ilicone elastomers are promising materials for dielectric elastomer transducers (DETs) due to their superior properties such as high efficiency, reliability and fast response times. DETs consist of thin elastomer films sandwiched between compliant electrodes, and they constitute an interesting class of transducer due to their inherent lightweight and potentially large strains. For the field to progress towards industrial implementation, a leap in material development is required, specifically targeting longer lifetime and higher energy densities to provide more efficient transduction at lower driving voltages. In this review, the current state of silicone elastomers for DETs is summarised and critically discussed, including commercial elastomers, composites, polymer blends, grafted elastomers and complex network structures. For future developments in the field it is essential that all aspects of the elastomer are taken into account, namely dielectric losses, lifetime and the very often ignored polymer network integrity and stability.

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The Current State of Silicone-Based Dielectric Elastomer Transducers.

Soft polymer networks have seen an explosion of recent developments motivated by new high tech applications in the biomedical field or in engineering. We present a candid and critical overview of the current understanding of the relation between the structure and molecular architecture of polymer networks and their mechanical properties, restricting ourselves to soft networks made of flexible polymers and displaying entropic elasticity. We specifically review and compare recent approaches to synthesize swollen hydrogels with enhanced toughness, resilient but tough unfilled elastomers and self-healing networks containing dynamic bonds. The purpose is less to draw a comprehensive catalogue of approaches than to identify and unify the underlying principles controlling toughening mechanisms and mechanical self-healing behavior and to point out remaining challenges.

50th Anniversary Perspective: Networks and Gels: Soft but Dynamic and Tough

In this review we highlight new developments in tough hydrogel materials in terms of their enhanced mechanical performance and their corresponding toughening mechanisms. These mechanically robust hydrogels have been developed over the past 10 years with many now showing mechanical properties comparable with those of natural tissues. By first reviewing the brittleness of conventional synthetic hydrogels, we introduce each new class of tough hydrogel: homogeneous gels, slip-link gels, double-network gels, nanocomposite gels and gels formed using poly-functional crosslinkers. In each case we provide a description of the fracture process that may be occurring. With the exception of double network gels where the enhanced toughness is quite well understood, these descriptions remain to be confirmed. We also introduce material property charts for conventional and tough synthetic hydrogels to illustrate the wide range of mechanical and swelling properties exhibited by these materials and to highlight links between these properties and the network topology. Finally, we provide some suggestions for further work particularly with regard to some unanswered questions and possible avenues for further enhancement of gel toughness.

Progress toward robust polymer hydrogels

Chemical modification of starch.

Establishing quantitative structure-property relationships for elastomeric materials requires independent information characterizing the network structure crucial to their rubberlike behavior. Such information can be obtained by using highly selective cross-linking reactions instead of the usual indiscriminate joining of chain segments (such as occurs in sulfur or peroxide cures or in high energy irradiations). The present review shows how such selectively prepared “model” elastomers can be used to elucidate the dependence of a variety of elastomeric properties on network structure.

The use of model polymer networks to elucidate molecular aspects of rubberlike elasticity

Elastomeric networks were prepared by tetrafunctionally end-linking mixtures of various proportions of relatively long and very short polydimethylsiloxane (PDMS) chains. The former had a number-average molecular weight of 18,500 and the latter either 660 or 220 g mole−1. The series of (unfilled) bimodal networks thus prepared were studied in elongation to the rupture point at 25°C, and in swelling equilibrium in benzene at room temperature. Elasticity constants characterizing the Gaussian regions of the stress–strain isotherms, and values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity. The isotherms also gave values of the elongation at which the modulus begins to increase anomalously because of limited chain extensibility, and values of the elongation and nominal stress at the point of rupture. These results were interpreted in terms of the known configurational characteristics of the constituent PDMS chains. Values of the energy or work required for rupture were used as an overall measure of the “toughness” of the networks. The very short chains were found to give a marked increase in toughness, through an increase in ultimate strength without the usual corresponding decrease in maximum extensibility. A variety of additional experiments will be required in order to elucidate the molecular origins of this important effect.

Model networks of end-linked polydimethylsiloxane chains. XI. Use of very short network chains to improve ultimate properties

End‐linking polymer chains by means of a multifunctional cross‐linking agent provides an ideal way for obtaining elastomeric networks of any desired distribution of chain lengths. In the present investigation, this technique was employed to give polydimethylsiloxane networks consisting of various proportions of relatively long and very short network chains. The stress–strain isotherms of these networks generally showed an anomalous increase in the modulus at high elongation, and the increase persisted at high temperatures and high degrees of swelling. This non‐Gaussian effect was quantitatively correlated with the limited extensibility of the network chains; specifcally, the increase in modulus was found to begin at approximately 60%–70% of the maximum extensibility of the network chains, and network rupture at 80%–90%. The elongation at which the increase becomes evident increases with decrease in the proportion of the very short chains, thus verifying the nonaffine nature of the deformation at high elongation. In addition, the elasticity constants characterizing the Gaussian regions of the isotherms, and the values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity, particularly with regard to the structure factor relating the modulus of an elastomer to the average length or molecular weight of the network chains.

Model networks of end‐linked polydimethylsiloxane chains. VII. Networks designed to demonstrate non‐Gaussian effects related to limited chain extensibility

Elastomeric networks were prepared by end-linking vinyl-terminated polydimethylsiloxane (PDMS) chains having number-average molecular weights of 11.3 × 103 g mol−1. The tetra-functional end-linking agent, Si[OSi(CH3)2H]4, was used in varying amounts smaller than that corresponding to a stoichiometric balance between its active hydrogen atoms and the chain vinyl groups. The number of dangling-chain irregularities thus introduced into the networks was directly determined by iodometric titration for unreacted vinyl groups. The (unfilled) PDMS networks thus obtained were studied in elongation to their rupture points at 25°C (a temperature sufficiently high to prevent complications from strain-induced crystallization), and in swelling equilibrium in benzene at room temperature. Small to moderately large proportions of dangling chains were found to have less of an effect on the elongation modulus than might be expected, and similarly a relatively small effect on the degree of equilibrium swelling. Most importantly, comparisons of constant values of the high deformation modulus show that dangling-chain irregularities decrease both the maximum extensibility of a network and its ultimate strength.

Model networks of end‐linked polydimethylsiloxane chains. XII. Dependence of ultimate properties on dangling‐chain irregularities

Model elastomeric networks were prepared by end‐linking hydroxyl‐terminated polydimethylsiloxane chains with a trialkoxy silane. Mixtures of various proportions of relatively long and very short chains were employed, since the resulting ’’bimodal’’ networks are of unique importance in characterizing non‐Gaussian effects related to limited chain extensibility. Stress–strain isotherms of these trifunctional networks gave values of the elongation at which the modulus begins to increase anomalously, and values of the elongation and modulus at the point of rupture. These results were interpreted in terms of calculated values of the maximum extensibilities of the chains, and were compared with previously reported results on the corresponding tetrafunctional networks in order to characterize the effects of crosslink functionality on these properties. In addition, the elasticity constants characterizing the Gaussian regions of the isotherms, and values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity, particularly with regard to how the elastic effectiveness of the network chains depends on chain length and extent of deformation.

Model networks of end‐linked polydimethylsiloxane chains. IX. Gaussian, non‐Gaussian, and ultimate properties of the trifunctional networks

Model networks were prepared by selective crosslinking through vinyl groups occurring as either chain ends or as side-groups on a poly(dimethylsiloxane) backbone. Iodometric titrations were used to determine the number of unreacted groups, thereby providing detailed information on the completeness of the reactions and the structure of the resulting networks. The end-linking reaction of the vinyl-terminated chains was generally found to be at least 95% complete. In the case of very high junction functionality, however, the extent of reaction was significantly lower, presumably because of steric interferences in the vicinity of the junctions, as was concluded in a previous investigation. Lower extents of reaction were also found in the case of vinyl groups located along the chains, probably because such groups are constrained by two chain sequences instead of one. The equilibrium elastomeric properties of both types of networks were interpreted using the structural information thus obtained and were found to be in good agreement with previous experimental results. They are also in satisfactory agreement with theory, without introduction of the highly questionable assumption of large contributions from interchain entanglements.

Chemical analysis of vinyl‐crosslinked poly(dimethylsiloxane) model networks and use of the resulting structural information in the interpretation of their elastomeric properties

M. A. Llorente

Papers

Model networks of end-linked polydimethylsiloxane chains. XI. Use of very short network chains to improve ultimate properties

Model networks of end‐linked polydimethylsiloxane chains. VII. Networks designed to demonstrate non‐Gaussian effects related to limited chain extensibility

Model networks of end‐linked polydimethylsiloxane chains. XII. Dependence of ultimate properties on dangling‐chain irregularities

Model networks of end‐linked polydimethylsiloxane chains. IX. Gaussian, non‐Gaussian, and ultimate properties of the trifunctional networks

Chemical analysis of vinyl‐crosslinked poly(dimethylsiloxane) model networks and use of the resulting structural information in the interpretation of their elastomeric properties