Behavior Of Elastomers Research Articles

Effect of isotropy and anisotropy: toward understanding the fracture behavior of magnetoactive elastomers

Magnetoactive elastomers (MAEs) exhibit magneto-mechanical coupling and typically contain micron-sized spherical ferromagnetic particles embedded in an elastomeric matrix. Anisotropic particle arrangement in MAEs, achieved by curing the material in a magnetic field, offers tailored and enhanced magneto-mechanical coupling compared to isotropic configurations. While tunable MAE properties such as the effective modulus and vibrational response have been relatively well studied, a thorough understanding of their fracture mechanisms, particularly in MAEs with microstructural anisotropy or composites with similar structures remains almost completely unexplored. Here, we characterize the fracture mechanisms of anisotropic and isotropic unmagnetized MAEs using experiments and simulations, focusing on the effects of spherical particles in anisotropic chain-like configurations. Specifically, we experimentally measure the fracture toughness of MAEs containing different volume fractions of particles, and with particles arranged both randomly and aligned at 0°, 45°, and 90° to the loading direction. Results show that anisotropic MAEs with particle chains aligned with the load direction can improve fracture toughness by up to almost 600%, whereas isotropic MAEs increase the fracture toughness by only 420%, compared to the pure elastomer. Scanning electron microscopy of the post-fractured surface reveals toughening mechanisms at micro-scale, such as chain waviness, particle agglomeration, and particle distribution. Simulations using a finite element method-based decoupled phase field-cohesive zone model on simplified geometries qualitatively support imaging interpretations. Overall, this work explains how internal geometry, including waviness in particle chains, chain alignment, and particle agglomerations affects fracture of MAEs and generally soft composites with chain-like geometries of spherical particles. These results have engineering applications in improving fracture properties of smart soft composites relevant to soft robotics, tunable vibration absorbers, and noise attenuators.

Phenomenological constitutive laws for the dissipative behaviour of highly compressible elastomers and their finite element implementation

This paper deals with the development of constitutive equations to model the mechanical behaviour of compressible elastomers. These materials are naturally incompressible but can be made compressible by the addition of hollow microspheres, for example. Such a material is referred to as syntactic foam. The CEA (French Commission for Atomic and Alternative Energies) employs such compressible materials as seals in complex structures to reduce the internal stresses in vulnerable components and prevent their failure. The behaviour of these structures is predicted by finite element simulations. It is important to know and model the mechanical behaviour of the seals. Like elastomers, they can undergo large deformations. The microspheres enable the material to undergo large volume change unlike pure elastomer that is nearly incompressible. This compressibility also intensifies dissipative phenomena encountered in elastomers such as viscosity or plasticity. Furthermore, the Mullins stress softening effect is also intensified even for loadings that only bring about volumetric changes. To model these behaviours, a phenomenological approach was developed based on the isochoric/volumetric decomposition of the deformation gradient. The method of intermediate dissipative configurations was employed to introduce multiple phenomena, including viscosity (with several characteristic times) and viscoplasticity, for these two parts of the deformation. The constitutive equations and their flow rules were implemented in Abaqus through a UMAT subroutine and using a numerical approach to define the tangent operator. The parameters of the behaviour law were identified using a model reduction technique known as shape manifold approach. The resulting model can be compared with experimental data.

Effect of ionic liquid on soft epoxy-amine electroactuators

Energy conversion represents a challenge in various fields of applications such as soft robotics or microfluidics technologies, particularly in terms of electromechanical coupling for actuators. The dielectric elastomers are a variety of electroactive polymers (EAP) which require high electric fields to be used as actuators. It is well-known that the presence of ionic impurities can have an influence of their electro-mechanical response under high electric field (i.e. above 1 MV/m). More precisely, it can be responsible for the bending of the sample under constant electric field. Here, we investigate the impact of a small content, i.e. from 0.1 wt% to 10 wt% of an imidazolium Ionic Liquid (IL) on the electromechanical response of a soft epoxy-amine network. The interest of this study therefore lies in its position at the frontier between dielectric polymers and ionic polymers. Dielectric spectroscopy revealed a significant increase of electric conductivity (≈2 orders of magnitude) when adding only 0.1 wt% of IL and up to 4 orders of magnitude with 10 wt% of IL at T = 20 °C for f = 10 Hz. It also evidenced the presence of the electrode polarization for all the samples doped with IL. The bending test carried out at E = 0.1 MV/m revealed no bending for pure epoxy-amine and a slow kinetics of bending for all the samples doped with IL with displacement evolving throughout the experiment (i.e. over 4 h). As evidenced by dielectric spectroscopy and bending tests, it is clear that the presence of a small quantity of ionic moieties (whether doping agent or impurity) can strongly modify the electromechanical behavior of elastomer commonly described as dielectric.

