Particle-filled Elastomers Research Articles

Properties and special phenomena of strain sensors made of carbon particle-filled elastomers

Abstract Elastomers with a percolative network of carbon particles are a frequently studied class of materials for applications requiring high elongation and compliant sensors. For novel applications such as soft robots or smart textiles, these have some advantages over traditional strain gauges. However, their functionality is not fully understood. In this work, such materials are investigated as strain sensors in terms of their dynamic behavior, and their current limitations are demonstrated. It becomes clear that such sensors exhibit a non-monotonic behavior under dynamic loads that differs significantly from that observed in quasi-static tests. Two strategies for improving sensor characteristics are derived, modeled, and experimentally tested using the results and an electro-mechanical network model. First, a melt-spinning process that orients the carbon nanotube particles in the process direction creates different degrees of anisotropy. Second, to generate a local negative transverse contraction, an additional auxetic support structure is used. While the resulting anisotropy is insufficient to improve sensor properties, the auxetic structure can significantly improve strain sensitivity.

The dependence of rubber tackiness on separation speed

ABSTRACT In the present work, the tack properties of a number of uncrosslinked polymers, oil-extended rubbers and particle-filled elastomers to a metal surface were investigated by first contacting a cylindrical metal probe with a flat-ended surface with an elastomer surface and then detaching the probe from the adhered substrate at various separation speeds. Our results reveal that there are simple and universal laws governing the tackiness behavior of these materials. Specifically, when the pulling speed (Vs ) is greater than a critical speed (Vc ), the maximum tack force (Fmax ) can be generally described by the following scaling relationships: Fma x ~ Vs 1/3 and Fmax ~ tmax −1/2, where tmax is the time when the maximum force is reached in the force-time profile. The chemical composition, polymer structure, plasticizer type and filler type in a rubber compound can affect the magnitude of the tacky force at a given separation speed, but have little or no effect on these scaling relationships that primarily exist at high separation speeds.

Covalent Hybrid Elastomers Based on Anisotropic Magnetic Nanoparticles and Elastic Polymers

Particle-filled elastomers are ubiquitous composite materials with applications ranging from durable materials, such as tires, to smart actuators and sensor materials. By employing magnetic nanoparticles as multifunctional, inorganic cross-linkers, PDMS-based hybrid elastomers with a novel unique architecture and a defined type of particle-matrix interaction are obtained—a direct covalent coupling between magnetic and elastic properties. The resulting particle-cross-linked elastomers possess application potential due to their defined magnetomechanical coupling and their large extensibility. As the filler phase, spindle-shaped hematite nanoparticles with a silica shell are used, and the swelling, thermal, magnetic, and mechanical properties of the resulting particle-cross-linked elastomers are systematically evaluated and compared to the properties of analogous yet conventionally cross-linked particle-filled elastomers of a similar composition. Some unique features are found for the hybrid elastomers, such as a large strain at break of up to εB ≈ 1700%, that are attributed to the exceptional architecture, combining a well-integrated network of long polymer chains that are interconnected by network nodes with a high cross-linker functionality formed by the anisotropic magnetic nanoparticles.

The nonlinear elastic response of filled elastomers: Experiments vs. theory for the basic case of particulate fillers of micrometer size

Standard particulate fillers utilized to enhance the macroscopic elasticity of elastomers — most notably, carbon black and silica particles — are inherently of nanometer size. Because their size is comparable to the typical lengths of the polymer chain segments between crosslinks in the embedding elastomers, there is a plurality of microscopic mechanisms by which such fillers provide macroscopic enhancement. Well-known among these is the so-called hydrodynamic effect, which has long been thought to become increasingly dominant as the size of the fillers increases from nanometer to micrometer or larger length scales. This paper reports a combined experimental/theoretical investigation aimed at critically examining such a belief by isolating — that is, by excluding the presence of other enhancing mechanisms, such as interphases and occluded rubber — and quantifying the hydrodynamic effect behind the enhanced nonlinear elastic response of a prototypical class of particle-filled elastomers: polydimethylsiloxane (PDMS) featuring various cross-link densities filled with an isotropic distribution of glass spherical particles of monodisperse micrometer size. The close agreement found between the experiments for a variety of filled PDMS elastomers with the predictions based on recently developed rigorous homogenization results corroborate that the observed enhancement in the nonlinear elastic response of the PDMS elastomers upon the addition of the glass particles is indeed solely due to the hydrodynamic effect. In addition to filling a lacuna in the basic experimental knowledge of filled elastomers, the findings reported here also have direct practical implications on the modeling of a number of emerging active filled elastomers, such as for instance magnetorheological elastomers, wherein the iron filler particles — in contrast to standard fillers — are typically micrometer in size.

