New polyethylene nanocomposites prepared by in-situ polymerization method using nickel a-diimine catalyst supported on organo-modified ZnAl layered double hydroxide

New nanocomposites based on polyethylene (PE) and organo-modified ZnAl layered double hydroxide (O-ZnAl-LDH) were prepared by an in-situ polymerization method. The late-transition-metal catalyst, bis(4,4′-methylene-bis-(2,6-diisopropylimino))acenaphthene nickel dibromide complex, was firstly supported on the O-ZnAl-LDH layers and subsequently initiated the polymerization of ethylene. Different from commonly used solution or melt intercalation method, such strategy could induce easily the nanosclae dispersion of the O-ZnAl-LDH layers in the PE matrix, as confirmed by the analyses from X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. From the curves of thermogravimetric analysis (TGA), it was found that the decomposition temperature of the nanocomposite with 12.9 wt% O-ZnAl-LDH could be 60.5 °C higher than that of pure PE when 30% weight loss was selected as a measuring point, showing an improved thermal oxidation stability. From the melt rheological tests, it was found that the resultant PE nanocomposites had obvious increases in the storage modulus and complex viscosity when compared with pure PE, showing an enhanced viscoelastic response.     

Electrical-thermal switching effect in high-density polyethylene/graphite nanosheets conducting composites

The nonlinear response in high-density polyethylene/graphite nanosheets conducting composites under increasing applied voltage is investigated. Under sufficient applied constant voltage, the resistance increases initially due to Joule heating effect and then eventually reaches a steady value with a characteristic thermal relaxation time τ _h , which decreases as the applied field increases. The switch value, namely the ratio of the resistance under steady condition to the resistance of sample in linear regime, gradually increases with the increasing applied field. The threshold voltage (V ₀) at which the resistance start to increase with time scales with linear regime resistance (R ₀) of the sample as with the exponent x = 0.78 ± 0.05. All the curves of R/R ₀ vs. V/V ₀ collapse to a similar curve with the function R/R ₀ = 1 + α(V/V ₀) ^θ . The results reveal that the threshold voltage value decreases with increasing graphite nanosheets content in the composites.     

Deformation mechanisms in oriented high-density polyethylene

The deformation of samples of oriented high-density polyethylene has been analysed in terms of three principal deformation mechanisms,fibrillar slip, lamella slip andchain slip. From a study of small- and wide-angle X-ray diffraction patterns it is possible to deduce which mechanism or mechanisms are operating in particular cases. Material prepared in three different ways has been examined and it appears that in all three cases the primary mechanism for plastic deformation is [001] chain slip.In oriented and annealed material with a well-defined lamella crystal structure it has been possible to show that the recoverable elastic deformation is primarily due to reversible lamella slip. In this material plastic deformation by chain slip starts at a well-defined critical resolved shear stress of about 15 MNm^–2.Deformation of oriented unannealed material, in which the crystal structure is not so well-defined, appears to be more complicated. In material prepared by cold drawing some of the plastic strain may be accounted for by permanent lamella slip. Fibrillar slip does not appear to be a major deformation mechanism in any of the three materials.     

Epitaxial crystallization of linear low-density polyethylene on high-density polyethylene

The oriented crystallization of linear low-density polyethylene (LLDPE) on high-density polyethylene (HDPE) has been investigated by transmission electron microscopy. From morphology and electron diffraction, it is confirmed that epitactic growth of LLDPE lamellae on the HDPE crystals takes place with an adoption of the HDPE crystal thickness at the interface and a continuous thinning of the LLDPE lamellae in the interface. This revised version was published online in November 2006 with corrections to the Cover Date.     

Toughness of high-density polyethylene in shear fracture

This paper evaluates the validity of a new test methodology for measuring shear fracture toughness (mode II) of high density polyethylene (HDPE). The methodology adopts Iosipescu test for the shear loading, and determines the toughness based on the essential work of fracture (EWF) concept. The results show that even under the Iosipescu loading, tensile deformation (mode I) is still involved in the fracture process, possibly due to the significant work hardening that HDPE develops during the plastic deformation. The study found that the mode II fracture toughness can be determined through data analysis using double linear regression, i.e., by extrapolating specific work of fracture to zero ligament length and zero ligament thickness. The paper demonstrates that the new test methodology can be used to evaluate mode II fracture toughness of ductile polymers like HDPE in which significant work-hardening may be involved in the fracture process. The paper also provides quantitative comparison of the fracture toughness for HDPE in mode II with its mode I counterpart.     