Effect of calcium silicate on foaming and anti‐shrinkage behavior of thermoplastic polyamide elastomers: Revelation of the open‐cell mechanism

AbstractThermoplastic polyamide elastomer (TPAE)/calcium silicate (CaSiO3) open‐cell foam with low shrinkage and high expansion ratio was prepared by using supercritical CO2 as a blowing agent. Adding CaSiO3 led to increased melt viscoelasticity and reduced crystallinity of TPAE. The primary mechanism of CaSiO3 promoting TPAE foam to form the open‐cell structure was discussed. The interaction between CaSiO3 and TPAE is fragile, making CaSiO3 easy to fall off the cell wall and form the open‐cell structure during foaming process. The increasing CaSiO3 content leads to increased cell density and thinned cell walls, thus increasing the tensile stress on cell wall. On the other hand, the agglomeration of CaSiO3 increases particle size, making it easier to fall off from the adhesion of TPAE matrix. The two aspects work together to improve the open‐cell content of TPAE foam up to 93.28%. The open‐cell structure is conducive to increasing the gas exchange rate and significantly decreasing foam shrinkage ratio. Finally, TPAE foam with a shrinkage ratio of only 2.68% and an expansion ratio of 18.77 times was obtained. In addition, open‐cell structure improves the adsorption performance of TPAE/CaSiO3 foam by 500%.

Effect of molecular and structural architecture of poly(epichlorohydrin‐co‐ethylene oxide‐co‐allyl glycidyl ether) on the properties of its elastomers

AbstractPoly(epichlorohydrin‐co‐ethylene oxide‐co‐allyl glycidyl ether) (GECO) elastomers have gained importance in recent years thanks to improved synthesis methods. The effect of curing agents, fillers, and ionizing radiation on the mechanical properties of GECO elastomers has been studied in detail, however, the effect of the molecular and structural architecture of GECO on their elastomers' properties is not well studied. The main aim of this study was to prepare GECO elastomers with a certain recipe and to examine the effects of molecular and structural parameters of these polymers on the cure characteristics, mechanical properties, temperature scanning stress relaxation, and damping properties. The results showed that among the three monomers, the ethylene oxide ratio was the more important parameter affecting the cure characteristics and cure degree of GECO elastomers. It was also observed that the energy absorption and damping properties vary with the ethylene oxide ratio, crosslink density, and long chain branching. The temperature scanning stress relaxation behavior of GECO elastomers was observed to have almost the same relative behavior, regardless of crosslink density and branching. The results suggest that the temperature‐induced relaxation behavior does not change with the type and ratio of the monomers that make up the GECO.

Dynamics and viscoelasticity of potato and corn starch bio-polymers reinforced with bentonite nanoclay

The influence of bentonite nanoclay on the viscoelasticity and dynamics of potato starch (PS) and corn starch (CS) films was investigated. Free standing films were plasticized with glycerol and displayed elastomeric behavior. The addition of bentonite (up to 4 wt%) increased the glass transition temperature Tg of PS/bentonite nanocomposites, however the Tg of CS/bentonite nanocomposites was reduced. On the other hand, bentonite increased the tensile modulus E’ of both type of nanocomposites. Dynamic mechanical analysis (DMA) showed that CS film displayed two mechanical relaxations, named α and α’, whereas PS film displayed only the α relaxation. The α relaxation was associated with the glass transition whereas the α’ relaxation appears to be associated to starch-rich domains due to lower miscibility of CS with glycerol. Rheology showed that bentonite increased the rubbery modulus Ec of the nanocomposites thus explaining the mechanical reinforcement. The starch nanocomposites exhibited exfoliated and intercalated morphologies (at low and higher bentonite content, respectively). Hence, nanoconfinement and domain separation appear to drive the thermo-mechanical properties and dynamics of the starch-based nanocomposites.