Volume growth during uniaxial tension of particle-filled elastomers at various temperatures – Experiments and modelling

A common presumption for elastomeric material behaviour is incompressibility, however, the inclusion of filler particles might give rise to matrix-particle decohesion and subsequent volume growth. In this article, the volumetric deformation accompanying uniaxial tension of particle-filled elastomeric materials at low temperatures is studied. An experimental set-up enabling full-field deformation measurements is outlined and novel data are reported on the significant volume growth accompanying uniaxial tension of two HNBR and one FKM compounds at temperatures of −18, 0, and 23 °C. The volumetric deformation was found to increase with reduced temperature for all compounds. To explain the observed dilatation, in situ scanning electron microscopy was used to inspect matrix-particle debonding occurring at the surface of the materials. A new constitutive model, combining the Bergström–Boyce visco-hyperelastic formulation with a Gurson flow potential function is outlined to account for the observed debonding effects in a numerical framework. The proposed model is shown to provide a good correspondence to the experimental data, including the volumetric response, for the tested FKM compound at all temperature levels.

An experimental and numerical study on the volume change of particle-filled elastomers in various loading modes

Laboratory tests show that there is a pronounced difference in the volumetric response between uniaxial tension and confined axial compression loading for commercial particle-filled hydrogenated nitrile butadiene rubber (HNBR) and fluoroelastomer (FKM) compounds. In uniaxial tension (UT), a volume increase of 5% and 20% for the HNBR and FKM respectively was present for a hydrostatic stress of less than 6 MPa, in addition, both compounds showed a clear hysteresis loop in the hydrostatic stress - volume ratio space. For confined axial compression (CAC) tests, on the other hand, the materials reached a 6–7% volume change for a hydrostatic stress of 140 MPa, and an elastic behavior was seen. This loading mode dependence of the volumetric response has severe implications for the constitutive representation of the materials. It is demonstrated that existing elastomer models, whereof many assume incompressible volumetric response, are unable to capture the behavior in both loading modes. To gain an increased understanding of the macroscopically obtained results, a tension in situ scanning electron microscopy study was performed. Matrix–particle debonding was observed to occur at the external surface of the materials, rendering a possible explanation for the loading mode dependent volumetric behavior. Finite element simulations of a single-particle model, incorporating a cylinder of matrix material with a spherical particle in its center, showed that the observed debonding can explain the experimental response of the materials in a qualitative manner.

Hugoniot-based equations of state for two filled EPDM rubbers

Particle-filled elastomers are commonly used as engineering components due to their ability to provide structural support via their elastic mechanical response. Even small amounts of particle fillers are known to increase the mechanical strength of elastomers due to polymer-filler interactions. In this work, the shock response of two filled (SiO2 or silica and KevlarTMfillers) ethylene-propylene-diene (EPDM) rubbers were studied using single and two-stage gas gun-driven plate impact experiments. Hugoniot states were determined using standard plate impact methods. Both filled-EPDM elastomers exhibit high compressibility under shock loading and have a response similar to adiprene rubber.

Novel elastic nanofibers of syndiotactic polypropylene obtained from electrospinning

Uniform nanofibers of syndiotactic polypropylene (sPP) with diameters between 140 and 350nm were successfully obtained from high-temperature solution electrospinning. Effects of sPP concentration and fiber-collecting method, either by a stationary plate or by a rotating disc, were investigated. The internal structure of the as-spun nanofibers was characterized via wide-angle X-ray diffraction (WAXD), small-angle scattering (SAXS), Fourier transform-infrared (FT-IR) spectroscopy, and differential scanning calorimetric (DSC) analysis. Based on the WAXD results, the as-spun nanofibers exhibited both all-trans mesophases and helical form I crystallites. Mesophase was the dominant phase of the sPP nanofibers obtained from the 5wt% solution, whereas form I was the dominant phase of those obtained from the 8wt% solution. For the more viscous solution, the enhancement of form I was attributed to the less effective stretching during processing, which induced oriented but amorphous chains in the sPP nanofibers deposited on the collector. Post crystallization of the un-crystallizing chains at ambient temperature that yielded more form I was evidenced by the in situ FT-IR probes. For the aligned nanofibers collected by a rotating disc, the amount of mesophase increased due to additional stretching force on the solidifying nanofibers. Under cyclic tensile deformation, the aligned sPP nanofibers exhibited low permanent set and intermediate hysteresis with a mechanical behavior similar to particle-filled elastomers.

