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Shish-kebab crystal is a crucial crystalline morphology in polymers, and its formation and evolution are paramount for achieving high-performance materials. In this study, using UHMWPE as an example, we employed precise temperature control in the gel molding process to prepare low-entangled UHMWPE gel films containing reserved shish crystals, Distinguished from other molding systems, gel molding, due to the presence of small-molecule solvents, not only reduces the entanglement degree of the system but also provides a favorable environment for molecular chain disentanglement, and thus is considered as a low-entanglement system that is easily disentangled. We investigated the structural evolution of the films at different stretching temperatures by employing in-situ WAXD, SAXS, USAXS, along with ex-situ SEM and DSC methods. Furthermore, the impact of entanglement degree on the evolution of crystal structures was explored through the change of molecular weight. 2D-USAXS results reveal the longer stripe scattering signals than previous studies, indicating a more pronounced ordered arrangement of shish crystals. Simultaneously, we discovered that only when molecular chains with an appropriate degree of entanglement were induced to disentangle by stretching with the presence of small-molecule solvents can the high orientation of crystals be facilitated, thereby promoting the evolution of crystal structure. Reserved shish crystals can induce crystal orientation during stretching and facilitate structural evolution. Building upon earlier investigations, this study innovatively through the dual-factor (the molecular weight and stretching temperature) control of the suitable entanglement degree and disentanglement conditions, allowing for reasonable control of the growth and evolution of shish-kebab crystals in low-entangled systems. During the stretching process of low-entanglement UHMWPE films with solvents, the disentanglement of molecular chains is facilitated. While the reserved shish crystals simultaneously drive the structure evolution of crystals. [Display omitted] • Low-entangled UHMWPE gel films with reserving shish crystals are prepared by precise temperature control. • Reserved shish crystals enhance the shish-kebab structural evolution during the stretching of low-entangled solvent films. • The appropriate degree of chain entanglement can contribute significantly to the evolution of crystal structures. [ABSTRACT FROM AUTHOR]

Ultrahigh molecular weight polyethylene (UHMWPE) is widely used in industrial, military, and civilian applications owing to its excellent properties. However, the high entanglement degree of UHMWPE melt leads to great difficulties in diffusion and crystallization, which causes poor interfacial strength and mechanical properties of pre-melted UHMWPE, restricting its high-quality processing and high-value recycling. To this end, we propose a rapid interface stitching technology with a pulsed vibration force driving free volume homogenization and chain segment orientation. For the first time, pulse vibration molding (PVM) is applied for the hot-pressing of pre-melted UHMWPE, successfully realizing the interface stitching and product strengthening. Compared with those of the convention molding (CM) samples at 180 °C, the interfacial adhesive fracture toughness of the adhesive samples processed using PVM is increased by 105.9 % and the work of tension of the recycled samples processed using PVM is also increased by 49.09 %, respectively. The fracture strength of the PVM recycled samples at 180 °C reaches 91.77 % of that of the pure samples and the yield strength is also higher, which significantly enhances the recycling of UHMWPE. The characterization results of the interfacial molecular chain entanglement and crystallization show that PVM strengthens the molecular chain diffusion and induces crystallization by promoting the orientation of chain segments, especially at the interface. Consequently, we believe that the proposed technology can play a significant role in the UHMWPE processing methods that involve surface healing. [Display omitted] • A rapid interface stitching technology with a pulsed vibration force driving free volume homogenization and chain segment orientation was proposed. • The processing-structure-properties relationships of pre-melted UHMWPE at different processing conditions were established. • The response law of the interfacial entanglement and crystal structure is proposed. • The interfacial strengthening and the high-value recycling of UHMWPE are realized at a low time cost. [ABSTRACT FROM AUTHOR]

Producing thinner polymer membranes has been a research hotspot to satisfy the increasing demand from diverse application scenarios in the industry. This work used biaxially stretching with an exceptionally high draw ratio near melting point to thin down the excessively petrolatum-swollen UHMWPE gel from millimeters to hundreds of nanometers. Of which the meso- and micro-structural evolution during stretching processing was systematically investigated, especially within the thickness range from microns to submicrons, threading through the micron-thickness boundary. These membranes consisted of oriented smooth fibrils without identifiable overgrowth lamellae but with two obvious melting points, being different from the conventional bead-fiber shape of the typical shish-kebab super-crystalline structures. Moreover, an ionic rejection phenomenon of submicron UHMWPE was observed, which was attributed to the surface potential that emerged when thinning down PE membrane below the threshold of 800 nm. It is the first report of emerging surface potential for the nonpolar neutral polymers as a function of reduced thickness within the submicron range. This work provides insight into the processing of submicron-thin polymer membranes, with the unique structure and properties distinguished from the bulk revealed and explained. These findings may also help the processing and structure-property analysis of submicron membranes of other polymer materials. PE membranes thinned down into submicron range. [Display omitted] • UHMWPE membranes have been thinned down into the submicron thickness range. • These membranes consist of biaxially oriented smooth fibrils without kebabs. • A rebound increase in fibril diameter and surface roughness upon drawing were first reported. • An increased surface potential during thinning of submicron PE was revealed. [ABSTRACT FROM AUTHOR]

