Engineers create polymer light-emitting devices that can be stretched like rubber

Stretchable electronics, an emerging class of modern electronic materials that can bend and stretch, have the potential to be used in a wide range of applications, including wearable electronics, "smart skins" and minimally invasive biomedical devices that can move with the body.

Today's conventional inorganic electronic devices are brittle, and while they have a certain flexibility achieved using ultrathin layers of inorganic materials, these devices are either flexible, meaning they can be bent, or they are stretchable, containing a discrete LED chip interconnected with stretchable electrodes. But they lack "intrinsic stretchabilty," in which every part of the device is stretchable.

Now, researchers at the UCLA Henry Samueli School of Engineering and Applied Science have demonstrated for the first time an intrinsically stretchable polymer light-emitting device. They developed a simple process to fabricate the transparent devices using single-walled carbon nanotube polymer composite electrodes. The interpenetrating networks of nanotubes and the polymer matrix in the surface layer of the composites lead to low sheet resistance, high transparency, high compliance and low surface roughness.
The metal-free devices can be linearly stretched up to 45 percent and the composite electrodes can be reversibly stretched by up to 50 percent with little change in sheet resistance.

Because the devices are fabricated by roll lamination of two composite electrodes that sandwich an emissive polymer layer, they uniquely combine mechanical robustness and the ability for large-strain deformation, due to the shape-memory property of the composite electrodes. This development will provide a new direction for the field of stretchable electronics.

This research was recently published in the peer-reviewed journal Advanced Materials .

More information: DOI: 10.1002/adma.201101986

Provided by University of California Los Angeles (news : web)

Total synthesis of anti-cancer marine product achieved

The concerted efforts of researchers from both PolyU and Peking University’s Shenzhen Graduate School have led to the first total synthesis of a natural marine product as a promising anti-cancer agent.

Under the leadership of Dr Ye Tao, Associate Professor of PolyU’s Department of Applied Biology and Chemical Technology, the concerted efforts of researchers from both PolyU and Peking University’s Shenzhen Graduate School have led to the first total synthesis of a natural marine product with anti-cancer properties: grassypeptolide. This breakthrough paves the way for the further development of anti-cancer drugs from grassypeptolide – a compound isolated from marine bacteria – which has emerged as a promising anti-cancer agent.

It is difficult to obtain grassypeptolide from natural sources, but the research team made its construction possible through a 17-step total chemical synthesis process. The team faced significant challenges in forming the 31-member ring of grassypeptolide and then introducing the two smaller thiazoline heterocycles – five-member rings containing sulphur and nitrogen – into that ring. The researchers constructed the 31-member macrocycle via a precursor with more favourable cyclization kinetics, and then introduced the thiazoline heterocycles at a later stage of synthesis to prevent them from undergoing side reactions.

The novel breakthrough has been reported in the authoritative Chemical Communications (Issue 40, Volume 46, 2010) and has been highlighted by Nature China.

This article was first appeared on PolyU Milestone, June 2011 edition.

 

University of Virginia researchers uncover new catalysis site

 

Mention catalyst and most people will think of the catalytic converter, an emissions control device in the exhaust system of automobiles that reduces pollution.

But catalysts are used for a broad variety of purposes, including the conversion of petroleum and renewable resources into fuel, as well as the production of plastics, fertilizers, paints, solvents, pharmaceuticals and more. About 20 percent of the gross domestic product in the United States depends upon catalysts to facilitate the chemical reactions needed to create products for everyday life.

Catalysts are materials that activate desired chemical reactions without themselves becoming altered in the process. This allows the catalysts to be used continuously because they do not readily deteriorate and are not consumed in the chemical reactions they inspire.

Chemists long ago discovered and refined many catalysts and continue to do so, though the details of the mechanisms by which they work often are not understood.

A new collaborative study at the University of Virginia details for the first time a new type of catalytic site where oxidation catalysis occurs, shedding new light on the inner workings of the process. The study, conducted by John Yates, a professor of chemistry in the College and Graduate School of Arts & Sciences, and Matthew Neurock, a professor of chemical engineering in the School of Engineering and Applied Science, will be published in the Aug. 5 issue of the journal Science.

Yates said the discovery has implications for understanding catalysis with a potentially wide range of materials, since oxidation catalysis is critical to a number of technological applications.

"We have both experimental tools, such as spectrometers, and theoretical tools, such as computational chemistry, that now allow us to study catalysis at the atomic level," he said. "We can focus in and find that sweet spot more efficiently than ever. What we've found with this discovery could be broadly useful for designing catalysts for all kinds of catalytic reactions."

