Microstructure Of Polyethylene Research Articles

Flexible Gd2O2S:Tb scintillators pixelated with polyethylene microstructures for digital x-ray image sensors

Flexible scintillators for digital x-ray image sensors were designed, fabricated and characterized. In these scintillaotrs, terbium-doped gadolinium oxysulfide (Gd2O2S:Tb) scintillator pixels were embedded into a polyethylene (PE) substrate. To evaluate the difference in the spatial resolution according to the pixel size, we designed three scintillators with pixels of different pitch sizes: 50 µm pitch size (P50), 100 µm pitch size (P100) and 200 µm pitch size (P200). Because of the high flexibility and good formability, polyethylene was used as the substrate of the scintillator. To fabricate nickel micromolds with high-aspect-ratio microstructures, two microfabrication techniques were employed: silicon dry-etching using a deep reactive ion etching (DRIE) process and nickel electroforming. The pixelated PE microstructures were fabricated by a hot embossing process. Because the solution-type Gd2O2S:Tb precursor can be handled at room temperature, Gd2O2S:Tb was used as the scintillator material. The measured sensitivities of the P50 and P100 models were, respectively, about 65% and 97% of that of the P200 model. The lower sensitivity values of the models with a small pitch size were due to two factors, such as the different pixel heights and the different fill factors. Because a scintillator with a small pixel size has a low fill factor, the sensitivity of the scintillator decreases as the pixel size decreases. The fill factors of the P50, P100 and P200 models were 36%, 49% and 56.25%, respectively. On the other hand, the spatial resolution of the scintillator increases as the pixel size decreases. Therefore, P50 gave the best spatial resolution among the designed models. The spatial frequency at 10% of the modulation transfer function (MTF) with P50 was 13.5 mm−1, while that with P200 was 10.0 mm−1. The resolution pattern and the tooth x-ray images obtained from a scintillator with a smaller pixel size was also clearer than that obtained from a scintillator with a larger pixel size. PE-based flexible Gd2O2S:Tb scintillators can be utilized directly in flexible x-ray image sensors.

Evaluation of thermal and mechanical properties of recycled polyethylene modified bitumen

This paper deals with the influence that polymer concentration exerts on the rheological and thermal properties and microstructure of recycled polyethylene modified bitumen. With this aim, a recycled polyethylene was used as modifying agent to improve the mechanical characteristics of a 150/200 penetration grade bitumen. The evolution of microstructure and thermal and rheological behaviour of the blends has been followed by optical microscopy, MDSC measurements, steady and oscillatory shear tests and dynamic mechanical thermal analysis (DMTA), respectively. Recycled polymer concentrations up to 15 wt. % lead to enhanced rheological properties of the modified bitumen, useful for paving applications. Larger recycled polymer concentrations yield modified binders suitable for roofing membranes.

New Nickel(II) Diimine Complexes and the Control of Polyethylene Microstructure by Catalyst Design

Starting from differently substituted boronic acids as versatile building block, new "ortho-aryl" alpha-diimine ligands a-h were synthesized in an easy, high-yielding route. Reaction of the complex precursor diacetylacetonato-nickel(II) with a trityl salt, like [CPh3] [B(C6F5)4] or [CPh3] [SbCl6], in the presence of the diimine ligands afford the monocationic, square planar complexes 2a-g in almost quantitative yields. Suitable crystals (2d',e,f,g) were submitted for X-ray diffraction analysis. A geometry model was developed to describe the orientation of ligand fragments around the nickel(II) center that influence the polymer microstructure. At elevated reaction temperature and pressure, and in the presence of hydrogen, 2a-e catalyze the homopolymerization of ethylene to give branched PE products ranging from HD- to LLD-PE grades. The polymerization results indicate the possibility of precise microstructure control depending on the particular complex substitution. Preliminary investigations on material density and mechanical behavior by uniaxial stretching until failure point toward new material properties that can result from the simple ethylene monomer by catalyst design.

