Crystallization of polyethylene under shear flow as studied by time resolved depolarized light scattering. Effects of shear rate and shear strain

We have studied structure formation during crystallization of polyethylene (PE) under shear flow using time resolved depolarized light scattering (DPLS) in order to elucidate the formation mechanism of the so-called shish-kebab structure. Two-dimensional (2D) DPLS pattern clearly showed streak-like scattering normal to the flow direction in the early stage during the crystallization after pulse shear, suggesting the formation of the shish-like structure in μm scale. In order to analyze the 2D DPLS pattern we defined measures for the acceleration of the crystallization rate and the degree of anisotropy and found that there are critical shear rates for both of the acceleration and the anisotropy at a given shear strain: the former is much smaller than the latter. We also determined the critical shear rate for the anisotropy as a function of the shear strain. Extrapolating to inverse of the infinite shear strain=0, we found the critical shear rate for the anisotropy at the infinite shear strain to be 1.5 s⁻¹. The results were discussed in relation to a competition between the relaxation rate of polymer chains and the orientation-induced crystallization rate.     

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

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

Formation of phase structure and crystallization behavior from disordered melt for ethylene-isoprene block copolymers and their blends

The structure formation and crystallization kinetics in crystallization from a disordered melt were investigated for a polyethylene-polyisoprene block copolymer (LEI) having M_n = 3.2 × 10⁴ and 53 wt% of polyethylene content and for its blends with the corresponding homopolymers, polyethylene (PE) and polyisoprene (PIp), using synchrotron small-angle X-ray scattering techniques (SAXS) and differential scanning calorimetry (DSC). For LEI copolymer and the blends, no microphase separation structure was observed in the molten state. In the crystalline state of the neat LEI, the first and higher order scattering peaks were clearly observed, in which the intensity of the higher order peaks was considerably strong. This unusual behavior of the higher order peaks was explained by the lamellar insertion model of Hama and Tashiro. From the analyses based on this model and one-dimensional electron density correlation function with a three phase model, the phase structure in the crystalline state of the neat LEI was concluded to be a regular lamellar structure consisting of crystalline lamella of PE block and amorphous layers of PE and PIp blocks. This phase structure was quite different from that reported previously for a polyethylene-polyisoprene block copolymer (HEI) with a higher molecular weight in which HEI crystallized with keeping the microphase separation structure in the melt. For the blends of LEI with PIp homopolymer, the phase structure is affected by the blend composition, while for the blends with PE homopolymer, the phase structure depended on the crystallization temperature as well as the molecular weight and composition of the added PE. The Avrami index was 2-3 for neat LEI, all blends and PE homopolymers.     

Molecular structure,crystallization behavior,and morphology of fractions obtained from an extrusion grade high-density polyethylene

An extrusion-grade of high density polyethylene (HOPE) (3 ethyl groups per 1000 carbons) has been divided into 16 fractions by preparative GPC and selective p-xylene extraction. The fractions, with molecular weights ranging from 900 to 1,000,000, have been studied by IR spectros-copy, DSC, WAXS, polarized microscopy, and small-angle light scattering (SALS), The average degree of chain branching (percent C₂H₅) is 0.5 percent for the part of the sample having a molecular weight lower than 10,000 and it decreases monotonically with increasing molecular weight, finally approaching 0.1 percent C2H5. A crystallinity depression with respect to linear PE equivalent to 20 percent/(percent C₂H₅) is recorded for all samples except for the very low molecular weight samples for which the crystallinity depression is much larger (30 to 35 percent/ (percent C₂H₅)). The unit cell volume increases with increasing percent C₂H₅, presumably due to the inclusion of ethyl groups in the crystals as interstitlals at 2gl kinks. The concentration of ethyl groups in the crystals (?_c) unanimously follows the relationship: ?_c(percent) = 0.32 + 0.25 log(percent C₂H₅) except for the low molecular weight fractions which have significantly lower values for ?_c. Our admittedly speculative explanation for this major discrepancy between high and low molecular weight samples is based on the idea that segments with ethyl groups close to chain ends have a greater difficulty in crystallizing than segments containing ethyl groups located at positions far from the chain ends. The fractions obtained from the extrusion-grade HDPE show a solidification temperature depression with respect to linear PE which can only be explained by the presence of chain branches in these samples. The depression is particularly pronounced for the low molecular weight samples as is expected from the data on molecular structure. Well-developed non-banded spherulites are observed in rapidly cooled (crystallized at about 35 K supercooling), low molecular weight samples (6,000 < M_w < 8,000)from the extrusion-grade HDPE in contrast to the axialites observed in linear PE of the same molecular weight and thermal treatment. This discrepancy in morphology has been related to the presence of ethyl groups in the extrusion grade HDPE fractions. Higher molecular weight samples (20,000 < M_w < 1,000,000)from the extrusion-grade HDPE and linear PE both display well-developed banded spherulites of similar nature as is expected due to the similarity in molecular structure of the two sets of sample.     

