Flame treatment of low-density polyethylene: Surface chemistry across the length scales

The relationship between surface chemistry and morphology of flame treated low-density polyethylene (LDPE) was studied by various characterization techniques across different length scales. The chemical composition of the surface was determined on the micrometer scale by X-ray photoelectron spectroscopy (XPS) as well as with time of flight secondary ion mass spectrometry (ToF-SIMS), while surface wettability was obtained through contact angle (CA) measurements on the millimeter scale. The surface concentration of hydroxyl, carbonyl and carboxyl groups, as a function of the “number” of the flame treatment passes (which is proportional to the treatment time) was obtained. Moreover, a correlation was found with chemical composition and polarity, emphasizing the role of oxygen-containing functional groups introduced during the treatment. Carboxyl functional groups were specifically identified by fluorescent labeling and the results were compared with the ToF-SIMS data. In addition, atomic force microscopy (AFM) was used to evaluate changes in surface topography and roughness on the nanometer to micrometer length scales. After flame treatment, water-soluble low molecular weight oxidized materials (LMWOM), which were generated as products of oxidation and chain scission of the LDPE surface, agglomerated into small topographical mounds that were visible in the AFM micrographs. After rinsing the flame treated samples with water and ethanol, bead-like nodular surface structures were observed. The ionization state of flame treated LDPE surfaces was monitored by chemical force microscopy (CFM). The effective surface pK_a values of carboxylic acid (-COOH) obtained by AFM were revealed by chemical force titration curves and the effective surface pK_a values were found to be around 6.     

Transient dynamics of packet-like space charge in low-density polyethylene at high temperatures

Among features observed in polyethylene-based materials under high fields, packet-like space charge are frequently reported but without complete theory explaining their formation and propagation. The present contribution reports transient dynamics of packet-like space charge in low-density polyethylene (LDPE) under multi-field coupling conditions based on pulsed electro-acoustic (PEA) method. The measurements were carried out under −125 kV/mm at 20 °C, 40 °C, and 60 °C. Space charge results reveal systematic occurrence of positive packet-like space charge in all samples generated at the anode and then moved toward the cathode. It is observed that higher temperatures contribute to the migration of packet-like space charge and field distortion. Based on the detailed injection model of packet-like space charge, it is shown that electrode injection plays a major role in the formation of positive packet-like space charge, the total amount and depth not only depend on the temperature, but also the negative charges dynamics. Packet-like space charge phenomena at different temperatures still feature NDM relationship with the nonlinear increase of migrate velocity as the rise of temperature.     

Effect of epitaxial crystallization on packet-like space charge characteristics in low-density polyethylene under multi-field coupling conditions

In order to study the effect of epitaxial crystallization on charge transport in low-density polyethylene (LDPE) under multi-field coupling conditions, three typical epitaxial crystallizations, namely disorder (glass substrate), crossover (isotactic polypropylene substrate), and parallel (polytetrafluoroethylene substrate), were prepared and denoted as LD-G, LD-iPP, and LD-PT, respectively. Packet-like space charge through samples was analyzed by the pulsed electro-acoustic (PEA) method. It is shown that different microscopic surface morphologies appeared in the LDPE samples with different epitaxial crystallizations, which, however, do not change the crystalline structure of the bulk. Packet-like space charge phenomena were observed and the distortion field increased with the temperature which could be attributed to the larger amount of charge injection in a shorter period. The differences of the amount and injection rate of the space charge were explained and verified considering the typical chain alignment of epitaxial crystallization, which, in our opinions, contributes to the pass over of positive charge in LD-iPP samples.     

Interfacial activity of styrene-butadiene block copolymers in low-density polyethylene/polystyrene blends

The effect of molecular structure of styrene-butadiene (SB) block copolymers on their interfacial activity in low-density polyethylene/polystyrene (LDPE/PS) (4/1) blends was studied. It was found that addition of some SB copolymers, which are localized in brittle PS particles, leads to a decrease in the blend impact strength in spite of the fact that these SB improve the toughness of both the blend components. Comparison with our previous results showed that the distribution of SB copolymers between the interface and bulk phases and their supermolecular structure in LDPE/PS (4/1) blends strongly differs from those in LDPE/PS (1/4) blends.     

