Terahertz Radiation or T-Rays
Sending tight bunches of electrons at nearly the speed of light through a magnetic field causes the electrons to radiate T-rays at a trillion cycles per second—the terahertz frequency that gives T-rays their name and that makes them especially useful for investigating biological molecules.
Invisible T-rays bear comparison with radio waves, microwaves, infrared light and X-rays. But unlike those much-used forms of radiated energy, up until recently T-rays have been little exploited—in part because no one knew how to make them bright enough.
T-rays are electromagnetic radiation of the safe, non-ionizing kind. They can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. They can penetrate fog and clouds. Their wavelength—shorter than microwaves, longer than infrared—corresponds revealingly with biomolecular vibrations.
For over a decade, scientists worldwide have been pressing the study of terahertz light and looking for better ways to generate and use it. An Aug. 16, 2002, Science magazine article, “Revealing the Invisible,” reported that “much research is being directed toward the development of T-ray sources and detectors, particularly for applications in medical imaging and security scanning systems.” The Web site of Dr. Xi-Cheng Zhang, a T-ray expert at Rensselaer Polytechnic Institute, predicts that “the future ‘killer application’ … will be in biomedicine.”
Overall, though, T-rays still constitute a gap in the science of light and energy. They inhabit a region of the electromagnetic spectrum remaining to be better understood—and much better exploited. Now that a way to generate them at high power has been demonstrated, T-rays can potentially extend and add widely to the wave-based technologies that have defined the last century and a half, from the telegraph, radio and X-rays to computers, cell phones and medical MRIs.
Image 1. T-rays in the Electromagnetic Spectrum. (Courtesy: Jefferson Lab)
Various lightsources around the world can produce Terahertz light and are beginning to develop research programs for Terahertz light.
But no matter how bright they are, T-rays can’t penetrate metal or water. So they can’t be used to inspect cargo containers on arriving ships or to diagnose conditions deep inside the human body. However, the growing awareness of T-rays’ usefulness is like what happened a century ago with X-rays—only T-rays will have a much wider range of applications. Scientists’ task is to develop those uses individually.
|Image 2. Potential Applications of T-rays (Courtesy: Jefferson Lab)
Tough flexible polymer blends
Document Type and Number:
United States Patent 5003004
A blend of a softer continuous phase polymer reinforced with harder acrylic star polymer particles having reactive functional groups with crosslinking ability built in or provided by a third polymer. The harder polymer has a Tg at least 10° C. above that of the softer polymer, and the acrylic star polymer is preferably made by group transfer polymerization. This blend provides coating compositions, films and bulk polymer with enhanced toughness and flexibility.
Simms, John A. (Wilmington, DE)
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|4048254||Blend of thermoplastic polymers with block radial butadiene-styrene polymers||September, 1977||Hilliev et al.||525/89|
|4417034||Living polymers and process for their preparation||November, 1983||Webster||526/190|
|4659783||Acrylic star polymers containing multifunctional monomers in the core||April, 1987||Spinelli||525/293|
1. A blend of at least two polymers, comprising about, by weight of blend solids,
(A) 10-60% of at least one acrylic star polymer comprising a crosslinked core which comprise
(i) a polymer derived from a mixture comprising
(a) 1-100% by weight of one or more monomers, each having at least two groups, ##STR16## (b) 0-99% by weight of one or more monomers, each having one group ##STR17## (ii) attached to the core, at least 5 arms comprising polymer chains derived from one or more monomers, each having one group, ##STR18## in each of which R is the same or different and is H, CH3, CH3 CH2, CN or CO2 R’ and Z’ is O or NR’, wherein R’ is C1-4 alkyl,
said arms contain reactive functional groups in an amount of at least about one such group per arm on the average, said star polymer having a primary glass transition temperature above 20° C.;
(B) 15-60% of at least one matrix-forming polymer selected from polyester, polyether, polyurethanes, and polysiloxanes different than that of polymer (A) and having a primary glass transition temperature below 10° C., a number average molecular weight of at least about 500, and a functionality of at least 2; and
(C) said blend having crosslinking capability, either in polymers (A) and (B), or in a third, crosslinking polymer.