Substituent Effects in (κ2-N,O)-Salicylaldiminato Nickel(II)−Methyl Pyridine Polymerization Catalysts: Terphenyls Controlling Polyethylene Microstructures

A series of (κ2-N,O)-salicylaldiminato Ni(II)−methyl pyridine complexes 7-pyr and 8-pyr derived from 3,5-diiodosalicyaldehyde (3a) and 3-(9-anthryl)salicylaldehyde (3b), and terphenylamines 2,6-(3,5-R-4-R‘-C6H2)2C6H3−NH2 (4a, R = CF3, R‘ = H; 4b, R = tBu, R‘ = H; 4c, R = tBu, R‘ = OH; 4d, R = Me, R‘ = H; 4e, R = Me, R‘ = MeO; 4f, R = MeO, R‘ = H; 4g, R = MeO, R‘ = MeO), was prepared by reaction of the respective salicylaldimine (5a−f, 6a−g) with [(tmeda)Ni(CH3)2] (tmeda = N,N,N‘,N‘-tetramethylethylenediamine) or [(pyridine)2Ni(CH3)2]. Complexes 7-pyr and 8-pyr are highly active single component catalysts for the polymerization of ethylene, producing a wide range of different polyethylene microstructures. While comparable complexes derived from 3a, 3b, 5-nitrosalicylaldehyde, 3-tert-butylsalicylaldehyde, 3,5-[3,5-(CF3)2C6H3]2-salicylaldehyde, and 2,6-[3,5-(CF3)2C6H3]2C6H3−NH2 afford polyethylenes with similar degrees of branching, variation of the terphenyl moieties in complexes 7-pyr and 8-pyr allows acce...

Effect of Coextrusion On Pp/Ldpe Bicomponent Blown Films

The effect of coextrusion on the microstructure of polypropylene (PP) and low-density polyethylene (LDPE) bilayer films was investigated by the use of Raman spectroscopy. The onset crystallization-time difference for PP versus LDPE components is an important parameter that controls the orientation and morphology of the coextruded films. Although the overall molecular orientation of PP and LDPE was not significantly different in multilayer films, single-layer LDPE films displayed some row-nucleation of crystals that was not observed in the LDPE layer in coextruded films. Also, there was a slight decrease of crystalline a-axis orientation for coextruded LDPE layer relative to that obtained for single-layer LDPE films.

Novel nickel(II)-based catalysts for the polymerization of ethylene

A series of Ni(II)-based bidentate α-diimine complexes bearing two alkyl (alkyl = methyl, ethyl and isopropyl) substituents on each imine aryl group were studied as precatalysts for the polymerization of ethylene. These new catalysts were observed to show high activity in combination with methyl aluminoxane (MAO) and produced high molecular weight polyethylenes. The effects of the steric bulk of ortho-aryl substituents of the ligand on the catalytic activity and the resulting polyethylene microstructure were investigated. Kinetics of polymerization was also studied by changing important parameters such as temperature and MAO concentration. The polymerization activity, polymer molecular weight and resulting polymer microstructure were drastically changed according to the catalyst structure modification and polymerization parameters.

Wear Mechanisms of Untreated and Gamma Irradiated Ultra-High Molecular Weight Polyethylene for Total Joint Replacements

Modification of the surface microstructure of ultra-high molecular weight polyethylene (UHMWPE) is essential for improving the wear resistance of orthopedic implants. A common approach is to cross-link the polymer by gamma irradiation. The objective of this study was to examine the tribological behaviors of untreated and gamma irradiated UHMWPE under physiologically relevant contact conditions. Emphasis was placed on the identification of the dominant wear mechanisms in the early stage of polymer wear. The irradiation dose exhibited a strong effect on the tribological properties of UHMWPE sliding against Co–Cr alloy in a bath of bovine serum. Transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM) were used to examine the microstructure and morphology of the worn surfaces. Regularly spaced folds with average spacing depending on the irradiation dose (i.e., cross-link density) formed on the wear tracks. Surface folding was related to plastic flow and the degree of mobility of the crystalline lamellae. SEM and TEM results elucidated the roles of the cross-link density and crystalline lamellae in the wear process. Based on the experimental evidence, a deformation model was obtained that provides explanation for the dependence of surface folding on the cross-link density and lamellae reorientation during sliding.

