Polyethylene Crystals Research Articles

Molecular dynamics study of axial tensile response of crystalline ultra-high molecular weight polyethylene under different loading conditions

Through molecular dynamics simulations, we investigated the effects of temperature on the axial tensile behavior of ultra-high molecular weight polyethylene (UHMWPE) crystals, considering the combined effects of transverse compression, strain rate, and molecular weight. The effects of temperature over the range of 100 K to 450 K is shown to reduce the mechanical properties. Existing chain end defects facilitate chain sliding and induce stress concentration in adjacent molecules, thereby reducing the strength and modulus of crystals. Lower molecular weight and higher temperatures promote chain sliding, while higher transverse compression and increased strain rates inhibit chain sliding, resulting in higher properties. Additionally, higher temperatures increase stress concentration due to thermal vibrations, which induce localized high stress conditions within polyethylene chains. The transition of the failure mode from chain sliding to bond breakage occurs at strain rates between 1012 s−1 and 1013 s−1, and is found to be independent of temperature, pressure, and molecular weight. The results are compared to the response of crystals without chain end defects. These insights contribute to a deeper understanding of the behavior of UHMWPE crystals under extreme loading conditions.

Direct observation of the fracture mechanism at polyurethane/polyolefin adhesive interfaces involving needle-like polyolefin crystals

Polyolefins, such as polyethylene and polypropylene, are low-surface-energy materials that are poorly adhesive owing to weak van der Waals (VDW) forces. While physical bonding offers an attractive alternative to conventional VDW-based adhesion, the development of adhesion methods based on this mechanism requires an understanding of the fracture mechanism of adhesive interfaces. Herein, we reveal the fracture mechanism associated with “nailing adhesion,” in which two polymers are tightly bound physically by needle-like polyolefin crystals at a polyurethane/polyolefin adhesive interface. The mechanism was directly observed by transmission electron microscopy using the modified copper-grid technique. The peel strength and fracture mechanism depended on the polyolefin type (polyethylene or polypropylene). During crack growth at the adhesive interface, needle-like polyethylene crystals deform and finally break, whereas needle-like polypropylene crystals snap with crazing. The deformation of needle-like polyethylene crystals enhances peel strength more than the crazing and snapping of needle-like polypropylene crystals.

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.

Improvement of thermal, mechanical, and electromagnetic interference shielding properties of polyethylene nanocomposites by introducing non‐covalently modified carbon nanotube and reactive polyethylene copolymer

AbstractWe herein report the enhancement of thermal, electrical, electromagnetic interference (EMI) shielding, and mechanical performance of polyethylene (PE) nanocomposites by the introduction of non‐covalently modified multi‐walled carbon nanotube (MWCNT) and ethylene‐glycidyl methacrylate (EGMA) copolymer. For this purpose, benzylacetic acid‐functionalized MWCNT (BA‐M) is prepared by ball milling, and it is melt‐mixed with EGMA to prepare a masterbatch. The masterbatch is then melt‐compounded with neat PE to manufacture a series of PE/EGMA/BA‐M nanocomposites with 0.5–5 wt% MWCNTs. Electron microscopic and spectroscopic analyses confirm the successful fabrication of non‐covalently functionalized BA‐M, the uniform dispersity of MWCNTs in the PE matrix, and the chemical reactions between the carboxylic acid groups of BA‐M and the epoxy group of EGMA in the nanocomposites. Scanning calorimetric data reveals that BA‐M acts as an efficient nucleating agent for the crystallization of PE. The excellent dispersity and robust interfacial bonding of BA‐M in the PE matrix result in improved electrical and EMI shielding properties in PE/EGMA/BA‐M nanocomposites. Furthermore, the tensile strength, initial modulus, and impact strength of the nanocomposites gradually rise as the BA‐M content increases. Thus, the PE/EGMA/BA‐M nanocomposite containing 5 wt% MWCNT achieves excellent performance of EMI shielding effectiveness of ~14.7 dB/mm, tensile strength of ~34.5 MPa, initial modulus of ~295.7 MPa, and impact strength of ~539.9 J/m, which represent enhancements of ~265%, ~40%, ~82%, and ~452%, respectively, compared to pristine PE.Highlights Non‐covalently modified MWCNT (BA‐M) was prepared by facile ball milling. PE nanocomposites with BA‐M and EGMA were prepared by masterbatch melt‐mixing. Excellent dispersity and interfacial bonding of MWCNT in the PE matrix were attained. Tensile strength and initial modulus of the nanocomposites were highly improved. EMI shielding effectiveness of the nanocomposite with 5 wt% MWCNT was ~14.7 dB/mm.

