It heals and grows together: Polymer with amazing self-healing properties

It heals and grows together: Polymer with amazing self-healing properties

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(PhysOrg.com) -- Sooner or later, a cut to the skin or a broken bone will heal on its own; however, a scratch to a car's paint or a tear in the wing of an airplane will not. Materials with self-healing properties could help extend the durability of products and make repairs easier.

Krzysztof Matyjaszewski and his co-workers at Carnegie Mellon University (Pittsburgh, USA) and Kyushu University (Japan) have now developed a polymer that can repair itself when irradiated with UV light -- over and over again. As the scientists report in the journal Angewandte Chemie, this is the first material in which capped covalent bonds repeatedly reattach, even allowing fully separated pieces to be fused back together.

Some previous solid self-healing materials contain tiny capsules that tear open to release a chemical agent when the material is damaged and have been able to repair themselves only one time. Other materials, including some gels, can repair themselves repeatedly but lack the covalent bonds that increase materials strength and stability.

In contrast, the new polymeric material produced by the American and Japanese team is stable and repairs itself again and again. The secret to their success is that the polymer is cross-linked through trithiocarbonate units. These are carbon atoms bonded to three sulfur atoms, two of which use their second bonding position to attach to another carbon atom. These groups have a special property: they can restructure under UV light. The light breaks one carbon–sulfur bond in the trithiocarbonate groups. This produces two radicals -- molecules with a free, unpaired electron. The radicals are very reactive and attack other trithiocarbonate groups to form new carbon–sulfur bonds while breaking others to form more free radicals. The chain reaction stops when two radicals react with each other.

The researchers were able to heal cut polymer fragments with irradiation—either immersed in liquid or in bulk. They only had to firmly press the cut edges together and irradiate them. The edges grew back together by means of the radical re-organization process described above.

The self-healing effect goes much further: even shredded polymer samples could simply be pressed together and irradiated to be fused into a continuous piece. The resulting object was in the shape of the cylindrical tube in which the procedure was carried out. This self-healing process can be carried out repeatedly on the same sample. The material is thus also interesting as a new recyclable product.

CSIRO grants global license for new polymer technology

CSIRO grants global license for new polymer technology

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CSIRO has signed a global licensing agreement for its patented RAFT technology. Reversible Addition-Fragmentation chain Transfer (or RAFT) technology is an elegant and powerful polymerisation process that has given rise to a new branch of polymer chemistry.

RAFT enables the development of very complex molecules that can be used for a wide range of products. The technology is already generating major improvements in the areas of coatings and paints, electroactive materials, fuel additives, biomaterials, polymer synthesis, personal care, drug delivery agents and car components.

About 3200 papers have been published on RAFT developments, coupled with over 200 patents granted to research and commercial institutions globally.

Monomer Polymer, a US company which specialises in manufacturing specialty monomers and sophisticated polymers has agreed to market the technology worldwide.

Monomer-Polymer and Dajac Labs CEO Stephen Bell said having access to RAFT technology will allow the company to ?undertake controlled radical polymerizations and consequently, create additional success and opportunity in material development?.

Additionally, Monomer-Polymer said that the licensing agreement would enable the company to strengthen their position as a key player in the synthesis, development and scale-up of specialty monomers and resulting polymer systems. A unique aspect of Monomer-Polymer?s chemistries is the internal expertise with organosilanes which should open up unique uses of RAFT to create polymers with organosilicon functionalities in the architecture.

?This new technology is creating global impact and has been licensed by a wide range of Australian and multinational companies,? CSIRO?s Business Development Manager for RAFT Kate Dawson said.

Monomer-Polymer and Dajac Labs is a manufacturer of specialty monomers and polymers used for coatings, adhesives, medical devices, dental resins, water treatment, academic and industrial R&D.

New surface may kill antibiotic-resistant staph bacteria with fluorescent light

New surface may kill antibiotic-resistant staph bacteria with fluorescent light

The prevalence of methicillin-resistant Staphylococcus aureus (MRSA) infections is well known, causing an estimated 19,000 deaths and $3-4 billion in healthcare costs per year in the U.S. What is less well known is that this increased infection and resistance rate has not been met with a simultaneous development of novel antimicrobial and antibiotic agents; in fact, only three classes of antibiotics have been developed since the 1950s.