Using a titanium dioxide substrate holding nanometer-size gold particles, U.Va. chemists and chemical engineers found a special site that serves as a catalyst at the perimeter of the gold and titanium dioxide substrate.

"The site is special because it involves the bonding of an oxygen molecule to a gold atom and to an adjacent titanium atom in the support," Yates said. "Neither the gold nor the titanium dioxide exhibits this catalytic activity when studied alone."

Using spectroscopic measurements combined with theory, the Yates and Neurock team were able to follow specific molecular transformations and determine precisely where they occurred on the catalyst.

The experimental and theoretical work, guided by Yates and Neurock, was carried out by Isabel Green, a U.Va. Ph.D. candidate in chemistry, and Wenjie Tang, a research associate in chemical engineering. They demonstrated that the significant catalytic activity occurred on unique sites formed at the perimeter region between the gold particles and their titania support.

"We call it a dual catalytic site because two dissimilar atoms are involved," Yates said.

They saw that an oxygen molecule binds chemically to both a gold atom at the edge of the gold cluster and a nearby titanium atom on the titania support and reacts with an adsorbed carbon monoxide molecule to form carbon dioxide. Using spectroscopy they could follow the consumption of carbon monoxide at the dual site.

"This particular site is specific for causing the activation of the oxygen molecule to produce an oxidation reaction on the surface of the catalyst," Yates said. "It's a new class of reactive site not identified before."

 

Novel coatings show great promise as flame retardants in polyurethane foam

Gram for gram, novel carbon nanofiber-filled coatings devised by researchers from the National Institute of Standards and Technology (NIST) and Texas A&M University outperformed conventional flame retardants used in the polyurethane foam of upholstered furniture and mattresses by at least 160 percent and perhaps by as much as 1,130 percent.

The impressive test results, reported in the journal Polymer, suggest that significant fire-safety advantages can be gained by coating polyurethane foam (PUF) with a club-sandwich-like arrangement of thin layers containing carbon nanofibers and polymers. The upshot, says NIST researcher Rick Davis, is that the experimental coating seems to create the equivalent of a "fire-resistant armor" on the porous foam.

Ignition of soft furnishings account for about 5 percent of residential fires, and the consequences are disproportionately high. These fires are responsible for a third of fire-caused deaths of civilians and 11 percent of property losses due to fires in homes.

The flammability of mattresses is regulated by federal law. A complementary rule to regulate the flammability of upholstered furniture has been proposed recently.

Several organizations, however, have challenged the health and safety of some flame retardants designed to protect against soft furnishing fires. And, a bill pending in California would ban the use of certain halogenated flame retardants in that state.

Today, recipes for making PUFs result in foams in which fire retardants are embedded in the interior. In contrast, the experimental technology uses the carbon nanofiber fire retardant as a coating that covers all the nooks and crannies on the sponge-like PUF surface. The new approach, says Davis, should be attractive to PUF manufacturers because the surface treatment has the potential to deliver a low flammability PUF without major change to the foam manufacturing process, thus saving time and money.

The NIST-Texas A&M team coated square samples of commercially available PUF with four bilayers of a carbon nanofiber-polymer combination. The average thickness of the coating was about 360 nanometers, increasing the mass of the foam by only 3 percent. By themselves, the carbon nanofibers accounted for 1.6 percent of the foam mass. Since the carbon nanofibers are only in the coating, all the carbon nanofibers are clumped like matted whiskers within the top 360 nanometers of the surface—assembled into the fire-blocking armor.

The team used a standard benchtop fire test to measure the fire performance of coated and uncoated PUF. The carbon nanofiber coatings reduced PUF flammability (measured as the peak heat release rate from an ignited specimen) by 40 percent. That result was more than 3 times better than achieved by putting the same carbon nanofibers in the foam (part of the foam recipe).

When compared at the same concentrations, the carbon nanofiber coating significantly outperforms three classes of commercially available flame retardants commonly used in PUF. Reductions in flammability achieved with the coating, according to the researchers, were 158 percent better than the reduction calculated for nonhalogens, 288 percent better than halogens, and 1,138 percent better than halogen-phosphorous flame retardants.

Additionally, the experimental coating "prevents the formation of a melt pool of burning foam, which in a real fire scenario, may further reduce the resulting fire threat of burning soft furnishings," the authors write.

More information: Polymer Volume 52, Issue 13, 8 June 2011, Pages 2847-2855. doi:10.1016/j.polymer.2011.04.023

Provided by National Institute of Standards and Technology (NIST)