Heterogeneous nucleation of polyethylene crystals on binary hexagonal nanoplatelets

Crystal nucleating agents offer an effective strategy for controlling the morphology, dimensional stability and rate of solidification of polymers during processing. Molecular dynamics (MD) simulation can shed light on nucleation behavior at the nanoscopic length and time scales over which nucleation occurs. In this work, crystal nucleation of a polyethylene oligomer, n-pentacontane, on three graphene-like substrates, hexagonal boron nitride (hBN), molybdenum disulfide (MoS2), and tungsten disulfide (WS2), was simulated, and the thermodynamic efficiencies of these substrates as nucleating agents were determined. Experimental measurements of heterogeneous nucleation of a high-density polyethylene on nanoparticles of these three graphene-like materials were performed using the method of dispersed microdroplets in an immiscible polystyrene matrix. Qualitative agreement between simulations and experiments was observed for trends in nucleation rate, J, and interfacial free energy difference, Δσ, with JhBN>JMoS2>JWS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$J_{\ ext{hBN}} > J_{\ ext{MoS}_{2}} > J_{\ ext{WS}_{2}}$$\\end{document}. The simulations are then used to gain additional insight into the mechanisms of nucleation. Epitaxy is confirmed in all systems, with small mismatches in lattice spacing being accommodated by strain in the oligomer crystal. However, epitaxy alone is insufficient to explain the observed trends. The strength of interaction between the nucleating agent and the polyethylene oligomer is found to be the strongest predictor of nucleating agent efficiency. The strength of interaction is in turn related to the density of interaction sites at the interface: hBN has the highest density, and thus the fastest nucleation rate.

Morphology and confinement effects on crystallization kinetics in polyethylene containing block copolymers

This paper investigates the morphology and crystallization kinetics of polyethylene (PE) containing block copolymers, differing in block number (di- vs. triblock copolymers) and composition (ABA triblock co- vs. ABC triblock terpolymers). In these block copolymers, the crystallizable PE block is linked to amorphous poly(methyl methacrylate) (PMMA) and/or polystyrene (PS) blocks, and for one triblock terpolymer, poly(ethylene oxide) (PEO) was introduced as an additional crystallizable block. The topological confinement exerted by the covalent linkage of PE to the other blocks (glassy PMMA and PS, PEO) in combination with the morphological confinement into different PE microdomains (cylinders, lamella) significantly influence the crystallization behavior, resulting in slower crystallization kinetics compared to pure PE. The strongest effects were observed for triblock terpolymers, where the lamellar PE domains are disrupted by PS cylinders or distorted PS domains. This research enhances our understanding of block copolymers with confined crystallizable segments, aiding in tailored material design.

Application of the NPK Method in the Crystallization Kinetics of High-Density Polyethylene

The Non-Parametric Kinetics (NPK) method, developed in 1998, by Serra, R., Sempere, J. and Nomen, R. has been used by them and other research groups to analyze the thermal behavior of a physical or chemical transformation and thermal stability. However, it was until 2021 that the method was applied for the first time in a cooling process for the determination of the crystallization kinetics of polypropylene, obtaining satisfactory results.
 The advantage of the NPK method is that it does not assume any model for the temperature function, nor for the conversion function to describe the crystallization kinetics. Through a mathematical process, the method provides two vectors: the vector u, which contains the information about the kinetic model, and the vector v, which contains the information about the temperature function.
 The aim of this work is to verify that the NPK method is applicable to the crystallization of high-density polyethylene by comparing the results with the most commonly used models to describe the crystallization kinetics of polymers. The fits obtained were very good and the vector u and v allow us to reproduce the original curves from DSC.

The Isothermal Melting Kinetics of Ultra-High Molecular Weight Polyethylene Crystals.

