Seals are a component in a lubricant system composed of an elastomer such as silicones, nitriles, fluoroelastomers, polyurethanes and EPDM. They ensure that lubricants remain within machinery while minimizing the presence of contaminants such as dust, dirt and water that can adversely affect performance.

In a previous TLT article, a new thermally conductive elastomer known as Thubber was discussed (1). This material represents an advancement because it exhibited 25 times the thermal conductivity compared to other elastomers. Thubber was prepared by suspending liquid metal droplets in a soft and highly deformable silicone elastomer. It may be useful in helping to dissipate heat from a lubricant system.

In developing new materials, researchers often look to emulate what is occurring in nature. One interesting example is the reptile known as the chameleon, which has the ability to change color depending upon a specific environmental factor. Another interesting aspect is that the chameleon’s skin stiffens rapidly with (during) deformation to prevent tearing.

Sergei Sheiko, George A. Bush Jr. Distinguished Professor of Chemistry at the University of North Carolina in Chapel Hill, N.C., says, “Many living species such as humans, chameleons and amphibians have skin that act in a manner of an elastomer. Upon stretching, the very soft skin exhibits intense strain-stiffening by two to three orders of magnitude, which makes breaking the skin nearly impossible.”

Further, structural coloration may occur due to the coherent scattering of light. Sheiko says, “Chameleons have the ability to change color because their skin microstructure can cause the diffraction of light leading to changes in constructive interference upon deformation.”

The skin properties are achieved in nature through the blending of two proteins: collagen and elastin. Collagen acts to resist deformation while elastin provides elasticity.

If a synthetic elastomer can be developed with this combination of tissue-like softness and strain-stiffening, then potential biomedical applications such as medical implants could become much more effective. The current types of polymeric materials available have not been able to exhibit both properties.

Sheiko says, “Working with polymeric materials is much more advantageous than monomers because the architecture of the material can be varied from linear to star-like shapes without changing the chemistry or the molecular weight. Previous attempts to use specific silicone rubbers and polymeric gels to generate tissue mimics that change color have not worked because these materials did not exhibit the desired strain-stiffening characteristics.”

Efforts to produce this elastomer through the use of polymer blends and conventional block-copolymers also has not proven to be effective. Sheiko says, “Mixing polymeric materials such as polybutadiene and polystyrene do not produce a desirable result because linear polymers are neither soft nor strain-stiffening.”

A new approach is needed to produce an elastomer to emulate the chameleon’s skin. Such a polymer has now been developed.

Triblock copolymers
Sheiko and his colleagues have developed an elastomer that changes color when stressed through the preparation of an ABA triblock copolymer where the A groups are linear polymer chains and the B group has the appearance of a bottlebrush. Polymers used in the A group include polymethylmethacry-late, polybenzyl methacrylate and poly [oligo (ethylene glycol) monomethyl ether methacrylate]. The bottlebrush segment of the triblock copolymer was prepared with polydimethysiloxane.

The triblock copolymer forms through a self-assembly process (see Figure 2). Sheiko says, “The bottlebrush segment represents approximately 90% of the polymer, while the linear A chains account for the remaining 10%. Though the blocks are immiscible, the fact that they are connected chemically through covalent bonds enables the triblock copolymer to act as both an elastomer and to change color when stretched.”

Figure 2. An ABA triblock copolymer, formed through a self-assembly process, changes color when stressed and may be useful as a seal material in the future. (Figure courtesy of the University of North Carolina.)

Each block component contributed a different characteristic to the overall copolymer. The bottlebrush segment was stiff yet provides bulk softness while the linear chains were flexible while yielding rigid materials.

The researchers evaluated the mechanical properties of the triblock copolymer though generation of stress-strain curves. Differential scanning calorimetry and atomic force microscopy were used to characterize the structure of the triblock copolymer.

A color change for the triblock copolymer is conducted in a similar manner to the chameleon skin through the diffraction of light between block phases. As the elastomer is stretched, the researchers observed a color change.

Sheiko says, “With the human population aging, we predict that the triblock copolymer will be quite valuable in biomedical devices. One of our future objectives will be to develop a triblock copolymer for which we can independently control the elastomeric properties and the stimulus needed to cause a color change.”

There might be a possibility to use this type of technology as a seal in a lubricant system. If the elastomer displays the proper mechanical properties, a color change may be used as a sensor to inform a maintenance engineer that the system is having a problem due to some factor such as contamination that could adversely affect the life of the lubricant. Such a visible warning could make this seal a valuable tool to predict when maintenance is required.

Additional information on the triblock copolymer can be found in a recent publication (2) or by contacting Sheiko at sergei@email.unc.edu.
REFERENCES
1. Canter, N. (2017), “Thubber: A new thermally conductive elastomer,” TLT, 73 (6), pp. 18-19.
2. Varnosfaderani, M., Keith, A., Cong, Y., Liang, H., Rosenthal, M., Sztucki, M., Clair, C., Magonov, S., Ivanov, D., Dobrynin, A. and Sheiko, S. (2018), “Chameleon-like elastomers with molecularly encoded strain-adaptive stiffening and coloration,” Science, 359 (6383), pp. 1509-1513.