Non-linear viscoelasticity and viscoplasticity of isotactic polypropylene

Observations are reported in tensile tests with constant cross-head speeds (ranging from 5 to 200 mm/min), relaxation tests (at strains from 0.02 to 0.08), creep tests (at stresses from 15.0 to 25.0 MPa) and recovery tests (after straining up to the maximal strains ranging from 0.04 to 0.12 and subsequent retraction) on isotactic polypropylene at room temperature. A constitutive model is derived for the time- and rate-dependent responses of a semicrystalline polymer at isothermal deformation with small strains. A polymer is treated as an equivalent heterogeneous network of chains bridged by temporary junctions (entanglements, physical cross-links and lamellar blocks). The network is thought of as an ensemble of meso-regions linked with each other. The viscoelastic behavior of the ensemble reflects thermally-induced rearrangement of strands (separation of active strands from temporary junctions and merging of dangling strands with the network). To describe the viscoplastic response, the entire plastic deformation is split into the sum of two components: one of them is associated with sliding of junctions in the non-affine network of chains, while the other accounts for coarse slip and fragmentation of lamellar blocks. Stress–strain relations and kinetic equations for the plastic strains are developed by using the laws of thermodynamics. The constitutive equations involve five material constants that are found by fitting the observations. Fair agreement is demonstrated between the experimental data and the results of numerical simulation.     

Combined thermodynamics approach for anisotropic, finite deformation overstress models of viscoplasticity

An anisotropic finite deformation thermodynamic framework is developed that admits most of the models within the class of unified viscoplasticity approaches, applying to representations of the behaviors of polymers, metals and their composites. The importance of changing natural configurations as a driver for dissipation is emphasized, as is the specification of constitutive behaviors via the energy and dissipation functions. In a sequential application of thermodynamic approaches, Caratheodory’s lemma is first applied to establish traditional potential relations for the stress and entropy, followed by application of the Coleman-Noll approach to deduce that evolution functions related to the rates of the independent variables must be associated with reversible, non-dissipative changes. The principal of maximum dissipation is lastly applied to deduce the flow rule, identify dissipative behaviors, and deduce interrelationships between the independently-specified energy and dissipation functions. A simplified case is described that illustrates these interrelationships and their compatibilities with the proposed framework.     

Reprocessable silane-crosslinked polyethylene: property and utilization as toughness enhancer for high-density polyethylene

Silane-crosslinked polyethylene (Si-XLPE) has been increasingly used as a replacement of polyvinyl chloride for use as residence water pipes and hydronic heating systems. Unfortunately, good management of Si-XLPE wastes originated in the manufacturing processes or from already used products is limited. This article presents a melt-recycling (remelting and remolding) of waste Si-XLPE and its use to produce the high-density polyethylene (HDPE) composition of improved thermal endurance, along with attractive balance of stiffness and toughness. Experiments were carried out with a series of Si-XLPE of different gel fraction and crosslink structure, either loose or tight, aiming at understanding the factors which dictate the reprocessability and properties of crosslinked waste. Tensile and impact properties, crystallization, and heat stability of the HDPE and waste blends were investigated. It is found that waste Si-XLPE materials of loosely network structure can be remelting and remolding as thermoplastic PE. Interestingly, the results showed that crosslink gel in the wastes could be as high as 70 %. Both tensile modulus and impact strength of the HDPE systematically increased with increasing loading and content of gel in the crosslinked materials. The addition of 50 % crosslinked waste led to a rise of heat distortion temperature of HDPE by 20 °C, without evitable change in the melting and crystallization temperatures.     

