Squeezing silicone polymers produces chemical energy, but raises doubts about implant safety

 A polymer is a mesh of chains, which slowly break over time due to the pressure from ordinary wear and tear. When a polymer is squeezed, the pressure breaks chemical bonds and produces free radicals: ions with unpaired electrons, full of untapped energy. These molecules are responsible for aging, DNA damage and cancer in the human body.

In a new study, Northwestern University scientists turned to squeezed polymers and free radicals in a search for new energy sources. They found incredible promise but also some real problems. Their report is published by the journal Angewandte Chemie.

The researchers demonstrated that radicals from compressed polymers generate significant amounts of energy that can be used to power chemical reactions in water. This energy has typically been unused but now can be harnessed when polymers are under stress in ordinary circumstances -- as in shoe soles, car tires or when compacting plastic bags.

They also discovered during the study that a silicone polymer commonly used in implants for cosmetic procedures releases a large quantity of harmful free radicals when the polymer is under only a moderate amount of pressure. These findings suggest the safety of certain polymer-based medical implants should be looked at more closely.

"We have established that polymers under stress create free radicals with overall efficiencies of up to 30 percent and shoot the radicals out into the surrounding medium where they can drive chemical reactions," said Bartosz A. Grzybowski, an author of the paper and the Kenneth Burgess Professor of Physical Chemistry and Chemical Systems Engineering. "These radicals can be useful or they can be harmful, depending on the situation."

Grzybowski and his team are the first to use this energy to drive chemical reactions by simply surrounding the compressed polymer with water containing desired reagents.

The radicals created in the polymer migrate toward the polymer/water interface where they produce hydrogen peroxide, which then can drive chemical processes.

"You can get a surprisingly large amount of chemical energy from a polymer under compression," Grzybowski said. "This energy is, in a sense, free for the taking. Under normal circumstances, the energy is virtually never retrieved from deformed polymers, which then age unproductively. But you could recharge a battery from the energy produced by walking or driving a car. And you could capture even more energy when compacting millions of plastic bags."

Grzybowski is also director of Northwestern's Non-Equilibrium Energy Research Center, which is funded by the U.S. Department of Energy.

"We are interested in new sources of chemical energy, and this energy from the simple breaking of polymers' bonds is not being used," he said. "By surrounding the polymer with a medium, such as water, we can produce environmentally friendly chemical energy. One direction we are pursuing is to use this energy to sanitize water in developing countries. This is possible because hydrogen peroxide produced by squeezed polymers kills bacteria."

The researchers confirmed that mechanical deformation -- moderate squeezing -- created free radicals in the polymers. They also determined the number of radicals produced in a polymer under pressure is approximately 1016 (10 to the 16th) radicals per cubic centimeter of polymer -- a substantial amount.

They next filled polymer tubes with water, squeezed the tubes and measured the total number of radicals that migrated into the surrounding solution. They found that nearly 80 percent of the radicals made the trip.

Grzybowski and his team demonstrated they can squeeze a polymer, such as what might be found in a shoe, tire or plastic bag, and get a mechanical-to-chemical energy conversion of up to 30 percent -- approaching the energy efficiency of a car engine.

The hydrogen peroxide produced when a polymer surrounded by water is squeezed can power a variety of chemical reactions, including fluorescence, nanoparticle synthesis and dye bleaching, the researchers showed.

To illustrate the process, they converted a Nike Air LeBron shoe into a "lightning shoe," where the air pockets in the polymeric sole are filled with a solution of a compound that lights up in the presence of radicals. After a person walked in the shoe for 30 minutes or more, enough radicals were created to generate a blue glow visible to the naked eye.

The researchers studied seven different polymers, including a number of particular public interest. Poly(dimethylsiloxane), a silicon-based material commonly used in medical implants, was one of them. In the lab experiments, the medium surrounding the polymer and the amount of pressure exerted on the material were similar to what would be found in the human body, Grzybowski pointed out.

"Our findings are somewhat worrisome since every polymeric implant in the human body experiences mechanical stresses and, as we now know, can produce harmful free radicals and liberate them into surrounding tissues, which may contribute to diseases such as cancer, stroke, myocardial infarction, diabetes and other major disorders," Grzybowski said. "With this knowledge, I am quite happy to have a metal implant in my knee, rather than a polymer implant.

"From a scientific perspective, our work proves yet again that a phenomenon can be useful or harmful depending on how we implement it," he said. "The same polymer can be a useful source of energy when outside of a human body, yet a potential risk hazard when implanted into it."

Story Source:

The above story is reprinted from materials provided by Northwestern University. The original article was written by Megan Fellman.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

H. Tarik Baytekin, Bilge Baytekin, Bartosz A. Grzybowski. Mechanoradicals Created in “Polymeric Sponges” Drive Reactions in Aqueous Media. Angewandte Chemie, 2012; DOI: 10.1002/ange.201108110

Scientists rediscover self-healing silicone mechanism from the 1950s

The researchers, grad student Peiwen Zheng and Professor Thomas J. McCarthy from the University of Massachusetts, have published a paper on the rediscovery of siloxane equilibration in a recent issue of the Journal of the American Chemical Society.

