Elastomeric Networks Research Articles

New Dielectric Elastomers with Variable Moduli

Dielectric elastomers have been widely investigated for muscle‐like soft actuators and capacitive sensors. Mechanical properties play a central role in the performances of the active material. Most elastomers have specific moduli pre‐determined by the polymers' molecular structures, which are not suitable for applications in changing working conditions as natural muscles are capable of. Here new dielectric elastomers are described exhibiting variable moduli controlled via thermal treatment. The elastomers contain furan‐maleimide Diels–Alder adduct moieties to administer the crosslinking densities of the elastomeric networks via reversible Diels–Alder/retro‐Diels–Alder cycloaddition reaction, resulting in changes in the elastomers' moduli. One of the synthesized elastomers has moduli that can be controlled between 0.17 and 0.52 MPa incrementally and reversibly. Capacitive strain sensors based on this elastomer can be operated in both rigid and soft modes to achieve variable sensing response up to 30% linear strain. Actuators were fabricated and operated in both high strain mode (35% actuation area strain at 65 MV m−1) and high force output mode (0.55 MPa at 104 MV m−1). The elastomers can exhibit a range of stress–strain outputs in similar fashion as muscle.

Mechanics of mechanochemically responsive elastomers

Mechanochemically responsive (MCR) polymers have been synthesized by incorporating mechanophores – molecules whose chemical reactions are triggered by mechanical force – into conventional polymer networks. Deformation of the MCR polymers applies force on the mechanophores and triggers their reactions, which manifest as phenomena such as changing colors, varying fluorescence and releasing molecules. While the activation of most existing MCR polymers requires irreversible plastic deformation or fracture of the polymers, we covalently coupled mechanophores into the backbone chains of elastomer networks, achieving MCR elastomers that can be repeatedly activated over multiple cycles of large and reversible deformations. This paper reports a microphysical model of MCR elastomers, which quantitatively captures the interplay between the macroscopic deformation of the MCR elastomers and the reversible activation of mechanophores on polymer chains with non-uniform lengths. Our model consistently predicts both the stress–strain behaviors and the color or fluorescence variation of the MCR elastomers under large deformations. We quantitatively explain that MCR elastomers with time-independent stress–strain behaviors can give time-dependent variation of color or fluorescence due to the kinetics of mechanophore activation and that MCR elastomers with different chain-length distributions can exhibit similar stress–strain behaviors but very different colors or fluorescence. Implementing the model into ABAQUS subroutine further demonstrates our model's capability in guiding the design of MCR elastomeric devices for applications such as large-strain imaging and color and fluorescence displays.

Self-healing dynamic bond-based rubbers: understanding the mechanisms in ionomeric elastomer model systems.

While it is traditionally accepted that the chain interactions responsible for the elastic response in an elastomeric network are ideally permanent and instantaneously active, the ongoing investigation of self-healing materials reveals that the introduction of self-healing properties into elastomers requires high mechanical integrity under dynamic load conditions, while on long timescales (or at extended temperatures), the chain and bond dynamics must allow for an intrinsic repair of micro cracks occurring during operation and aging. Based on an acrylate-based amorphous ionomer model system with pendant carboxylate groups allowing the systematic variation of the composition and the nature of the counter ion, we demonstrate the interrelation between the morphological, thermal, and mechanical properties, and identify the prerequisites and tools for property adjustment and optimization of self-healing efficiency. While the ion fraction is directly related to the effective network density and elastic performance, the crossover frequency between viscous and elastic behavior is influenced by the nature of the counter ion. In order to achieve reliable elastic response and optimal damage repair, the ion fraction in these systems should be in the range of 5 mol% and the chain dynamics should be appropriate to allow for excellent self-healing behavior at moderate healing temperatures.

Automation Highlights from the Literature

Automation Highlights from the Literature

PEROXIDE CROSS-LINKING OF EPDMs HAVING HIGH FRACTIONS OF ETHYLENIC UNITS

ABSTRACT The considerable amount of research in the literature has practically allowed the elucidation of the mechanism of peroxide cross-linking of ethylene–propylene–diene–monomer rubber (EPDM), which occurs through a radical chain reaction initiated by the thermal decomposition of the peroxide molecule. According to this radical chain reaction, all types of labile hydrocarbon bonds (i.e., allylic, methynic, and methylenic CH bonds) would be exposed to alkoxy radicals and involved in the formation of the elastomeric network. However, for high fractions of ethylenic units (typically ≥60 mol.%), simple chemical kinetics and thermochemical analyses have shown that the radical attack would essentially occur on the methylenic CH bonds. Starting from this assertion, a simplified mechanistic scheme has been proposed for the three commercial EPDMs under study. The corresponding kinetic model, derived from this new scheme by using the basic concepts of the chemical kinetics, provides access to the changes in concentration of the main reactive chemical functions (against exposure time), among which are double bonds and changes in cross-linking density. The validity of these predictions has been eventually successfully verified by five distinct analytical techniques frequently used for studying the cross-linking of rubbers.