Shish crystal is the key structure to achieve low-dimensional ultra-high molecular weight polyethylene (UHMWPE) products with high performance. UHMWPE gel film containing shish crystals was prepared by precise temperature control in the gel molding, and its structural evolution in the stretching flow field at different temperatures was investigated by in-situ wide angle X-ray diffraction (WAXD)/small angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). It was found that shish crystals accelerated the structural evolution in both room and high temperature stretching, promoting more highly oriented crystals along the stretching direction. In room temperature stretching, the lamellae were deflected vertically to the stretching direction and gradually accumulated leading to a stabilization of the crystallinity index and the degree of orientation of the crystals along the stretching direction. However, in high temperature stretching, this intermediate orientation was broken and the transition to shish-kebab crystals continued, leading to a sharp increase in the crystallinity index and the degree of orientation of the crystals along the stretching direction. The reserved shish crystals promote more highly oriented crystals along the stretching direction in both room temperature and high temperature stretching, thus accelerating the structural evolution. [Display omitted] • Shish crystals are reserved in UHMWPE gel film by precise temperature control. • Reserved shish crystals accelerate the crystal structural evolution. • Increasing the stretching temperature promotes the generation of highly oriented crystals. [ABSTRACT FROM AUTHOR]

Poly (vinyl alcohol) (PVA) modification to nonpolar polymers has been intensively investigated due to the performance improvement in membrane separation, tissue engineering, and other applications. Slightly deviated from the conventional route, no covalent bonding by crosslinking or hydrogen bonding is needed to anchor PVA onto the target matrix. This work harnessed van der Waals interaction only to achieve the modification. Owing to the high surface energy and aspect ratio of ultra-high molecular weight polyethylene (UHMWPE) ultrathin membranes, the physical binding between PVA and PE was found stable and easy to establish. This interaction is initiated by the entropy-driven hydrophobic effect at the liquid-solid interface to reduce surface energy. By encapsulation of PVA on the fibrils, the PE hydrophilicity could be overturned (from 134.6° to 48.4°) with micro-structures remaining almost intact at the low loading of modification (0.001% conc. solution dip-coating). The resultant hydrophilic ultrathin PVA-PE membranes showed high transparency, high mechanical strength (380 MPa in tensile max. stress), and high porosity with only 200 nm thickness in a freestanding state. This work provides a facile approach to switch the hydrophilicity of nonpolar polymer ultrathin membranes by establishing vdW physical bonding only. By trace-adsorption, this method could also be helpful in fine-tuning nanostructures of the target polymer matrix by a resolution of nanometer level. [Display omitted] • The PVA-PE ultrathin composite was prepared without crosslinking and hydrogen bonding. • The surface hydrophilicity of polyethylene was switched at an exceptional low loading of PVA. • PE's porous structures retained almost intact after the homogeneous trace-adsorption of PVA. • This physical modification of PVA could also serve to fine-tune the porous nanostructures. [ABSTRACT FROM AUTHOR]

It is highly interesting to prepare polyethylene with ultra-high molecular weight under homogeneous conditions by nickel based neutral P^O type catalysts. In this contribution, ligand remote electronic effect was explored in some sterically hindered phosphine-phenolate based neutral nickel catalysts Ni1 – Ni3 to afford UHMWPE. The maximal activity of these catalysts was found to be 1.33 × 106 g mol−1 h−1, which is quite active. The nickel catalysts retained considerable activity at high temperature of 120 °C showing their good thermal stability. More importantly, all these catalysts are capable of generating UHMWPE with maximum M n of 2.51 × 106 g mol−1 by Ni2. The minimum distance between the Ni center and the oxygen of the 2,6-dimethoxy-1,1′-biphenyl unit attached with the phosphorous atom in Ni1 was recorded as 3.154 Å, which exactly matches with the van der Waals radii of oxygen and nickel. This provides an effective shield around the metal by preventing chain transfer and resulting to enhance the molecular weight of polymer. Combined with steric the remote substituents effect also crucial both to modulate activity and molecular weight of polymers. 2,6-dimethoxyaryl unit attached on phosphorous provides an effective shield at the axial place of metal to block the chain transfer process. [Display omitted] • The catalysts Ni1-Ni3 were found highly active with activity up to 1.33 x 106 g·mol-1·h-1. • All catalysts remain active even at 120°C showing their good thermal stability whereas previously reported catalysts of this class usually decompose at 60-80°C. • All the catalysts generate UHMWPE at 60°C with maximum M n up to 2.51 x 106 g·mol-1 by Ni2 , representing a major breakthrough in this class of catalysts. • The minimum distance from the Ni to the oxygen of the 2,6-dimethoxy-1,1'-biphenyl unit to the metallacycle in Ni1 recorded as 3.154 Å which exactly matches with the van der Walls radii. • Catalysts also show considerable incorporation of challenging fundamental polar monomer such as methyl acrylate. [ABSTRACT FROM AUTHOR]

It is an efficient way to introduce ortho-aryl dibenzosuberyl substituents into late-transition metal catalysts for retarding chain transfer in olefin polymerization. Here, we further found that dibenzosuberyl substituents could also suppress chain transfer and thus increase the molecular weight in ethylene polymerization with bis(imino)pyridyl Fe(II) catalysts. A series of bis(imino)pyridyl ligands with unilateral diarylmethyl or dibenzosuberyl moiety on the ortho-positions of N -aryl rings and their corresponding Fe(II) catalysts were synthesized and characterized. All these bis(imino)pyridyl Fe(II) catalysts exhibited high polymerization activity and could generate high weight-average molecular weight polyethylenes with very broad molecular weight distributions in most cases. Among them, the dibenzosuberyl Fe(II) catalysts could generate ultra-high weight-average molecular weight (M w = 2038–2137 kg/mol) polyethylenes at low temperatures, significantly higher than the diarylmethyl catalysts. The obtained polyethylenes are all highly linear with almost no branching distribution. Bis(imino)pyridyl Fe(II) catalyst with dibenzosuberyl substituents possesses outstanding ability to suppress the chain transfer in ethylene polymerization, producing ultra-high weight-average molecular weight (M w = 2038–2137 kg/mol) polyethylenes with very broad molecular weight distributions. [Display omitted] • 1, All these bis(imino)pyridyl Fe(II) catalysts exhibited high polymerization activity. • 2, The dibenzosuberyl Fe(II) catalyst could generate ultra-high weight-average molecular weight polyethylenes. • 3, The obtained polyethylenes are all highly linear with almost no branching distribution. [ABSTRACT FROM AUTHOR]