Flame Retardancy and Properties of Thermoplastic Vulcanizates from Dibutyl Phosphate-Bound Natural Rubber/Ethylene Vinyl Acetate Blends

ABSTRACT Dibutyl phosphate-bound epoxidized natural rubber (DBNR) was synthesized from natural rubber latex via epoxidation using a 20 mol% intermediate. Successful formation of epoxidized natural rubber (ENR-20) and DBNR was confirmed by FTIR analysis. Thermoplastic vulcanizates (TPVs) were prepared by dynamically vulcanizing blends of DBNR and ethylene-vinyl acetate copolymer (EVA). The 50/50 DBNR/EVA TPV exhibited higher mixing torque during vulcanization, indicative of increased shear stress, shear rate, and tensile strength. This was attributed to finer dispersion of vulcanized rubber domains in the EVA matrix, leading to enhanced interfacial adhesion. Elongation at break decreased with increasing EVA content, while hardness and tension set increased, suggesting reduced elasticity at higher EVA proportions. Under heat aging, TPVs experienced degradation of mechanical properties and increased tension set, indicating loss of elastomeric behavior. Significantly, DBNR exhibited a low burning rate of 0.2 mm/s, imparting inherent flame retardancy. The burning rate of DBNR/EVA TPVs decreased with increasing DBNR content, demonstrating improved flame retardancy. Overall, DBNR/EVA TPVs exhibited favorable mechanical properties and flame retardancy, making them suitable for applications requiring flame-retardant elastomeric materials.

Compressive behavior and visco-hyperelastic constitutive of polyurethane elastomer over a wide range of strain rates

Polyurethane elastomers (PUEs) will experience different strain rates in different application scenarios. Therefore, it is of great significance to study the mechanical properties of PUE under a wide range of strain rates and establish a constitutive model that considers strain rates with high accuracy and few parameters. In this study, the quasi-static and dynamic compression tests of two types of PUEs (PUE55 and PUE85) were carried out, and investigated the strain rate effect of the materials. Based on the Mooney-Rivlin hyperelastic model and the Prony series, a compressible visco-hyperelastic constitutive model for PUE was established. Different from the conventional constant relaxation time in Prony series, two relaxation times that vary exponentially with principal stretch were proposed based on the relaxation test to describe the strain rate effect of the material at low and high strain rate respectively. In addition, using the visco-hyperelastic constitutive model to obtain the model inputs of the Simplified rubber/foam model in LS-DYNA, the impact process of the Metal/PUE composite projectile was reproduced under different impact conditions through the finite element simulation. Simulation results verified the visco-hyperelastic model in generating numerical model material parameters and the rationality of the Simplified rubber/foam model in describing PUEs.

Fully Bio-Based Blends of Poly (Pentamethylene Furanoate) and Poly (Hexamethylene Furanoate) for Sustainable and Flexible Packaging.

In the present study, bio-based polymeric blends have been prepared for applications in the field of sustainable food packaging, starting from two furan-based homopolymers, poly(hexamethylene 2,5-furanoate) (PHF) and poly(pentamethylene 2,5-furanoate) (PPeF). PHF and PPeF were synthesized by two-step melt polycondensation-a solvent-free synthetic strategy-and then binary physical mixtures, PHF/PPeF, with different weight compositions were prepared by dissolution in a common solvent. The blends were processed into compression-moulded films, and molecular, morphological, structural, thermal, and mechanical characterizations were subsequently carried out. Blending did not negatively affect the thermal stability of the parent homopolymers, and good compatibility between them was observed. This strategy also allowed for the modulation of the chain rigidity as well as of the crystallinity, simply by acting on the relative weight amount of the homopolymers. From a mechanical point of view, the presence of PPeF led to a reduction in stiffness and an increase in the elongation at break, obtaining materials with an elastomeric behaviour. Evaluation of the gas barrier properties confirmed that the good barrier properties of PHF were preserved by blending. Finally, lab-scale composting tests confirmed a greater weight loss of the mixtures with respect to the PHF homopolymer.