The isothermal melting behaviors of ultra-high molecular weight polyethylene (UHMWPE) with different entangled states (i.e., nascent and melt-crystallized samples) are studied. For two kinds of UHMWPE samples, the result shows that the relative content of survived crystals (Xs) exponentially decreases with time and reaches a constant value. It is suggested that such a melting behavior is related to the observed nonlinear growth of crystals induced by the kinetically rejected entanglements accumulated at the growth front. Additionally, the exponential decay of Xs with time provides a characteristic melting time (τ) for the melting process. Compared to the melt-crystallized UHMWPE, the τ value of nascent UHMWPE is generally longer even in a higher temperature range, which is mainly because the former has a larger entanglement density difference. Furthermore, these observations demonstrate that UHMWPEs with different entangled states have an analogous melting mechanism since they exhibit a similar melting activation energy (≈1300kJ mol-1).

Bottlebrush Elastomers with Crystallizable Side Chains: Monolayer-like Structure of Backbones Segregated in Intercrystalline Regions.

Bottlebrush (BB) elastomers with water-soluble side chains and tissue-mimetic mechanical properties are promising for biomedical applications like tissue implants and drug depots. This work investigates the microstructure and phase transitions of BB elastomers with crystallizable polyethylene oxide (PEO) side chains by real-time synchrotron X-ray scattering. In the melt, the elastomers exhibit the characteristic BB peak corresponding to the backbone-to-backbone correlation. This peak is a distinct feature of BB systems and is observable in small- or medium-angle X-ray scattering curves. In the systems studied, the position of the BB peak ranges from 3.6 to 4.8 nm in BB elastomers. This variation is associated with the degree of polymerization of the polyethylene oxide (PEO) side chains, which ranges from 19 to 40. Upon crystallization of the side chains, the intensity of the peak decays linearly with crystallinity and eventually vanishes due to BB packing disordering within intercrystalline amorphous gaps. This behavior of the bottlebrush peak differs from an earlier study of BBs with poly(ε-caprolactone) side chains, explained by stronger backbone confinement in the case of PEO, a high-crystallinity polymer. Microstructural models based on 1D SAXS correlation function analysis suggest crystalline lamellae of PEO side chains separated by amorphous gaps of monolayer-like BB backbones.

Revealing the Effect of the Molecular Weight Distribution on the Chain Diffusion and Crystallization Process under a Branched Trimodal Polyethylene System.

With the increasing demand for high-end materials, trimodal polyethylene (PE) has become a research hotspot in recent years due to its superior performance compared with bimodal PE. By means of molecular dynamics (MD) simulations, we aim to expound the effect of the molecular weight distribution (MWD) on the mechanism of nucleation and crystallization of trimodal PE. The crystallization rate is faster when short-chain branching is distributed on a single backbone compared to that on two backbones. In addition, as the content of high molecular weight backbone decreases, the time required for nucleation decreases, but the crystallization rate slows down. This is because low molecular weight backbones undergo intra-chain nucleation and crystallize earlier due to the high diffusion capacity, which leads to entanglement that prevents the movement of medium or high molecular weight backbones. Furthermore, crystallized short backbones hinder the movement and crystallization of other backbones. What is more, a small increase in the high molecular weight branched backbone of trimodal PE can make the crystallinity greater than that of bimodal PE, but when the content of high molecular weight backbone is too high, the crystallinity decreases instead, because the contribution of short and medium backbones to high crystallinity is greater than that of long backbones.

Investigation of Crystallization, Morphology, and Mechanical Properties of Polypropylene/Polypropylene-Polyethylene Block Copolymer Blends.

Polyethylene (PE)-based elastomers are the ideal choice for enhancing the compatibility of polypropylene/polyethylene (PP/PE) blends and improving the mechanical properties of PP-based materials. However, the issue of blend systems lies in the interplay between the crystallization processes. Therefore, we investigated the crystallization behavior during the cooling process of a new generation of PP/PE block copolymers (PP-b-PE) and random polypropylene (PPR, a copolymer of propylene and a small amount of ethylene or an alpha-olefin) blends using in-situ X-ray diffraction/scattering and differential scanning calorimetry (DSC) techniques. We also conducted mechanical performance tests on PPR/PP-b-PE blends at room temperature and low temperature (-5 °C). The results indicate that during the cooling process, the PP phase of PP-b-PE will follow the PPR to crystallize in advance and form a eutectic mixture, thereby enhancing the compatibility of PP/PE. Moreover, the PPR/PP-b-PE blend will form stable β-(300) crystals with excellent mechanical properties. Due to the improved compatibility of PP/PE with PP-b-PE, PE crystals are dispersed within PP crystals, providing bonding that improves the toughness of PPR under the low stiffness failure conditions of PPR/PP-b-PE blends, thereby enhancing their impact performance at low and room temperatures. This research has great significance for both recycling waste plastics and enhancing the low-temperature toughness of PPR.

