多壁碳纳米管-高聚物纳米复合材料的抗蠕变性能

通过双螺杆挤出机和模压成型设备制备了两种不同长径比的多壁碳纳米管(MWNT)增强的聚丙烯(PP)纳米复合材料.实验表明,通过添加1%体积含量的MWNT,聚丙烯的抗蠕变性能得到很大提高,即长时间加载后,基体的蠕变变形量和蠕变率均显著降低.同时,在特定载荷下,纳米复合材料的蠕变寿命比纯基体提高了10倍.几种载荷传递机理导致了材料抗蠕变性能的增强:(1)碳纳米管和基体之间较好的界面性能,(2)碳纳米管限制了基体内无定型分子链的活动性,以及(3)碳纳米管的较高的长径比.差分热扫描(DSC)的结果显示了材料蠕变前后结晶的变化和载荷传递机理分析是一致的.这些实验结果显示,在不增加成本的基础上可以大大提高抗蠕变的聚合物纳米复合材料的工程应用.     

聚合物纳米复合材料蠕变性能研究进展

聚合物在室温甚至低温条件下的蠕变被认为是制约其更广泛应用的主要瓶颈之一.实验研究发现添加很低含量(1{\%}重量或体积含量)的纳米颗粒,在基本上不影响基体其他力学和物理性能的前提下,能够大幅度提高聚合物的耐蠕变性能; 另外和静态性能相比较,蠕变和松弛等特性对于聚合物微观结构的变化和分子链的相互作用更加敏感,能够在新型纳米复合材料(如多层级纳米复合材料)的力学设计中为我们提供更加丰富的微观结构及其相互作用的信息.本文综述了多种形貌纳米颗粒(包括金属氧化物、碳纳米管、层状纳米黏土等)对聚合物耐蠕变性能影响的研究现状和进展.讨论了纳米颗粒的种类、形貌和含量,以及外部应力水平和温度等因素对聚合物基体材料蠕变性能的影响规律;分析了目前一些常用的模拟和预测蠕变行为的模型, 并利用这些模型,结合纳米复合材料特点, 对蠕变实验结果进行了模拟和预测;结合多层级纳米复合材料的实验研究结果,阐述了蠕变条件下纳米复合材料分子链间相互作用的特点;进而探讨了纳米颗粒影响聚合物蠕变性能的机理,展望了该领域研究的发展态势.      

纳米颗粒增强镍基MEMS器件材料的蠕变性能研究

利用同步辐射LIGA微铸复合工艺,将纳米氧化物增强颗粒复合到微电子机械系统(MEMS)结构材料中。制作了专用夹具,采用微力材料试验机测量了纳米Al2O3颗粒增强镍基复合材料的强度为1GPa;将恒加载速率/载荷法和恒载荷法相结合,利用纳米压痕仪测量了该材料的室温蠕变速率敏感指数m。结果表明,LIGA复合技术得到的纳米颗粒增强镍基复合材料具有较高的强度;MEMS器件材料在室温下会发生蠕变;材料在相同压深下最大载荷不随加载速率而改变,加载段粘弹性和粘塑性变形极少;主要由局部高应力导致压痕蠕变;材料的蠕变速率敏感指数m值为0.004,说明纳米Al2O3颗粒可有效增强基体材料的抗蠕变能力;且不同恒.P/P下获得的m值基本相同,表示此种材料对加载速率不敏感。     

蠕变压痕实验的计算机模拟

采用有限元的方法对双相材料的蠕变压痕实验进行了数值模拟，在有限元数值解的基础上，定义了相应于传统单轴蠕变实验的“等效应力”和“等效应变”，建立了蠕变压痕实验同传统单轴蠕变实验之间的关系，给出了确定薄膜蠕变应力指数和蠕变常数的方法；同时数值解的结果表明，实验中通过控制压痕深度不超过薄膜厚度的 ５％～１０％，忽略基体的硬化指数对确定薄膜性能的影响存在一定的误差，但基体的弹性模量对确定薄膜的蠕变性能影响不大。     