Implicit algorithm for finite deformation hypoelastic–viscoplasticity in fcc metals

An implicit objective stress update algorithm is proposed for a hypoelastic–viscoplastic model. A thermal/dynamic yield function, which is derived based on the thermal activation analysis and dislocation interaction mechanisms, is used, along with the Consistency approach and the framework of additive viscoplasticity, in deriving the proposed model for fcc metals. The corotational formulation approach is utilized in developing the proposed model in the finite deformation field. For the case of the Newton–Raphson iteration method, a new expression for the consistent (algorithmic) tangent stiffness matrix of rate‐dependent metals is derived by direct linearization of the stress update algorithm. Finite element simulations are performed by implementing the proposed viscoplasticity constitutive models in the commercial finite element program ABAQUS. Numerical implementation for a simple tensile problem is used for validating the material parameters of the OFHC Copper under low and high strain rates and temperatures. The numerical results of the adiabatic true stress–true strain curves compare very well with the experimental data. The effectiveness of the present approach is tested by studying strain localization in a simple plane strain problem. Results indicate excellent performance of the present framework in describing the strain localization problem and in obtaining mesh‐independent results. Copyright © 2006 John Wiley & Sons, Ltd.     

新型抗菌聚乙烯塑料制备及性能研究

采用熔融共混法制备得到具有负离子释放功能的低密度聚乙烯(LDPE)/稀土复合矿粉(Eli)、LDPE/Eli/吡啶硫铜锌(ZPT)(LDPE/Eli/ZPT)两种LDPE抗茵塑料.通过对其结晶行为、抗菌性能及抗菌持久性系统研究表明:Eli的加入对LDPE有异相成核作用;Eli用量为1份时,LDPE/Eli复合材料负离子释放量为790 ions/co,与公园中负离子释放量相当,抗菌率达到45%以上;ZPT用量为0.15份时,LDPE/Eli/ZPT复合体系对大肠杆菌(E. coli)抗菌率达到98.21%,对金黄色葡萄球菌(S. aureus)抗菌率达到96.15%,经过30天的水洗后,其抗菌率仍能保持82.14%(E coli)和81.25%(S.aureus).     

Dimensional stability and mechanical behaviour of wood–plastic composites based on recycled and virgin high-density polyethylene (HDPE)

This paper investigated the stability, mechanical properties, and the microstructure of wood–plastic composites, which were made using either recycled or virgin high-density polyethylene (HDPE) with wood flour (Pinus radiata) as filler. The post-consumer HDPE was collected from plastics recycling plant and sawdust was obtained from a local sawmill. Composite panels were made from recycled HDPE through hot-press moulding exhibited excellent dimensional stability as compared to that made from virgin HDPE. The tensile and flexural properties of the composites based on recycled HDPE were equivalent to those based on virgin HDPE. Adding maleated polypropylene (MAPP) by 3–5 wt% in the composite formulation significantly improved both the stability and mechanical properties. Microstructure analysis of the fractured surfaces of MAPP modified composites confirmed improved interfacial bonding. Dimensional stability and strength properties of the composites can be improved by increasing the polymer content or by addition of coupling agent. This project has shown that the composites treated with coupling agents will be desirable as building materials due to their improved stability and strength properties.     

Stress and strain‐driven algorithmic formulations for finite strain viscoplasticity for hybrid and standard finite elements

This work deals with the formulation and implementation of finite deformation viscoplasticity within the framework of stress‐based hybrid finite element methods. Hybrid elements, which are based on a two‐field variational formulation, are much less susceptible to locking than conventional displacement‐based elements. The conventional return‐mapping scheme cannot be used in the context of hybrid stress methods since the stress is known, and the strain and the internal plastic variables have to be recovered using this known stress field. We discuss the formulation and implementation of the consistent tangent tensor, and the return‐mapping algorithm within the context of the hybrid method. We demonstrate the efficacy of the algorithm on a wide range of problems. Copyright © 2009 John Wiley & Sons, Ltd.     

Structure–Relaxivity Relationships of Magnetic Nanoparticles for Magnetic Resonance Imaging

Magnetic nanoparticles (MNPs) have been extensively explored as magnetic resonance imaging (MRI) contrast agents. With the increasing complexity in the structure of modern MNPs, the classical Solomon–Bloembergen–Morgan and the outer‐sphere quantum mechanical theories established on simplistic models have encountered limitations for defining the emergent phenomena of relaxation enhancement in MRI. Recent progress in probing MRI relaxivity of MNPs based on structural features at the molecular and atomic scales is reviewed, namely, the structure–relaxivity relationships, including size, shape, crystal structure, surface modification, and assembled structure. A special emphasis is placed on bridging the gaps between classical simplistic models and modern MNPs with elegant structural complexity. In the pursuit of novel MRI contrast agents, it is hoped that this review will spur the critical thinking for design and engineering of novel MNPs for MRI applications across a broad spectrum of research fields.     

Elastic–viscoplastic behavior of plain-woven GFRP laminates: Homogenization using a reduced domain of analysis

In this study, a method for reducing the domain of analysis is developed for the homogenization analysis of plain-woven laminates. To this end, it is first shown that the internal structures of plain-woven laminates satisfy point-symmetry on the assumption that the laminates have the in-phase or out-of-phase laminate configuration of plain fabrics. The point-symmetry is then utilized for the boundary condition of unit cell problems, reducing the domain of analysis to 1/4 and 1/8 for the in-phase and out-of-phase laminate configurations, respectively. Using the present method, the in-plane elastic–viscoplastic deformation of plain-woven GFRP laminates is analyzed based on the homogenization theory of nonlinear time-dependent composites. Moreover, the in-plane uniaxial tensile tests of a plain-woven GFRP laminate at a constant strain rate are performed at a room temperature. It is thus shown that the present analysis successfully predicts the in-plane elastic–viscoplastic behavior of plain-woven GFRP laminates. It is also shown that the laminate configurations of plain fabrics exert an influence on the viscoplastic behavior of laminates, not their elastic behavior.     

Viscoelasticity of Maxwell's model and non‐Newtonian viscosity revisited

The viscosity of fluids and melts is an important characteristic to steer processes and reactions of materials, to use lubricants and to fabricate diverse products of glass. To steer such fabrication processes reliably and free of failures one necessarily needs to understand and use the true data of the viscosity. The forces applied to measure the viscosity act also to accelerate parts of the samples and particularly to deform them elastically. To evaluate the viscosity one must necessarily consider such ‘side reactions’. Cylindrical samples under longitudinal deformation are demonstrated to deform rather elastically than by viscous flow upon application of the load, e. g. Neglecting this effect the ’viscosity’ apparently decreasing with increasing load may be misinterpreted as non‐Newtonian viscosity. In fact, it represents the change from viscous to elastic behaviour with decreasing time interval of the application of force. Furthermore, production of entropy during deformation for measurement has to be taken into account adequately. The sample warms up during heavy deformation. If the viscosity depends strongly on the temperature, one must take into account that temperature and viscosity change with the intensity of the load. Then the so called ’shear thinning’ is rather ’sheer heating’. This is demonstrated quantitatively with data of the viscosity as a function of the load, which have recently been published using capillary rheometers.     

Cyclic deformation behavior of double-slip-oriented copper single crystals I: coplanar double slip orientation on 011-1̄11 side of the stereographic triangle

The cyclic deformation behaviors of [2̄33] coplanar double-slip-oriented and [4̄ 18 41] single-slip-oriented copper single crystals were investigated at constant plastic shear strain amplitude γ_pl in the range of about 10⁻⁴–10⁻² at ambient temperature in air. It was revealed that the cyclic deformation behavior of copper single crystal oriented on the 011-1̄11 side is distinctly dissimilar from that on the 001-1̄11 and 001-011 sides in the stereographic triangle. The plot of initial hardening rate θ_0.2 against γ_pl of [2̄33] crystal exhibits two regions as presented for single-slip-oriented crystals. The critical strain amplitude (≈3.5×10⁻³), corresponding to the occurrence of the secondary hardening stage in the cyclic hardening curve of the [2̄33] crystal, was found to be an intermediate value between that for single-slip-oriented single crystals and polycrystals. The result shows that the cyclic hardening behavior of the [2̄33] crystal, as compared with that of single-slip-oriented crystals, is more close to that of polycrystals. Instead of a clear plateau, the cyclic stress–strain (CSS) curves of the [2̄33] crystals shows a quasi-plateau over the range of about 3.0×10⁻⁴–2.0×10⁻³, which would be greatly attributed to the mode of dislocation interactions between slip systems operating in the crystal. The habit plane of two types of deformation bands DBI and DBII, formed in the cyclically deformed [2̄33] crystal, are perpendicular to each other strictly, and they develop with increasing applied strain amplitude.     

