Preparation, Characterization,and Mechanism Of A Novel Magneto-sensitive Smart Soft Materials:Magentorheological Plastomers

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.