Preparation and Charaterization of Poly(1,4–Butanediol-Citrate) Elastomers and CdTe/Poly(1,4–Butanediol-Citrate) Composite Bioelastomers

Poly (1, 4–butanediol-citrate) elastomer (PBC) network polyester bioelastomer and high fluorescent CdTe/PBC nanocomposite bioelastomer were obtained by melting polycondensation with citric acid and monomer 1, 4-Butylene glycol as monomers. The structures and properties of the products were characterized by FTIR,DSC and fluorescent spectra, etc. It shows that the glass transition temperature of the PBC and CdTe/PBC were all lower than 0°C. Both PBC and CdTe/PBC were amorphous, transparent and highly flexible. They also have good hydrophilicity, degradation, and stability in dimensions. The biodegradation and water uptake of elastomers PBC and CdTe/PBC could be modulated by adjusting the monomers ratio and post-polymerization time. CdTe/PBC has high fluorescence and is expected to be useful in soft tissue engineering with the function of fluorescence biomarker.

New semi-biodegradable materials from semi-interpenetrated networks of poly(ϵ-caprolactone) and poly(ethyl acrylate).

Semi-degradable materials may have many applications. Here poly(ethyl acrylate) and poly(ϵ-caprolactone) were combined as semi-interpenetrated networks, and thoroughly characterized in terms of final composition, interactions between components, wettability, and mechanical properties. PCL modulates the mechanical properties of the PEA elastomeric network. Cultures of fibroblasts and adipose-tissue derived stem cells showed excellent biological performance of the materials. The results are relevant for applications seeking materials leaving a permanent supporting skeleton after the partial degradation, as in patches for cardiac regeneration or in abdominal wall meshes.

Novel elastomeric fibrous networks produced from poly(xylitol sebacate)2:5 by core/shell electrospinning: Fabrication and mechanical properties

Fabrication of nonlinear elastic materials that resemble biological tissues remains a challenge in biomaterials research. Here, a new fabrication protocol to produce elastomeric fibrous scaffolds was established, using the core/shell electrospinning technique. A prepolymer of poly(xylitol sebacate) with a 2:5mol ratio of xylitol:sebacic acid (PXS2:5) was first formulated, then co-electrospun with polyvinyl alcohol (PVA – 95,000Mw). After cross-linking of core polymer PXS2:5, the PVA shells were rinsed off in water, leaving a porous elastomeric network of PXS2:5 fibres. Under aqueous conditions, the PXS2:5 fibrous scaffolds exhibited stable, nonlinear J-shaped stress–strain curves, with large average rupture elongation (76%) and Young׳s modulus (~1.0MPa), which were in the range of muscle tissue. Rupture elongation of PXS2:5 was also much higher when electrospun, compared to 2D solid sheets (45%). In direct contact with cell monolayers under physiological conditions, PXS2:5 scaffolds were as biocompatible as those made of poly-l-lactic acid (PLLA), with improvements over culture medium alone. In conclusion, the newly developed porous PXS2:5 scaffolds show tissue-like mechanical properties and excellent biocompatibility, making them very promising for bioengineering of soft tissues and organs.

Two-way shape memory polymer with “switch–spring” composition by interpenetrating polymer network

In the present work, a novel molecular strategy was employed to fabricate a two-way shape memory polymer (2WSMP), which is an interpenetrating network (IPN) with elastomeric and crystalline components leading to a “switch–spring” composition. In this 2WSMP the switch, a PCL crystalline network, is responsible for the reversible shape shrinkage. While the spring, the compressed PTMEG elastomeric network, provides the push force for shape expansion. This IPN polymer has the desired two-way shape effect (2WSME) as proved by DMA.

Highly Stretchable and Sensitive Strain Sensor Based on Silver Nanowire–Elastomer Nanocomposite

The demand for flexible and wearable electronic devices is increasing due to their facile interaction with human body. Flexible, stretchable and wearable sensors can be easily mounted on clothing or directly attached onto the body. Especially, highly stretchable and sensitive strain sensors are needed for the human motion detection. Here, we report highly flexible, stretchable and sensitive strain sensors based on the nanocomposite of silver nanowire (AgNW) network and PDMS elastomer in the form of the sandwich structure (i.e., AgNW thin film embedded between two layers of PDMS). The AgNW network-elastomer nanocomposite based strain sensors show strong piezoresistivity with tunable gauge factors in the ranges of 2 to 14 and a high stretchability up to 70%. We demonstrate the applicability of our high performance strain sensors by fabricating a glove integrated with five strain sensors for the motion detection of fingers and control of an avatar in the virtual environment.

Polymer mechanochemistry: Flex, release and repeat.

