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Polymer Science and Engineering - مطالب ابر material

Nanostructured and Functionalized Materials & Devices

Materials for biomedical applications

Molecular functionalization techniques are used to tailor substrates for biomedical applications. Examples include microbicidal surfaces which inhibit biofilm formation; high strength bone cements with bactericidal biopolymers or antibiotic-conjugated monomers which achieved higher antibacterial efficacy longer than the present cements; and magnetic nanoparticles for bioimaging and tumor targeting. In a separate development, the adsorption disruption of oriented liquid crystal molecules on a patterned surface was developed into a new label-free optical method for the simultaneous detection of multiple glycine oilgomers using sample size as little as 2 μL.

Materials for energy applications

Research in energy is focused on the design and synthesis of new and alternative materials for energy supply, transformation, storage, delivery and end-use. There are strong coordinated efforts in developing catalysts for fuel production, and for the non-oil based route to chemicals production. Ceramic membranes are used to produce oxygen from air. Electrochemical energy conversion is another focused research area covering a number of technological areas: anode materials for lithium-ion batteries, catalysts and polymer electrolyte membranes for direct alcohols fuel cells, and materials for supercapacitors.

Materials for optoelectronic applications

Many optical devices based on 3D photonic crystals, such as optical switches, low-threshold lasers and light-emitting diodes, and waveguide, require the exact placement of artificial defects embedded in the interior of the photonic crystals. We have recently embedded artificial line-defects in a 3D photonic crystal using a combination of “bottom-up” self-assembly method and the conventional “top-down” technique. The new technique circumvents some of the problems in the self-assembly approach to fabricating functional photonic devices from photonic crystals.

Polymer and molecular electronics

Molecular memories based on polymers and organic materials have the advantages of simplicity in structure, drive-free read and write capability, good scalability, 3-D stacking ability, low-cost potential, and a large capacity for data storage. By combining molecular design with novel synthesis approaches, several polymer/molecular memories, including flash (rewritable) memory, write-once read-many-times (WORM) memory and dynamic random access memory (DRAM) have been realized. All these devices exhibit stable states with high ON/OFF current ratios (104-107), and perform up to 108 read cycles under ambient conditions.

Self-assembly of nanomaterials

A real-world functional material (e.g. solid catalyst) is a highly organized multi-component materials system. This modern view calls for the development of new strategies promoting the self-assembly of various functional components. For example, catalytic metals such as Au and Co can be introduced to the exterior surfaces or interior spaces of photosensitized metal oxide systems to enhance their functions as preparative nano-reactors. In another development colloidal and interfacial polymerizations are used to produce hydrophilic-lipophilic polymer composite membranes for separation; and micro-spheres and porous continuous media for catalyst immobilization and storage of energetic materials.

reference : http://www.chbe.nus.edu.sg/research/materials#


برچسب ها: materials، polymers، material، nanomaterials، nano،

تاریخ : شنبه 11 دی 1395 | 05:47 ب.ظ | نویسنده : Arash Sadeghi | نظرات


Properties Of Graphene

 Graphene is , basically, a single atomic layer of graphite; an abundant mineral which is an allotrope of carbon that is made up of very tightly bonded carbon atoms organised into a hexagonal lattice. What makes graphene so special is its sp2 hybridisation and very thin atomic thickness (of 0.345Nm). These properties are what enable graphene to break so many records in terms of strength, electricity and heat conduction (as well as many others). Now, let’s explore just what makes graphene so special, what are its intrinsic properties that separate it from other forms of carbon, and other 2D crystalline compounds?

Fundamental Characteristics

Before monolayer graphene was isolated in 2004, it was theoretically believed that two dimensional compounds could not exist due to thermal instability when separated. However, once graphene was isolated, it was clear that it was actually possible, and it took scientists some time to find out exactly how. After suspended graphene sheets were studied by transmission electron microscopy, scientists believed that they found the reason to be due to slight rippling in the graphene, modifying the structure of the material. However, later research suggests that it is actually due to the fact that the carbon to carbon bonds in graphene are so small and strong that they prevent thermal fluctuations from destabilizing it.

Electronic Properties

One of the most useful properties of graphene is that it is a zero-overlap semimetal (with both holes and electrons as charge carriers) with very high electrical conductivity. Carbon atoms have a total of 6 electrons; 2 in the inner shell and 4 in the outer shell. The 4 outer shell electrons in an individual carbon atom are available for chemical bonding, but in graphene, each atom is connected to 3 other carbon atoms on the two dimensional plane, leaving 1 electron freely available in the third dimension for electronic conduction. These highly-mobile electrons are called pi (π) electrons and are located above and below the graphene sheet. These pi orbitals overlap and help to enhance the carbon to carbon bonds in graphene. Fundamentally, the electronic properties of graphene are dictated by the bonding and anti-bonding (the valance and conduction bands) of these pi orbitals.