“We have been working on silicone materials from a couple of different perspectives,” Zheng told PhysOrg.com. “When we rediscovered the forgotten unusual properties of silicones and combined them with today’s research interests, we found that the silicone material with the siloxane equilibration was an obvious candidate for a self-healing material.”

The researchers performed several experiments to test the theoretical predictions from papers published in the early 1950s, as well as to extend some of the experiments performed at that time. In one experiment, Zheng and McCarthy prepared a siloxane-based mixture containing a cross-linking agent and a catalyst. Then they poured the solution into molds of various shapes, such as cylinders, disks, and dog bones. After heating the molds at 90 °C (194 °F) for four hours, the researchers removed clear, rubbery silicone shapes from the molds. The scientists described these silicone samples as “living polymer networks.”

“The silicone network is at a chemically anionic equilibrium,” Zheng explained of the term, “where the reactive center will cleave and reform a covalent siloxane bond.” These bonds are reversible, which enables the two sides of a crack to reconnect under the right conditions.

To demonstrate the self-healing ability, the researchers cut a 1-cm-long cylindrical sample in half using a razor blade. Then they rejoined the two pieces by wrapping them together with Teflon plumbing tape and heating them in an oven at 90 °C (194 °F) for 24 hours. When the researchers retrieved the sample and removed the tape, they found that the silicone cylinder had completely healed. Then they bent the cylinder by hand until it broke again - significantly, it broke in a different location than where it had been cut. The scientists repeated this experiment on different shaped objects with the same results.

In another experiment, the researchers molded a silicone dog bone, which they cut into multiple pieces. Then they rearranged the pieces to fit into a mold of a dog. Heating the sample resulted in a silicone dog with no visible fractures or weak spots where the pieces had been fitted together.

The researchers also quantified the strength of the healed samples in comparison with the original samples using fracture toughness measurements. The data for the two types of samples was indistinguishable, indicating exceptional self-healing.

The researchers explained that, in principle, any cross-linked dimethylsilicone elastomer (only one type was used here) can be converted into a living elastomer by the addition of basic catalysts. This possibility opens up many different routes for synthesizing a variety of self-healing silicione-based materials. While the samples used here required applied heating to self-heal, the researchers predict that samples in a sealed, high-temperature environment would self-heal “autonomically,” or automatically. Zheng explained that self-healing materials, with some improvements, could lead to a variety of applications.

“It can be developed into self-healing coatings on auto vehicles or countertops,” she said. “It is also a ‘plastic’ elastomer which can be used in molding to form desired shapes and patterns. The concept of a self-healing silicone can be used to guide the preparation of elastomers with gradient modulus, Janus elastomers, reversible surface patterns when filled with magnetic particles, and super tough materials which can chemically relax stress.”

More information: Peiwen Zheng and Thomas J. McCarthy. “A Surprise from 1954: Siloxane Equilibration Is a Simple, Robust, and Obvious Polymer Self-Healing Mechanism.” Journal of the American Chemical Society. DOI: 10.1021/ja2113257

? 2011 PhysOrg.com

Wacker Commissions Silicone Rubber Compounding Plant in India

 The Munich-based chemical group Wacker has commissioned a compounding plant at its joint venture site Wacker Metroark Chemicals Pvt. Ltd. near Kolkata, India, for the manufacture of ready-to-use silicone elastomers. The plant is designed for an annual production volume of several thousand metric tons and can be expanded in stages as demand requires. It is intended to supply India’s strongly growing economy with high-quality silicone compounds faster and more flexibly. Several million euros are planned for investment in this plant.

“India is one of our fastest-growing sales markets,” says Dr. Bernd Pachaly, head of Wacker’s Engineering Silicones business unit. “The new SILMIX® facility is constructed according to Wacker standards and will produce silicone compounds for the Indian electronics, automotive and medium and high-voltage insulator market as well as for other industries.” During the start-up event in Kolkata, Pachaly’s deputy Dr. Johann Schuster pointed out that Wacker is one of the leading manufacturers of silicone elastomers for electric insulators. “By manufacturing such products locally, we can serve the market much more flexibly and significantly reduce lead times while still delivering the expected high product quality.” The SILMIX® plant has been designed so that production capacity can be adjusted to meet current market requirements in a short period of time. Schuster: “If necessary, we can respond very quickly to additional demand.”

Soumitra Mukherjee, Managing Director of Wacker Metroark Chemicals (WMC), praised the investment as an important milestone for the joint venture. “Wacker and Metroark have been working together with great success for over a decade. With our range of high quality silicone products, we are already market leaders in many industries in India today.” Mukherjee pointed out that the plant includes a state-of-the-art lab for quality control. With its new compounding plant in Kolkata, WACKER is excellently positioned. “This way it is possible to test and develop high quality silicone compounds for the Indian market in India itself,” emphasized Mukherjee. “This is a major competitive advantage for our customers, who are often under great pressure to deliver quality on time.”

For the manufacture of silicone compounds, the crosslinker, pigment and other additives are usually mixed into the rubber base. Blends such as these are used in a number of industries, including the automotive sector, the electrical industry and the heavy current industry.