【作者】 许阳光；

【导师】 龚兴龙； 宣守虎；

【作者基本信息】 中国科学技术大学 ， 固体力学， 2014， 博士

【摘要】 磁流变塑性体是一类将微米级软磁性颗粒均匀分散在塑性高聚物基体中制备而成的新型磁敏智能软材料。这种材料在无外加磁场条件下类似于橡皮泥,可以被改变成任意形状且当外力撤去后可以保持其形状不变。由于高聚物基体的塑性特征,磁流变塑性体克服了传统的磁流变液的颗粒沉降问题,同时继承了磁流变液中磁性颗粒可移动的特性。当施加外部磁场后,材料内部的磁性颗粒在磁场力作用下可以沿着磁场方向取向排列,直至形成稳定的具备链状(或柱状)微结构的各向异性材料,且这种链状或柱状颗粒微结构还会沿着新的磁场方向重新排列。当撤去外部磁场之后,链状(或柱状)颗粒微结构可以继续保持在基体中,这又类似于磁流变弹性体中颗粒链可以被固化在基体中的特性。磁流变塑性体的出现丰富了磁流变材料的种类,填补了磁流变液和磁流变弹性体之间的空白。磁流变塑性体内部灵活的磁致颗粒取向排列的过程可以极大地改变其磁流变性能,也使其成为研究磁流变机理的理想材料。对磁流变塑性体的磁流变性能和磁致阻抗性能的表征有助于深入地理解磁流变材料微结构演化和宏观物理性能之间的联系,也为这类材料的优化设计打下了基础。此外,在对磁流变塑性体进行系统的表征中还发现了许多独特的物理性能,如磁控阻尼性能,高磁敏性,灵活的颗粒自组织性和优异的自愈性等,这使得磁流变塑性体在缓冲器,阻尼器,传感器等领域具备很大的应用潜力。为了了解磁流变塑性体动态力学性能的微结构依赖性,在振荡剪切模式下对磁流变塑性体采用应变幅值扫描和频率扫描。扫描过程中发现了分别由应变幅值和振荡频率引起的非线性现象,并对不同的非线性机理进行了讨论。讨论结果认为,应变幅值引起的非线性是由于微结构被破坏引起的。有意思的是,虽然微结构的破坏会立即对动态力学性能产生影响,但却不会马上导致应变幅值引起的非线性的产生。当应变幅值超过0.1%时,储能模量随应变幅值增加而急剧减小,但当应变幅值超过1%时,明显的非线性才会产生。与角频率的平方成正比的惯性项会导致频率非线性的产生。这种非线性只有当振荡频率大于21.54Hz时从应力应变曲线中才能观察到。当确定了磁流变塑性体的线性粘弹性区间后,我们对其在振荡剪切模式下的磁流变性能进行了系统地表征。颗粒含量为80wt%的各向异性磁流变材料表现出优异的磁控动态力学性能：最高磁致模量达到6.54MPa；相对磁流变效应为532%；相比之前报道的磁流变弹性体,这种材料具备更高的磁流变性能。通过调节外部磁场损耗因子可以从1左右降低到0.03。此外我们还发现磁流变塑性体的动态力学性能受溶剂的影响很大,其在无磁场下物理状态也可以通过调节溶剂的含量实现从类固态向类液态转变。通过分析添加不同含量溶剂的磁流变塑性体之间磁流变性能的巨大差异和磁场作用下颗粒在不同基体中的运动情况对深入地理解颗粒和基体之间的力磁耦合机理并促进类固态磁流变胶的应用都非常有价值。研究磁流变塑性体在剪切模式下的蠕变回复行为可以深入分析其在常应力作用下的变形机理。实验结果说明磁场和温度对磁流变塑性体的时间相关力学性能有很大的影响。特别地,磁流变塑性体的蠕变应变在930mT磁场条件下随着温度的增加而降低,这种现象与无场条件下得出的结论相反。我们提出一个假设来解释磁流变塑性体这种有趣的温度效应。此外我们还发现不同的颗粒分布的磁流变塑性体的蠕变回复曲线也表现出很大的差别,这从另一个角度反映了磁流变塑性体的颗粒预结构化过程。之前的研究表明,挤压增强效应会极大地增加磁流变液的屈服应力,研究磁流变材料在挤压模式下的力学性能对于磁流变机理的研究和实际应用都具有很大的意义。然而目前尚未有关于磁流变胶在挤压模式下的力学性能研究的报道。基于此,我们对磁流变塑性体的挤压流动行为(包括准静态压缩拉伸行为和振荡挤压行为)进行了系统地实验研究。结果发现准静态压缩拉伸过程都可以被分成弹性变形,应力松弛和塑性流动三个区间。实验结果表明压缩屈服应力和拉伸屈服应力对磁场,颗粒分布和颗粒含量都十分敏感。由于高分子基体的存在,磁流变塑性体在挤压模式下的屈服应力要高于磁流变液。我们发现磁流变塑性体在振荡挤压模式下的滞回曲线是不对称的,这种不对称性来自于拉伸和压缩行为之间的差异。另外,磁流变塑性体振荡挤压行为也受磁场和颗粒含量的影响,但是颗粒分布对其滞回曲线的影响并不明显。电化学阻抗谱方法也是研究材料微结构演化的有效手段,于是我们还用阻抗谱方法研究了磁流变塑性体的磁致微结构机理。通过比较两种不同颗粒分布(各向同性和各向异性)的磁流变塑性体的阻抗谱,提出了一个平衡电路模型来分析不同的阻抗响应。我们发现各向异性的磁流变塑性体的阻抗谱对磁场十分敏感。在磁场作用下,电子扩散效应将被抑制。进一步地,磁流变塑性体在磁场作用下的导电行为为基体中弹性的存在提供了直接证据。另外,我们还研究了颗粒链方向对不同颗粒含量的各向异性磁流变塑性体的导电性的影响。以此为基础,我们发展了一种等效方法来定量地表征磁流变塑性体的各向异性程度。更多还原

【Abstract】 Magnetorheological plastomer (MRP) is a kind of novel magneto-sensitive smart soft materials by evenly dispersing micrometer sized soft magnetic particles into plastic polymer matrix. MRP is similar to the plasticene in the absence of magnetic field, which can be changed to various shapes and the shapes can be retained after removing the external force. Due to the plasticity of polymer matrix, MRP overcomes the particle settling problem and inherits the moveable characteristics of magnetic particles in conventional MR fluids. When an external magnetic field is applied to MRP, the magnetic particles can move to form chain-like (or column-like) microstructures along with the magnetic field direction, which makes MRP change from isotropic to anisotropic. The particle chains (or columns) will rearrange driven by the magnetic force if the magnetic field direction is changed. After the magnetic field is removed, the chain-like (or column-like) microstructures can still retain in the matrix, which is analogous to the characteristics of MR elastomers that the particle chains can be solidified in the matrix. This