Morphologies, structures, and properties on blends of triblock copolymers and linear low-density polyethylene

Abstract Focusing on the study of the phase separation behavior of triblock copolymer and linear low-density polyethylene (LLDPE) systems helps to understand the influence of microstructure on the properties of poly(vinylcyclohexane)-b- poly(ethylene)-b-poly(vinylcyclohexane) (PVCH-PE-PVCH/LLDPE) blends. We prepared a series of blends of LLDPE and PVCH-PE-PVCH and explained their compatibility from the microstructure. The research findings indicate that despite having similar block compositions, PVCH-PE-PVCH with a higher molecular weight exhibits significantly stronger phase separation and crystallization ability compared to PVCH-PE-PVCH with lower molecular weight. In PVCH-PE-PVCH/LLDPE blends, the addition of 10 %, 20 %, and 30 % LLDPE induces earlier crystallization and crystal phase separation of polyethylene (PE) fragments. In addition, compared to the lower molecular weight of PVCH-PE-PVCH, the higher molecular weight of PVCH-PE-PVCH exhibits a higher tendency for independent crystallization and shows significant crystal phase separation during the cooling crystallization process when blended with LLDPE. The PE segments in the lower molecular weight of PVCH-PE-PVCH can more easily enter the nanoscale domain of LLDPE. Impact fracture electron microscopy also reveals better compatibility between the lower molecular weight of PVCH-PE-PVCH and LLDPE compared to the higher molecular weight of PVCH-PE-PVCH. Furthermore, the blends of lower molecular weight of PVCH-PE-PVCH and LLDPE exhibit a greater growth rate in elongation at break.

Investigation of the Properties of Polyethylene and Ethylene-Vinyl Acetate Copolymer Blends for 3D Printing Applications.

3D printing of polyolefins, such as polyethylene (PE) and polypropylene (PP), is of great practical interest due to the combination of high properties of these materials. However, the use of these materials in 3D printing is associated with many problems due to their high rate of crystallization, which causes shrinkage and warpage of the printed object. In this regard, blends of PE and ethylene-vinyl acetate copolymer (EVA) of various compositions were investigated for 3D printing. It was found that with an increase in the concentration of EVA, an increase in the pseudoplastic effect and amorphization of PE occurs. It has been shown that with an increase in the EVA content, the degree of crystallinity of PE decreases slightly (by 11% at a content of 80% EVA); however, a significant decrease in the rate of crystallization of PE is observed (by 87.5% at the same EVA concentration). It was found that PE and EVA are completely compatible in the amorphous phase and partially compatible in the crystalline phase, which leads to a slight decrease in the melting point of PE. The introduction of EVA also leads to a significant increase in impact strength: the maximum value is achieved at a 50/50 ratio, which is five times the value of the initial PE and two times the value of the initial EVA. At the same time, it was revealed that EVA leads to a gradual decrease in the elastic modulus and strength of PE, the change of which generally obeys the additivity rule. The resulting printing filaments are characterized by a certain ovality due to their shrinkage, which decreases with increasing EVA content and reaches a minimum value at a PE/EVA ratio of 30/70. This composition also demonstrates the lowest shrinkage of the printed sample and higher processability during printing.

Reassessing chain tilt in the lamellar crystals of polyethylene

Semicrystalline polymers are extensively used in various forms, including fibres, films, and bottles. They exhibit remarkable properties, e.g., mechanical and thermal, that are governed by hierarchical structures comprising 10–20-nm-thick lamellar crystals. In 1957, Keller deduced that long polyethylene (PE) chains fold to form thin single lamellar crystals, with the molecular chains perpendicular to the flat faces of the crystals (the chain-folding model). Chains inclining to the perpendicular orientation in single crystals have since been reported, along with their effects on the physical properties of PE. For bulk specimens, the chain tilt angle (φ) has been investigated only for model samples with well-annealed internal structures. However, for briefly annealed specimens, the φ values of lamellae and their origins are controversial owing to the disordered lamellar morphology and orientation. Herein, we report the direct determination of molecular-chain orientations in the lamellar crystals of high-density PE using a state-of-the-art electron-diffraction-based imaging technique with nanometre-scale positional resolution and provide compelling evidence for the existence of lamellar crystals with different inner-chain orientations. Clarifying the nanoscale variation in lamellar crystals in PE can allow precise tuning of properties and expedite resource-saving material design.

