Silicone liquid crystal stiffens with repeated compression

ERJ staff report (BC)

Houston, Texas – Scientists at Rice University have discovered that silicone in a liquid crystal phase becomes 90% stiffer when the material is gently and repeatedly compressed. Their research could lead to new strategies for self-healing materials or biocompatible materials that mimic human tissues.

The research has been reported in the peer-reviewed journal Nature Communications.

Silicone in its liquid crystal phase is somewhere between a solid and liquid state. Rice polymer scientist Rafael Verduzco was intrigued to see a material he thought he knew well perform in a way he didn’t expect: “I was really surprised to find out, when my student did these measurements, that it became stiffer,” he says. “In fact, I didn’t believe him at first.”

“The molecules in a liquid crystal elastomer are like rods that want to point in a particular direction,” Verduzco said. “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. It’s surprising that by a relatively gentle but repetitive compression, you can work out all the [molecular] entanglements and knots to end up with a sample where all the polymer rods are aligned.”

X-ray diffraction analysis showed that samples heated to 70°C slipped out of the liquid crystal phase and did not stiffen, Verduzco said. The stiffening effect is therefore reversible, as heating and cooling a stiffened sample will allow it to relax back into its original state within hours.

Verduzco plans to compress silicones in another phase, called smectic, in which the polymer rods align in layers. “People have been wanting to use these in displays, but they’re very hard to align. A repetitive compression may be a simple way to get around this challenge,” he said.

Since silicones are biocompatible, they can also be used for tissue engineering. Soft tissues in the body like cartilage need to maintain strength under repeated compression and deformation, and liquid crystal elastomers exhibit similar durability, Verduzco said.

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