Liquid Crystal Silicone May be Key to Self-Healing Materials

Silicone is a resilient and useful material, returning to its original shape even after being squeezed. Yet now, researchers have found another use for it. They've discovered that during its liquid crystal phase, silicone becomes 90 percent stiffer when compressed. The findings could lead to new strategies for self-healing materials.

The liquid crystal phase of silicone is somewhere between a solid and liquid state. This makes it useful for many different applications. In previous studies, Rice graduate student Brent Carey subjected a nano-infused polymer to process called repetitive dynamic compression. This process involved compressing the material 3.5 million times over a week. It turned out that, surprisingly, the material became tougher-just like muscles after a workout.

In order to see whether this same process would work on other materials, researchers decided to use liquid crystal silicones without nanotubes. Silicones are actually made of long, flexible chains that are knotted together like a bowl of spaghetti. In conventional silicones, the chains are randomly oriented. Yet the class of silicones that the researchers studied possessed chains that organize themselves into rod-shaped coils.

Before testing this material, the researchers chemically attached liquid crystal molecules, similar to the ones used in LCD displays, to the silicones. They then dynamically compressed the material for 16 hours in order to see if it would show the same results as the previous experiment.

It turned out the material not only became stiffer, but also retained its new shape for weeks after the experiment.

"The molecules in a liquid crystal elastomer are like rods that want to point in a particular direction," said Rafael Verduzco, a Rice polymer scientist, in a news release. "In the starting sample, the rods are randomly oriented; but when the material is deformed, they rotate and eventually end up pointing in the same direction. This is what gives rise to the stiffening."

The researchers didn't only find that the material stiffened, though. They also discovered that samples that were heated slipped out of the liquid crystal phase and did not stiffen, which means the effect is reversible.

The discovery could mean that silicones, which are biocompatible, could be used for tissue engineering in the future. Soft tissues in the body, such as cartilage, need to maintain strength under repeated compression and deformation. It seems like the unique property of liquid crystal elastomers is perfect for the job.

The findings are published in the journal Nature Communications.