Crystallization, structure and properties of PP-b-PE block copolymer

In recent years, a commercialized polypropylene (PP) and polyethylene (PE) block copolymer has been synthesized using chain-shuttling polymerization, but its molecular structure and properties are still limitedly understood. In this work, we analyzed the chain composition, mechanical properties, and structural transformations occurring during the deformation of commercial PP-PE block copolymer obtained via chain-shuttle technology. The chain composition and crystallization properties of PP-PE block copolymers obtained by chain shuttling technique were analyzed using nuclear magnetic resonance (NMR), temperature-rising elution fractionation (TREF), and differential scanning calorimetry (DSC) techniques. The sample has a mass ratio of 49:51 of PE chains to PP chains, but the crystallinity of the two components differs significantly. The PP displays limited crystallinity, indicating short PP chain segments. Some PE sequences are incorporated into the PP matrix as random copolymers, resulting in elastomeric behavior. We investigated the In-situ structural changes of crystals during stretching using synchrotron radiation 2D X-ray diffraction/scattering. In the elastic deformation region, the strain has a significant influence on the orientation of PE crystals but has little effect on the orientation of PP crystals. After the yield point, the strain-induced PE crystal failure resulting in a decrease in stress. In the strain hardening region, the orientation of PE induces recrystallization, forming a layered structure with tensile orientation. At the same time, the PP crystals are disoriented under the action of strain until they break. The results from this work provide significant insight into the design of PP-PE block copolymer products to meet the performance requirements needed for engineering applications.

Surface-enhanced nucleation in immiscible polypropylene and polyethylene blends: The effect of polyethylene chain regularity

In blends where polyethylene (PE) is the minor component dispersed in an isotactic-polypropylene (PP) matrix, an enhancement of the crystallization rate of PE was found when the crystallization temperature of the matrix phase is increased by means of a self-nucleation protocol. Such an effect is interpreted as the result of the epitaxial nucleation of PE droplet domains at the interface with the PP matrix (Macromolecules, 2021, 54, 19, 9100–9112). This study extends the findings on immiscible blends of a PP matrix and various industrially produced PE grades as the dispersed phase. Eight different PEs with varying molecular architecture, and thus density and melting temperatures, were mixed with PP in a 20/80 PE/PP weight ratio. The possible existence of surface nucleation was probed by applying a self-nucleation thermal protocol to the PP matrix and recording the concomitant variation of PE crystallization temperature (Tc). Different behaviours are observed for various PEs. In particular, those with relatively high density display a clear increase in crystallization temperature, over 3.0 °C, when the Tc of the matrix is increased (i.e., as PP lamellar thickness increased), indicating an efficient surface nucleation mechanism. Metallocene-made LLDPE also shows a small increase, about 1.7 °C, while PEs with lower densities, below 927 kg/m3, metallocene PEs or LDPE display no meaningful change in Tc with matrix self-nucleation. It was demonstrated that a threshold value of chain regularity is required to trigger the surface nucleation of PE on PP. Only polymers with a density above approximately 920 kg/m3 and melting temperatures exceeding about 115 °C can efficiently nucleate onto the PP substrate. It is postulated that the presence of a high amount of branches or comonomers along the PE chains hinders the epitaxial matching between PE and PP.

Molecular Simulations of Controlled Polymer Crystallization in Polyethylene.

Multilamella polymer crystals are grown from the melt for the first time, in molecular dynamics simulations of a united-monomer model, with in excess of 1500000 united-monomers. Two-component systems comprised of equal weight fractions of 2000 united-monomer long chains and 200 united-monomer short chains are considered, with varying numbers of short butyl branches placed along the long chains. Utilizing two different cooling protocols, continuous-cooling and self-seeding, drastically different multilamella structures are revealed, which depend heavily on the branch content and crystallization protocol used. By self-seeding, well-aligned multilamella crystals are grown, which more clearly reveal the subtle alterations an increasing number of branches create on the size and shape of the crystallites in the early stages of spherulite formation. Under continuous cooling, this observation is almost completely obscured. At maximum thickness, chain portions as long as 100 united-monomers (200 carbons) are extended inside the crystalline lamella.