Fractographic study of high-density polyethylene pipe

High-density polyethylene (HDPE) pipe is now being used as an alternative to medium-density polyethylene (MDPE) for gas, water, sewage and waste-water distribution systems. Laboratory tests appear to show that HDPE is more able to suppress rapid crack propagation (RCP), whilst remaining sufficient resistance under the operational circumstances that lead to the type of slow crack growth observed in service failures. There have been many fractographic studies on MDPE pipe materials, actual pipe and fittings, but little on HDPE. A fractographic study of the type of HDPE pipe in current production has been undertaken. For these tests, whole pipe sections were subjected to either static or dynamic internal (water) pressurization fatigue loading. Failure mechanisms are discussed based on the fracture morphologies resulting from these tests. A further argument for good resistance of HDPE pipe to RCP is suggested. © 1998 Chapman & Hall     

高密度聚乙烯波纹管爆破振动动力响应尺寸效应

为研究爆破振动作用下高密度聚乙烯（high-density polyethylene,HDPE）波纹管动力响应的尺寸效应,采用现场预埋管道的爆破试验及动力有限元数值计算相结合的方法,通过振动监测结果分析HDPE管道动力响应数值计算模型的可靠性;在此基础上,建立相同条件下不同管径HDPE管道的数值模型,研究尺寸效应影响下的埋地管道动力响应特征。研究结果表明：在爆破振动作用下,管道各截面迎爆侧的振速和von-Mises有效应力均大于管道背爆侧,管道振速峰值出现于管道底部;随着管径的增大,管道振速与有效应力会随之减小;管道有效应力与峰值速度之间、管道振速与地表振速之间均具有函数关系;根据HDPE管道相关规范中最大的允许压力,可得到管径为40,50,60,80和100 cm的HDPE波纹管（空管）在类似岩土层条件下地表爆破控制振速为分别为22.5,20.91,19.70,17.91,16.64 cm/s。     

The lamellar structure of oriented high-density polyethylene

The lamellar structure of a high-density polyethylene oriented to give a single-crystal type of texture has been studied by electron microscopy and X-ray diffraction. Structures shown by the Kanig technique [6, 7] and a replication technique of electron microscopy are consistent with one another and with the small-angle X-ray diffraction patterns of this type of material, but the electron microscope observations show local regions of highly misoriented lamellae which are not detectable by X-ray diffraction. In regions where the lamellar orientation is that expected from the X-ray patterns the lamellae are wide and irregularly wavy.     

Double torsion fracture testing of high-density polyethylene

The fracture of polyethylene has been studied extensively using conventional testing geometries such as three-point bending (TPB) and single-edge notch tension (SENT). These geometries are of limited utility for studying crack growth, because the crack speed is constantly changing and the crack front is in the centre of the specimen. Double torsion (DT) is a fracture geometry that suffers neither of these disadvantages, yet has only received limited attention in the literature. Its use has been limited to highly brittle materials such as glass, ceramics, thermosetting plastics and PMMA. In contrast to these materials, high-density polyethylene (HDPE) is an inherently ductile polymer. Before the advantages of DT can be exploited for testing HDPE, it is first necessary to demonstrate the validity of DT fracture measurements performed on such a ductile material. In this paper it is shown that at moderate rates of loading and at temperatures below 0C, valid double torsion fracture results can be obtained for an ethylene 1-butene copolymer. A novel technique for specimen preparation and a simple method for accurately monitoring crack growth are also described.     

Measurement of internal stresses in chemically cross-linked high-density polyethylene

Residual stresses in injection-moulded high-density polyethylene bars have been assessed by the stress-relaxation procedure of Kubát and Rigdahl, and also by the layer removal technique. Stresses were found to be tensile in the interior and compressive near to the surface. The introduction of cross-links through moisture-activated alkoxysilyl groups grafted onto the polyethylene molecules was found to increase the value of the internal stress parameter derived from the relaxation experiments and also to produce higher stress values by the layer removal analysis. Some of the relaxation curves have been analysed in addition by the technique due to Li and the significance of the internal stress parameter, _i obtained by the two different relaxation analyses is discussed.     

Relation of local plasticity and fractability in high-density polyethylene

Within the framework of the cluster model of the structure of the amorphous state of polymers the article examines the relation between plasticity and fractability of the fracture surfaces of high-density polyethylene. It shows that the model under consideration yields good agreement between the experimental and the theoretical data.Translated from Problemy Prochnosti, No. 8, pp. 38–41, August, 1995.     

Polymerically modified layered silicates: an effective route to nanocomposites

Polymer/clay nanocomposites have been under an extensive investigation for about 15 years. Traditional methods to modify the clay are usually limited to small organic cations, preferably containing long alkyl chain(s), which are exchanged with the inorganic cations in the clay gallery. This article provides a comprehensive review on the strategies for clay modification using polymeric surfactants or polycations: from the synthesis of such surfactants, through the preparation of the polymerically modified clays, and to the fabrication of the respective polymer nanocomposites and their properties.     

Lamellar separation during the deformation of high-density polyethylene

Upon tensile straining at low deformation rates (=2×10^–5 sec^–1), spherulitic linear polyethylene behaves reversibly for extensions up to 40%. In stress relaxation experiments on unloaded specimens the stress increases with time. Samples kept under constant strain ( = 40%) over four months show macroscopic cracking. Microstructural investigation was performed using low- and wide-angle X-ray diffraction and transmission electron microscopy. These investigations reveal a very inhomogeneous deformation within those lamellar stacks for which the crystalline lamellae lie normal to the tensile axis. The deformation in that case is similar to what has been observed for elastic hard fibres. A two-mechanism model to explain the macroscopic observation on the basis of the microscopic observations is developed.