Nascent powder particles of Ultra High Molecular Weight Polyethylene have been covered by multi-walled carbon nanotubes and then processed by means of sintering. The prelocalization of carbon nanotubes in the interfacial region between powder particles of UHMWPE allowed reaching an electrical percolation threshold as low as 0.04 wt%. It is also a possible way to reinforce specifically the interfacial regions with a very small amount of fillers. Indeed, the nucleation effect of the nanotubes is very strong on the strain-induced crystallization in the melting state of the composite. The tensile experiments above the melting point allowed by the extremely broad rubbery plateau of the UHMWPE show a significant increase in strain-hardening due to the addition of CNTs from 0.02 wt%. In the solid state the elastic properties, such as yield stress, and the crystallinity can also be increased for higher CNT concentrations despite the presence of aggregates which reduce the elongation at break. [ABSTRACT FROM AUTHOR]

New activation systems for the bis[ N -(3- tert -butylsalicylidene)pentafluoroanilinato] Ti (IV) dichloride catalyst (FI) for the synthesis of ultra-high molecular weight polyethylene (UHMWPE) with reduced entanglement density are explored. Together with the well-studied FI catalyst-methylaluminoxane (MAO) catalytic system, different aluminoxanes, namely polymethylaluminoxane-improved performance (PMAO), modified methylaluminoxane type 12 (MMAO12) and type 3A (MMAO3A) have been used. The catalyst activity increases with the addition of a co-catalyst modifier (2,6-di- tert -butyl-4-methylphenol, BHT). For an example when using MMAO3A as co-catalyst in combination with BHT the catalyst activity increases by tenfold. The synthesized UHMWPEs have been characterized via rheology, differential scanning calorimetry and uniaxial solid-state deformation to evaluate the influence that the different co-catalysts have on the catalyst activation and the entangled state of the polymer. Entanglement density increases when PMAO is used as a co-catalyst influencing the rheological response of the polymer melt and melting kinetics of nascent powder. Mechanical properties increase when MMAO12 is used in place of MMAO3A as co-catalyst. However, the earlier reported mechanical properties of UHMWPE synthesized using the FI/MAO are found to be better than those obtained using the co-catalysts investigated in this study. [ABSTRACT FROM AUTHOR]

The extremely high molecular weight and molecular entanglement have rendered ultra-high molecular weight polyethylene (UHMWPE) superior properties. However, poor inter-particle diffusion makes it difficult to pelletize UHMWPE powders for easy processing/handling. A novel tapered die with air cooling was proposed to pelletize UHMWPE and a UHMWPE blend with 5 wt% high-density polyethylene (HDPE) for compression molding and material characterization. The tensile strength of samples prepared with the tapered die outperformed those by the regular die or from virgin powders. The addition of HDPE further improved the tensile strength. Fourier transform infrared spectroscopy (FTIR) revealed little chain scission and the lowest amount of oxidation from the tapered die. Intrinsic viscosity (IV) measurements confirmed the negligible chain scission and showed an increment in IV for the UHMWPE/HDPE blend. Multi-angle light scattering with size-exclusion chromatography (SEC-MALS) indicated no change in the UHMWPE molecular weight distribution, but some crosslinking in the blend. Polarized optical microscopy (POM) showed that the HDPE and extrusion led to a finer UHMWPE domain size and better fusion between UHMWPE and HDPE. The combined effect of enhanced molecular chain diffusion, improved consolidation, and lower oxidation using the tapered die led to 40% improvement of tensile strength for the UHMWPE/HDPE blend. [Display omitted] • Neat UHMWPE powders were successfully pelletized using a regular twin-screw extruder. • A novel tapered die with air cooling led to noticeable improvement in tensile strength. • Blending 5 wt% HDPE with UHMWPE by the tapered die improved the tensile strength by 40%. • The increased tensile strength and molecular weight are attributed to the crosslinking and enhanced molecular diffusion. [ABSTRACT FROM AUTHOR]

Abstract: Ionic liquids (ILs) with long alkyl substituted groups, including 1-docosanyl-3-methylimidazolium bromide (IL-1) and 1-docosanyl-3-methylimidazolium hexafluorophosphate (IL-2), were synthesized and used to modify the surface of carbon nanofibers (CNF). The nanocomposite film prepared by solution-blending of ionic liquid modified CNF (i-CNF) and ultrahigh molecular weight polyethylene (UHMWPE) displayed better toughness when compared with pure UHMWPE even at very low concentrations (e.g. 0.4 wt%). The effect of ionic liquids on the elongation-to-break ratio of this nanocomposite system was investigated. The ionic liquid with hexafluorophosphate as the anion was more efficient to increase the toughness of UHMWPE due to the improved compatibility of IL with UHMWPE in the polymer matrix than that of the bromide. The rheological behavior of molten nanocomposites revealed that the storage modulus and the complex viscosity decreased with increasing ionic liquid content in the high frequency region. However, a reverse trend was observed when the frequency was less than 0.05 s−1. In-situ monitoring in the change of crystallinity of the nanocomposite during tensile deformation suggested a mechanism of sliding between UHMWPE crystal regions and the surface of carbon nanofibers. [Copyright &y& Elsevier]