Heterogeneous Nucleation of High-Density Polyethylene Crystals on Graphene within Microdomains

In polymer processes, nucleating agents are often used to control the kinetics of crystallization, but their application remains largely a matter of trial and error. Thermodynamically, the efficiency of a nucleating agent can be quantified by the difference in substrate/crystal, crystal/melt, and substrate/melt interfacial energies, Δσ. In this work, the efficiency of graphene nanoplatelets (GNP) as nucleating agents for high-density polyethylene (HDPE) is investigated. HDPE nucleates and crystallizes rapidly, so Δσ can be especially difficult to measure experimentally. To overcome this difficulty, blends of HDPE+GNP are confined to microdomains so that crystallization becomes nucleation limited. Two methods of microdomain formation are employed. In the first, HDPE+GNP is melt-blended with an immiscible matrix of polystyrene (PS), and crystallization is characterized using differential scanning calorimetry (DSC); in the second, dispersions of HDPE+GNP in toluene are sprayed onto PS substrates, and crystallization is characterized with polarized optical microscopy (POM). Heterogeneous nucleation rates at several crystallization temperatures and for several GNP loadings were measured by these two methods and found to give excellent agreement across GNP loadings. The value of Δσ for HDPE+GNP is calculated to be 0.83 ± 0.18 erg/cm2. This value is only 2.8 times larger than that reported for HDPE nucleated heterogeneously on a HDPE fiber, a nearly ideal nucleating agent for HDPE, and much smaller than many of the best nucleating agents reported for other polymers. We conclude that GNP is an efficient nucleating agent for HDPE.

CRYSTAF, DSC and SAXS Study of the Co-Crystallization, Phase Separation and Lamellar Packing of the Blends with Different Polyethylenes.

The crystallization of polyethylene (PE) blends is a highly complex process, owing to the significant differences in crystallizability of the various PE components and the varying PE sequence distributions resulting from short- or long-chain branching. In this study, we examined both the resins and their blends through crystallization analysis fractionation (CRYSTAF) to understand the PE sequence distribution and differential scanning calorimetry (DSC) to investigate the non-isothermal crystallization behavior of the bulk materials. Small-angle X-ray scattering (SAXS) was utilized to study the crystal packing structure. The results showed that the PE molecules in the blends crystallize at different rates during cooling, resulting in a complicated crystallization behavior characterized by nucleation, co-crystallization, and fractionation. We compared these behaviors to those of reference immiscible blends and found that the extent of the differences is related to the disparity in crystallizability between components. Furthermore, the lamellar packing of the blends is closely associated with their crystallization behaviors, and the crystalline structure varies significantly depending on the components' compositions. Specifically, the lamellar packing of the HDPE/LLDPE and HDPE/LDPE blends is similar to that of the HDPE component owing to its strong crystallizability, while the lamellar packing of the LLDPE/LDPE blend is approximately an average of the two neat components.

Effects of combined melt stretching and fast cooling fields on crystallization of high-density polyethylene

In many industrial processes, polymer melt undergoes complex flow-cooling environments before being solidified into the final products. Investigating polymer crystallization under processing conditions is a necessary step to predict and customize the structures and properties of semicrystalline polymers. Herein, the crystallization behavior of high-density polyethylene (HDPE) under combined melt stretching and fast cooling fields was studied through a homemade extensional rheometer. A series of HDPE samples were prepared by cooling the stretched melt at different cooling rates (0.02–220 °C/s) immediately after flow, or after waiting for different times of isothermal crystallization at 128 °C. Microstructural characterizations confirm four different structural states related to the evolution of shish-kebabs after melt stretching, namely generation of shish precursors, shish crystal growth, kebab lamellar growth, and perfection of shish-kebabs, which diversify greatly the structures and morphologies of the final solidified samples. By prolonging the isothermal crystallization time at high temperature, the fast cooling-induced lamellae keep decreasing in thickness. This supports the occurrence of molecular segregation caused by shish-kebabs growth at high temperature, which reduces the molecular weight of the remaining un-crystallized melt. In addition, quantitative analysis of crystalline orientation manifests that the lamellar growth during isothermal process takes strictly the shish as a template and adopts a high orientation, while the fast cooling-induced lamellae are only related to the chains conformation of un-crystallized melt prior to cooling. This work would be essential to advance the in-depth understanding on the actual industrial processing of semicrystalline polymeric materials.