Combined research over the last 50 years has proved that at the Dirac point in graphene, electrons and holes have zero effective mass. This occurs because the energy – movement relation (the spectrum for excitations) is linear for low energies near the 6 individual corners of the Brillouin zone. These electrons and holes are known as Dirac fermions, or Graphinos, and the 6 corners of the Brillouin zone are known as the Dirac points. Due to the zero density of states at the Dirac points, electronic conductivity is actually quite low. However, the Fermi level can be changed by doping (with electrons or holes) to create a material that is potentially better at conducting electricity than, for example, copper at room temperature.

Tests have shown that the electronic mobility of graphene is very high, with previously reported results above 15,000 cm2·V−1·s−1 and theoretically potential limits of 200,000 cm2·V−1·s−1 (limited by the scattering of graphene’s acoustic photons). It is said that graphene electrons act very much like photons in their mobility due to their lack of mass. These charge carriers are able to travel sub-micrometer distances without scattering; a phenomenon known as ballistic transport. However, the quality of the graphene and the substrate that is used will be the limiting factors. With silicon dioxide as the substrate, for example, mobility is potentially limited to 40,000 cm2·V−1·s−1.

Mechanical Strength

Another of graphene’s stand-out properties is its inherent strength. Due to the strength of its 0.142 Nm-long carbon bonds, graphene is the strongest material ever discovered, with an ultimate tensile strength of 130,000,000,000 Pascals (or 130 gigapascals), compared to 400,000,000 for A36 structural steel, or 375,700,000 for Aramid (Kevlar). Not only is graphene extraordinarily strong, it is also very light at 0.77milligrams per square metre (for comparison purposes, 1 square metre of paper is roughly 1000 times heavier). It is often said that a single sheet of graphene (being only 1 atom thick), sufficient in size enough to cover a whole football field, would weigh under 1 single gram.

What makes this particularly special is that graphene also contains elastic properties, being able to retain its initial size after strain. In 2007, Atomic force microscopic (AFM) tests were carried out on graphene sheets that were suspended over silicone dioxide cavities. These tests showed that graphene sheets (with thicknesses of between 2 and 8 Nm) had spring constants in the region of 1-5 N/m and a Young’s modulus (different to that of three-dimensional graphite) of 0.5 TPa. Again, these superlative figures are based on theoretical prospects using graphene that is unflawed containing no imperfections whatsoever and currently very expensive and difficult to artificially reproduce, though production techniques are steadily improving, ultimately reducing costs and complexity.

Optical Properties

Graphene’s ability to absorb a rather large 2.3% of white light is also a unique and interesting property, especially considering that it is only 1 atom thick. This is due to its aforementioned electronic properties; the electrons acting like massless charge carriers with very high mobility. A few years ago, it was proved that the amount of white light absorbed is based on the Fine Structure Constant, rather than being dictated by material specifics. Adding another layer of graphene increases the amount of white light absorbed by approximately the same value (2.3%). Graphene’s opacity of πα ≈ 2.3% equates to a universal dynamic conductivity value of G=e2/4ℏ (±2-3%) over the visible frequency range.

Due to these impressive characteristics, it has been observed that once optical intensity reaches a certain threshold (known as the saturation fluence) saturable absorption takes place (very high intensity light causes a reduction in absorption). This is an important characteristic with regards to the mode-locking of fibre lasers. Due to graphene’s properties of wavelength-insensitive ultrafast saturable absorption, full-band mode locking has been achieved using an erbium-doped dissipative soliton fibre laser capable of obtaining wavelength tuning as large as 30 nm.

In terms of how far along we are to understanding the true properties of graphene, this is just the tip of the iceberg. Before graphene is heavily integrated into the areas in which we believe it will excel at, we need to spend a lot more time understanding just what makes it such an amazing material. Unfortunately, while we have a lot of imagination in coming up with new ideas for potential applications and uses for graphene, it takes time to fully appreciate how and what graphene really is in order to develop these ideas into reality. This is not necessarily a bad thing, however, as it gives us opportunities to stumble over other previously under-researched or overlooked super-materials, such as the family of 2D crystalline structures that graphene has born.

reference : geraphenea.com 



طبقه بندی: شیمی،
برچسب ها: Geraphenea، geraphen، polymer، properties، material، electricity، carbon،

تاریخ : چهارشنبه 8 دی 1395 | 12:16 ق.ظ | نویسنده : Arash Sadeghi | نظرات

The story of Graphene

If you've ever drawn with a pencil, you've probably made graphene. The world's thinnest material is set to revolutionise almost every part of everyday life.

Fascination with this material stems from its remarkable physical properties and the potential applications these properties offer for the future. Although scientists knew one atom thick, two-dimensional crystal graphene existed, no-one had worked out how to extract it from graphite.