flexible particle alignment process can greatly change the magnetorheological performance of MRP, which is very important to the investigation on the magnetorheological mechanism.The apearence of MRP enriches the varieties of MR materials, and fills in the gap between MR fluids and MR elastomers. The flexible rearrangement process of particles induced by magnetic field in MRP can ont only change the MR performance, but also make it an ideal material to investigate the MR mechanism. The characterizations on MR properties and magneto-induced impedance properties of MRP are helpful for further understanding the relashionship between microstructure evolution of MR materials and physical properties, in the meantime, laying the foundation of optimal design of MR materials. Furthermore, a lot of unique properties, such as magneto-controllable damping property, high magneto-sensitivity, flexible particle self-assembling property, and excellent self-healing capacity, and so on, have been found when systematically characterizing on MRP, which may enable MRP possesses great application protential in the areas of absorber, damper, and sensor.To fully understand the microstructure dependent dynamic mechanical properties of MRP, the strain amplitude sweep and oscillatory frequency sweep were applied to MRP under oscillatory shear rheometry. Nonlinear phenomena induced by strain amplitude and oscillatory frequency were found and the mechanisms for different nonlinearity were discussed, respectively. It is believed that the strain-dependent nonlinearity is attributed to the destruction of microstructure. Interestingly, the destruction of microstructure will not cause strain-dependent nonlinearity immediately, though the dynamic properties are very sensitive to the microstructure alteration of MRP. When the strain amplitude exceeds to0.1%, the storage modulus shows a sharply decreasing trend with the increasing of strain amplitude. However, the nonlinearity appears after the strain amplitude is increased to1%. The inertia item which is proportional to the square of angular frequency will result in frequency-dependent nonlinearity and this nonlinearity can only be found from the strain-stress hysteresis loops when the oscillatory frequency is larger than21.54Hz. After the linear viscoelastic (LVE) range is determined, the magnetorheological properties of MRP under oscillatory shear rheometry were systematically characterized. The anisotropic MRP with80%particle weight fraction shows excellent magneto-controllable dynamic mechanical performance:the maximum magneto-induced storage modulus is6.54MPa, the relative MR effect reaches as high as532%, and the loss factor can be changed from1to0.03by adjusting external magnetic field. We also found that the dynamic mechanical properties of MRP are greatly influenced by the solvent that added in the PU matrix, the physical state of MRP in the absence of magnetic field can also be easily switched from solid-like to liquid-like by adjusting the solvent content. By analyzing the huge differences in magnetorheological performance of MRP with different solvent content and the movements of iron particles in different polymer matrixes under the external magnetic field