Abstract: Temperature dependence of lamellae orientation of a branched low molecular weight polyethylene (B-LMWPE)/ultra-high molecular weight polyethylene (UHMWPE) blend film has been studied by using transmission electron microscopy (TEM), two-dimensional wide angle X-ray diffraction (2D-WAXD) and two-dimensional small angle X-ray scattering (2D-SAXS). The film was prepared by solution casting under a controlled temperature gradient (T-gradient). TEM and X-ray scattering observations demonstrate that no clear lamellar crystal exists in B-LMWPE-rich phase while a large amount of lamellae are found in UHMWPE-rich phase. And these lamellae show a significant temperature dependence of preferred orientation. Results of 2D-WAXD and 2D-SAXS indicate that the lamellae stack regularly and preferentially parallel to the film surface at lower temperatures. At the temperatures near to the transition region, namely near to the gelation temperature, a part of the lamellae become to orient in an irregular way. At higher temperatures, almost all of the lamellae preferentially orient along the normal to the film surface and they randomly distribute around that direction. It's worth noting that the preferred orientation of the lamellae may influence the drawability of B-LMWPE/UHMWPE blend films. [Copyright &y& Elsevier]

Abstract: In a previous communication we showed improvement in the wear resistance and toughness of cross-linked ultrahigh molecular weight polyethylene (UHMWPE) for total joint implants by radiation cross-linking after high temperature melting (HTM). In this study, we hypothesized that introduction of vitamin E into UHMWPE before high temperature melting could improve the oxidative stability of these UHMWPEs with low wear and high toughness. Vitamin E was blended with UHMWPE powder at concentrations of 0.1 and 0.2 wt% and consolidated, followed by melting at 300 and 320 °C for 5 h, and subsequent irradiation with electron beam to 150 kGy. These vitamin E/UHMWPE blends showed improved tensile and impact toughness and good wear resistance in comparison with the radiation cross-linked vitamin E/UHMWPE blends. Aggressive accelerated aging with or without pro-oxidant lipids showed that vitamin E-blended, high temperature melted and subsequently irradiated UHMWPE had good oxidation resistance. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Inorganic particles are often used to enhance the wear resistance of UHMWPE, but the mechanism of the "enhancement effect" of inorganic particles improving the wear resistance of UHMWPE is ambiguous. In this paper, boron carbide particles(B 4 C) were used to improve the wear resistance of UHMWPE, it was found that the wear resistance of UHMWPE was increased by up to 46% with appropriate filler loading of B 4 C. Subsequently, the mechanism of B 4 C improving the wear resistance of UHMWPE was studied and illustrated from the perspective of structure-property relationship. The B 4 C particles act as exotic entanglement points, forming an entanglement network of particle-coupling agent-matrix, improving the degree of entanglement of molecular chains in the amorphous region of UHMWPE, thus improving the shear resistance of the matrix and reducing the structure damage caused by strong shearing during friction. Meanwhile, the highly entangled molecular chains prevented the transition of the interphase to the crystalline region, thus increasing the content of the interphase of UHMWPE and consequently alleviating the structural damage during the friction. Furthermore, B 4 C particles played the role of load bearing, which enhanced the compression resistance of UHMWPE then reduced the structure damage caused by compression in the normal upward during friction. This paper could be helpful to better understand the role of fillers on the wear resistance of UHMWPE and provide theoretical guidance to improve the wear resistance of UHMWPE and other materials. [Display omitted] • Mechanism of B 4 C improving the wear resistance of UHMWPE was studied according to the structure-property relationship. • The complex wear process was studied by dividing it into shear damage and compression damage. • The increasing of the interphase of UHMWPE/B 4 C was explained by Flash-DSC annealing experiments. [ABSTRACT FROM AUTHOR]

Abstract: The dissipative particle dynamics (DPD) simulation method has been used to study mesophase formation of the binary UHMWPE/PP and ternary UHMWPE/PP/PEG blends. The effects of shear rates and volume fractions of each of the blend components on end-to-end distances of UHMWPE, diffusivities and mesoscale morphologies of the blends have been investigated in detail. As compositions of the UHMWPE/PP and UHMWPE/PP/PEG blends vary, the mesoscale simulations have predicted the ordered structures with defined morphologies of lamellas, perforated lamellas, hexagonal spheres, and body-centered-cubic spheres. Micelle-like melted structures between totally disordered and the ordered phases have also been found in the UHMWPE/PP (10/90) blends. Immiscibility property of UHMWPE, PP and PEG induces the phase separation and exhibits different mesoscpic morphologies at different shear rates and volume fractions. Taking the shear rates dependence of mesophase into account, the change in morphology of the UHMWPE/PP/PEG blends with shear rate is also well studied in this work. As a function of PP concentration, the end-to-end distances of UHMWPE are found to decrease with the increase of PP concentration. This effect is more prominent for a high amount of PP. [Copyright &y& Elsevier]

Abstract: The majority of total joint replacements employs ultra-high molecular weight polyethylene (UHMWPE) for one of the bearing components. These bearings may fail due to the stresses generated in the joint during use, and fatigue failure of the device may occur due to extended or repeated loading of the implant. One method of analysis for fatigue failure is the application of fracture mechanics to predict the growth of cracks in the component. Traditional analyses use the linear elastic stress intensity factor K to describe the stresses near a loaded crack. For many materials, such as metals, it is the range of stress intensity, ΔK, that determines the rate of crack propagation for fatigue analysis. This work shows that crack propagation in UHMWPE correlates to the maximum stress intensity, K max, experienced during cyclic loading. This K max dependence is expected due to the viscoelastic nature of the material and the absence of crazing or other cyclic load dependent crack tip phenomena. Such a dependence on a non-cyclic component of the stress allows cracks to propagate under load with little or no fluctuating stresses. Consequently, traditional fatigue analyses, which depend on the range of the stress to predict failure, are not always accurate for this material. For example, significant static stresses that develop near stress concentrations in the component locking mechanisms of orthopedic implants make such locations likely candidates for premature failure due the inherent underestimate of crack growth obtained from conventional fatigue analyses. [Copyright &y& Elsevier]