[ . . . ]
In attempts to achieve tough flexible polymers, especially reinforced elastomeric finishes with large amounts of acrylics for durability, it has now been found that the limitations due to molecular weight of the acrylic polyol can be essentially overcome by using highly branched acrylic polymers which can reasonably be referred to as functional star polymers. These polymer blends contain from about 15 to 60% of an acrylic of such high molecular weight that it remains as a second phase in the cured film and with at least on T g above use temperature, leading to reinforcement of the other, usually soft and elastic phase. The branches of the new star polymers are below the entanglement molecular weight for the acrylic, leading to relatively low solution viscosity and web free spraying, yet in the cured film the benefits of high molecular weight such as phase separation and reinforcing ability are retained.
It is possible that the star and soft phases both contain the same groups and that they become coreactive in the presence of a catalyst. Oxirane copolymerized in the presence of acid, or trialkoxysilyl which coreacts in the presence of water are suitable. If the crosslinking polymer is a separate polymer, it can be one such as an isocyanate or a melamine formaldehyde polymer.
The preferred star may contain both terminal and randomly placed functional groups. It may be most economical, but possibly not optimum to have the functional groups only on the outer end of the arm.
The flexible enamel vehicle of the invention is reinforced, i.e., toughened, by means of a functionally substituted acrylic star polymer preferably made by group transfer polymerization techniques, described in the above-identified WO patent publication, and summarized below and later in this specification.
Group transfer polymerization is a process in which the polymerization of monomers having carbon-carbon double bonds is initiated by certain initiators of the formula Q-Z, wherein Z is an activating substituent that becomes attached to one end of the growing polymer molecule and where Q is a group that continuously transfers to the other end of the growing polymer molecule as more monomer is added. The group Q is thus an active site than can initiate further polymerization of more monomer. The polymer molecule having the group Q is referred to as a “living” polymer, ad the group Q as a “living” group-transfer-initiating site.
Details of the group transfer polymerization process as it is applied to the preparation of large acrylic star polymers can be found in the aforementioned WO patent publication. The acrylic star polymers prepared by this method comprise (a) a core derived from a multifunctional monomer having at least two polymerizable double bonds, e.g., a di- or triacrylate or di- or trimethacrylate; (b) at least five polymeric arms attached to the core, e.g., polymer chains derived from a methacrylate polymer; and (c) “living” group transfer sites, e.g., the –Si(CH 3 ) 3 group, on the core and/or the arms. The “living” polymer may be deactivated, if desired, by contacting it with an active proton source such as an alcohol or water. In the aforementioned WO patent publication, various core-forming and arm-forming monomers, group transfer initiators, and catalysts are described as well as arm-first, core-first, and arm-core-arm techniques for preparing the star polymers. Preferably, arm-first techniques are used in the present invention.
The present invention involves the discovery that acrylic star polymers which bear reactive groups such as hydroxyl, carboxyl, epoxy, trialkoxysilyl, primary and secondary amino, hydroxyamide, alkoxyamide, i.e., groups capable of reacting with themselves, or with appropriate crosslinking agents such as nitrogen resins or di- and polyisocyanates, or flexible coreactants and preferably hydroxyl groups randomly and/or terminally situated along the star arms, are capable of reinforcing or enhancing the toughness of soft, flexible binders for enamels, resulting in blend compositions having a substantially improved hardness/flexibility balance over a wide range of temperatures. The ability of star polymers, even at concentration levels as low as 10-15% based on film weight, to reinforce such binders without deleteriously affecting the coating’s flexibility has been found not to be a characteristic of star polymers in general, but rather to be dependent upon the presence of the described reactive groups therein. Star polymers having no functionality lead to blend embrittlement. Moreover, the favorable hardness/flexibility balance achieved with the blend coatings vehicles of the invention has not been found in crosslinked star polymers per se, but requires a blend of two components having two different glass transition temperatures. While it is not intended that this invention be limited by theoretical considerations, it is believed that the unique properties achieved with blend vehicles containing substituted star polymers are due to the high degree of uniformity of dispersion of a relatively hard dispersed phase in a softer continuous phase. Much prior effort in two-phase materials concerns the well-known rubber-toughened plastics which have a soft elastomeric internal phase within a continuous phase of higher-modulus material. There is evidence that blends containing substituted star polymers have a two-phase structure wherein rigid internal inclusions are present in a continuous rubbery matrix.
