Weibull Analysis of Electrical Breakdown Strength as an Effective Means of Evaluating Elastomer Thin Film Quality

To realize the commercial potential of dielectric elastomers, reliable, large‐scale film production is required. Ensuring proper mixing and subsequently avoiding demixing after, for example, pumping and coating of elastomer premix in an online process is not facile. Weibull analysis of the electrical breakdown strength of dielectric elastomer films is shown to be an effective means of evaluating the film quality. The analysis is shown to be capable of distinguishing between proper and improper mixing schemes where similar analysis of ultimate mechanical properties fails to distinguish.

     

The stability of polysiloxanes incorporating nano-scale physical property modifiers

Abstract

Reported here is the synthesis and subsequent characterization of the physical and chemical properties of novel polysiloxane elastomers modified with a series of polyhedraloligomericsilsequioxane (POSS) molecular silicas. The physical properties of the formulated nanocomposite systems have been characterized with a combination of dynamic mechanical analysis (DMA), broadband dielectric spectroscopy (BDS) and confocal Raman microscopy. The results of the physical property characterization demonstrate that the incorporation of low levels (1–4% by wt.) of POSS particles into the polysiloxane network leads to significant improvements in the mechanical properties of the elastomer and significantly alters the motional chain dynamics of the system as a whole. The results of studies performed to assess the long-term stability of these novel nanocomposite systems have demonstrated that POSS physical property modifiers can significantly alter the thermal stability of polysiloxane elastomers. Physically dispersed POSS has also been shown in some cases to be both mobile and disruptive within the polysiloxane networks, agglomerating into domains on a micron scale and migrating to the surface of the elastomers. This work demonstrates both the potential of POSS nanoparticles as physical property modifiers and describes the effects of POSS on the physical and chemical stability of polysiloxane systems.     

Electromechanical instability in anisotropic dielectric elastomers

In this paper, we analyze the electromechanical instability of anisotropic dielectric elastomers. When an electric field and biaxial stress are applied to a dielectric elastomer, the homogeneous deformation of the dielectric elastomer may be unstable and pull-in instability and bifurcation instability may occur. Based on the previous investigations on the incompressible anisotropic elastic solids and dielectric elastomers, we outline the theory of anisotropic dielectrics. The electromechanical instability is considered for a thin layer of a dielectric elastomer sandwiched between two compliant electrodes. Analytic solutions are obtained for the classic neo-Hookean model of anisotropic materials. The results show that the stability of the dielectric elastomer is remarkably enhanced by anisotropy parameter.     

A Liquid‐Metal–Elastomer Nanocomposite for Stretchable Dielectric Materials

Stretchable high‐dielectric‐constant materials are crucial for electronic applications in emerging domains such as wearable computing and soft robotics. While previous efforts have shown promising materials architectures in the form of dielectric nano‐/microinclusions embedded in stretchable matrices, the limited mechanical compliance of these materials significantly limits their practical application as soft energy‐harvesting/storage transducers and actuators. Here, a class of liquid metal (LM)–elastomer nanocomposites is presented with elastic and dielectric properties that make them uniquely suited for applications in soft‐matter engineering. In particular, the role of droplet size is examined and it is found that embedding an elastomer with a polydisperse distribution of nanoscale LM inclusions can enhance its electrical permittivity without significantly degrading its elastic compliance, stretchability, or dielectric breakdown strength. In contrast, elastomers embedded with microscale droplets exhibit similar improvements in permittivity but a dramatic reduction in breakdown strength. The unique enabling properties and practicality of LM–elastomer nanocomposites for use in soft machines and electronics is demonstrated through enhancements in performance of a dielectric elastomer actuator and energy‐harvesting transducer.     

Dielectric elastomers – numerical modeling of nonlinear visco‐electroelasticity

Dielectric elastomer actuators that can directly turn electrical energy into mechanical energy belong to the group of electroactive polymers. This type of electroelastic material exhibits large displacement characteristics and is able to change its mechanical behavior in response to the application of an electric field. Dielectric actuators are made out of elastomers which in general show viscoelastic behavior. To take this time dependent effect into account, the deformation gradient is multiplicatively decomposed. The paper is focused on the numerical modeling of soft dielectric elastomers. The theoretical foundation and the consistent finite element implementation is outlined based on the laws of electricity and elasticity. Furthermore, numerical examples of the nonlinear visco‐electroelasticity model are shown. Copyright © 2012 John Wiley & Sons, Ltd.     