聚乙二醇/磺化聚苯醚质子交换膜的制备与性能研究

采用溶液共混浇铸法制备了一系列的聚乙二醇/磺化聚苯醚(PEG/SPPO)共混膜SPPO的红外光谱图显示了磺酸基团的成功引入;X-射线衍射图表明,加入柔性的聚乙二醇,SPPO的有序程度降低;膜的离子交换容量(IEC值)、质子传导率和吸水率等理化性能结果表明:PEG/SPPO共混膜的质子传导率和离子交换容量虽然较纯SPPO膜有所下降,但是最大也分别达到了1.80mequiv.g-1和0.061S/cm(PEG/SPPO=10/90,90℃),仍然可以满足质子交换膜的使用要求,而复合膜的吸水率和溶胀率较SPPO膜大幅降低。整体来说,复合膜的各项综合性质良好,有望应用于直接甲醇燃料电池。     

On the strengthening effect of increasing cycling frequency on fatigue behavior of some polymers and their composites: Experiments and modeling

Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinforced injection molded polymer composites was investigated by conducting load-controlled fatigue tests at several stress ratios (R = −1, 0.1, and 0.3) and at several temperatures (T = 23, 85 and 120 °C). A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in DMA tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson–Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together. An analytical fatigue life estimation model was also used to consider the strengthening effect of frequency in addition to mean stress, fiber orientation, and temperature effects on fatigue life.     

多级拉伸挤出对纳米有机蒙脱土/高密度聚乙烯复合材料形态结构及性能的影响

采用多级拉伸挤出工艺和传统挤出工艺制备了纳米有机蒙脱土/高密度聚乙烯（OMMT/HDPE）复合材料,利用广角X射线衍射（WAXD）、TEM、DSC和小角X射线散射（SAXS）等手段对比研究了两种不同工艺制备OMMT/HDPE复合材料的形态结构和性能。结果表明：多级拉伸挤出作用能够细化OMMT的颗粒尺寸,促进OMMT在HDPE基体中的分散;随多级拉伸挤出次数的增加,OMMT的分散性越好,OMMT颗粒的长径比增加,从而使OMMT/HDPE复合材料的初始成核速率和结晶速率明显提高,诱导复合材料形成取向串晶（Shish-kebab）结构,显著提高OMMT/HDPE复合材料的拉伸强度;当OMMT∶HDPE分别为1∶100和5∶100时,多级拉伸挤出OMMT/HDPE复合材料的拉伸强度相对于传统挤出OMMT/HDPE复合材料的拉伸强度分别提高了50%和356%。     

A multiplicative finite strain finite element framework for the modelling of semicrystalline polymers and polycarbonates

This paper presents a phenomenological model for the simulation and analysis of stress‐induced orientational hardening in semicrystalline polymers and polycarbonates at finite strains. The notion of intermediate (local) stress‐free configuration is used to develop a set of constitutive equations, and its relation to the multiple natural (stress‐free) configurations in the class of materials being considered here is discussed. A hyperelastic stored energy function, written with respect to the intermediate stress‐free configuration is presented to model the finite elastic response. It is then combined with the J₂‐flow theory to model the finite inelastic response. The isochoric constraint during inelastic deformation is treated via an exact multiplicative decomposition of the deformation gradient into volume‐preserving and spherical parts. The numerical solution algorithm is based on the use of operator splitting technique that results in a product formula algorithm with elastic‐predictor/inelastic‐corrector components. Numerical results are presented to show the behaviour of the model. Copyright © 2000 John Wiley & Sons, Ltd.     