Toughness is a different parameter than softness. One material can be softer than another, yet still be more brittle. When a particulate phase is tougher than and adherent to the matrix, cracks forming in the matrix tend to be stopped and the stresses absorbed in the particles. Toughness is the tendency to resist the formation and propagation of cracks and can be defined as the property of absorbing energy before fracture. With consolidated bodies rather than coatings, it is usually represented by the area under a stress-strain curve, usually measured in K joules/m 2 . Toughness involves both ductility and strength and thus is the opposite of the combined parameters of brittleness and lack of strength. Although the concept of toughness is more often encountered in the arts and sciences of consolidated materials such as molding resins, or metal sheet and bar, it is helpful in understanding the behavior of coatings, particularly in the context of the present invention.
Various tests can be used to determine the relative toughness of a material. The most relevant tests for purposes of the present invention are tests which show the relative degree of toughness in a coating. Thus, a suitable test would be forming a coating of the material to be tested on a glass substrate which has been soaped and rinsed, leaving a soap film residue. The coating can then be floated or peeled off the substrate and a stress-strain curve obtained by a tensile test of the coating itself.
The glass transition temperature or T g is one of the most important characteristics of amorphous (as contrasted to crystalline) polymers. Amorphous polymers which are crosslinked and have only one T g which is the rule when the material contains only one phase, will be hard and tough only in the narrow transition region between glass and rubber behavior. This range is usually no more than about 20° C. wide, so this normally limits coatings to behavior that is either glassy or rubbery or they undergo a major change in hardness in their use temperature range, which is not desirable. Nonacrylic polymers can partly circumvent this difficulty by using strong, noncovalent bonds between chains to increase their hardness in the rubbery plateau. Thus polyureas and polyurethanes can be fairly hard rubbers and for this reason have been candidates for use in coatings for both metal and rubber. Acrylic-containing systems on the other hand do not offer the opportunity for strong, noncovalent bonds between chains. In the present invention, the presence of two phases is used to broaden the area of tough, flexible behavior. The film is flexible when the use temperature is above the glass transition of either phase, and is relatively hard and tough when it is used below the glass transition of the higher T g phase, but above the lower T g .
Carbon Nanotube Muscles Strong as Diamond, Flexible as Rubber
For the next installment of the Terminator franchise, Hollywood might skip the polymimetic liquid alloys — they’re so 2003 — and turn to the laboratory of Ray Baughman, who has created a next-generation muscle from carbon nanotubes.
Baughman and his colleagues have produced a formulation that’s stronger than steel, as light as air and more flexible than rubber — a truly 21st century muscle. It could be used to make artificial limbs, “smart” skins, shape-changing structures, ultra-strong robots and — in the immediate future — highly-efficient solar cells.
“We can generate about 30 times the force per unit area of natural muscle,” said Baughman, director of the NanoTech Institute at the University of Texas at Dallas.
Carbon nanotubes have fascinated material scientists since the early 1990s, when researchers started to explore the ultra-light, ultra-strong cylindrical molecules. Though bulk manufacturing difficulties have slowed the development of commercial applications, carbon nanotubes are already used in bicycle components, and in prototypes of airplanes, bulletproof clothing, transistors, and ropes that might someday be used to tether a space elevator. (On a historical note, carbon nanotube-infused steel was used to made Damascus blades, renowned as history’s sharpest swords, though the technique has been lost.)
Baughman became interested in carbon nanotubes while designing artificial muscles from energy-conducting polymers. He figured he could do the job better with linked carbon nanotubes. First he made haphazard tangles of fibers activated by charged liquids. Then he experimented with more structurally-consistent configurations, and other methods of delivering the charge.
His latest muscle, described Thursday in Science, is made from bundles of vertically aligned nanotubes that respond directly to electricity. Lengthwise, the muscle can expand and contract with tremendous speed; from side-to-side, it’s super-stiff. Its possibilities may only be limited by the imaginations of engineers.
“This apparently unprecedented degree of anisotropy” — direction-dependent physical properties — “is akin to having diamond-like behavior in one direction, and rubber-like behavior in the others,” wrote John Madden, a University of British Columbia material scientist, in an accompanying commentary.
Baughman’s muscles rely on the tendency of an electric charge to make carbon nanotube fibers repel or attract each other, depending on their configuration.
Natural muscles, said Baughman, contract at a maximum rate of 10 percent per second. In the same amount of time, his latest nanotube sheaths can contract by 40,000 percent. Because it responds to an electrical current rather than ion movement in electrolytic liquids, it’s far more efficient than his old formulations.