Investigation on elastomer behaviour when exposed to conventional and sustainable aviation fuels

Investigation on elastomer behaviour when exposed to conventional and sustainable aviation fuels

A model for capturing the rate-dependent mechanical behaviour of liquid crystal elastomers

A modelling framework is adopted and adapted here to model the rate-dependent deformation behaviour of liquid crystal elastomers (LCEs). On using an isotropic hyperelastic strain energy function WI1,I2 of binomial form, previously employed for capturing the (quasi-static) deformation of polydomain LCEs, an extension is presented for application to the rate-dependent behaviour of LCEs by incorporating the deformation rate into the model parameters. That is, the model parameters of the core hyperelastic strain energy function W are considered to evolve with, i.e., be a function of, the deformation rate. A specific measure of deformation rate ɛ̇ is utilised, and a particular functional dependency of the hyperelastic model parameters on ɛ̇, of a skewed exponential form, is devised and presented. The ensuing rate-dependent model Wr is then applied to a range of extant experimental datasets, encompassing various LCE specimens and across different deformation rates, under uniaxial loading and including both tension and compression deformations. The model is shown to capture and predict the data favourably. Given the simplic of devising and implementing the presented modelling framework, the flexibility it provides for incorporating different choices of strain energy W function as well as the functional form(s) of the dependency of the model parameters on the deformation rate, and the accuracy of the modelling results, the application of this modelling approach to capturing the rate-dependent deformation behaviour of LCEs is hereby presented.

Vat photopolymerization of silica reinforced styrene-butadiene rubber elastomeric nanocomposites

Compared to their unfilled counterparts, elastomeric nanocomposites offer tailorable combinations of properties, including improved mechanical properties, increased durability, and enhanced thermal and electrical conductivity. However, the high viscosity caused by the presence of both the high molecular weight elastomer and the inclusion of fillers complicates additive manufacturing (AM) of elastomeric nanocomposites. This is especially challenging in vat photopolymerization (VP) AM, which requires resins that possess low viscosity (< 10 Pa·s) and exhibit sufficient storage modulus (∼ 104 Pa) when photocured. To address this process limitation, the authors have previously demonstrated decoupling the relationship between viscosity and molecular weight via VP of a photocurable composite latex resin. In this approach, a photocurable matrix is photocured as a scaffold around the latex particles, which provides sufficient modulus for the printed "green" part. Following a dehydration post-processing step, the latex particles coalesce to provide the final elastomeric mechanical properties without compromising feature resolution.In this work, the authors explore the effects of the addition of silica nanoparticles (10, 15, and 20 wt.%) on (i) the printability of photocurable styrene-butadiene rubber (SBR) latex resin and (ii) the mechanical properties of the resultant printed composites. The printable photocurable silica-SBR colloidal resin is realized by balancing the viscosity of the resin, the modulus of the cured green body, the silica nanoparticles content for reinforcement, and the high SBR content for elastomeric behavior. Specifically, the photocurable colloidal resin's viscosity is controlled by formulation design, while high silica nanoparticle content in the final part is achieved through evaporation of unreactive diluent. Characterization of the printed SBR nanocomposites with 20 wt.% silica reinforcement elucidated significant enhancements in the tensile strength (53 % increase), Young's modulus (671 % increase), and Shore A hardness (108 % increase) compared to printed SBR without filler reinforcement. To the authors' knowledge, this is the first report of 3D printing rubber nanocomposite using VP.

Data-Driven Identification unravels multiaxial mechanical response of a carbon-black filled elastomer during ageing

Under environmental exposure, the mechanical properties of elastomers change due to ageing, all while enduring mechanical service loading conditions. The influence of ageing on the multiaxial mechanical response of elastomers remains an understudied question, lacking exploration in both experimental evidence and modelling proposals. The present study describes an experimental/numerical approach to characterize the multiaxial behaviour of elastomers with consideration of ageing. This technique associates complex experimental tests conducted with a hexapod device, with a Data-Driven Identification (DDI) algorithm. Practically, heterogeneous strain fields are measured by Digital Image Correlation (DIC), and the corresponding stress and energy fields are calculated by DDI. These fields are visualized through three-dimensional maps, encompassing kinematical quantities and strain energy density. These maps convincingly capture the stiffening induced by ageing, in different deformation modes. Finally, the coupling between ageing and multiaxiality is foregathered in a material database that can be fitted for further modelling purposes.