Space Charge Accumulation and Micro-Structure of Cross-linked Polyethylene

In this paper, laser induced pressure pulse (LIPP) method and electrostatic force microscopy (EFM) method are utilized to investigate the distribution of space charge in cross-linked polyethylene (XLPE), and the action on the groups of XLPE by the accumulative charge are investigated by infrared spectroscopy (IR) method. It was found that space charge in the sample has obvious influence on the vibration of chemical group, especially on group OH, group CH3 and group CH2. Group vibration affected considerably by space charge in XLPE sample locates on the interface between crystalline and amorphous domains. The experimental results also indicate that positive charge compared to negative charge has a different effect on bands.

Constitutive modelling of viscoelastic unloading of short glass fibre-reinforced polyethylene

The simulation, by traditional uni-dimensional rheological models, of viscoelastic unloading to zero stress after tensile testing of short glass fiber-reinforced polyethylene and its pure matrix is poor. The models significantly underestimate recovery rates, even with small amounts of strain. The use of a finite number of relaxation times does not sufficiently increase recovery rates during unloading when models are generated from the responses of materials under load. Similar results and observations are obtained using rate jumps in loading and unloading. 3D models developed using local state methods require that an additional recovery potential should be used. This observation seems to prove that it is necessary to take into account the evolution of the polyethylene microstructure while the stress is applying, thus justifying the existence of the additional potential. The similar situation that exists for pure polyethylene means that the phenomenon must not be confused with material damage.

Effect of cocatalyst on the chain microstructure of polyethylene made with CGC‐Ti/MAO/B(C6F5)3

AbstractThis investigation studied the solution polymerization of ethylene in Isopar E in a semibatch reactor using CGC‐Ti as catalyst and methylalumoxane (MAO) and tris(pentaflourophenyl)borane [B(C6F5)3] as cocatalysts. The effects of cocatalyst type and amount on the chain microstructure were investigated. 13C NMR and gel permeation chromatography were used to determine the long‐chain branching (LCB) content and molecular weight distribution (MWD), respectively, of the samples. It was observed that higher concentrations of MAO increased the LCB content and decreased the molecular weight of the polymer. On the other hand, increasing the amount of B(C6F5)3 lowered the LCB content, increased the molecular weight, and broadened MWD significantly. We believe that this approach can be used as an efficient way to control the microstructure of polyolefins made with these catalytic systems. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3055–3061, 2004

Zirconocene catalyst well spaced inside modified montmorillonite for ethylene polymerization: role of pretreatment and modification of montmorillonite in tailoring polymer properties

Zirconocene catalyst was heterogenized inside an organosilane-modified montmorillonite (MMT) pretreated by calcination and acidization, for supported catalyst systems with well-spaced α-olefin polymerization active centers. The varied pretreatment and modification conditions of montmorillonite are efficient for supported zirconocene catalysts in control of polyethylene microstructures, in particular, molecular weight distribution. In contrast to other supported catalyst systems, Cp 2ZrCl 2/modified montmorillonite(MMT-7)-supported catalysts with a distinct interlayer structure catalyzed ethylene homopolymerization and copolymerization with 1-octene activated by methylaluminoxane (MAO), resulting in polymers with a bimodal molecular weight distribution (MWD).