To address this need, scientists at the University of New Mexico are working on a new type of antimicrobial surface that is inhospitable to MRSA but won't harm people or animals. Their results will be presented today at the AVS 57th International Symposium & Exhibition, which takes place this week at the Albuquerque Convention Center in New Mexico.

The new polymer-type material, "conjugated polyelectrolyte" (CPE) with an arylene-ethynylene repeat-unit structure, has been effective at killing Gram-negative bacteria, enabling its use in a wide range of potential applications. For instance, certain "light-activated" CPEs are inert toward bacteria in the absence of light, and display bacteria-killing activity with the addition of light. This opens up many potential applications, including the possibility of using these polymers as antibacterial countertops that may be sterilized using regular fluorescent lights.

Until recently, it was unknown if the CPEs would exhibit similar biocidal activity toward mammalian cells. In-vitro testing performed on these CPEs at the University of New Mexico is an important first step in determining whether they are harmful to humans at concentrations envisioned in potential applications. In a poster presented today at the AVS Conference, Kristin Wilde will present the results.

Tiny particles can deliver antioxidant enzyme to injured heart cells

Tiny particles can deliver antioxidant enzyme to injured heart cells

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Fluorescent green polyketal particles stay in the heart three days after injection. Credit: Michael Davis

Researchers at Emory University and the Georgia Institute of Technology have developed microscopic polymer beads that can deliver an antioxidant enzyme made naturally by the body into the heart.

Injecting the enzyme-containing particles into rats' hearts after a simulated heart attack reduced the number of dying cells and resulted in improved heart function days later.

Michael Davis, PhD, is presenting the results Sunday evening at the American Heart Association Scientific Sessions in Orlando. Davis is assistant professor in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University.

The enzyme in the particles, called superoxide dismutase (SOD), soaks up toxic free radicals produced when cells are deprived of blood during a heart attack. Previously scientists have tried injecting SOD by itself into injured animals, but it doesn't seem to last long enough in the body to have any beneficial effects.

"Our goal is to have a therapy to blunt the permanent damage of a heart attack and reduce the probability of heart failure later in life," Davis says. "This is a way to get extra amounts of a beneficial antioxidant protein to the cells that need it."

The simulated heart attacks caused a 20 percent decrease in the ability of the rats' hearts to pump blood that was completely prevented by the particles, he says.

The particles are made of a material called polyketals, developed by Niren Murthy, PhD, assistant professor of biomedical engineering at Georgia Tech and Emory. The polyketals encase the enzyme and are taken up by cells within the heart. There the particles slowly release the enzyme.

The microparticles break down into nontoxic components in the body -- an advantage over other biodegradable polymers like PLGA (polylactic-co-glycolic acid), already approved for use in sutures and grafts. When polymers such as PLGA are made into particles for drug delivery, they can induce inflammation.

Davis and his colleagues have also used the polyketal microparticles to encase anti-inflammatory drugs. This is the first report on the antioxidant enzyme-containing particles' use in a model of heart attack.

Emory and Georgia Tech scientists have also used SOD-containing particles to treat mice engineered to have a deficiency in SOD in the lung.

Although the SOD particles had a protective effect when the heart was examined three days after the simulated heart attack, the beneficial effects weren't as strong three weeks later. The rats' hearts still had a 35 percent improvement compared to untreated animals, Davis says. Combining them with microparticles containing the anti-inflammatory drugs proved to provide an additional boost.

"This is likely because it is important to scavenge free radicals at early time points, but inflammation becomes more important later on," he says.

More information: http://www.nature. … mat2299.html

Source: Emory University (news : web)

On the Death of Polymers: Revisiting Termination Rate Coefficients in Radical Homopolymerization

On the Death of Polymers: Revisiting Termination Rate Coefficients in Radical Homopolymerization

(PhysOrg.com) -- Although radical polymerization is used in the synthesis of about half the world's polymers, details of exactly what is going on in the reaction soup in complex industrial settings have been sketchy at best. As the materials enter our lives as, for example, drugs, coatings, fibers and solar cells, controlling their reactions and therefore their properties is extremely important.

Scientists in New Zealand have recently addressed a fundamental part of this story by considering termination rate coefficients for a couple of very common reactions, using results from new analytical techniques to revisit our old understanding. They found the way the small polymers (oligomers) in the system move and their speed, i.e., their diffusion behavior, to be the critical factor. This work is published in a special issue of Macromolecular Chemistry and Physics, devoted to radical polymerization.

