Getting smarter - advances in smart elastomers
James Busfield at Queen Mary University of London and James Adams at the University of Surrey, both in the UK, describe advances in smart elastomers.
Some types of smart elastomers have been known for some time. These include materials such as magneto-rheological elastomers, where properties of stiffness or damping can be modified or controlled by appying a magnetic field, or piezoresistive electric elastomers where the strain in the sample can be measured by examining the change in the electrical conductivity.
Magneto-rheological elastomers typically contain shaped metallic fillers that can be orientated through the magnetic field. Piezoresistive materials have conducting fillers at a volume fraction close to the percolation threshold – when the filler forms a continuous conducting network. Strain application can distort this network and can then be used as a load or pressure sensor. Although the capabilities of smart elastomers have been demonstrated in scientific literature, their uptake is not yet exploited. The technology is developing rapidly and it is anticipated that manufacturers will want to use them in standard components such as bearings and seals for additional functionality at little cost. The potential is that parts such as a seal can also detect leaks, or a bridge bearing can detect an overload situation. Over the next decade, industries such as automotive or aerospace will start to demand more functionality from existing elastomer components without imposing a weight or cost penalty.
In December 2009, the Rubber in Engineering Group of The Polymer Society of IOM3, organised a technical meeting on smart elastomers.
Adam Graham from Peratech Ltd, Brompton-on-Swale, UK, described a quantum tunnelling composite (QTC) made of a conducting metal incorporated in an insulating polymer. Here the metal particles have a highly fractal surface morphology (see graph above) within an elastomeric polymer matrix. This QTC demonstrates piezoresistive behaviour, showing a high resistance in the unstrained state which reduces sharply under mechanical loading (see graph below). When loaded the flow of additional current results in heating which is sufficient to allow the sample to expand, with the potential application of creating a switch or current limiting device.
Work undertaken at Queen Mary University of London, UK, was discussed by Martyn Bennett from Artis, Melksham, UK. He described a piezoresistive elastomer where a composite is made from rubber filled with a conducting carbon black filler.
Earlier work using similar composites had shown that when carbon black is at a volume fraction around the percolation threshold, the resistivity changes with strain. However, the precise resistivity versus strain behaviour is non-linear and irreversible for conventional carbon black fillers. A measuring device, deriving strain directly from a measure of the resistivity, requires that the behaviour is reversible and reproducible from cycle to cycle. This has been achieved by replacing the conventional carbon blacks with another of a higher surface area, such as Printex XE2. For the first time, reversible electrical resistivity dependence with strain is reported for a carbon black-filled elastomer. Unlike the Peratech materials, this type of composite displays a marked increase in resistance with strain. This behaviour offers the possibility of a flexible load, pressure or switch sensor.
Professor Peter Foot from Kingston University, UK, demonstrated a different approach by blending a conductive polymer with a conventional rubber material to create a conductive large strain polymer with potential for strain sensing or other smart applications.
Liquid crystal elastomers (LCEs) were discussed by James Adams from the University of Surrey, UK. On a molecular level liquid crystals (LCs) consist of rigid rod-like molecules that have random orientations at high temperature, but form the aligned nematic phase by aligning in a common direction below the isotropic-nematic transition temperature. In this phase transition, the aligned rods gain translational entropy at the expense of their orientational entropy. In LC displays, the alignment direction of the rods is manipulated using an electric field.
Liquid crystal phase transition can be converted into a mechanical response by connecting the rod-like molecules to a polymer network to form an LCE. The polymer network is sufficiently mobile on a microscopic level for the rod-like molecules to change their orientation. At high temperatures the rods are isotropically oriented, and the polymer backbone is on average spherical. When the elastomer is cooled, the LC rods align and cause the polymer backbone to adopt a cigar shape. Macroscopically, the rubber expands on cooling and contracts on heating by as much as several hundred per cent. This shape change is completely reversible.
An alternate stimulus for an actuator could be light, and this has been achieved in LCEs by incorporating dye molecules into the rubber. The molecules are rod shaped, but when illuminated by the correct wavelength of light, their conformation changes from a rod-like state (with a trans-double bond) to a kinked rod. These will disorder the LC alignment the same way as heating. This has led to rubber photo-actuators that may prove beneficial as microscopic fluid pumps in devices such as a lab-on-a-chip.
The LCEs use the nematic LC phase, and were first made by Heino Finkelmann. They have been the focus of theoretical and experimental research. However, there are many different phases of LCEs.
Some rod-like molecules arrange into layers on cooling, creating the smectic LC phase. They are technologically significant because some smectic phases have a low enough symmetry to have a permanent electric dipole moment, and can be manipulated with an electric field. The smectic layers are difficult to compress and have a high elastic modulus.
In smectic LCEs, rods in the layered phase are crosslinked to a polymer network. They are elastically interesting because of the embedded smectic layers. The rods deform almost like a 2D rubber when stretched perpendicular to the layer normal, and show threshold behaviour when stretched parallel to the layer normal. This is due to buckling of the smectic layers, which is similar to other layered materials such as geological rock strata, but in this case, the layers are on the nanometre scale. If they are stretched parallel to the layer normal, they buckle and the system softens. This is a reversible transition. However, there is the potential for actuation using an electric field, which has also been demonstrated. The degrees of freedom obtained with the rod-like molecules in LCEs make them different from conventional rubbers, and potentially important in high-tech applications.
Further information: James Busfield
These talks were presented at a meeting of the Rubber in Engineering Group of The Polymer Society, December 2009, London.