蒙脱土-SiO2/低密度聚乙烯复合材料结晶行为及电树枝化特性

为了进一步改善低密度聚乙烯（LDPE）的耐电树枝化性能,以有机化蒙脱土（MMT）和表面改性的SiO₂为纳米填料,采用熔融共混法制备了MMT-SiO₂/LDPE多元复合材料。利用FTIR表征了纳米填料与LDPE基体间分子链相互作用。研究了纳米MMT和SiO₂对LDPE结晶行为、结晶形态及耐电树枝化性能的影响。结果表明：纳米MMT和SiO₂均通过改性剂长链与LDPE基体分子链以物理纠缠的形式混合在LDPE中。纳米SiO₂异相成核形成小的晶体结构,与分散在无定形区的纳米MMT均对电树枝的发展具有阻挡作用,二者相互协同使MMT-SiO₂/LDPE多元复合物材料中电树枝的发展路径更加曲折,因此MMT-SiO₂/LDPE多元复合物材料耐电树枝性能优于MMT/LDPE和SiO₂/LDPE复合材料。     

A numerical investigation of the potential of rubber and mineral particles for toughening of semicrystalline polymers

The impact strength of many semicrystalline polymers can be improved by the dispersion of second-phase rubber particles. A criterion for the effect of this practice is based on the average interparticle matrix ligament thickness. The critical interparticle distance, below which a substantial toughness increase can be observed, is considered to be an intrinsic material property of the matrix. A toughening mechanism has recently been suggested which considers a layer of transcrystallized material around well-dispersed particles, having a reduced yield strength in certain preferentially oriented directions, thereby opening the possibility of using mineral fillers. In this work, the potential of toughening of semicrystalline polymeric material by local anisotropy in combination with soft rubber and hard mineral filler particles is investigated. The matrix material is modeled within the framework of anisotropic Hill plasticity with a rate dependent and hardening yield stress. Various particle/matrix interface conditions are used to study the role of debonding and cavitation. The presence of debonded moderately stiff or hard fillers is found to affect the shear yielding effect of local anisotropy that was found for voided material.     

An analytical–numerical framework for the study of ageing in fibre reinforced polymer composites

In the last few years there has been available a growing amount of information on composites ageing that considers effects such as temperature, oxidation, UV radiation, permanent loading, etc. In this paper, an analytical–numerical framework adequate for the compilation, interpretation and application of experimental data to actual engineering analysis and design is discussed. The formulation proposed includes elastic anisotropic relations; ageing viscoelastic anisotropic constitutive equations in terms of state variables; age-adjusted failure and degradation criteria; all in a setting of large displacements with small strains. The essential differences between hardening and softening ageing processes are described together with the constitutive relations adequate for each case. These equations are written in a form suitable for numerical analysis in a finite elements context, using an incremental-iterative solution procedure that accounts for post-critical effects. Several examples, including elastic, viscoelastic and failure behaviour in a large displacement context are included to check the algorithms.     

Effect of semicrystalline polymers on self-emulsifying solid dispersions of nateglinide: in vitro and in vivo evaluation

This study was undertaken to improve solubility and bioavailability of nateglinide by preparation of stable self-emulsifying solid dispersions (SESDs). The influence of semicrystalline polymers (poloxamer 407, gelucire 50/13) and method of preparation on dissolution behavior, in vivo performance and stability of nateglinide SESDs were investigated. After optimization, SESDs were prepared at 1:5 weight ratio of nateglinide and polymer individually. All the SESDs were characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. Aqueous solubility of nateglinide was enhanced by 91.82-fold. The SESDs (poloxamer 407-based solid dispersions) achieved rapid and complete drug release (～100% within 45?min) at pH 2. The improved dissolution appeared to be well correlated with the enhanced bioavailability of nateglinide in rabbits. After oral administration of SESDs (poloxamer 407-based solid dispersions), C_max and AUC of nateglinide were increased by ～2.92 and 1.77-folds, respectively, signifying the effectiveness of solid dispersions to improve the bioavailability of nateglinide. Stability during storage was established to show prevention of recrystallization. In conclusion, SESDs with poloxamer 407 in solvent method appeared to be an economic and promising technique to improve the dissolution, bioavailability, and stability of nateglinide.