The nanotube bundles retain their properties at temperatures ranging from the -320 degree Fahrenheit of liquid nitrogen to the 2800 degree Fahrenheit melting point of iron.
The first applications, said Baughman, will likely be as wrappers for solar cells, with nanotubes conducting electricity and rapidly changing shape in order to produce optimally light-sensitive arrangements.
“We’ve characterized the activity and performance,” he said. “Now we want to use them.”
Citations: “Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles.” By Ali E. Aliev, Jiyoung Oh, Mikhail E. Kozlov, Alexander A. Kuznetsov, Shaoli Fang, Alexandre F. Fonseca, Raquel Ovalle, Márcio D. Lima, Mohammad H. Haque, Yuri N. Gartstein, Mei Zhang, Anvar A. Zakhidov, Ray H. Baughman. Science, Vol. 323 Issue 5921, March 19, 2009.
“Stiffer Than Steel.” By John D. W. Madden. Science, Vol. 323 Issue 5921, March 19, 2009.
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Scientists Have Made Wonder Steel-Strong Flexible Plastic for Robocop Armors
Based on nanotechnology and the pattern of seashells
By Stefan Anitei, Science Editor
5th of October 2007, 08:23 GMT
Science fiction materials have turned real. Steel strong plastic has been created by a team at the University of Michigan by imitating the brick-and-mortar molecular structure of the seashells. But unlike steel, the new material is lighter and transparent, even if not stretchy enough.
The new plastic is built of layers of clay nanosheets and a water-soluble polymer similar to white glue.
“Nevertheless, its further development could lead to lighter, stronger armor for soldiers or police and their vehicles. It could also be used in microelectromechanical devices, microfluidics, biomedical sensors and valves and unmanned aircraft,” said co-author Nicholas Kotov, engineering professor.
Individual nanomaterial building blocks like nanotubes, nanosheets and nanorods are extremely strong, but previous larger materials made of them were comparatively weak.
“When you tried to build something you can hold in your arms, scientists had difficulties transferring the strength of individual nanosheets or nanotubes to the entire material. We’ve demonstrated that one can achieve almost ideal transfer of stress between nanosheets and a polymer matrix.” said Kotov.
The novel composite plastic was achieved with a machine that places one nanomaterial layer after another, consisting of an arm that hovers over a wheel of vials of various fluids.
Now, the arm employed a glass the size of a stick of gum on which the novel material was made. The glass was dipped by the arm into the glue-like polymer solution and then into a fluid representing a dispersion of clay nanosheets. When the layers dried, the operation was repeated.
300 layers of each glue-like polymer and the clay nanosheets resulted in a piece of plastic as thick as a plastic wrap. This is exactly how clams built layer by layer the mother of pearl, one of the most resistant natural materials, built from minerals (basically calcium carbonate).
The glue-like polymer (polyvinyl alcohol) had the role of sticking together the nanosheets. The nanoglue-nanosheet structure resulted in cooperative hydrogen bonds, inducing “the Velcro effect”: when broken, the bonds reformed rapidly in a new place. This explains the strength of the material and an important contribution is brought by the brick alternating arrangement of the nanosheets.
“When you have a brick-and-mortar structure, any cracks are blunted by each interface. It’s hard to replicate with nanoscale building blocks on a large scale, but that’s what we’ve achieved.” said Kotov.
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29th January 2007
The Ford is supporting the LEGO League for secondary schools. The league is the result of an alliance between education group FIRST (For Inspiration and Recognition of Science and Technology) and the global LEGO toy brand. Ford’s UK product development centre in Dunton, Essex, and BP, Ford’s partner on fuels, lubricants and future technologies, both sponsor FIRST LEGO League.
The annual FIRST LEGO League provides participating nine to 16-year-olds with a challenge and LEGO equipment to use for set tasks. They had to explore breakthroughs in the innovative area of microscopic-sized nanotechnology and demonstrate their new-found knowledge by designing, building and programming the robots at the final.
Up to 3,500 children in teams of 10 embarked on the 2006 FIRST LEGO League challenge which kicked off in September. Many had the advice and support of Ford engineers and apprentices acting as mentors. December regional heats were held at Ford’s Dunton technical centre, the Southampton Ford Transit plant and Halewood, where Ford transmissions and vehicles for Ford Motor Company-owned Jaguar and Land Rover are built.