纤维增强金属基复合材料整体蠕变性能的细观力学分析

高温下金属基复合材料的蠕变主要由基体蠕变和界面扩散蠕变两部分构成,以往的研究中常常只考虑其中一种蠕变机理,从而导致得到的规律具有较大的局限性．本文提出了一种可预测金属基复合材料整体蠕变性能的细观力学方法,同时考虑了基体蠕变和界面扩散蠕变两种蠕变机理,导出了具有张量形式并满足不可压缩性的界面扩散蠕变应变表达式．采用Mori-Tanaka法和自洽法二者结果的平均以便更准确地计算纤维中的应力,揭示了两种蠕变机理相互影响的竞争关系．研究了恒定双轴荷载下的总体蠕变和固定位移约束下的应力松弛这两种常见蠕变问题,探究了基体蠕变与界面扩散蠕变两种蠕变机理在总蠕变中发挥的作用,考察了不同加载条件和不同纤维体积分数对复合材料整体蠕变行为的影响．     

碳纳米管增强Nb-Si基复合材料界面应力传递的研究

基于均匀化理论,建立了碳纳米管增强Nb-Si基复合材料的代表体积元模型,并采用剪切滞后模型对碳纳米管增强Nb-Si基复合材料界面上的应力分布和传递机制进行了研究,探讨了分子间作用力、杨氏模量比β、长径比α、体积分数?等对其应力分布和传递机制的影响。结果表明,复合材料界面应力分布的变化主要集中在碳纳米管的两端,最大的应力都是分布在加载端或拔出端,然后向另一端传递;分子间作用力、杨氏模量比、长径比、体积分数等参数对界面应力的传递均有一定的影响,其中长径比和体积分数的影响最明显,体积分数为0.02时拔出端的界面剪切应力值相对于体积分数为0.0025时增大幅度达到近7倍,而长径比从200减小到50时,其应力传递距离增大了近8倍。     

纳米碳酸钙对聚丙烯/碳纤维复合材料力学性能的影响

通过熔融共混的方法分别制备了聚丙烯/碳纤维、聚丙烯/纳米碳酸钙及聚丙烯/纳米碳酸钙/碳纤维复合材料,并研究了碳纤维及纳米碳酸钙对聚丙烯材料增韧增刚的效果。结果表明,少量的碳纤维能够大幅提高聚丙烯材料的弯曲模量,并有一定的增韧效果;表面改性后的纳米碳酸钙能在聚丙烯基体中均匀分散,并大幅提高聚丙烯的缺口冲击韧性,在30%添加量下(重量比)提高幅度达到5.6倍,同时模量也有一定程度提高。将改性后的纳米碳酸钙和碳纤维同时加入聚丙烯基体中,结果显示:在保证纳米碳酸钙均匀分散的前提下,复合材料的缺口冲击强度和弯曲模量可以同时得到大幅度地提升,实现了聚丙烯基复合材料的高刚高韧改性。     

界面特性对短纤维金属基复合材料蠕变行为的影响

基于短纤维增强金属基复合材料(MMC)的单纤维三维模型(三相)，利用粘弹性有限元分析方法对影响金属基复合材料的蠕变行为的因素进行了较为系统的分析。研究中主要讨论了界面特性和纤维取向角对金属基复合材料的蠕变性能的影响。研究结果发现，界面特性诸如厚度、模量和应力指数都对纤维最大轴应力和稳定蠕变率产生影响：稳态蠕变率随界面模量的增大而逐渐减小，当高于基体模量时基本保持不变；纤维轴应力的变化与蠕变率正好相反。稳态蠕变率随界面厚度、应力指数的增加而增大；而轴应力则随之减小。同时不同的纤维取向也影响金属基复合材料蠕变时的轴应力分布和稳态蠕变率。     

二氧化硅纳米颗粒对碳纤维与环氧树脂基体粘合强度的增强

纤维与基体的粘合强度是决定纤维增强高分子复合材料性能的关键因素。本文采用横向纤维柬拉伸实验的方法研究了碳纤维与经过纳米颗粒改性的环氧树脂基体间的粘合强度。平均直径为25纳米的二氧化硅纳米颗粒用特殊的溶胶-凝胶法引入环氧基体（由 Hanse ChemieAG提供），可以达到很高的含量，同时保持较为理想的分散状态。实验结果表明，二氧化硅纳米颗粒对于碳纤维与改性环氧基体的粘合强度有显著的增强效应。当纳米颗粒含量为14v01．％时，横向纤维柬拉伸的断裂强度相比纯环氧基体提高了104％。通过对横向纤维柬拉伸样品断裂面的扫描电镜观察，以及二氧化硅纳米颗粒改性环氧树脂基体材料的力学性质的测量，可以发现横向纤维柬拉伸的断裂强度与改性环氧基体本身的断裂韧性之间存在良好的相关性。由此可推测纳米颗粒对环氧树脂基体材料的增韧是碳纤维与基体间界面增强的一个重要原因。     

