretched-network is responsible for shish-kebab formation in high density polyethylene melt. With strain rates ε̇ larger than a specific value, the critical strain ε* required to induce shish formation turns out to be a constant of about 1.57, which ensure full extension of chain segments with critical entanglement molecular weight locked between two adjacent entanglement points. The results clearly demonstrate that the formation of shish-kebab in polymer melt stems from stretched network instead of coil−stretch transition.