Formation of phase structure and crystallization behavior in blends containing polystyrene–polyethylene block copolymers

The microphase separation structure in the molten state and the structure formation in crystallization from such ordered melt were investigated for the blends of polystyrene–polyethylene block copolymers (SE) with polystyrene homopolymer (PS) and polyethylene homopolymer (PE) and for the blends consisting of two kinds of SE with different copolymer compositions from each other, using synchrotron small-angle X-ray scattering techniques (SAXS). The copolymer compositions of SE block copolymers employed were 0.34, 0.58 and 0.73 wt. fraction of PE, and their melt morphologies were cylindrical, lamellar and lamellar, respectively. Macrophase separation or the morphology change in the melt occurred depending on the molecular weight and the blend composition, as reported so far. In crystallization from such macrophase-separated and microphase-separated melts, the melt morphology was completely kept for all the blends. Crystallization behavior was also investigated for the blends. The crystallization within the spherical and cylindrical domains surrounded by glassy PS was not observed for SE/PS blends. In the crystallization from the macrophase-separated melt, two exothermal peaks were observed in the DSC measurements, while a single peak was observed for other blends. For the blends with PS, the degree of crystallinity was depressed and the apparent activation energy of crystallization was high, compared to those for the corresponding neat SE. For SE/PE and SE/SE blends, those were changed depending on the blend composition.     

Photocross-linking of low-density polyethylene. III. Supermolecular structure studied by SALS

The supermolecular structure of photocross-linked polyethylene (XLPE) has been studied by small-angle light scattering (SALS). The data show that the spherulitic structure of XLPE gradually deteriorates with increasing degree of cross-linking and increasing irradiation temperature from well-developed spherulites to rodlike aggregates and disordered lamellar structures. A photocross-linked sample of PE has lower crystallinity, smaller crystallites, and smaller spherulites than does the original sample. At high degrees of cross-linking, the SALS patterns show little or no spherulitic structure. Results with photocross-linked polyethylene demonstrate that the overall effect of cross-linking on the morphological structure is similar to that of an increase in molecular weight of the polymer. © 1993 John Wiley & Sons, Inc.     

Self-assembly and crystallization behavior of a double-crystalline polyethylene-block-poly(ethylene oxide) diblock copolymer

The self-assembly and crystallization behavior of a well-defined low molecular weight polyethylene-block-poly(ethylene oxide) (PE-b-PEO) diblock copolymer was studied. The number-average degrees of polymerization for the PE and PEO blocks were 29 and 20, respectively. The molecular weight distribution was 1.04 as determined by size-exclusion chromatography. The PE-b-PEO sample exhibited two melting points at 28.7 and 97.4 °C for the PEO and the PE crystals, respectively. The crystallization of the PE blocks was unconfined, while the crystallization of the PEO blocks was confined between pre-existing PE crystalline lamellae, as demonstrated by simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) studies. In the fully crystalline state, both PE and PEO blocks formed extended-chain crystals with PE chains tilted ∼22° from the lamellar normal and PEO chains parallel to the lamellar normal, as evidenced by two-dimensional WAXD study of shear-oriented samples. Regardless of hydrogen bonding among hydroxyl chain ends in the PEO blocks, interdigitated, single-crystalline layer morphology was observed for both PE and PEO crystals. The partial crystalline morphology, where the PE crystallizes and the PEO is amorphous, had the same overall d-spacing as the fully crystalline morphology. A double-amorphous PEO layer sandwiched between neighboring PE crystalline layers was deduced based on a chain conformation study using Fourier transform infrared. The confined crystallization kinetics for PEO blocks was investigated by differential scanning calorimetry, which could be explained by a heterogeneous nucleation mechanism. The slower crystallization rate in the PEO-block than the same molecular weight homopolymer was attributed to the effects of nanoconfinement and PEO chains tethered to the PE crystals.     