高温环境下纤维复合材料蠕变损伤的细观机理研究

首先利用复合材料纤维断裂单胞模型，编制蠕变损伤子程序，对单胞模型进行蠕变损伤分析。分析了纤维／基体弹性模量比对蠕变变形、蠕变损伤以及应力场的影响。从计算结果发现，蠕变损伤首先在纤维断裂尖端起始，然后沿着一定的角度向基体外围延伸，直至完全损伤，而且纤维／基体模量比对高温环境下的复合材料蠕变损伤产生很大的影响；纤维与基体的模量相差越大，复合材料越容易变形，抵抗蠕变变形的能力就越小，蠕变损伤越严重。经过对不同韧性的基体材料进行研究，发现基体韧性低的复合材料蠕变损伤明显高于高韧性基体复合材料，表明低韧性基体复合材料抵抗蠕变破坏的能力较低。     

Creep and recovery of polypropylene/carbon nanotube composites

The creep and recovery of polypropylene/multi-walled carbon nanotube composites were studied. It was found for thermoplastics in general that the creep strain reduces with decreased temperature, and with enhanced content of carbon nanotubes. The incorporation of nanotubes improved the recovery property remarkably, especially at high temperature. The unrecovered creep strain of nanocomposites with content of 1 and 2.8 vol.% carbon nanotubes decreased by 53% and 73% compared to that of polymer matrix. To understand the mechanisms, the Burger’s model and Weibull distribution function were employed since the variations in the simulating parameters illustrated the influence of nano-fillers on the creep and recovery performance of the bulk matrix. To further study the recovery properties, the particular contribution of each Burger’s element to the total deformation was obtained and the recovery percentage was calculated. The time-temperature-superposition-principle was applied to predict the long-term creep behavior.     

多壁碳纳米管/环氧树脂纳米复合材料的摩擦磨损性能研究

采用浇铸法,利用超声分散制备了多壁碳纳米管(MWNTs)/环氧树脂(EP)纳米复合材料,研究了MWNTs的添加量及分散程度对复合材料表面形貌和摩擦磨损性能的影响,并探讨了影响MWNTs/EP复合材料摩擦磨损性能的因素.结果表明:随着MWNTs加入量的提高(1%～4%),复合材料的摩擦系数和磨损率均呈现降低趋势,摩擦系数由0.60降到0.22,磨损率由1.11×10-4mg/(N·m)降为2.22×10-5mg/(N·m);在MWNTs添加量(1%)相同的情况下,MWNTs分散程度高的复合材料的摩擦性能更好.纯环氧树脂与45#钢对摩时发生粘着磨损和疲劳剥落,而由于MWNTs的增强和自润滑作用,MWNTs/EP复合材料的粘着磨损和疲劳剥落显著减轻.     

Mechanics of deformation of single- and multi-wall carbon nanotubes

An effective continuum/finite element (FE) approach for modeling the structure and the deformation of single- and multi-wall carbon nanotubes (CNTs) is presented. Individual tubes are modeled using shell elements, where a specific pairing of elastic properties and mechanical thickness of the tube wall is identified to enable successful modeling with shell theory. The incorporation and role of an initial internal distributed stress through the thickness of the wall, due to the cylindrical nature of the tube, are discussed. The effects of van der Waals forces, crucial in multi-wall nanotubes and in tube/tube or tube/substrate interactions, are simulated by the construction of special interaction elements.The success of this new CNT modeling approach is verified by first comparing simulations of deformation of single-wall nanotubes with molecular dynamics results available in the literature. Simulations of final deformed configurations, as well strain energy histories, are in excellent agreement with the atomistic models for various deformations. The approach was then applied to the bending of multi-wall carbon nanotubes (MWNTs), and the deformed configurations were compared to corresponding high-resolution images from experiments. The proposed approach successfully predicts the experimentally observed wavelengths and shapes of the wrinkles that develop in bent MWNTs, a complex phenomenon dominated by inter-layer interactions. Presented results demonstrate that the proposed FE technique could provide a valuable tool for studying the mechanical behavior of MWNTs as single entities, as well as their effectiveness as load-bearing entities in nanocomposite materials.     