Electromechanical instability and snap-through instability of dielectric elastomers undergoing polarization saturation

By applying a voltage, electric charge will be induced on the surface of dielectric elastomers. Generally, the charge increases with the level of voltage. When the voltage reaches to a certain value, the charge would not increase any more due to the polarization saturation of dielectric materials. In this paper, a thermodynamic constitutive model, combined both the nonlinear dielectric and hyperelastic behavior as dielectric elastomers undergoing polarization saturation, has been developed. Analytical solutions have been obtained for situations incorporating strain-stiffening effect, electromechanical instability and snap-through instability. The numerical results reveal the marked influence of the extension and polarization saturation limits of elastomer material on its electromechanical instability and the snap-through instability. The developed constitutive model would be helpful in future research of dielectric elastomer based high-performance transducers.     

Dielectric constant enhancement in a silicone elastomer filled with lead magnesium niobate–lead titanate

Lead magnesium niobate–lead titanate (PMN–PT) ferroelectric powder was used to develop a particulated composite based on a silicone elastomer matrix, with improved dielectric permittivity. The filler was characterised by X-ray diffraction and scanning electron microscopy. Complex dielectric permittivity (10–10⁸ Hz) and tensile mechanical properties (elastic modulus and ultimate stress) of composites at various filler contents (up to 30% by vol.) were compared with those of the neat silicone elastomer. Both the dielectric constant and loss factor regularly increased with the filler content. The elastic modulus increased with a lower rate than that of the dielectric constant. Even though the addition of filler resulted in a detriment of both toughness, ultimate stress and elongation at break, a good stretchability was still retained, as elongation ratios greater than 3 were possible at the highest filler content. Several dielectric models were compared to the experimental data and the best match was achieved by the Bruggeman model, which can be used as a predictive rule for different volume contents of filler.     

Partially crystallized elastomer as a nanocomposite model

Data on the influence of crystallization on the mechanical properties of elastomers — the elastic modulus, the relaxation properties, in particular, restorability in compression, and the tensile strength — have been generalized. These data have been compared to those on the influence of active fillers and a much higher crystallization efficiency has been shown. The size of single crystals has been evaluated for most crystallizable rubbers. It has been inferred that the nanosize of single crystals of elastomers and their direct bond with the elastomer matrix influence the mechanical properties of elastomer materials. In considering a partially crystallized elastomer as a nanocomposite model, one can formulate requirements imposed on efficient nanofillers for elastomer materials. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 5, pp. 19–23, September–October, 2005.     

填充PPC/NBR共混弹性体研究(Ⅰ)加工条件和力学性能

本文研究了经碳黑填充、补强的聚丙撑碳酸酯（ＰＰＣ）／丁腈橡胶（ＮＢＲ）共混弹性体的加工条件和力学性能。发现加入ＰＰＣ能改善填充ＮＢＲ弹性体加工性，ＰＰＣ含量在１０％时填充弹性体力学强度存在最佳值。用马来酸酐改性能有效地提高填充共混弹性体耐油和热氧老化稳定性。     

Thermoelectromechanical stability of dielectric elastomers undergoing temperature variation

In this paper, the influence of both temperature and deformation on dielectric constant is considered during the establishment of free energy function of dielectric elastomers. A constitutive model of the thermodynamic systems undergoing adiabatic process is derived to study its thermoelectromechanical stability. The relations between different work conjugated parameters of dielectric elastomer are theoretically described, including the relations between nominal electric field and nominal electric displacement, entropy and temperature. Under different temperatures and electric fields, the allowable energy range of dielectric elastomer is calculated. Furthermore, the electric-induced variation of dielectric elastomer’s temperature and entropy is also studied under various principal planar stretch ratios. These simulation results should offer assistances in guiding the design and fabrication of excellent actuators featuring dielectric elastomers.     

Synthesis of cross-linked polyurethane elastomers with fluorescein linkages

The reaction of polyurethane prepolymers with fluorescein (free acid) and glycerin or castor oil-cross-linker has been performed in N,N-dimethylformamide and in such way a cross-linked elastomer material has been obtained. Polyurethane prepolymers were prepared by reaction of the poly(1,4-butane)diols (Terathane 1400) with aliphatic 1,6 hexamethylene diisocyanate. The above polymerization reactions lead to the formation of novel aromatic–aliphatic polyurethane elastomers having fluorescein moieties in the main chain. The resulting novel cross-linked polyurethanes have excellent mechanical properties, especially in the castor oil polyurethane elastomer formulations and show light-violet fluorescence property and could be used as photoactive polymers for labeling materials. These polyurethanes were characterized through IR, TGA, DSC, and mechanical analyses. Some aspects of the fluorescent emission in fluorescein-polyurethane elastomers were studied.     