Poly(alkylene 2,4-furanoate)s: The potential of structural isomerism for outstanding sustainable food packaging and unexpected evidence of self-healing microstructure

2,5-furandicarboxylic acid is an extremely appealing renewable chemical building block because of its potential to replace the petrochemical and industrially widespread terephthalic acid via the synthesis of poly(alkylene 2,5-furanoate)s (2,5-PAF). The recent interest in its structural isomer, 2,4-furandicarboxylic acid (2,4-FDCA), opened the study of poly(alkylene 2,4-furanoate)s (2,4-PAF). In this work, 2,4-FDCA was polymerized with linear glycols of increasing chain length, via a solvent-free polycondensation reaction, obtaining high molecular weight 2,4-PAF. Namely, poly(trimethylene 2,4-furanoate) (2,4-PTF), poly(pentamethylene 2,4-furanoate) (2,4-PPeF) and poly(hexamethylene 2,4-furanoate) (2,4-PHF). These polyesters were compression molded into films and subjected to NMR, GPC, WAXS, PLOM, TGA and DSC analyses. The functional properties for food packaging applications were evaluated by mechanical and gas permeability tests. 2,4-PAF had tunable mechanical properties, depending on the glycol used, and in some cases, the mechanical behavior of a thermoplastic elastomer and shape recovery after break. In particular, 2,4-PPeF had outstanding gas barrier properties, while DSC analyses on 2,4-PHF showed an endothermic phenomenon attributed to the isotropization of a partially-ordered phase: it was possible to demonstrate that this phase was disrupted during tensile tests and slowly recovered over time, at room temperature. Overall, the results offer new insights into the structure-property relationships of poly(alkylene 2,4-furanoate)s and display their great potential for the production of biobased, monomaterial, easily recyclable and sustainable food packaging.

Versatile data-adaptive hyperelastic energy functions for soft materials

Applications of soft materials are customarily linked to complex deformation scenarios and material nonlinearities. In the bioengineering field, soft materials typically mimic the low stiffness of biological matter subjected to extreme deformations. Computational frameworks surge as a versatile tool to assist the design of functional applications. The constitutive model lies at the core of such frameworks. In this regard, the customary extreme non-linear behavior of elastomers poses an additional challenge to thoroughly capture the material behavior. Here, data-driven methodologies hold considerable promise for enhancing constitutive modeling when contrasted with phenomenological approaches. In this investigation, we introduce a versatile data-adaptive method tailored to the modeling of hyperelastic soft materials at finite strains. Specifically, our method substitutes an a priori chosen model for the strain energy function by a flexible interpolant defined on a discretized invariant space. Within this framework, the interpolation values assume the role of material parameters and are determined through finite element model updating to conform to measured experimental data — comprising full-field displacements coming from Digital-Image-Correlation and global reaction forces. We validate the method on uniaxial experimental tests of soft elastomers, encompassing ELASTOSILTM, DOWSILTM, and V HBTM. Overall, we aim to establish a new route for the construction of hyperelastic energy functions, untethered from any predefined existing models or assumptions regarding the shape of the energy.

Modelling of Static and Dynamic Elastomer Friction in Dry Conditions

Understanding the tribological behavior of elastomers in dry conditions is essential for sealing applications, as dry contact may occur even in lubricated conditions due to local dewetting. In recent decades, Persson and co-authors have developed a comprehensive theory for rubber contact mechanics and dry friction. In this work, their model is implemented and extended, particularly by including static friction based on the bond population model by Juvekar and coworkers. Validation experiments are performed using a tribometer over a wide range of materials, temperatures and speeds. It is shown that the friction model presented in this work can predict the static and dynamic dry friction of various commercial rubber materials with different base polymers (FKM, EPDM and NBR) with an average accuracy of 10%. The model is then used to study the relevance of different elastomer friction contributions under various operating conditions and for different roughness of the counter surface. The present model will help in the development of novel optimized sealing solutions and provide a foundation for future modeling of lubricated elastomer friction.

