Polymer engineering is generally an engineering field that designs, analyses, and/or modifies polymer materials. Polymer engineering covers aspects of petrochemical industry, polymerization, structure and characterization of polymers, properties of polymers, compounding and processing of polymers and description of major polymers, structure property relations and applications. Polymer materials and polymer solar cells are some of the most important technological inventions.
The basic division of polymers into thermoplastics and thermosets helps define their areas of application. The latter group of materials includes phenolic resins, polyesters and epoxy resins, all of which are used widely in composite materials when reinforced with stiff fibres such as fibreglass and aramids. Since crosslinking stabilises the thermosetting matrix of these materials, they have physical properties more similar to traditional engineering materials like steel. However, their very much lower densities compared with metals makes them ideal for lightweight structures.
A thermoplastic is a plastic that melts to a liquid when heated and freezes to a brittle, very glassy state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene). Thermoplastic polymers differ from thermosetting polymers as they can, unlike thermosetting polymers, be remelted and remoulded. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene and polypropylene. Moreover, thermoplastics have relatively low tensile moduli, but also have low densities and properties such as transparency which make them ideal for consumer products and medical products.
Thermosetting plastics (thermosets) are polymer materials that irreversibly cure form. The cure may be done through heat (generally above 200 degrees Celsius), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing. Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC's).
Elastomer is a polymer with the property of elasticity. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, and is preferred when referring to vulcanisates. Each of the monomers which link to form the polymer is usually made of carbon, hydrogen, oxygen and/or silicon. Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. At ambient temperatures rubbers are thus relatively soft (E~3MPa) and deformable. Their primary uses are for seals, adhesives and molded flexible parts.
Certain polymers have been fabricated that have the ability to "self-heal", i.e. to repair themselves after cracking. The polymer is embedded with a microcapsulated healing agent and a catalyst. When a crack approaches the microcapsules, the microcapsule is ruptured and the healing agent enters the crack plane due to capillary forces. The surrounding catalyst facilitates the polymerization of the healing agent, thus sealing the crack.
Electrostatic fiber spinning or ‘electrospinning’ is a novel process for forming fibers with submicron scale diameters through the action of electrostatic forces. When the electrical force at the interface of a polymer liquid overcomes the surface tension, a charged jet is ejected. The jet initially extends in a straight line then undergoes a vigorous whipping motion caused by the electrohydrodynamic instability. As the solvent evaporates, the polymer is collected onto a grounded mesh or plate in the form of a non-woven mat with high surface area to mass ratio (10–1000 m2/g). These non-woven mats are finding uses in filtration, protective clothing and biomedical applications.
 Fong H, Reneker DH. In: Salem DR, editor. Structure formation in polymeric fibers. Munich: Hanser Gardner Publications, Inc.; 2001. p.225–46  Shin YM, Hohman MM, Brenner MP, Rutledge GC. Appl Phys Lett 2001;78:1149–51  Shin YM, