Polymer scientists at Rice University, Houston, TX, discovered that the liquid crystal phase of silicone, which is partway between a solid and a liquid, becomes up to 90 percent stiffer when repeatedly compressed. Their research could lead to new strategies for self-healing materials or biocompatible materials that mimic human tissues, they say.
Conventional silicones are made of long, flexible chains that are knotted and oriented randomly. But, in a different type of silicone, known as a liquid crystal elastomer, the chains organize themselves into rod-shaped coils. When the material is compressed statically, it snaps back into its original shape since knots between the chains prevent it from changing shape. However the researchers found that when it was gently and repetitively compressed for 16 hours, the silicone held its new shape for weeks and was much stiffer than the original material.
Before testing, they researchers chemically attached liquid crystal molecules to the silicones. X-ray diffraction images determined that the rods had aligned under compression. The images also showed that samples heated to 70 degrees Celsius slipped out of the liquid crystal phase and did not stiffen, but heating and cooling a stiffened sample allowed it to relax back into its original state within hours.