Characterization of compositional heterogeneity in the polyethylene prepared with bis(imino)pyridyl iron(II) precatalyst and triethylaluminum

Analytical tools including solvent gradient elution fractionation (SGEF), GPC, ¹³C NMR, and differential scanning calorimetry (DSC) are integrated for the characterization of compositional heterogeneity in the polyethylene (PE) prepared with the LFeCl₂/AlEt₃ catalytic system. The results indicate that at least two different kinds of catalytic species are present in ethylene polymerization. One active species generating branched PE gives low molecular weight; another kind of active species gives high molecular weight PE with high linear structure. The amount of branch decreases with increasing the molecular weights, and the small proportion of the branched PE shows low molecular weight with vinyl‐terminated end group, indicating that the branched PE is generated from the catalytic species giving low activity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Phase behavior and structure formation for diblock copolymers composed of side-chain liquid crystalline and glassy amorphous components

Phase behavior and structure formation in liquid crystallization of a side-chain liquid crystalline (LC) block copolymers composed of poly[11-(4′-cyanophenyl-4″-phenoxy)undecyl acrylate] (PA₁₁OCB) and polystyrene (PSt) were investigated by using a time-resolved small-angle X-ray scattering technique (SAXS), differential scanning calorimetry and polarizing optical microscopy. PA₁₁OCB homopolymer formed smectic (Sm) liquid crystal. Liquid crystallization behavior of the block copolymers depended on the molecular weight and the block composition. When molecular weight was relatively low, order-disorder transition (ODT) was observed. In cooling of such block copolymers, liquid crystallization seemed to wait for the formation of LC-rich microphase by ODT. For the block copolymers with relatively high molecular weight, liquid crystallization slightly enlarged the domain spacing without changing the microphase separation structure in the melt. The order of the LC phase was lowered with decreasing dimensionality of the LC microdomains, that is, the LC blocks formed smectic liquid crystal in the matrix or lamellar microphase while liquid crystallization in the cylindrical microdomains did not show smectic but maybe nematic liquid crystal. Moreover, the LC blocks within the spherical microdomains did not liquid crystallize. From the 2-D SAXS with applying shear flow, the Sm layers were orientated perpendicularly to the interface of the microphase separation. The relation between the layer thickness of the LC phase and the molecular weight suggested that the main chain was extended normally to the interface of the microphase separation.     

Effect of rotation extrusion and molecular weight on ductile failure of polyethylene pipes under hydrostatic pressure

ABSTRACT

In this study, two kinds of polyethylene (PE) with different molecular weight were processed into the pipes via rotation extrusion and the failure behaviours under hydrostatic pressure as well as molecular relaxation were investigated. The experimental results showed that the high molecular weight PE exhibited slower relaxation behaviour with higher relaxation activation energy to facilitate the formation of shish-kebab under flow field, while for the low molecular weight one, the stretching molecules easily relaxed back coil state and spherulites were prone to form. Therefore, the low molecular weight PE pipes via convention and rotation extrusions had similar isotropic spherulite morphology and short failure times. In the case of high molecular weight PE, during the convention extrusion, polymer melts flowed along the axial direction to induce the alignment of shish-kebab accordingly, which went against to the hoop stress in hydrostatic pressure. With the rotation of mandrel and die, the direction deviated from the axis so that PE pipe exhibited better resistance to the hoop stress. The failure time was 182?h, 264% longer than the convention-extruded one. Accordingly, a new strategy to prepare high hydrostatic pressure PE pipe under cooperative effects of rotation extrusion and long molecular relaxation time was proposed.     

Precursor of shish-kebab in isotactic polystyrene under shear flow

We performed polarized optical microscope (POM), depolarized light scattering (DPLS) and small- and wide-angle X-ray scattering measurements on the structure formation process or the crystallization process of isotactic polystyrene (iPS) under shear flow below and above the nominal melting temperature T_m. It was found that an anisotropic oriented structure termed here as a string-like object was formed in μm scale even above the nominal melting temperature and stable for more than 24 h, but melted at around 270 °C far above the nominal melting temperature. The string-like object acts as a nucleation agent for the folded chain lamella crystal (or the kebab), and was assigned to a precursor of the shish-kebab. We also examined the shear rate dependence of the structure formation to find a critical shear rate for the formation of the string-like object, suggesting the relaxation of the chains plays an important role in the formation of the structure. Based on the results we have discussed the inner structure of the string-like object.     