基于氢键作用的超分子弹性体材料的研究进展

超分子弹性体是指在分子(包括小分子和大分子)间通过氢键、离子键、配位键等非共价键结合的,在常温下具有与传统弹性体材料类似高弹性和流变特性的一类新型材料,同时由于其特殊的分子键作用方式,超分子弹性体还具有与传统弹性体不同的特性,如可逆热塑性、自修补性、热敏性和良好的加工性等。文中综述了近年来国内外在氢键型超分子弹性体的合成、制备与表征等方面的研究进展,并指出其发展趋势。     

Electromechanically driven variable-focus lens based on transparent dielectric elastomer

Dielectric elastomers with low elastic stiffness and high dielectric constant are smart materials that produce large strains (up to 300%) and belong to the group of electroactive polymers. Dielectric elastomer actuators are made from films of dielectric elastomers coated on both sides with compliant electrode material. Poly(3,4-ethylenedioxythiophene) (PEDOT), which is known as a transparent conducting polymer, has been widely used as an interfacial layer or polymer electrode in polymer electronic devices. In this study, we propose the transparent dielectric elastomer as a material of actuator driving variable-focus lens system using PEDOT as a transparent electrode. The variable-focus lens module has light transmittance up to 70% and maximum displacement up to 450. When voltage is applied to the fabricated lens module, optical focal length is changed. We anticipate our research to be a starting point for new model of variable-focus lens system. This system could find applications in portable devices, such as digital cameras, camcorder, and cell phones.     

Characterisation and micromechanical modelling of the elasto-viscoplastic behavior of thermoplastic elastomers

The first objective of this study is to characterise the physico-chemical and mechanical properties of thermoplastic elastomers (TPE) and their constituents. In parallel with the experimental study, a model describing the mechanical behaviour of such materials at room temperature and without damage is proposed. The composite materials studied in the present work are processed by blending particles of vulcanized rubber ethylene-propylene-diene (EPDM) into an isotactic polypropylene (PP) matrix. These particles, obtained from a recycling process, have an average diameter of 70 $\mathrm{\mu }$m. The constitutive equation for TPE composites is developed within the framework of a self-consistent micromechanical approach which considers the mechanical behaviour of each phase. A preliminary analysis of various TPE in linear elasticity justifies the choice of a morphological pattern for the model, which views elastomer particles as embedded in the thermoplastic matrix. In the non-linear domain, the PP matrix is modelled by means of an elastoviscoplastic model whose parameters are fitted using tensile and instrumented spherical micro-indentation tests. The elastomer exhibits viscoelastic behaviour. Having determined the material parameters by inverse analysis, the proposed micromechanical model is compared with tensile and bending tests performed before damage initiation and for various elastomers contents.     

Ionomeric modification of metallocene-based polyolefinic elastomers with varied pendant chain length and its influence on physico-mechanical properties

A new class of ionomer was developed through sulfonation of the metallocene-based poly(ethylene–octene) elastomer, followed by its neutralization with zinc acetate. Ionomeric products were characterized through Fourier transform infrared (FT-IR) spectroscopy, small angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) studies. The effect of pendant chain length on the ionomeric modification was also studied through the variation of the comonomer. The impact of these ionomeric modifications on various physico-mechanical properties was thoroughly investigated by using differential scanning calorimetry, mechanical, dynamic mechanical, and thermogravimetric analysis. The resultant properties were correlated with structures of the modified elastomers. Ionomerisation of the base elastomers causes a significant improvement in the stress–strain, as well as the thermal properties compared to the corresponding pristine elastomer. Anjan Biswas is on leave from Government College of Engineering and Leather Technology, LB Block, Sector III, Saltlake City, Kolkata-700098, India.     

Preparation and properties of adjacency crosslinked polyurethane---urea elastomers

Adjacency crosslinked polyurethane--urea (PUU) elastomers with different crosslinking density were prepared by using hydroxyl-terminated liquid butadiene-nitrile (HTBN), toluene diisocyanate (TDI) and chain extender 3,5-dimethyl thio-toluene diamine (DMTDA) as raw materials, dicumyl peroxide (DCP) as initiator, and N,N'-m-phenylene dimaleimide (HVA-2) as the crosslinking agent. The influences of the crosslinking density and temperature on the structure and properties of such elastomers were investigated. The crosslinking density of PUU elastomer was tested by the NMR method. It is found that when the content of HVA-2 is 1.5%, the mechanical properties of polyurethane elastomer achieve optimal performance. By testing thermal performance of PUU, compared with linear PUU, the thermal stability of the elastomers has a marked improvement. With the addition of HVA-2, the loss factor tanδ decreases. FT-IR spectral studies of PUU elastomer at various temperatures were performed. From this study, heat-resistance polyurethane could be prepared, and the properties of PUU at high temperature could be improved obviously.     