Crystallization and morphology in blends of isotactic polypropylene and linear polyethylene

Thermooptical, wide-angle x-ray diffraction and morphological investigations of blends of isotactic polypropylene with linear polyethylene revealed an increased crystallization rate of the polyethylene blend component, compared to crystallization of polyethylene alone. Crystallization behavior of the polyethylene component was markedly dependent on the blend thermal history and on the circumstances of the polyethylene phase—whether it was disperse or continuous. The higher crystallization rate of the polyethylene component was related to the presence of various types of heterogeneous crystallization nuclei in the blend and to the stabilizing action of the solid polypropylene matrix on minute polyethylene crystallites which survived above melting temperature.     

Omnidirectional photonic band gaps in one-dimensional gradient refractive index photonic crystals considering linear and quadratic profiles

By using the transfer matrix formalism, in this work it is presented the study of the optical properties of 1D photonic structures constructed with M periods of bilayers of dielectric material and slabs with gradient refractive index (GRIN) profile of two types: linear and quadratic. By varying the profile parameters, preserving the average value of the refractive index for the GRIN slab, the results show the formation of new photonic band gaps whose bandwidths depends on the slope and the curvature of the linear and quadratic profile respectively. Also, it can be observed the formation of omnidirectional photonic bandgaps for the TE and TM polarizations, one for the linear profile and three for the quadratic one, for which their bandwidths depend linearly on the slope and the curvature of the GRIN profiles. It is expected that the presented results could be useful in the construction of optical devices based in their optical response under oblique incidence.     

线性低密度聚乙烯中空间电荷陷阱的能量分布与空间分布的关系

使用激光感应压力波法和热刺激放电技术,系统地研究了直流高压作用下线性低密度聚乙烯(LLDPE)半导性电极试样中空间电荷的形成和演变及电荷陷阱分布和退极化过程.在直流高压作用下试样中空间电荷的分布明显地表现为两电极同极性电荷快速对称注入的特征,半导性电极与LLDPE的界面近乎呈现欧姆接触特征.LLDPE中的电荷陷阱分布表现出体内为浅陷阱、表层为深陷阱的特征.半导性电极与LLDPE薄片间的压合条件或电极材料对LLDPE表层的掺杂显著地影响表层陷阱的能量分布,导致表层中较深陷阱的深度和密度减小、较浅陷阱的密度增大.在整个短路退极化过程中,试样中正、负电荷的中心分别向距它们较近的电极迁移,而在开路退极后期则表现为与短路时不同的行为、被表层深陷阱再俘获的电荷脱阱后向背电极迁移. 关键词： 线性低密度聚乙烯 空间电荷 陷阱分布 热刺激放电     

Molecular origin of shear thickening in transient polymer networks: A molecular dynamics study

This paper proposes a simple model of transient networks of telechelic associating polymers for molecular simulations and reports the main results obtained by molecular dynamics on the rheological properties of the transient networks. The steady shear viscosity obtained by the non-equilibrium molecular dynamics simulation exhibits shear thickening at moderate shear rates and shear thinning at larger shear rates. The behavior is similar to that observed in experiments of telechelic associating polymers. By analyzing the distribution function of the end-to-end vector of bridge chains as a function of the shear rate, we find that shear thickening is mainly caused by the stress from the bridge chains highly stretched by shear flow. We also find that fracture of the transient network occurs in the shear-thinning regime at high shear rates.     

Solid-state 13C NMR and synchrotron SAXS/WAXS studies of uniaxially-oriented polyethylene