Crystallization behavior of poly(β-propiolactone)-block-polyethylene copolymers with varying polyethylene crystallinities

The crystallization behavior of poly(β-propiolactone)-block-polyethylene (PPL-b-PE) copolymers with high PE crystallinities χ_PE (>0.30) has been examined using time-resolved synchrotron small-angle X-ray scattering and Fourier transform infrared spectroscopy, where the PE block crystallized first and subsequently the PPL block crystallized on quenching from a strongly segregated melt. The crystallization of PE blocks destroyed the lamellar microdomain structure (LMS) existing in the melt to form the crystalline lamellar morphology (CLM), and then PPL blocks crystallized within CLM. This morphology formation was compared to our previous results for the crystallization of PPL-b-PE copolymers with low χ_PE (0.12 < χ_PE < 0.26), where the crystallizability of PE blocks was not sufficiently large to destroy LMS. As a result, PE blocks crystallized promptly within LMS to reinforce and stabilize it against the subsequent crystallization of PPL blocks, yielding the confined crystallization of both blocks within LMS. We summarize these results including the case of χ_PE = 0, and propose three mechanisms of morphology formation occurring in PPL-b-PE copolymers according to χ_PE (i.e., high, low, or zero).     

Influence of high molecular weight component on the hierarchical crystalline structures of injection‐molded bars of polyethylene

A small amount of high molecular weight molecules can have a dramatic influence on the flow‐induced crystallization kinetics and orientation of polymers. To elucidate the effects of the high molecular weight component under a real processing process, we prepared model blends in which high density polyethylene with a high molecular weight and wide molecular weight distribution was blended with a metallocene polyethylene with a low molecular weight and very narrow molecular weight distribution. To enhance the shear strength, gas‐assisted injection molding was utilized in producing the molded bars. The hierarchical structures and orientation behavior of the molded bars were intensively explored by using scanning electron microscopy and two‐dimensional wide‐angle X‐ray diffraction, focusing on effects of the high molecular weight component on the formation of the shish kebab structure. It was found that there exists a critical concentration of high molecular weight component for the formation of a shish kebab structure. The threshold was about 5.5–7.0 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. Moreover, the rheological properties of molten polyethylene melts were studied by dynamic rheological measurements and a critical characteristic relaxation time for shish kebab formation was obtained under the processing conditions adopted in this research. © 2013 Society of Chemical Industry     

Preparation of high modulus polyethylene sheet by the roller-drawing method

The high density polyethylene (HDPE) sheets were drawn through a pair of heated rollers. The process, referred to as roller drawing, was found to be useful for producing high modulus and high strength HDPE sheets. The higher draw ratio could be obtained for the HDPE sheet with lower molecular weight and narrower molecular weight distribution. The Young's modulus and the breaking strength reached 43 GPa and 0.67 GPa, respectively, at the highest draw ratio. The measurements of wide-angle X-ray diffraction (WAXD) pole figures revealed that the crystallographic a-, b-, and c-axes were oriented to the normal direction (ND), the traverse direction (TD), and the drawing direction (DD), respectively. The small-angle X-ray scattering (SAXS) of the roller-drawn HDPE sheets with draw ratio higher than 7 exhibited two intensity maxima on the meridian, suggesting the presence of the two-phase structure in which crystalline and amorphous regions are stacked alternately along DD. The relationship between mechanical properties and microstructure was discussed on the basis of the concept of the formation of amorphous tie molecules in the interfibrillar and intercrystallite regions.     

Molecular mass characteristics of polyethylene produced with supported vanadium‐magnesium catalysts

Highly active supported vanadium‐magnesium catalysts (VMC) produce polyethylene (PE) with broad and bimodal molecular mass distribution (MMD) in comparison with the famous titanium‐magnesium catalysts (TMC). The effect of hydrogen as an efficient chain‐transfer agent on the MMD of PE has been studied. Increasing hydrogen concentration causes a considerable broadening of MMD of PE due to the shift of the low molecular weight peak on the MMD curve. At the same time, the high molecular weight shoulder stays at the same position even at high hydrogen concentration. This means that VMC contain two types of active centre. One type is very reactive in the chain‐transfer reaction with hydrogen. These centres produce low molecular weight PE in polymerization in the presence of hydrogen. The other type of active centre is not active in chain transfer with hydrogen. These centres produce high molecular weight PE ((1–3) × 10⁶) and hydrogen does not affect the position of the high molecular weight shoulder. MMD data were used to analyze the kinetics of the chain‐transfer reaction with hydrogen and to calculate the rate constants of this reaction. Copyright © 2005 Society of Chemical Industry     

Domain-boundary structure of styrene-isoprene block copolymer films cast from solutions. III. Preliminary results on spherical microdomains