Abstract: The evaluation of microstructure and crystal morphology in injected-molded bar becomes much complicated because of the existence of a shear gradient and a temperature gradient from the skin to the core of the samples. To understand the relationship between shear rate-molecular weight and oriented structure of injection molded bar, in this work, the hierarchy structure and the effect of molecular weight on the formation of shish-kebab structure were investigated by examining the lamellar structure of injection molded samples of high density polyethylene (HDPE) with different melt flow index (MFI), layer by layer, along the sample thickness. To enhance the shear effect, so-called dynamic packing injection molding (DPIM), in which the melt is firstly injected into the mold and then forced to move repeatedly in a chamber by two pistons that move reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part, was used to obtain the molded bar. Furthermore, a small amount of ultra-high molecular weight polyethylene (UHMWPE) was added into HDPE to explore the effect of UHMWPE on the crystal morphology and orientation. Our results indicated (1) that the overall orientation in the molded bar increased with decreased MFI, and a small amount of UHMWPE could enhance substantially HDPE orientation; (2) at the skin, there existed intertwined lamellae constituting an interlocked lamellar assembly, a typical shish-kebab structure gradually developed from the subskin-layer to the core, with increased shish content toward the center, but in the core was a spherulite-like superstructure with randomly distributed lamellae; (3) UHMWPE played an important role not only in the formation of shish, but also in the transformation from spherulite to shish-kebab oriented structure for HDPE with a low molecular weight (high MFI). [Copyright &y& Elsevier]

Abstract: The effects of the addition of UHMWPE particles, of nominal 〈80μm〉 size, on the fracture toughness, flexural modulus and strength of composites made with dimethacrylate resins (60/40wt/wt BisGMA–TEGMA) were investigated as a function of volume fraction of UHMWPE (0–60vol%) and particle surface treatment. Interfacial shear strengths (τ) were measured via microbond shear strength tests using Spectra900™ (UHMWPE) fibers and BisGMA–TEGMA beads. τ increased by a factor of 4 compared with untreated UHMWPE, and surface treated particles improved the mechanical properties of the composite. Fracture toughness (K IC) and flexural modulus (E) increased with increased volume fraction of UHMWPE, with maximum K IC/E increases (at 60vol%) of 238%/25% compared with the neat resin. SEM images showed debonding as well as yielding and fibrillation of the UHMWPE particles, suggesting that these were significant toughening mechanisms. [Copyright &y& Elsevier]

Abstract: Radiation-induced crosslinking has been shown to have a beneficial effect on the wear resistance of ultra high molecular weight polyethylene (UHMWPE). Since we postulate that crosslinking takes place through reactions involving terminal double bonds, unsaturated additives were added to UHMWPE in this study to enhance crosslinking. UHMWPE specimens soaked in 1,7-octadiene, methylacetylene and ethylene, respectively, were irradiated with electron beam to different doses in single or multiple passages. FTIR spectroscopy was used for the chemical characterisation of the crosslinked polymer. Tensile tests were performed with all samples in order to monitor changes in the mechanical properties. Gel fraction measurements proved that crosslinking took place in all the irradiated samples, but 1,7-octadiene turned out to be the most effective additive for the present purpose, exhibiting a good efficiency in enhancing crosslinking. The FTIR analyses showed that consumption of vinyl double bonds is proportional to the irradiation dose, with an efficiency which increases with increasing their initial concentration, confirming the hypothesis of the involvement of these groups in the crosslinking process. In the case of UHMWPE with 1,7-octadiene irradiated to high doses in multiple passages, the results of the tensile tests indicated a significant decrease in both elongation at break (up to 65%) and ultimate stress (up to 25%). A possible explanation of the diminished strength of the crosslinked material has also been proposed. [Copyright &y& Elsevier]

Abstract: Shear-induced crystallization of isotactic polypropylene (iPP) within the oriented scaffolds of noncrystalline ultrahigh molecular weight polyethylene (UHMWPE) was investigated by means of in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The study was carried out using iPP/UHMWPE blends under isothermal crystallization at 145°C (i.e., above the melting point of polyethylene) and step shear (shear rate=60s−1, duration=5s) conditions. The oriented and isotropic iPP crystalline phases were extracted from the 2D WAXD pattern, and their kinetics data were evaluated with the Avrami equation. The dominant component in the oriented iPP phase was a kebab structure, whose nanostructure dimensions were determined by a novel SAXS analysis scheme. The minor non-crystalline but oriented UHMWPE component played a key role in the nucleation of iPP, which could be explained in terms of mutual diffusion at the interface, resulting in a significant increase in the relaxation time of iPP chains. As a result, after shear, the interfacial iPP chains could also retain their orientation and formed oriented nuclei to initiate the kebab growth. [Copyright &y& Elsevier]