We report solid-state ¹³C NMR and synchrotron wide-and small-angle X-ray scattering experiments (WAXS, SAXS) on metallocene linear low density polyethylene films (e.g., Exceed™ 1018 mLLDPE; nominally 1 MI, 0.918 density ethylene-hexene metallocene copolymer) as a function of uniaxial draw ratio, λ. Combined, these experiments provide an unambiguous, quantitative molecular view of the orientation of both the crystalline and amorphous phases in the samples as a function of draw. Together with previously reported differential scanning calorimetry (DSC), gas transport measurements, transmission electron microscopy (TEM), optical birefringence, small angle X-ray scattering (SAXS) as well as other characterization techniques, this study of the state of orientation in both phases provides insight concerning the development of unusually high barrier properties of the most oriented samples (λ=10). In this work, static (non-spinning) solid-state NMR measurements indicate that in the drawn Exceed^TM films both the crystalline and amorphous regions are highly oriented. In particular, chemical shift data show the amorphous phase is comprised increasingly of so-called “taut tie chains” (or tie chains under any state of tautness) in the mLLDPE with increasing draw ratio – the resonance lines associated with the amorphous phase shift to where the crystalline peaks are observed. In the sample with highest total draw (λ=10), virtually all of the chains in the non-crystalline region have responded and aligned in the machine (draw) direction. Both monoclinic and orthorhombic crystalline peaks are observed in high-resolution, solid-state magic-angle spinning (MAS) NMR measurements of the oriented PE films. The orientation is comparable to that obtained for ultra-high molecular weight HDPE fibers described as “ultra-oriented” in the literature. Furthermore, the presence of a monoclinic peak in cold-drawn samples suggests that there is an appreciable internal stress associated with the LLDPE. The results are confirmed and independently quantified by Herman's Orientation Function values derived from the WAXS measurements. The degree of orientation approaches theoretically perfect alignment of chains along the draw direction. We deduce from this observation that a high fraction of the non-crystalline chains are either tie chains that directly connect adjacent lamellae or are interlocking loops from adjacent lamellae. In either case, the chains are load-bearing and are consistent with the idea of “taut tie chains”. We note that transmission electron micrographs recorded for the ultra-oriented Exceed showed the lamellae are often appreciably thinner and shorter than they are for cast or blown Exceed 1018. Combined with higher crystallinity, the thinner lamellae statistically favor more tie chains. Finally, the remarkably large decrease in permeability of the λ=10 film is primarily attributed to the high degree of orientation (and loss of entropy) of the amorphous phase.     

Branched crystal morphology of linear polyethylene crystallized in a two-dimensional diffusion-controlled growth field

The branched crystal morphology of linear polyethylene formed at various temperatures from thin films has been studied by atomic-force microscopy (AFM), transmission electron microscopy (TEM), electron diffraction (ED) pattern and polymer decoration technique. Two types of branched patterns, i.e. dendrite and seaweed patterns, have been visualized. The fractal dimension d _f = 1.65 of both dendrite and some of seaweed patterns was obtained by using the box-counting method, although most of the seaweed patterns are compact. Selected-area ED patterns indicate that the fold stems tilt about 34.5^° around the b-axis and polymer decoration patterns show that the chain folding direction and regularity in two (200) regions are quite different from each other. Because of chain tilting, branched crystals show three striking features: 1) the lamella-like branches show two (200) regions with different thickness; 2) the crystals usually bend towards the thin region; 3) the thick region grows faster by developing branches, thus branches usually occur outside the thick region. The branched patterns show a characteristic width w, which gives a linear relationship with the crystallization temperature on a semilogarithmic plot. Received 15 March 2002 and Received in final form 29 April 2002     

Evolution of TiO2 coating layers on lamellar sericite in the presence of La and the pigmentary properties

TiO₂-coated sericite powders were prepared by the chemical deposition method starting from lamellar sericite and TiCl₄ in the presence or absence of La³⁺ cations. After calcination at 900 °C for 1 h, the resultant TiO₂ nanoparticles on the sericite surfaces existed in anatase phase. The light scattering indexes of the TiO₂-coated lamellar sericite powders were dozens of times higher than that of the naked lamellar sericite powders. The presence of La³⁺ in the deposition solution was beneficial to the formation of the small-sized anatase TiO₂ nanoparticles, resulting in the formation of the dense and uniform island-like TiO₂ coating layers in a large range of the weight ratios of TiO₂ to sericite from 5% to 20%. The TiO₂-coated lamellar sericite powders prepared in the presence of La³⁺ had higher light scattering index than that prepared in the absence of La³⁺. XPS analysis shows that when La³⁺ cations were absent in the reaction solution, TiO₂ coating layers anchored at the sericite surface via the Ti-O-Si and Ti-O-Al bonds. The presence of La³⁺ cations caused the formation of Si-O-La and Al-O-La bonds at the sericite surface and Ti-O-La bond at the surface of TiO₂ coating layers. After coating TiO₂ on the sericite surface, the yellowness of the TiO₂-coated sericite powders obviously increased and the brightness slightly decreased.     