Biocompatible composites of fibrous nanohydroxyapatite embedded in a polydimethylsiloxane

Silicone elastomers have the potential to be a valuable biomaterial due to their mechanical and chemical properties, easy processing, and high gas permeability. Some inherent properties of the pure silicone implant such as high hydrophobicity and low load bearing capacity can be problematic for biomedical applications. The issues were addressed by fabricating hydroxyapatite nanofiber/polydimethylsiloxane nanocomposites. The morphology of nanocomposite structures was visualized by high resolution transmission electron microscopy and field emission scanning electron microscopy. Improved mechanical strength and compliance of the prepared nanocomposite structures were obtained by frequency sweep and creep measurements. Surface hydrophilicity of polydimethylsiloxane was enhanced by hydroxyapatite nanofiber incorporation into the polymer matrix. The cytotoxicity and biocompatibility of the structures were analyzed using breast epithelial cells (MDA MB 231 cell line). These studies showed that the nanocomposite scaffold did not leach any cytotoxic material and showed better cell adhesion and cell proliferation compared to the unfilled elastomer.     

催化剂对异佛尔酮二异氰酸酯制备的脂肪族聚氨酯弹性体结构和性能的影响

采用一步法通过异佛尔酮二异氰酸酯与聚丙二醇和1,4-丁二醇反应合成了脂肪族聚氨酯（PU）弹性体。考察了催化剂的种类和含量对PU弹性体结构和性能的影响。结果表明,以辛酸亚锡为催化剂时,PU弹性体的软段相和硬段相间的相分离程度最明显且分子量最低,导致其力学性能最差;以辛酸铋为催化剂时,PU弹性体软段相和硬段相的相容性较好且...     

高强度自修复聚氨酯弹性体的制备及性能

针对自修复材料力学性能和自修复性难以兼顾的问题,采用传统预聚体法,引入含双硫结构的交联剂,制备得到既具有一定力学强度、又具有良好自修复性的聚氨酯弹性体。采用红外光谱进行化学结构表征;采用邵氏硬度计进行硬度测定;采用万能力学试验机考察了不同条件下的自修复效率;通过三维视频显微镜和拍照记录弹性体自修复过程;采用热重分析仪对样品进行热性能表征。结果表明,双硫键被成功引入弹性体中,弹性体邵氏硬度大多可达50 HA以上。升高温度和延长时间都能提高弹性体的自修复效率:24 h时,自修复效率从25 ℃的31.3%升高到80 ℃的99.5%;在80 ℃下,自修复效率从2 h的48.4 %提高到24 h的99.5%。双硫交联剂质量分数的增加也有利于自修复,弹性体的自修复效率从PUSS 3的54.5%提高到PUSS 6的99.5%。热重分析显示,弹性体的热稳定性随双硫质量分数的增加而略有下降,但所有弹性体的5%热失重温度都高于265.0 ℃。     

The Synergy of Hydrogen Bond and Entanglement of Elastomer Captures Unprecedented Flaw Insensitivity Rate

Elastomers are regarded as one of the best candidates for the matrix material of soft electronics, yet they are susceptible to fracture due to the inevitable flaws generated during applications. Introducing microstructures, sacrificial bonds, and sliding cross-linking has been recognized as an effective way to improve the flaw insensitivity rate (R_insen). However, these elastomers still prone to failure under tensile loads with the presence of even small flaws. Here, this work reports a polybutadiene elastomer with unprecedented R_insen via the synergy of hydrogen bond and entanglement. The resulting polybutadiene elastomer exhibits a R_insen ≈1.075, which is much higher than those of reported elastomers. By molecular chain interaction and molecular chain conformation analysis, this work demonstrates that the synergistic effect of hydrogen bond dissociation and entanglement slip in the polybutadiene elastomers during stretching leads to the high R_insen. Using polybutadiene elastomer as matrix of thermal interface materials, this work demonstrates effective heat transfer for strain sensor and electronic devices. In addition, cytocompatibility of the elastomers is verified by cell proliferation and live/dead viability assays. The combination of outstanding biocompatible and excellent mechanical properties of the elastomers creates new opportunities for their applications in electronic skin.