Surface modification of ultra high molecular weight polyethylene (UHMWPE) is induced by ion implantation of different ions at 300 keV energy. The aim of this work was the chemical, physical and mechanical characterization of the modified material in order to know deep inside about the effect of the ion beam upon the polymer.The irradiated surfaces are investigated by Raman spectroscopy, infrared absorption and micro-hardness analysis, scanning electron microscopy. Pin on disc measurements valuated the wear of the UHMWPE against a stainless steel probe; wear resistance increases of about 76% after the ion implantation. This result can be attributed to the ion bombardment inducing a micro-hardness increasing in the irradiated layers due to the high carbon surface concentration and cross-linking effects in the polymeric chains. The irradiated UHMWPE surfaces may find special applications to the field of the mobile prosthesis. [Copyright &y& Elsevier]

High performance in multiple properties is still a challenge to address in the case of polymer composites with engineered polymers. Herein, reduced graphene oxide was dispersed in an ultra high molecular weight polyethylene matrix through a novel and optimized process to produce nanocomposites with high performance at low nanofiller contents. The graphene layers assisted the formation of long-range ordered lamellar structures and microfibers between the crystals. We found that there is a fracture transition from fragile to ductile depending on the impact speed. This transition occurs at 0.25 wt% of rGO content, which is extremely useful for several high-performance applications that require a good combination of impact and wear resistance during operation. Graphene enhances the wear resistance performance of UHMWPE matrix, leading to a reduction of 40% of volume loss in abrasive wear. The results indicate an advance in the classification of the UHMWPE from engineering polymers to high performance polymers. [Display omitted] • The carbon bidimensional nanomaterial contributes to the formation of the lamellar morphology of the polymer matrix. • The graphene reinforces the plastic deformation resistance of the polymer matrix. • Crack propagation is limited by graphene. • Polymer nanocomposite surface display modified wear mechanism from cutting to the plowing. [ABSTRACT FROM AUTHOR]

To address the issue of polymer molecular weight in olefin polymerization, versatile strategies have been utilized in the benchmark α-diimine nickel catalysts. In this contribution, one cooperative strategy that combines the concerted double-layer steric strategy with the rotation-restricted strategy was highlighted in the newly prepared α-diimine nickel catalysts. By using this strategy, these nickel catalysts with the flexible restraint and the rigid restraint exhibited enhanced molecular weight than the freely rotated one in ethylene polymerization. More strikingly, the preferred nickel catalyst had the ability to produce virtually linear polyethylene (0.8 brs/1000C, T m = 130.2 °C) with both ultrahigh molecular weight of 7097 kg mol−1 and ultrahigh catalytic activity. Even at high temperature of 90 °C, an ultrahigh molecular weight of 1103 kg mol−1 was accessible. [Display omitted] • A cooperative strategy of combining concerted double-layer steric strategy and rotation-restricted strategy. • Flexible and rigid restraints of freely rotated phenyl rings enhanced molecular weight. • Preferred Ni catalyst enabled ultrahigh molecular weight polyethylene in ultrahigh activity. • Virtually linear UHMWPE (7097 kg mol−1, 0.8 brs/1000C, T m = 130.2 °C) was possible. [ABSTRACT FROM AUTHOR]

Low efficiency in the fabrication of ultra-high molecular weight polyethylene (UHMWPE) with excellent mechanical properties has limited its wide application, owing to its ultra-high melt viscosity. Present processing methods including compression molding (CM) and high velocity compaction (HVC) can't be used for manufacturing UHMWPE products possessing outstanding mechanical strength, toughness and wear resistance simultaneously. A novel molding process of pulse vibration molding (PVM) first utilizes application of pulsed vibration pressure to UHMWPE powders pre-heated close to melting temperature. This method effectively promotes consolidation at particle interfaces at low molding temperatures, while melt flow in a specific direction during the PVM process accelerates a further fusion at particle interfaces, thus obtaining UHMWPE products with orientation induced by melt flow during the PVM process (i.e. OPVM). The crystallinity and lamellae thickness of PVM-UHMWPE molded at 170 °C for 30 min (PVM-170-30) were significantly greater than those of CM-UHMWPE prepared at 210 °C for 60 min (CM-210-60), consequently yield strength, Young's modulus, and wear resistance of PVM-170-30 were much higher. Inside OPVM-UHMWPE molded at 170 °C for 30 min (OPVM-170-30), orientation of molecular chains along the flow direction generated by melt flow resulted in high crystallinity and formation of numerous shish-kebab structures at particle interfaces. Compared with those of CM-210-60, yield strength, Young's modulus, elongation at break and work-to-failure of OPVM-170-30 increased from 21.5 ± 0.4 MPa, 292 ± 3 MPa, 505 ± 20% and 114 ± 3 kJ/m2 to 23.0 ± 0.2 MPa, 334 ± 8 MPa, 571 ± 17% and 132 ± 6 kJ/m2, and its wear rates and wear index decreased from 3.8 ± 0.3% and 184 ± 15 to 2.8 ± 0.2% and 142 ± 6, respectively. The outstanding performance of OPVM-170-30 was originated from high crystallinity and formation of self-reinforcing shish-kebab superstructures at particle interfaces. The present results suggested OPVM-170-30 has high potential in applications requiring high wear resistance, mechanical strength, and fracture toughness, such as artificial joint. [Display omitted] • Pulsed vibration pressure effectively promoted UHMWPE particles consolidation at low temperature. • Crystallinity and lamellae thickness of PVM-UHMWPE were larger than those of CM-UHMWPE. • Orientation induced by melt flow resulted in high crystallinity and formation of shish-kebab structures. • Orientation improved mechanical strength, toughness and wear resistance. [ABSTRACT FROM AUTHOR]