Effect of position-dependent effective mass on linear and nonlinear optical properties of a cubic quantum dot

In this paper, we first obtain an analytic relation for studying the position-dependent effective mass in a GaAs/Al_xGa_1−xAs cubic quantum dot. Then, the effect of position-dependent effective mass on the intersubband optical absorption coefficient and the refractive index change in the quantum dot are studied. Our numerical calculations are performed using both a constant effective mass and the position-dependent effective mass. We calculate the linear, nonlinear and total intersubband absorption coefficient and refractive index change as a function of the incident optical intensity and structural parameters such as dot length. The results obtained from the present work show that spatially varying electron effective mass plays an important role in the intersubband optical absorption coefficient and refractive index change in a cubic quantum dot.     

Measurement and analysis of fission rates in a spherical mockup of uranium and polyethylene

Measurements of the reaction rate distribution were carried out using two kinds of Plate Micro Fission Chamber （PMFC）. The first is a depleted uranium chamber and the second an enriched uranium chamber. The material in the depleted uranium chamber is strictly the same as the material in the uranium assembly. With the equation solution to conduct the isotope contribution correction, the fission rate of 238U and 235U were obtained from the fission rate of depleted uranium and enriched uranium. Then, the fission count of 238U and 235U in an individual uranium shell was obtained. In this work, MCNP5 and continuous energy cross sections ENDF/BV.0 were used for the analysis of fission rate distribution and fission count. The calculated results were compared with the experimental ones. The calculation of fission rate of DU and EU were found to agree with the measured ones within 10% except at the positions in polyethylene region and the two positions near the outer surface. Because the fission chamber was not considered in the calculation of the fission counts of 238U and 235U, the calculated results did not agree well with the experimental ones.     

Structural, luminescent and thermal properties of blue SrAl2O4:Eu, Dy phosphor filled low-density polyethylene composites

The performance of nanophase luminophors is usually compromised by environmentally induced degradation. In this study, composites of low density polyethylene (LDPE) with various concentrations of the blue-emitting europium and dysprosium co-doped strontium aluminate (SrAl₂O₄:Eu²⁺,Dy³⁺) phosphor were investigated. The blue long-lasting phosphorescence of the composites was observed in the dark after removal of the excitation light. X-ray diffraction analysis revealed the presence of the SrAl₂O₄ phase in the composites. PL spectra of the composites have two sets of peaks, major broad bands peaking at about 4855 Å and minor ones at wavelengths between 4115 and 4175 Å, attributed to the 4f-5d transition of Eu²⁺. DSC and TGA results show that the introduction of the phosphor in LDPE matrix caused a slight reduction in the crystallinity of LDPE but a significant increase in the stability of the composites.     

Phase transition properties of polyethylene glycol-cellulose blends and their miscibility in mixed solvents

The phase transition properties of blends of polyethylene glycol (PEG) with cellulose (CELL) prepared from solution in N,N-dimethylacetamide/lithium chloride (DMAC/LiCI) and those from solution in dimethylsulfoxide/paraform- aldehyde (DMSO/PF) were found to be completely different. The differences of the phase transition properties were probably related to the different miscibilities of these two polymers in the two solvent systems. In DMAC/LiCl, the miscibility of CELL and PEG was limited; the composite obtained exhibited a solid- liquid phase transition and had a small phase transition enthalpy. However, in DMSO/PF, these two polymers had a high level of miscibility; the composite obtained exhibited a solid-solid phase transition and had a large phase enthalpy. It is suggested that the differences of miscibility and the phase transition properties were caused by the different dissolving mechanisms of CELL and the different interactions in these two solutions.     