Effects of Bond Roll Pattern and Temperature on the Microstructure and Properties of Polyethylene Nonwovens

This paper investigates the effects of bond roll pattern and bonding temperature on the microstructure, tensile properties, flexural rigidity, and abrasion of polyethylene (PE) nonwovens. Bond roll variables include percent bond area, bond point concentration, and side-wall angle. Temperature greatly affects nonwoven mechanical properties. Of the various bond roll variables, bond point concentration significantly affects the elongation and flexural rigidity of PE nonwovens, whereas the side-wall angle affects tenacity. Microstructure analysis indicates that PE nonwovens fail by disintegration of the bond area rather than by fibers breaking at the bond perimeter. A one-dimensional, unsteady-state heat conduction model of the area under a bond point confirms that partial melting occurs in the center of thermally bonded areas.

Catalyst/cocatalyst nuclearity effects in single-site olefin polymerization. Significantly enhanced 1-octene and isobutene comonomer enchainment in ethylene polymerizations mediated by binuclear catalysts and cocatalysts.

This Communication describes the implementation of a new binuclear homometallic organotitanium "constrained geometry catalyst" (CGC), (mu-CH2CH2-3,3'){ (eta5-indenyl )[1-Me2Si (tBuN)](TiMe2)}2[EBICGC(TiMe2)2; Ti2], together with the bifunctional activators (Ph3C+)2[1,4-(C6F5)3BC6F4B(C6F5)3]2- (B2) and new bisborane 1,4-(C6F5)2BC6F4B(C6F5)2 (BN2) in ethylene + alpha-olefin copolymerization processes. Specifically examined are the comonomers 1-octene and poorly responsive isobutene. Large increases in comonomer enchainment efficiency into the polyethylene microstructure are observed versus the corresponding mononuclear catalyst [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe2 (Ti1) + Ph3C+B(C6F5)4- (B1) or B(C6F5)3 (BN) under identical polymerization conditions. In ethylene + 1-octene copolymerization, 11 times more 1-octene incorporation is observed for Ti2 + B2 vs Ti1 + B1. In ethylene + isobutene copolymerization, 5 times more isobutene incorporation is observed for Ti2 + BN2 vs Ti1 + BN.

Interfacial microstructure and withstand voltage of polyethylene for power cables

In order to increase the electrical breakdown strength and treeing inception voltage of cross-linked polyethylene (XLPE) insulated power cable, surfactant in combination with titanate coupling agent, as an additive, has been mixed with the semiconducting layer, which is surrounded by polyethylene insulation, to chemically improve the dielectric properties of polyethylene interface. By means of infrared and X-ray diffraction spectra analyses, a change in the crystalline orientation angle and the density of polyethylene is observed inside the improved polyethylene interfacial layer. Additionally, it is assumed that a thin diffusion layer is formed, that results in a decrease in the electric field stress on the polyethylene interface and then an increase in the withstand voltage of polyethylene.

Effect of dynamic vulcanization on the microstructure and performance of polyethylene terephthalate/elastomer blends

AbstractBlends of polyethylene terephthalate (PET) and ethylene‐ethyl acrylate‐maleic anhydride terpolymer (E‐EA‐MAH) were dynamically crosslinked in a one‐step extrusion process. An amine‐terminated glycol reacting with MAH moieties was used as the crosslinking agent. The effect of blend composition and dynamic crosslinking on the microstructure and mechanical properties were investigated. Blend ratios ranging from 80:20 to 20:80 PET/E‐EA‐MAH were studied. The region of phase inversion was located for uncrosslinked and dynamically crosslinked blends. The rheological characterization was also carried out for these blends in comparison with the neat materials. After dynamic crosslinking, the phase inversion is shifted from the 30–40% range to the 70–80% range of elastomer content. This shift is induced by the increase of viscosity and elasticity of the network formed. Dynamically crosslinked blends show significant improvements in impact strength but also exhibit a decrease in elongation at break.