The microdomain structure of styrene-isoprene A-B type block copolymers having a nearly constant fraction of polyisoprene block segments (13 ～ 22 wt percent) was investigated by small-angle X-ray scattering (SAXS) as a function of the molecular weights of the copolymers. The styrene-rich block copolymers all have spherical microdomains of polyisoprene block segments dispersed in a matrix of polystyrene block segments. The size of the spherical domains increases with increasing molecular weight of the polyisoprene segment with a power of ca. 0.6. The thickness of the domain-boundary interphase arising from a partial mixing of the incompatible segments at the domain-boundary interface is also estimated by analyzing a systematic deviation of the SAXS intensity distribution from Porod's law at large scattering angles on the basis of the infinite slit-height approximation. The results indicate the interfacial thickness to be about 20Å and to be almost independent of the molecular weight of the block copolymers studied. The applicability of the infinite-slit height approximation in the analysis of SAXS intensity distributions at large scattering angles is also discussed in an approximate fashion.     

Crystallization of double crystalline block copolymer/crystalline homopolymer blends: 1. Crystalline morphology

The crystalline morphology formed in binary blends of poly(ε-caprolactone)- block-polyethylene (PCL-b-PE) copolymers and PCL homopolymers has been examined using synchrotron small-angle X-ray scattering (SR-SAXS) and differential scanning calorimetry (DSC) as a function of the homopolymer fraction in the blend. The PE block crystallized first on quenching from a lamellar microdomain structure to set a hard lamellar morphology (PE lamellar morphology) in the blend, followed by the crystallization of PCL chains (i.e., PCL homopolymers + PCL blocks). Two binary blends were studied by considering the miscible state of PCL homopolymers in the microdomain structure: when the PCL homopolymers were uniformly mixed with PCL blocks, they formed a mixed crystal. When the PCL homopolymers were localized between PCL blocks in the microdomain structure, DSC results suggested the possible formation of separate PCL crystals in the PE lamellar morphology. The effect of the advance crystallization of PE blocks on the subsequent crystallization of PCL chains was discussed as compared with the crystalline morphology formed in PCL-block-polybutadiene copolymer/PCL homopolymer blends, where the crystallization of PCL chains started directly from a microdomain structure without forming the hard lamellar morphology.     

Shear-induced epitaxial crystallization in injection-molded bars of high-density polyethylene/isotactic polypropylene blends

As a continuation of our previous works on exploring shear-induced epitaxial crystallization of polyolefin blends during practical molding processing [Na et al. Polymer 2005; 46, 819 and 5258], the present study focused on the importance of molecular weight on the formation of epitaxial structure in injection-molded bars of high-density polyethylene (HDPE)/isotactic polypropylene (iPP) blends. By choosing two kinds of HDPE and two kinds of iPP with high molecular weight or low molecular weight, four blends with different molecular weight combinations can be designed. After making the blends via melt mixing, the injection-molded bars were prepared in a so-called dynamic packing injection molding equipment where repeated shearing was imposed on the melts during the solidification stage. Crystal structure and orientation were estimated mainly through 2D-WAXD. Our results indicated that an appropriate matching of low molecular weight HDPE and high molecular weight iPP was more favorable for epitaxial crystallization than other component matches. The effects of orientation and epitaxy on the re-crystallization behaviors of polyolefin blends have been elucidated in detail through PLM experiments. Moreover, epitaxy has been proved to play a positive effect in determining the ultimate mechanical properties of injection-molded bars.     

聚乙烯/石蜡油共混体系动态流变行为和相容性

将分子量相差较大的两种聚乙烯(PE)分别与石蜡油(LP)组成共混体系,通过旋转流变仪、偏光显微镜(PLM)、差示扫描量热仪(DSC)和扫描电子显微镜(SEM)研究了温度和LP含量对共混体系动态流变行为的影响,并对两相共混体系的相容性及微孔结构进行了表征.结果表明,随着温度的升高和LP含量的增加,PE/LP共混体系的储能...     

An in-situ X-ray scattering study during uniaxial stretching of ionic liquid/ultra-high molecular weight polyethylene blends

An ionic liquid (IL) 1-docosanyl-3-methylimidazolium bromide was incorporated into ultra-high molecular weight polyethylene (UHMWPE) and formed IL/UHMWPE blends by solution mixing. The structure evolution of these blends during uniaxial stretching was followed by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. During deformation at room temperature, deformation-induced phase transformation from orthorhombic to monoclinic phase was observed in both IL/UHMWPE blends and neat UHMWPE. The elongation-to-break ratios of IL/UHMWPE blends were found to increase by 2-3 times compared with that of pure UHMWPE, while the tensile strength remained about the same. In contrast, during deformation at high temperature (120 °C), no phase transformation was observed. However, the blend samples showed much better toughness, higher crystal orientation and higher tilting extent of lamellar structure at high strains.