are valuable for thoroughly understanding the mechanical-magnetic coupling mechanism between magnetic particles and polymer matrix and promoting the application of MR gels. The deformation mechanism under constant shear stress can be further understood by studying the creep and recovery behaviors of MRP. The experimental results suggested that the time-dependent mechanical properties of MRP are highly influenced by the magnetic field and temperature. Especially, the creep strain of MRP trends to decrease with increasing temperature under a930mT magnetic field and this phenomenon is opposite to the results obtained in the absence of magnetic field. Finally, a hypothesis was proposed to explain the interesting temperature effect on the creep behaviors of MRP. In addition, it is found that great discrepancies were presented in creep curves for isotropic and anisotropic MRP, which must be ascribed to the different particle distributions. Previous researches indicated squeeze-strengthen effect will greatly enhance the yield stress of MR fluids. The investigation on the mechanical properties of MR materials under squeeze mode is valuable for the study of MR mechanism and practical applications. However, there is no report about the mechanical behaviors of solid-like MR gels under squeeze mode have been found. Therefore, the squeeze flow behaviors (including compressive, tensile, and oscillatory squeezing behaviors) of MRP are systematically investigated. Both compression and tension processes can be classified as elastic deformation region, stress relaxation region, and plastic flow region. The experimental results demonstrate that both yield compressive stress and yield tensile stress are sensitive to magnetic field, particle distribution, and particle concentration. The yield stress of MRP under squeeze mode is higher than that of MR fluids due to the existence of polymer matrix. Asymmetry of hysteresis loop is found under oscillatory squeeze mode and this asymmetry originates from the differences between compressive and tensile behaviors. The oscillatory squeeze behaviors of MRP are also influenced by magnetic field and particle concentration but the influence of particle distribution is not so obvious.Electrochemical impedance spectroscopy (EIS) is an effective method to investigate the microstructure evolution of materials, so an impedance spectroscopy (IS) method was employed to investigate the magneto-induced microstructure mechanism of MRP. The IS of MRP with two typical particle distributions (isotropic and anisotropic) were compared and an equivalent circuit model is proposed to analyze to different impedance responses. It was found that the IS of anisotropic MRP is quite sensitive to the magnetic field and the electron diffusion effect will be restricted in the presence of magnetic field. Furthermore, the conduction behavior of MRP in the presence of magnetic field reveals the existence of elasticity in the polymer matrix. The influence of particle chain direction on the conductivity of anisotropic MRP with different particle contents was also investigated. Based on the experimental results, an equivalent method is developed to quantitatively characterize the anisotropy of MRP.更多还原