Micromechanics and macromechanics of carbon nanotube-enhanced elastomers

The effects of carbon nanotubes on the mechanical behavior of elastomeric materials is investigated. The large deformation uniaxial tension and uniaxial compression stress-strain behaviors of a representative elastomer are first presented. This elastomer is then reinforced with multi-wall carbon nanotubes (MWNTs) and the influence of weight fraction of MWNTs on the large deformation behavior of the resulting composite is quantified. The initial stiffness and subsequent strain-induced stiffening at large strains are both found to increase with MWNT content. The MWNTs are also found to increase both the tensile strength and the tensile stretch at break. A systematic approach for reducing the experimental data to isolate the MWNT contribution to the strain energy of the composite is presented. A constitutive model for the large strain deformation behavior of MWNT-elastomer composites is then developed. The effects of carbon nanotubes are modeled via a constitutive element which tracks the stretching and rotation of a distribution of wavy carbon nanotubes. The MWNT strain energy contribution is due to the bending/unbending of the initial waviness and provides the increase in initial stiffness as well as the retention and further enhancement of the increase in stiffness with large strains. The model is shown to track the stretching and rotation of the CNTs with macroscopic strain as well as predict the dependence of the macroscopic stress-strain behavior on the MWNT content for both uniaxial tension and uniaxial compression.     

Modeling deformation behavior of polymers with viscoplasticity theory based on overstress

The nonlinear strain rate sensitivity, multiple creep and recovery behavior of polyphenylene oxide (PPO), which were explored through strain rate-controlled experiments at ambient temperature by Khan [The deformation behavior of solid polymers and modeling with the viscoplasticity theory based overstress, Ph.D. Thesis, Rensselaer Polytechnic Institute, New York], are modeled using the modified viscoplasticity theory based on overstress (VBO). In addition, VBO used by Krempl and Ho [An overstress model for solid polymer deformation behavior applied to Nylon 66, ASTM STP 1357, 2000, p. 118] and the classical VBO are used to demonstrate the improved modeling capabilities of VBO for solid polymer deformation. The unified model (VBO) has two tensor valued state variables, the equilibrium and kinematic stresses and two scalar valued states variables, drag and isotropic stresses. The simulations include monotonic loading and unloading at various strain rates, multiple creep and recovery at zero stress. Since creep behavior has been found to be profoundly influenced by the level of the stress, the tests are performed at different stresses above and below the yield point. Numerical results are compared to experimental data. It is shown that nonlinear rate sensitivity, nonlinear unloading, creep and recovery at zero stress can be reproduced using the modified viscoplasticity theory based on overstress.     

Stress relaxation and creep of polymer gels in solvent under uniaxial and biaxial deformations

Stress relaxation and creep of polymer gels in solvent under various deformation modes such as uniaxial, strip-biaxial, and equibiaxial were theoretically investigated. The magnitudes of relaxed stress and the creep at equilibrium under each deformation mode were derived by a thermodynamic consideration of gel system. Combining a constitutive equation of gel with the equation of motion of polymer network, the stress and strain under each deformation mode have been formulated. The theory proposed here was applied to the rectangular gels under various deformations to calculate the stress relaxation and creep behavior of polymer networks in solvent. Two methods different in treatment of swelling under the constant strain or stress were employed for the calculation: one is based on the assumption that the swelling proceeds isotropically, and the other considers the anisotropic swelling process. The results obtained by the two methods mainly differ in the diffusion mode determining the swelling behavior. The possibility of undershoot of relative strain in load-free direction in the creep is also expected.     

Axisymmetric compressive buckling of multi-walled carbon nanotubes under different boundary conditions

The paper studies the axisymmetric compressive buckling behavior of multi-walled carbon nanotubes (MWNTs) under different boundary conditions based on continuum mechanics model. A buckling condition is derived for determining the critical buckling load and associated buckling mode of MWNTs, and numerical results are worked out for MWNTs with different aspect ratios under fixed and simply supported boundary conditions. It is shown that the critical buckling load of MWNTs is insensitive to boundary conditions, except for nanotubes with smaller radii and very small aspect ratio. The associated buckling modes for different layers of MWNTs are in-phase, and the buckling displacement ratios for different layers are independent of the boundary conditions and the length of MWNTs. Moreover, for simply supported boundary conditions, the critical buckling load is compared with the corresponding one for axial compressive buckling, which indicates that the critical buckling load for axial compressive buckling can be well approximated by the corresponding one for axisymmetric compressive buckling. In particular, for axial compressive buckling of double-walled carbon nanotubes, an analytical expression is given for approximating the critical buckling load. The present investigation may be of some help in further understanding the mechanical properties of MWNTs.     