The people responsible, Greg Russell and his colleagues at the University of Canterbury, are experts in polymer kinetics. Russell explains, ?The majority of chemists simply try to bring about reactions by mixing different chemicals together under different conditions. However it is also important, especially for those who make chemical products on a large scale, to have precise quantitative descriptions of the speeds at which reactions occur. Chemical kinetics is the field of work that develops such descriptions. It is therefore an area where chemistry and mathematics intersect.?

He goes on to say, ?Arguably the hardest nut to crack in the radical polymerization scheme has been the termination reaction. In layman's terms, termination is the fundamental reaction whereby a polymer molecule stops growing larger. A reasonable analogy is human death, the process which ceases human life and thus prevents a human's age from mounting and mounting. In radical polymerization this reaction is diffusion controlled in rate, which means that its speed is determined by how fast the molecules move.? This speed of movement can depend on many factors such as how long the molecule is, the number of obstacles around the polymer, the temperature of the system, and so on. ?This is the origin of the complexity of the termination reaction, and is the reason why, after over 60 years of intensive study, it is still not fully understood, not nearly.?

In this work Russell revisited some of the earliest questions about termination. ?Recent years have seen the development of highly specialized techniques for measuring termination rate coefficients under precisely controlled conditions. I have taken this information and attempted to see whether it is consistent with systems where many different termination reactions occur at once, as is the case in commercial processes. For the monomer styrene I find there is consistency, but for methyl methacrylate there is not.?

In trying to explain this result, he eliminated most of the conventional views, and came to the conclusion that the answer lies with the oligomers in each system, which seem to have slightly different diffusional behavior.

Philipp Vana works at the University of Göttingen, where Greg Russell is currently on sabbatical. He specializes in radical polymerization and serves on the Advisory Board of Macromolecular Chemistry and Physics. He is the Guest Editor of the special issue. In his view, ?Russell?s paper is especially exciting, as it demonstrates that the information gathered by modern and advanced methods is useful to reevaluate the results obtained by older methods. Completely new insights can be extracted by such an approach.? He adds that Russell?s work not only adds new information to the field, but also presents a nice view of the complete picture, ?which helps us to understand the complete history of science instead of getting a short snapshot of the present.?

This historical perspective of the field is especially pertinent as the special issue focused on the kinetics and mechanism of radical polymerization was prepared in order to honor Michael Buback, who turns 65 this year and who, according to the editor Vana, ?undisputably is one of the doyens in this field.? Recent years have seen the invention of new controlled polymerization methods and Vana says it is of ?vital importance to fundamentally understand these new techniques in order to exploit their full potential for material design. The newly invented polymerization technologies also provided new avenues for unlocking the secrets of the conventional processes. Many important questions could be answered recently and it seems to be justified to resumé at this stage and to identify, which major questions need further attention.?

Radical predictions in polymer chemistry

Radical predictions in polymer chemistry

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Credit: iStockphoto.com/Martin McCarthy

Free radicals may have a bad reputation for causing ozone depletion and premature aging, but they are in fact extremely useful for making novel materials, particularly polymers. Skilled chemists can link up a wide range of free radicals into long-chain polymers using a technique called atom transfer radical polymerization (ATRP). Using different metal catalysts, one can precisely control the rates of polymerization and termination and therefore the architecture and functionality of the polymer.

Fabio di Lena and Christina Chai at the A*STAR Institute of Chemical and Engineering Sciences have now performed the first-ever theoretical modeling of copper-catalyzed ATRP to explain quantitatively how radical polymerization rates are influenced by molecular structures and properties—laying out a critical roadmap for the production of next-generation polymer materials.

The success of an ATRP reaction depends on how well the metal catalyst generates and deactivates organic radicals by intermittently stealing or giving up electrons. If radical production is too fast, the polymerization stops, while sluggish activation or deactivation makes it hard to produce high-quality polymers. Unfortunately, fine-tuning ATRP rates is tricky because researchers must simultaneously optimize many diverse factors such as catalyst and radical geometries, solvents and reaction conditions.

To solve this problem, di Lena and Chai turned to computer-aided molecular design, a technique widely employed in the pharmaceutical drug discovery. They first performed theoretical calculations to extract hundreds of numerical parameters or ‘molecular descriptors’ corresponding to specific structural and chemical properties for a series of ATRP copper catalysts and organic radicals. They then conducted sophisticated statistical analyses on the data to reveal subsets of principal descriptors that had the most influence over polymerization rates.