That was until it was isolated in 2004 by two researchers at The University of Manchester, Prof Andre Geim and Prof Kostya Novoselov. This is the story of how that stunning scientific feat came about and why Andre and Kostya won the Nobel Prize in Physics for their pioneering work.




طبقه بندی: شیمی،
برچسب ها: graphene، geraphen، polymer، material، physics،

تاریخ : چهارشنبه 8 دی 1395 | 12:08 ق.ظ | نویسنده : Arash Sadeghi | نظرات

What Are Synthetic Polymers?

Check out these images of useful, everyday items. Do you notice anything that they have in common? For one, all these compounds are super strong, cheap, and easy to make. Secondly, they are all examples of this video's topic: synthetic polymers! But what is a synthetic polymer?

Let's break the term apart to discover the definition. To start, a compound that is synthetic is man-made and produced by chemical reactions. Synthetic compounds may be made as exact replicas of naturally occurring compounds like vitamin C, or they may be unique compounds like plastic.

To talk about polymers, imagine a paperclip chain. If you've got time (and lots of paperclips), you can just make one instead of thinking about it! A paperclip chain is like a polymer. It is a long, strong chain made of many paperclips hooked together. By definition, a polymer is a compound that is made of many small repeating units bonded together. In our case, the small repeating units are the paper clips.

We use the scientific term 'monomer' to describe the small, repeating units used to make up a polymer. Polymers usually consist of tens of thousands of monomers, all bonded together. Huge molecules like these are often referred to as macromolecules. Our world is loaded with naturally occurring polymers, like cellulose (the stuff in plant fibers), DNA (the molecule that contain our genes), and silk.

Now, we can put our two terms together! A synthetic polymer is a man-made macromolecule that is made of thousands of repeating units. Sometimes these polymers are straight-chained, like our paperclip chain example, and consist of one long chain of monomers bonded end to end.

Sometimes polymers are both straight-chained and branched. This means that neighboring chains will bond with each other and make vast, net-like structures. This type of bonding between chains is called crosslinking.

Synthetic polymers are lightweight, hard to break, and last a long time. They are quite cheap to make and easy to form into shapes.

One of the most common and versatile polymers is polyethylene. It is made from ethylene (also known as ethene) monomers. In polymer form, the double bond between the carbons is lost and a chain is formed between repeating units of two carbons, each bonded to two hydrogens.

Polymer chain
Polymer chain

Sometimes for brevity's sake, the polymer chain is represented like the image you see here, with a large pair of parentheses around the monomer. You'll notice that there is an n in the bottom right hand corner outside the parenthesis. This n can represent any number. It could be 5 or 10,000! Often times, it is just left as a simple n to show it is a polymer of varying length.

Polyethylene is used to make plastics of all sizes and shapes, from piping to bottles to toys. And if you've ever dealt with these pesky things, then you know polyethylene!

Examples

Polyethylene has a pretty popular cousin, named polyethylene terephthalate (abbreviated PET or PETE). You might recognize PET from our intro! PET is commonly used for packaging liquids, especially sodas. PET is also used to make plastics that need to tolerate extreme temperatures.

PET is a great example of a thermoplastic. Thermoplastics are solid until heated to a certain temperature. When they get to that special temperature, they can be molded into any shape. Once they cool, their shape is set. Thermoplastics can be melted down once they are used up or no longer needed, and reshaped! This process is known as recycling.

Maybe you've seen these symbols on some plastics? They tell you several things. First, the item is a thermoplastic. Also, the number represents the type of polymer. And lastly, this symbol lets you know that this is very recyclable! Next time you are using something in a plastic bottle, look for one of these symbols. Then when you're finished using the plastic bottle, make sure to recycle it




طبقه بندی: اطلاعات پلیمری،
برچسب ها: polymer، monomers، material، plastic، synthetic، polymers،

تاریخ : سه شنبه 7 دی 1395 | 12:43 ق.ظ | نویسنده : Arash Sadeghi | نظرات

What Are Polymers

What do DNA, a plastic bottle, and wood all have in common? Give up? They are all polymers!

Polymers are very large molecules that are made up of thousands - even millions - of atoms that are bonded together in a repeating pattern. The structure of a polymer is easily visualized by imagining a chain. The chain has many links that are connected together. In the same way the atoms within the polymer are bonded to each other to form links in the polymer chain.

The molecular links in the polymer chain are called repeat units that are formed from one or more molecules called monomers. The structure of the repeat unit can vary widely and depends on the raw materials that make up the polymer. For example, polyethylene, the polymer used to make a wide variety of plastic bags and containers, has a very simple repeat unit, two carbons that are bonded to one another to form a single link.




طبقه بندی: اطلاعات پلیمری،
برچسب ها: polymer، chain، molecular، monomer، material،

تاریخ : سه شنبه 7 دی 1395 | 12:38 ق.ظ | نویسنده : Arash Sadeghi | نظرات
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