A team of scientists at Rice University‘s Verduzco laboratory in Houston have discovered that the liquid crystal phase of silicone becomes 90 percent stiffer when silicone is gently and repeatedly compressed — research that could lead to new strategies for self-healing materials or biocompatible materials that mimic human tissues.
The lab, headed by polymer scientist Dr. Rafael Verduzco, the Louis Owen Assistant Professor in the Department of Chemical and Biomolecular Engineering at Rice, conducts research involving polymers, with the specific goal of utilizing the self-assembly in polymeric materials to improve organic solar cells, engineering surface properties, and drug encapsulation and delivery.
In a Rice News release, senior media relations specialist in Rice University’s Office of Public Affairs Mike Williams notes that if you squeeze a piece of silicone, it will quickly return to its original shape, as squishy as ever. However, Dr. team have discovered that the liquid crystal phase of silicone becomes 90 percent stiffer when silicone is gently and repeatedly compressed. Their research could lead to new strategies for self-healing materials or biocompatible materials that mimic human tissues. A paper (Nature Communications 4, Article number: 1739 doi:10.1038/ncomms2772) on the research entitled “Dynamic self-stiffening in liquid crystal elastomers” appeared this month in Nature’s online journal Nature Communications.
In the paper’s abstract, corresponding author Aditya Agrawal — a 3rd Year Rice graduate student, and co-authors Brent Carey, a Rice alumnus – now a scientist at Owens Corning; graduate student Alin Chipara; Yousif Shamoo, a professor of biochemistry and cell biology; Pulickel Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of mechanical engineering and materials science, chemistry and chemical and biomolecular engineering; and Walter Chapman, the William W. Akers, Professor of Chemical and Biomolecular Engineering, and Dr. Verduzco — all of Rice; and Prabir Patra, an assistant professor of mechanical engineering at the University of Bridgeport with a research appointment at Rice, explain that biological tissues have the remarkable ability to remodel and repair in response to disease, injury and mechanical stresses, qualities that synthetic materials, lacking the complexity of biological tissues, do not have. Consequently, man-made materials that respond to external stresses through a permanent increase in stiffness are uncommon, but the researchers report their observation that polydomain nematic liquid crystal elastomers increase in stiffness by up to 90% when subjected to a low-amplitude (5%), repetitive (dynamic) compression. Elastomer stiffening is influenced by liquid crystal content, the presence of a nematic liquid crystal phase and the use of a dynamic as opposed to static deformation. Stiffening under dynamic compression has not been previously observed in liquid crystal elastomers and the co-authors suggest that the discovery may be useful for development of self-healing materials or for the development of biocompatible, adaptive materials for tissue replacement.
Silicones are made of long, flexible molecular chains that are entangled and knotted together like bowls of spaghetti. In conventional silicones the chains are randomly oriented, but the Verduzco Lab group studied a special type of silicone known as a liquid crystal elastomer. In these materials, the chains organize themselves into rod-shaped coils. When the material was compressed statically, like squeezing a piece of Jell-O or stretching a rubber band, it snapped right back into its original shape. The entanglements and knots between chains prevent it from changing shape. But when dynamically subjected to 600,000 cycles of compression over the course of 16 hours, the silicone held its new shape for weeks and, surprisingly, was much stiffer than the original material.
Silicone in its liquid crystal phase is somewhere between a solid and liquid state, which makes it an extremely handy material for many things — for example home, marine, and industrial elastomer sealants. Consequently Dr. 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 told Mr. Williams. “In fact, I didn’t believe him at first.”
The researchers had intended to quantify results seen a few years ago by former Rice graduate student and co-author of the Nature paper Brent Carey, who subjected a nanotube-infused polymer to a process called repetitive dynamic compression in which an astonishing 3.5 million compressions (five per second) over a week toughened the material, similarly to human muscles after a workout, by 12 percent. What Dr. Verduzco and study lead author and Rice graduate student Aditya Agrawal came across was a material that shows an even stronger effect. Williams notes that they had originally planned to study liquid crystal silicone/nanotube composites similar to what Carey tested, but decided to look at liquid crystal silicones without the nanotubes first, Dr. Verduzco observing: “It’s always better to start simple.”
“The molecules in a liquid crystal elastomer are like rods that want to point in a particular direction,” Dr. Verduzco continues in the Rice report. “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 entanglements and knots to end up with a sample where all the polymer rods are aligned.”
Before testing, the researchers chemically attached liquid crystal molecules — similar to those used in LCD displays — to the silicones. While they couldn’t see the rods, X-ray diffraction images showed that the side groups – and thus the rods – had aligned under compression. “They’re always coupled. If the side group orients in one direction, the polymer chain wants to follow it. Or vice versa,” Dr. Verduzco explains, noting that the X-rays also showed that samples heated to 70 degrees Celsius slipped out of the liquid crystal phase and did not stiffen, and the stiffening effect is reversible, since heating and cooling a stiffened sample will allow it to relax back into its original state within hours.
The next stage of research will be 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,” Dr. Verduzco told Williams, and 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.
Dr. Rafael Verduzco studied at Rice University as an undergraduate, receiving a bachelor’s degree in chemical engineering in 2001. He earned a master’s degree in 2003 and doctorate in chemical engineering in 2007 from the California Institute of Technology studying liquid crystal polymer gels. Dr. Verduzco then went on to a postdoctoral position in the Center for Nanophase Materials Sciences at the Oak Ridge National Laboratory in Tennessee, where he studied bent-core liquid crystals, water-soluble dendrimers for drug delivery, and conjugated polymeric materials for organic electronics.
The research was supported by an IBB Hamill Innovations Grant, the Robert A. Welch Foundation, the National Science Foundation and the National Institutes of Health, through the National Institute of Allergy and Infectious Diseases.