In this study, we proposed a unique continuous two-step two-roll mill processing method to prepare high-performance self-reinforced polyethylene materials of high-density polyethylene (HDPE)/ultra-high molecular weight polyethylene (UHMWPE). The first step of high temperature melting is to disperse UHMWPE uniformly in HDPE matrix, and the second step of directional extension upon low-temperature is to significantly improve the structural regularity. The special crystal structures containing interlocked shish-kebab formed and the mechanical properties, barrier property, creep resistance of HDPE/UHMWPE were improved dramatically. Moreover, we found that the effective UHMWPE content is 0.02～0.1%, especially when the content was 0.08%, the tensile strength is 98.2 MPa, nearly double than pure HDPE, and the oxygen permeability is only 18.3% of pure HDPE. So, our study suggests an efficient, eco-friendly strategy for the manufacturing of high-performance all-polyethylene materials, which is of significance for extending the application fields of polyethylene. [Display omitted] • Preparing high-performance self-reinforced polyethylene materials. • The strategy is a unique continuous two-step, two-roll mill processing method. • The most effective UHMWPE content is within 0.08～0.1 wt%. • The reinforcements in property are consistent with interlocked shish-kebab. [ABSTRACT FROM AUTHOR]

Multifunctional polymer foam becomes the tendency due to the single-functional foam is difficult to adapt the various complicated environments. However, preparing multifunctional porous ultra-high molecular weight polyethylene (UHMWPE) material with high porosity is still challenge. Thus, it is desirable to develop a simple and effective method that could integrate multiple functions into polymer foam. Herein, a multifunctional UHMWPE foam was fabricated via a facile thermally induced phase separation method. The foam displays a low density of 8.23 mg cm−3, high porosity of 99.2% and robust superhydrophobicity (water contact angle of 152°), thus exhibiting excellent adsorption capacity in oil-water separation under different extreme environments. Moreover, the thermal conductivity of the foam goes down to 11.6 mWm−1K−1, which endows the foam with great thermal insulation performance under various temperature conditions. Thus, this foam can meet the demands of various practical applications. Image 1 • Multifunctional UHMPE foam was fabricated via thermally induced phase separation method. • The foam displays an ultra-light property (8.23 mg cm−3) and robust superhydrophobicity (water contact angle of 152°). • It shows excellent adsorption capacity in oil-water separation under different extreme environments. • The foam with 11.6 mWm−1K−1 thermal conductivity exhibits great thermal insulation performance. [ABSTRACT FROM AUTHOR]

We report a new nanoconfinement phenomenon whereby surface confinement leads to the amorphization of polyethylene oxide (PEO) in porous ultrahigh molecular weight polyethylene membrane (UHMWPE). The surface confinement was achieved through a monolayer attachment of PEO onto the UHMWPE nanofibril surfaces‒pore bounding edges‒in the membrane via amphiphilic interactions of polypropylene oxide (PPO) blocks in the triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO). Full amorphization of PEO is corroborated through the lack of phase transitions under differential scanning calorimetry (DSC), and liquid-like vibrational absorption responses under Fourier transform infrared spectroscopy (FTIR). Monolayer confinement of PEO in the membrane is quantified using a combination of Brunauer-Emmett-Teller (BET), time-of-flight secondary ion mass spectrometry (ToF-SIMS) molecular depth profiling and scanning electron microscopy (SEM). The total amount of PEO confined in the porous membrane is 7.7 wt% agreeing with that expected from the total BET surface area of the UHMWPE membrane; ToF-SIMS depth profiling using argon cluster ion reveals the homogeneous PEO distribution along the membrane depth; and surface topology analysis by SEM shows no obvious topological changes after PEO confinement. Image 1 • Surface-confinement of PEO on nanofibrils of UHMWPE membrane leads to amorphization. • Unique confinement crystallization mechanisms from the monolayer confinement of PEO. • Stable monolayer confinement of PEO on polyethylene nanofibrils via van der Waals. • Amorphization of PEO is highly desirable in solid-state battery application. [ABSTRACT FROM AUTHOR]

Crystallinity (X c) is a crucial crystallographic factor that related closely to mechanical properties in linear polyethylene or high density polyethylene (HDPE), whose molecular weight (MW) is at a 105 level. However, as a typically intractable-processing polymer, the structure-property relationship concerned X c is still ambiguous for ultrahigh molecular weight polyethylene (UHMWPE) with M W﹥1.5 × 106. The reason is that high entanglement density hinders chain mobility in the course of UHMWPE crystallization, thus crystalline degree in UHMWPE molding parts is difficult to be artificially modulated within a wide scope. In this study, low chain-entanglement status of UHMWPE melt was achieved by (1) utilizing lowly entangled nascent UHMWPE synthesized via the single-site Ziegler-Natta catalyst, and (2) decreasing the sintered temperature. It is impressive that partial disentanglement reserved in the melt is favorable for obtaining higher crystallinity. Meanwhile, an isothermal crystallization treatment was additionally adopted to further improve the capacity of regulating crystallization after the traditional two-step sintering procedure. By a combination of varying the initial entanglement in nascent powders, sintering temperature, isothermal crystallization duration, and molecular weight of UHMWPE, the X c of the compression-sintered products has been successfully adjusted in the relatively broad range of 29%≦ X c ≦ 65%, which offered an opportunity to validly survey the correlation between mechanical property and crystallinity. Particular attention was paid to the yield strength. A linear master straight-line can be plotted as yield strength vs. X c for all of the samples inspected, indicating crystallinity also plays a dominant role on mechanical performances of UHMWPE sintering parts, as same in the case of HDPE. However, the mechanical reinforcement induced by increment of X c in UHMWPE is more efficient than in HDPE. The yield strength of UHMWPE is much higher than that of HDPE at the same X c , indicating the amorphous phase or entanglements in UHMWPE is crucial to determine the mechanical property. These findings are of significance for guiding the manufacturing of high-performance UHMWPE materials via crystallization manipulation. Image 1 • UHMWPE melt with high crystalline capability was achieved. • The X c of the sintered UHMWPE products has been successfully adjusted in a broad range. • A good linear relation between yield strength and X c is identified for the UHMWPE sintering parts. • The mechanical reinforcement induced by increment of X c in UHMWPE is more efficient than HDPE. [ABSTRACT FROM AUTHOR]