Measurements and model predictions of transient elongational rheology of polymeric nanocomposites

Transient elongational rheology of two commercial-grade polypropylene (PP) and the organoclay thermoplastic nanocomposites is investigated. A specifically designed fixture consisting of two drums (SER Universal Testing Platform) mounted on a TA Instruments ARES rotational rheometer was used to measure the transient uniaxial extensional viscosity of both polypropylene and nanoclay/PP melts. The Hencky strain rate was varied from 0.001 to 2 s^− 1, and the temperature was fixed at 180°C. The measurements show that the steady-state elongational viscosity was reached at the measured Hencky strains for the polymer and for the nanocomposites. The addition of nanoclay particles to the polymer melt was found to increase the elongation viscosity principally at low strain rates. For example, at a deformation rate of 0.3 s^− 1, the steady-state elongation viscosity for polypropylene was 1.4 × 10⁴ Pa s which was raised to 2.8 × 10⁴ and 4.5 × 10⁴ Pa s after addition of 0.5 and 1.5 vol.% nanoclay, respectively. A mesoscopic rheological model originally developed to predict the motion of ellipsoid particles in viscoelastic media was modified based on the recent developments by Eslami and Grmela (Rheol Acta 47:399–415, 2008) to take into account the polymer chain reptation. We show that the orientation states of the particles and the rheological behavior of the layered particles/thermoplastic hybrids can be quantitatively explained by the proposed model.     

Rheological properties and percolation in suspensions of multiwalled carbon nanotubes in polycarbonate

This paper is concerned with several issues related to the rheological behavior of polycarbonate/multiwalled carbon nanotube nanocomposites. The composites were prepared by diluting a masterbatch of 15 wt.% nanotubes using melt-mixing method, and the dispersion was analyzed by SEM, TEM, and AFM techniques. To understand the percolated structure, the nanocomposites were characterized via a set of rheological, electrical, and thermal conductivity measurements. The rheological measurements revealed that the structure and properties were temperature dependent; the percolation threshold was significantly lower at higher temperature suggesting stronger nanotube interactions. The nanotube networks were also sensitive to the steady shear deformation particularly at high temperature. Following preshearing, the elastic modulus decreased markedly suggesting that the nanotubes became more rigid. These results were analyzed using simple models for suspensions of rod-like particles. Finally, the rheological, electrical, and thermal conductivity percolation thresholds were compared. As expected, the rheological threshold was smaller than the thermal and electrical threshold.     

Combined torsional buckling of multi-walled carbon nanotubes coupling with axial loading and radial pressures

This paper reports the results of an investigation on combined torsional buckling of multi-walled carbon nanotubes (MWNTs) under combined torque, axial loading and radial pressures based on the continuum mechanics model, which takes into account the effect of the van der Waals interaction between adjacent tubes. A buckling condition is derived for determining the critical buckling torque and associated buckling mode. In particular, for combined torsional buckling of double-walled carbon nanotubes, an explicit expression is obtained and some detailed results are demonstrated. According to the innermost radius-to-thickness ratio, MWNTs are classified into three types: thin, thick, and (almost) solid. Numerical results are worked out for the critical buckling torque and associated buckling mode for all the three types of MWNTs subjected to various axial stresses (axial tensile stresses or axial compressive stresses), internal pressures, and external pressures. It is shown that, the axial tensile stress or the internal pressure will make the MWNTs resist higher critical buckling torque, while the axial compressive stress or external pressure will lead to a lower critical buckling torque. The effect of axial stress (axial tensile stress or axial compressive stress) on the critical buckling torque of MWNTs is very small for all the three types of MWNTs, while the effect of the internal pressure or external pressure is related to the types of MWNTs, which is strong for the thin MWNTs, moderate for the thick MWNTs, and small for the solid MWNTs. Numerical results also indicate that, the associated buckling mode is unique and dependent on the structure of MWNTs. Especially, for combined torsional buckling of MWNTs with very small axial stress and radial pressures, the buckling mode is just the one for the corresponding pure torsional buckling.