Mullins effect recovery of a nanoparticle‐filled polymer

AbstractParticle‐filled elastomers often exhibit a reduction in peak stress after the initial extension under cyclic straining, a phenomenon known as the Mullins effect. In reported literature, long rest periods and elevated temperatures are often necessary conditions to even partially restore stiffness. The stress softening of the polymer nanoparticle composite discussed in this article appeared to be completely reversible in a comparatively short time span (less than 17 h) at room temperature. Although this material acted elastomeric at slow strain rates, significant stiffening, and a pronounced yielding behavior was observed during the first strain cycle at higher strain rates. Subsequent cycling of the material revealed much softer behavior and an absence of the yielding phenomenon, although relatively short rest periods allowed the material to regain its original behavior. The Mullins effect recovery phenomenon was investigated by introducing rest periods of various durations following loading and unloading cycles. Both single and multiple loadings were used in the tests conducted to evaluate recovery. A mechanism of stress recovery was proposed based on the reduction of entanglements and weakening particle‐matrix interactions. New entanglements and particle‐chain interactions were considered major contributors to the recovery of the stiffness. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

Homogenization of magnetostrictive particle-filled elastomers using an interface-enriched reproducing kernel particle method

A formulation is proposed for homogenization of magnetostrictive particle-filled elastomers (MPFE) based on an interface-enriched reproducing kernel particle method. A variational equation for obtaining the local fluctuating deformation of MPFE is introduced. The magnetostrictive effect in the metal inclusion is modeled as an eigen-deformation. An interface-enriched reproducing kernel approximation with embedded derivative discontinuities on the material interface is presented. This approach does not require additional degrees of freedom in the approximation of displacement field for the interface conditions compared to the conventional reproducing kernel approximation. Microscopic solution and homogenized constitutive behavior of uniaxial tension and simple shear deformation of MPFE are presented.

Micromechanics-based hyperelastic constitutive modeling of magnetostrictive particle-filled elastomers

An effective hyperelastic constitutive model is developed for particle-filled elastomer composites based on the microstructural deformation and physical mechanism of the magnetostrictive particles embedded in the hyperelastic elastomer matrix. Two types of loading conditions are considered––magnetic field and mechanical load. Magnetic eigenstrains are prescribed on the particles due to effect of magnetostriction. The effective constitutive relation of the composites during infinitesimal deformation can be established based on Eshelby's micromechanics approach. Since the elastomers normally exhibit finite hyperelastic deformation, the corresponding hyperelastic constitutive law of the composites is constructed in terms of the strain energy densities of the constituents.

Effects of Particle Interaction and Size Variation on Damage Evolution in Filled Elastomers

A micromechanical analysis of damage evolution (interfacial debonding) in particle-filled elastomers addresses the effect of the interactions between particles and of variation in filler size. The composite is treated as an assembly of two constituents in a finite-element model. It is shown that the interaction between particles controls the damage evolution: (1) For high volume fraction, a relatively small change in particle size has a surprisingly large effect on the local material response; (2) for large differences in particle sizes (e.g., bimodal distribution), damage occurs at interfaces between large particles and matrix, with limited damage occurring at small particles. While these effects of particle interaction and size variation are smoothed out in a large ensemble of particles, it is foreseeable that they are an important factor in a failure process such as macroscopic crack propagation, which spans scales considerably larger than the maximum particle size. Specifically, one thus expects that in the vicinity of a macroscopic crack the large particles become sites for small cracks which coalesce into larger ones and join up with the macro crack, while small particles operate primarily so as to locally stiffen the matrix without incurring significant damage in their vicinity.

Analysis of Interfacial Failure in Particle-Filled Elastomers

The evolution of microdamage (interfacial dewetting) in highly filled elastomers under consideration of high deformation gradients is examined. The interface between hard (rigid, two-dimensional) inclusions embedded in an elastomer characterized by a three-term Ogden (rate insensitive) model, and the elastomer matrix is represented by a cohesive-zone type interfacial model to follow the whole process of interfacial dewetting and its effect on the global (multiphase) material response in a plane strain setting. The analysis is carried out through a mixed finite element formulation for hyperelasticity, incorporating interface elements. We consider the effects of particle geometry and loading conditions on the process of interfacial failure. The results indicate that the distributed failure process is highly unstable and depends heavily on the size, shape, orientation and interactions of inclusions as well as the global loading conditions. The overall material behavior of the model agrees qualitatively with experimental observation.

Micellar morphology and its plastic deformation behaviour in ultra-high molecular weight polyethylene

Highly oriented samples from ultra-high molecular weight and normal high-density polyethylene (UHMWPE, HDPE) were prepared under the same experimental conditions. The morphology of the UHMWPE, investigated by transmission electron microscopy, was found to be oriented micellar, while the HDPE samples had a shishkebab morphology. Tensile test experiments under cyclic loading conditions exhibit a strain-hardening effect (increase in stress for the onset of plastic deformation) for HDPE, while the UHMWPE shows a decrease in Young's modulus, which is attributed to similar molecular mechanisms as for the Mullins effect in particle-filled elastomers.