Infrared studies of drawn polyethylene part I. Changes in orientation and conformation of highly drawn linear polyethylene

Infrared spectroscopy is applied to discuss the orientation, the crystallinity, and the conformation of chain segments in the amorphous regions in drawn high-density polyethylene. The orientation of the crystals as well as the crystallinity are derived from the dichroism and the absorbance, respectively, of the band at 1894 cm-¹. The orientation and some aspects about the conformation of the chain segments in the amorphous regions can be obtained from the bands in the 1400-1300 cm-¹ region (gauche) and at 1078 cm-1 (gauche and trans). The dichroic studies show a high degree of orientation increasing with draw ratio λ for the chain segments in the crystals, but a low orientation reaching saturation at λ between 5 and 10 for those in the amorphous regions. The experiments indicate a change in crystallinity during the drawing process which depends on the thermal treatment of the undrawn sample. In the amorphous regions the number of CH₂ groups in gauche conformations decreases up to λ between 10 and 15 and remains nearly constant with further drawing. Since the sum of gauche and trans conformations remains unchanged, it can be deduced that the number of loops decreases and that of tie molecules increases with draw ratio.     

Raman characterisation of conventional and cross‐linked polyethylene in acetabular cups run on a hip joint simulator

Five sets of differently sterilised conventional ultra‐high molecular weight polyethylene (UHMWPE) and cross‐linked polyethylene (XLPE) acetabular cups were run for 5 million cycles on a hip joint simulator in order to evaluate their wear behaviour in relation to material properties (PE grade, conventional or cross‐linked) and sterilisation method (ethylene oxide (EtO) treatment or γ‐irradiation). Gravimetric measurements revealed that conventional UHMWPE wore significantly more than XLPE. The differences in wear behaviour could be partly related to the orthorhombic contents obtained by Raman spectroscopy in the unworn areas of the cups: XLPE cups showed a significantly higher crystallinity degree than the UHMWPE specimens. Raman analysis showed that wear testing did not significantly modify the orthorhombic content of any of the tested acetabular cups. However, the set of cups that showed the highest weight loss, i.e. γ‐sterilised PE GUR1020, appeared the most homogeneously polished upon wear testing; from a molecular point of view, only this set of cups showed a significant increase of the I₁₁₃₀/I₁₀₆₀ intensity ratio, suggesting the occurrence of chain orientation. On the other hand, XLPE cups, despite the lowest weight loss undergone, showed a decrease in the amorphous content upon wear testing as well as a limited orthorhombic → monoclinic transformation, which did not appear detrimental. Copyright © 2011 John Wiley & Sons, Ltd.     

多元线性回归方程与光学塑料的配方设计研究

本文根据折射率加和性原理，利用多元线性回归方法，得出了光学塑料的折射率与其组成的线性方程，由此可以设计在光学系统的设计中所需折射率的光学塑料配方。该方法的折射率误差小于５‰，而且有较好的重复性。     

Generation of low-density plasma by two-step photoionization of barium and its evolution in electric field

Two-color, two-step resonant photoionization has been used to produce low-density (∼10⁶ – 10⁷cm⁻³) barium photoplasma in an atomic beam. The two-photon induced specific absorption coefficient at 355nm for the photoionization process has been measured. The motion of the finite size photoplasma bounded by vacuum is experimentally studied in a static electric field in parallel-plate electrode configuration. It is observed that at sufficiently high electric fields all electrons leave the plasma and the evolution of the left over ion bunch is understood on the basis of independent particle motion.     

Propagation and Interaction of Ion-Acoustic Solitary Waves in a Two-Dimensional Plasma Consisting of Isothermal Electrons and Hot Ions

We study the propagation and interaction of ion-acoustic solitary waves in a simple two-dimensional plasma by using the extended Poincare Lighthill-Kuo perturbation method. We consider the interaction between two ion-acoustic solitary waves with different propagation directions in such a system, and obtain two Korteweg-de Vries equations for small but finite amplitude solitary waves along both ξ and η trajectories. The effects of the ratio of ion temperature σ the ratio of heat capacity γ and the colliding angle a on the amplitude, the width of the new nonlinear wave created by the collision between two solitary waves are studied. The effects of these parameters on both the colliding solitary waves are examined as well. It is found that all the above-mentioned parameters have significant effects on the properties of these nonlinear waves.