A novel method of a hybrid porous structure preparation in UHMWPE was developed. A porous structure consisted of large 50–900 μm and medium-size 50 nm-4 μm macropores together with mesopores. The polymodal structure was prepared via water leaching of UHMWPE-NaCl composite followed by o-xylene treatment at a temperature of 99–103 °C with further supercritical drying in CO 2. Scanning electron micrograph showed the formation of additional porous structure on the large macropores' walls surface formed by lamellar crystals. The size and quantity of the lamellar crystals depended on the heating temperature and could be varied over a wide range. The formation of the hybrid porous structure in UHMWPE was accompanied by a significant increase in the crystallinity degree (up to 81%) and specific surface area (up to 65 m2/g). The mechanical behavior of the UHMWPE with hybrid porous structure was investigated by DMA. The change in the UHMWPE crystal structure led to a change in the UHMWPE relaxation behavior and to an increase of damping ability. Image 1 • The new method allows creation of hybrid porous structure in UHMWPE. • Hybrid porous structure of UHMWPE consists of pores ranging from mesopores to 900 μm in size. • Medium-size macropores and mesopores on the large macropores' surface are formed by lamellar crystals. [ABSTRACT FROM AUTHOR]

Aerogels were prepared via supercritical CO 2 (SC CO 2) extraction of a solvent – hexadecane or o-xylene – from the swollen ultra-high molecular weight polyethylene (UHMWPE, GUR 4120). This protocol produced highly porous flaky structures containing a polymodal system of pores. The specific surface area of the porous UHMWPE samples lay between 15 and 104 m2/g, the density was 0.03–0.07 g/cm3, and the porosity was 93–97% depending on the swelling solvent amount. The degree of crystallinity reached 94%. Dynamic mechanical analysis of prepared aerogels was performed. All the samples demonstrated high damping ability. Tan δ value of 0.42–0.48 at 90–110 °C was achieved. UHMWPE aerogels revealed high thermal conductivity up to 2.3 W/(m·К). Image 103 • The aerogels' porosity was 93–97% depending on the swelling solvent amount. • Specific surface area varied from 15 to 104 m2/g, density varied from 0.03 to 0.07. • The degree of crystallinity varied from 59 to 94%. • High damping ability – tan δ value of 0.42–0.48 at 90–110 °C was achieved. • UHMWPE aerogels revealed high thermal conductivity up to 2.3 W/(m·К). [ABSTRACT FROM AUTHOR]

Due to the linear and ultra-long molecular chains, ultra-high molecular weight polyethylene (UHMWPE) has been playing a vital role in many fields. Based on more and more demands for special UHMWPE resins, the relationship between microstructure of nascent powder and its corresponding physical properties was investigated in this work, and one special powder with high degree of short-chain branch was fully contrasted with one general-purpose UHMWPE resin. Based on series characterization results, it revealed that short-branched chains led to a decrease on the entanglement density and brought a significant increase in the processing performance and mechanical properties. In addition, the short-branched chains also made a huge difference on crystal morphology of nascent UHMWPE powders. By correlating high-resolution scanning electron microscopy, DSC curves and Raman spectroscopy characteristics, the slurry polymerization mechanism of UHMWPE was deduced, and the corresponding performance of nascent UHMWPE powders was fully explained. Image 10674 • Short-branched chains brought UHMWPE resin a distinct crystal morphology that consisted of fewer fibrils than linear UHMWPE. • Short-branched chains reduced the entanglement density of UHMWPE nascent powder. • Short-branched chains improved the process-ability and mechanical properties of UHMWPE resin. [ABSTRACT FROM AUTHOR]

Due to its extraordinary molecular weight feature (M w > 106 g/mol), ultra high molecular weight polyethylene (UHMWPE) plays as an appropriate reinforcement for polyethylene. However, ultrahigh molecular weight also causes remarkably high melt viscosity, which hinders the solvent-free processing of UHMWPE. Here, we report a self-reinforcement all-polyethylene material, high density polyethylene (HDPE)/UHMWPE via a melt processing of high-speed shear extrusion. A critical reinforced content of UHMWPE, 0.1 wt%, has been discovered. After reaching such content, the tensile strength and Young's modulus of HDPE can be reinforced remarkably at 500 rpm. Thanks to the high shear, UHMWPE is uniformly dispersed and to form reinforcement network in the HDPE matrix. Interestingly, UHMWPE long chains enable the appearance of lamellae orientation and shish-kebab crystalline morphology in the injection molded parts, which are responsible for the mechanical reinforcement behavior observed. Our current work offers a convenient, solvent-free processing strategy to manufacture super polyolefin blends containing UHMWPE. Image 1 • All-polyethylene material is mechanically reinforced by dispersing UHMWPE via solvent-free high-speed shear extrusion. • UHMWPE is uniformly dispersed in HDPE via increasing the extruder rotation speed to a certain high level, 500 rpm. • A critical content of UHMWPE, 0.1wt%, has been discovered to realize effective mechanical reinforcement. • Lamellae orientation and shish-kebab crystalline morphology are responsible for the mechanical reinforcement behavior observed. [ABSTRACT FROM AUTHOR]