Mechanical Behaviors of Copolymer Elastomers: Experimental Characterization and Constitutive Modeling

We combine experimental analysis and modeling to investigate the complex mechanical behaviors of copolymer elastomers. Experimental studies are conducted to analyze the impact of monomer ratio, strain rate, and loading mode on the response of copolymer elastomers, which reveal the distinct hyperelastic, viscoelastic, damage and self-recovery behaviors of the materials. Based on the experimental observations, we develop a finite deformation hyperelastic-viscoelastic constitutive model, which incorporates multiple discrete relaxation processes. For the hyperelastic process, an extended eight-chain model is employed. Different from the classic viscoelastic model, the strain hardening and damage/recovery mechanisms are also included in the relaxation processes. The model successfully describes the complex mechanical responses observed in elastomers, including quasi-static loading, strain rate effects, and stress relaxation phenomena. Additionally, the model captures the behavior of elastomers during loading-unloading cycles in a reasonable manner. The work offers an efficient theoretical tool for the design and practical application of elastomers.

A nonequilibrium constitutive equation for tensile behavior of polyolefinic thermoplastic vulcanizates

Polyolefinic thermoplastic vulcanizates (TPVs) composed of iPP (isotactic polypropylene) and crosslinked EPDM (ethylene-propylene-diene rubber) prepared by dynamic vulcanization process exhibit rubber domains dispersed in the iPP matrix, even though the EPDM component is the major component. Here we proposed a kinetic nonequilibrium constitutive equation that considers the transition of the mechanical nature from plasto-rigid deformation to rubbery deformation during uniaxial extension to explore the origin of the thermoplastic elastomer behavior of TPV (thermoplastic vulcanizate) materials. Fitting of the constitutive equation to the experimental stress‒strain curves quantitatively revealed the critical transition point of the mechanical properties. The transition shifted to higher strains with decreasing EPDM content. In situ wide-angle X-ray diffraction profiles monitored during the uniaxial extension confirmed the mechanical transition process.

Multiscale finite element modeling of origami-inspired dual matrix deployable composite with visco-hyperelastic hinge

Dual matrix deployable composites with viscoelastic materials are prone to experiencing relaxation in their stored strain energy, leading to potential degradation in their deployment performance. This study presents a multiscale FE modeling approach to investigate the folding and deployment behavior of origami inspired dual-matrix deployable composite with visco-hyperelastic hinge. A typical dual-matrix origami composite fold was manufactured using a hot compression molding technique and hand layup method and folding tests were carried out. The visco-hyperelastic behavior of elastomer used in the folding region was characterized. Experiments were carried out on elastomer under rate dependent loading, single-step, multi-step, and cyclic tests and material parameters for constitutive model were identified. The homogenized properties of hybrid laminate and elastomer laminate were obtained from XCT-driven FE based homogenization of real RVE. First, the multiscale FE model was validated with the experimentally-measured moment-curvature diagram. Next, the proposed multiscale FE model was employed to simulate the influence of modulus relaxation of pure elastomer on the deployment behavior of a simple rectangular fold and an origami-inspired waterbomb base structure. The variation of moment-curvature, and relaxation in strain energies during stowage under controlled and free deployment are reported. The effect of incomplete folding and stowage durations on strain energy and deployment time was investigated. In the end, the influence of the loading rate effect on the bending stiffness relaxation of pure elastomer and elastomer laminate fold was investigated.

Generalized continuum theory for nematic elastomers: Non-affine motion and characteristic behavior

We develop a physically-motivated mechanical theory for predicting the behavior of nematic elastomers — a subset of liquid crystal elastomers (LCEs). We begin with a statistical description of network geometry that naturally incorporates independent descriptors for the mesogens, which create the nematic phase, and the polymer chains, which are assumed to not deform affinely with global deformations. From here, we develop thermodynamically consistent constitutive laws based on classical continuum mechanics principles and ultimately provide simple governing equations with transparent physical interpretation. We found that our framework converges identically to two previously developed mechanical theories, including the well-known neo-classical theory, when considering the extreme ends of our parametric space. We then explore the new predictive capabilities of our model inside these two extremes and illustrate its unique predictions at finite strains, which are distinct in form from other theories. We validate our model using published experimental data from four monodomain nematic liquid crystal elastomers.