Next, the team combined their chemical intuition with stringent testing to further narrow the list of descriptors. Finally, biology-inspired artificial intelligence techniques called genetic function algorithms were used to produce mathematical models that relate ATRP rates to algebraic combinations of descriptors like energy levels, molecular volumes and bond lengths. According to di Lena, these models are striking because they agree with the generally accepted mechanistic picture of ATRP and can provide unprecedented predictive insights.

“This method should facilitate the design of new ATRP catalysts by screening, in a virtual way, hundreds of metal complexes at time,” says di Lena. “Labs will only need to prepare the most promising candidates, saving time and money.” Di Lena is also confident that the method will become a powerful tool for developing polymers with tailored properties and functions.

Stretched molecule ends up shorter than it started (w/ Video)

Stretched molecule ends up shorter than it started (w/ Video)

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A ring-like polymer called gDFC was stretched (middle) and sprung back to shape smaller than it began

(PhysOrg.com) -- Crazy bands are cool because no matter how long they've been stretched around a kid's wrist, they always return to their original shape, be it a lion or a kangaroo.

Now a Duke and Stanford chemistry team has found a polymer molecule that's so springy it snaps back from stretching much smaller than it was before.

Duke graduate student Jeremy Lenhardt and associate professor Stephen Craig have been systematically hunting through a library of polymers in search of a molecule that might be useful for "self-healing" materials. They hope to find a polymer that can trigger a chemical reaction when it is stretched and enable a material to build its own repairs.

Imagine a sheet of Saran Wrap that could fix a microscopic puncture before the hole ever got big enough to see. This would require that the polymer molecules immediately around the tear could somehow jump into action and perform new chemistry to build bridges across the hole.

To stretch polymers and see what happens to them, Lenhardt uses an apparatus that pumps up and down on a solution filled with polymers, pressurizing it and depressurizing it 20,000 times a second which causes tiny bubbles to form fleetingly. The void created by the bubbles exerts a tug on the ends of some of the polymers in the solution and stretches them, if only for a billionth of a second.

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After being stretched by ultrasonication, a ring-shaped gem-difluorocyclopropane (gDFC) forms a transition state 1,3-diradical, and then snaps back to shape smaller than it began. Duke graduate student Jeremy Lenhardt narrates.

"Think of two rafts going down a river with a rope between them," Craig explained. "As the first raft enters a rapids and accelerates forward, that rope - the polymer - gets pulled taught and stretches."

Over and over Lenhardt ran the experiment, characterizing different polymer species that became more reactive when stretched, potentially able to do "stress-induced chemistry."

Then, while looking at polymers that contained tiny ring-shaped molecules called gem-difluorocyclopropanes (gDFC), he was surprised to find that some of these molecules emerged from the stretching noticeably shorter than when they went in.

"I ran up to his office," Lenhardt said. " 'Steve, something funny is going on here. Look at this!' " A technique called nuclear magnetic resonance had revealed the shapes of the rings after pulling and shown that they were, in fact, shorter.

But not only were the gDFCs snapping back smaller than they started, it also appeared that before snapping back they were actually trapped in an unusual stretched state far longer than normal, a reactive state called a 1,3-diradical.

Normally, as a molecule goes through a reaction, it passes through a special point known as a transition state, and stays there for only ten to a hundred femtoseconds, "a tenth of a millionth of a millionth of a second," Craig said. This makes it extraordinary hard to actually watch chemistry happen, so chemists usually can only infer what happens at the transition state by what they've seen before and after.

Work by their Stanford collaborators showed that the trapped 1,3-diradicals are in fact one type of these usually fast-moving transition states, but in Lenhardt's experiments they were essentially stopped in their tracks and trapped for nanoseconds, tens of thousands of times longer than usual.

This might be a window for watching transition states in action, Craig said. "We can trap these things long enough to probe new facets of their reactivity."

Lenhardt has begun doing just that, stretching the polymers to learn more about these transition states and seeing if he can watch other molecules by using this technique as a sort of stop-action camera.

"Every chemical reaction has a high energy state that you have to guess at," Lenhardt said. "But maybe, in some cases, you don't have to guess anymore."

The team's findings appear Aug. 27 in Science.