Characteristic Morphology and Its Properties of Melt-Crystallized Polyethylene Banded Spherulites on Scanning Electron Microscope

In spite of commercial importance of polyethylene(PE), morphologies of melt crystallized PE have been rarely studied. Although, it was possible to observe polyethylene microstructure, they were based on the observation performed almost exclusively on thin films which were obtained either by casting samples between glass plates, or by microtoming the materials with transmission electron microscope(TEM). Despite the usefulness of TEM for detailed investigation of the polyethylene, it is not suitable for the structure characterization of the bulk samples, for examples, the characterization of a thick testpiece and commercial products of polyethylene itself. For such bulk specimen or thick sample, scanning electron microscope (SEM) is more useful, if the appropriate and reliable sample preparation method is applied.In this study, we introduce a chemical etching technique for the investigation of melt crystallized polyethylene microstructure by SEM. Especially in this work we studied the characteristic properties and shapes of polyethylene banded spherulite which come from the melt s.

Effect of polymerization temperature and pressure on the microstructure of Ni-diimine-catalyzed polyethylene: parameter identification for Monte-Carlo simulation

The degree of short chain branch (SCB) formation during ethylene polymerization catalyzed with nickel-diimine/trimethylaluminum systems is a function of the relative rates of polymerization and catalyst chain walking. The proper choice of polymerization temperature (30–50°C) and ethylene pressure (1.0–18.4 kgf cm −2 ) allows one to tailor make the SCB content of the polyethylene produced with these catalysts. Within such range, polyethylene can be prepared with 10–100 branches per 1000 carbon atoms in the backbone. The unique branch distribution, which mimics the one obtained in the copolymerization of ethylene and α-olefins, is described in detail using Monte-Carlo simulation of polymerization and chain walking. The four probability parameters used in the model were correlated to polymerization temperature, T, and ethylene concentration, [ E]. With this model, it is possible to predict polyethylene microstructure as a function of T and [ E]. The dependence of each parameter on these two polymerization conditions is helpful to clarify the mechanism of branch formation with these intriguing catalyst systems.

Peroxide‐initiated comonomer grafting of styrene and maleic anhydride onto polyethylene: Effect of polyethylene microstructure

Peroxide‐initiated comonomer grafting of styrene and maleic anhydride onto polyethylene: Effect of polyethylene microstructure

DSC Evidence for Microstructure and Phase Transitions in Polyethylene Melts at High Temperatures

Thermal/microstructural transitions were found at 208 and 227 °C in commercial polyethylenes of types HDPE, LDPE, and LLDPE. DSC data were obtained for a range of heating and cooling rates, over a broad high-temperature regime (30− 280 °C) that spans the melt processing regime. Six samples, from four resin suppliers, were tested; all were of moderate molecular weight (M̄w = (36−85) × 103). Results are discussed in terms of molecular order and transitions between different degrees of ordering. No transitions were detectable in highly branched metallocene samples. The DSC results are in agreement with our previous rheology findings of order and high-temperature transitions in polyethylene melts.

Peroxide-initiated comonomer grafting of styrene and maleic anhydride onto polyethylene: Effect of polyethylene microstructure

Maleic anhydride has been grafted onto various polyethylenes (PEs) using 2,5-dimethyl-2,5-(di-t-butylperoxy)hexane as a free radical initiator in the presence of styrene as a comonomer. Three polyethylenes, differing systematically in their levels of terminal unsaturation and branching, were selected to investigate the effect of these microstructural characteristics on the course of both grafting and crosslinking. It was observed that when polyethylenes containing high levels of terminal unsaturation were reacted in the presence of peroxide or peroxide–maleic anhydride, crosslinking events were enhanced. When styrene was added as comonomer to the reaction medium to eliminate these undesirable side reactions, crosslinking was still observed with those polyethylenes that contained a high concentration of terminal unsaturation. This is attributed to a low reactivity between styrene and the allylic radical generated on the polyethylene backbone, which is believed to be responsible for the increased crosslinking. However, in the presence of high concentrations of styrene, crosslinking was eliminated for PEs containing high degrees of branching. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 96–107, 2001