Force-induced covalent bond changes in mechanophore-linked polymers typically require large, irreversible material deformation, limiting successive activation cycles. Now, repeated force-induced reactions have been achieved by incorporating flex-activated mechanophores into elastomeric networks.

Molecular modeling approach to the prediction of mechanical properties of silica‐reinforced rubbers

ABSTRACTRecently, we have suggested a nanomechanical model for dissipative loss in filled elastomer networks in the context of the Payne effect. The mechanism is based on a total interfiller particle force exhibiting an intermittent loop, due to the combination of short‐range repulsion and dispersion forces with a long‐range elastic attraction. The sum of these forces leads, under external strain, to a spontaneous instability of “bonds” between the aggregates in a filler network and attendant energy dissipation. Here, we use molecular dynamics simulations to obtain chemically realistic forces between surface modified silica particles. The latter are combined with the above model to estimate the loss modulus and the low strain storage modulus in elastomers containing the aforementioned filler‐compatibilizer systems. The model is compared to experimental dynamic moduli of silica filled rubbers. We find good agreement between the model predictions and the experiments as function of the compatibilizer's molecular structure and its bulk concentration. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40806.

Sawtooth Tensile Response of Model Semiflexible and Block Copolymer Elastomers

We study via coarse-grained molecular modeling the elastic response of semiflexible elastomer networks with idealized diamond connectivity. Under strain-driven uniaxial deformation, these networks are found to exhibit first a perfect soft elastic behavior (including a buckling instability preceding hardening) up to moderate extension ratios (<2.9), followed by a sawtooth-shaped stress–strain curve for larger deformations at the nanoscale. This unique tensile response follows from a deformation mechanism that entails the successive creation and distortion of ordered (smectic CA) chain domains in concert with cross-link segregation and layering; such events arise from the proclivity of the chains to align (due to the chain stiffness) and to disintersperse (due to the network regularity and lack of trapped entanglements). Multiple segregation states hence occur for different strains, producing multiple minima in the stress. These regular networks also exhibit marked anisotropic elastic behavior and shape-mem...

Synthesis of Poly(Dimethylsiloxane) Networks Functionalized with Imidazole or Benzimidazole for Copper(II) Removal from Water

Functionalized elastomeric networks were obtained via a polycondensation reaction between poly(dimethylsiloxane) (PDMS) containing Si(CH3)2OH as end groups and an imidazole- or benzimidazole-modified alkoxysilane. The structure of the polymeric materials was characterized via attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and solid-state 13C and 29Si NMR spectroscopy, which indicated the presence of silsesquioxane units that acted as nodes on the PDMS chains and linked the functional groups. Thermogravimetric analysis (TGA) of the product indicated high thermal stability with an initial weight-loss temperature around 450 K. The capacity for the removal of copper(II) from aqueous solution was estimated from adsorption isotherms, yielding values of 35.74 and 66.09 mg g-1, respectively, for benzimidazole-and imidazole-functionalized materials. The major adsorption models were evaluated to fit the removal data of copper(II) and the model elaborated by Sips was determined to provide the best fit for both prepared materials.

Adsorption of cadmium ions on thiol or sulfonic-functionalized poly(dimethylsiloxane) networks

Thiol or sulfonic-functionalized poly(dimethylsiloxane) elastomeric networks were prepared from 3-mercaptopropyltrimethoxysilane or the oxidized silane. The characterization of these materials using infrared spectroscopy (IR) and 13C/29Si nuclear magnetic resonance (NMR) suggested that the materials were functionalized with the aforementioned groups and that the networks are composed of linear segments of PDMS crosslinked by nodes of silsesquioxanes, which contain T3 and/or T2 type silicon. The thermogravimetric analysis of the polymeric networks revealed that they exhibit good thermal stability. The adsorption capacities for cadmium ions in ethanolic solutions were 0.33 and 0.89mmolg−1 for the thiol- and sulfonic-functionalized PDMS networks, respectively. In water solution the material with thiol group do not adsorbed Cd (II), however, the network with sulfonic group adsorbed 0.70mmolg−1. In ethanol, the linearization of the adsorption isotherms revealed that the Langmuir model describes the interaction between the adsorbate–adsorbent, in water, the Freundlich adsorption model described the metal adsorption for the sulfonic-functionalized PDMS.

Influence of cross-linker and tin (II) bis-2-ethylhexanoate on compression set characteristics of poly(dimethylsiloxane) elastomer networks

We have investigated the influence of cross-linker and catalyst loading on the compression set properties of poly(dimethylsiloxane) (PDMS) networks. Solid PDMS networks were prepared from the condensation reaction between hydroxyl-terminated PDMS resins and tetraethoxysilane cross-linker whilst in the presence of tin(II) bis-2-ethylhexanoate catalyst. The relationship between cross-linker loading and mechanical properties was assessed using uniaxial tension, solvent swelling and compression set testing. Two series of networks were prepared with either high or low catalyst loading. Solvent swelling studies highlight the influence of excess catalyst on the polymer network structure and the contribution of residual 2-ethylhexanoic acid to compression set. Networks prepared with lower catalyst loadings displayed increased cross-link density, improved tensile strength and improved compression set resistance. Gas-phase Fourier Transform-Infra Red (FTIR) analysis of the head space produced during heating the PDMS network in a sealed environment indicates that a minimum post-cure regime of 72 h at 115 °C under light vacuum is required to remove the condensation bi-products from network synthesis. Thermogravimetric Analysis-Mass Spectrometry (TGA-MS) combined with Gas Chromatography-Mass Spectrometry (GC-MS) of extracts in toluene show the decomposition of 2-ethylhexanoic acid occurs during high temperature thermal treatment which affords networks with improved compression set resistance.

Water-Swelling Rubber Containing Small Amount of Nanofillers with Enhanced Water Swelling Durability

A series of water-swelling rubbers (WSR) were prepared by directly mixing room temperature vulcanization-2 components silicone rubber (RTV-2 SR) with superabsorbent polymer (SAP) and nanofiller (s) (halloysite nanotube or/and CaCO3). The effects of component on its mechanical behaviours and water-absorbent properties such as degree of swelling, swelling ratio, and weight loss ratio were investigated. The equilibrium swelling ratio increased with increasing amounts of water-absorbent resin. Well-dispersed nanofiller (s) increased the mechanical properties of WSR and prevented SAP breaking off from the elastomer network to effectively enhance the water-swelling durability of WSR.

Tactile Feedback Display with Spatial and Temporal Resolutions

We report the electronic recording of the touch contact and pressure using an active matrix pressure sensor array made of transparent zinc oxide thin-film transistors and tactile feedback display using an array of diaphragm actuators made of an interpenetrating polymer elastomer network. Digital replay, editing and manipulation of the recorded touch events were demonstrated with both spatial and temporal resolutions. Analog reproduction of the force is also shown possible using the polymer actuators, despite of the high driving voltage. The ability to record, store, edit, and replay touch information adds an additional dimension to digital technologies and extends the capabilities of modern information exchange with the potential to revolutionize physical learning, social networking, e-commerce, robotics, gaming, medical and military applications.

Tribological properties and morphology of bimodal elastomeric nitrile butadiene rubber networks

Bimodal nitrile butadiene rubber (NBR) was examined in this study. The molecular structure was determined by dynamic mechanical analysis and transmission electron microscopy. The relationship between the structure and the mechanical properties related to elastomeric tribological properties was investigated. The properties and the mechanisms of friction and wear of bimodal elastomeric NBR networks were also analyzed. The lubricating characteristics of bimodal NBR networks were revealed based on the mechanisms of friction and wear. Results show that bimodal NBR networks are similar to bimodal polydimethylsiloxane networks. The form and density of the network structure can be controlled from elastomeric networks to thermosetting resin networks. The mechanical properties of bimodal NBR networks, such as elasticity, elongation at break, fatigue characteristic, tensile strength, elastic modulus, and thermal stability can be precisely controlled following the variation in network structure. The friction, wear, and lubrication of bimodal NBR networks can be clearly described according to the principles of tribology. Common elastomers cannot simultaneously reduce friction and wear because of the different mechanisms of friction and wear; however, bimodal elastomer networks can efficiently address this problem.

All-silicone prestrain-locked interpenetrating polymer network elastomers: free-standing silicone artificial muscles with improved performance and robustness

We present a novel all-silicone prestrain-locked interpenetrating polymer network (all-S-IPN) elastomer for use as a muscle-like actuator. The elastomer is fabricated using a combination of two silicones: a soft room temperature vulcanizing (RTV) silicone that serves as the host elastomer matrix, and a more rigid high temperature vulcanizing (HTV) silicone that acts to preserve the prestrain in the host network. In our novel S-IPN fabrication procedure we co-dissolve the RTV and HTV silicones in a common solvent, cast thin films, and allow the RTV silicone to cure before applying prestrain and finally curing the HTV silicone to lock in the prestrain. The free-standing prestrain-locked silicones show a performance improvement over standard free-standing silicone films, with a linear strain of 25% and an area strain of 45% when tested in a diaphragm configuration. We show that the process can also be used to improve electrode adhesion and stability as well as improve the interlayer adhesion in multilayer actuators. We demonstrate that, when coupled with carbon nanotube electrodes, fault-tolerance through self-clearing can be observed. We use the fault-tolerance and improved interlayer adhesion to demonstrate stable long-life (>30 000 cycles at >20% strain) actuation and repeated high-performance actuation (>500 cycles at ∼40% strain) of prestrained free-standing multilayer actuators driving a load.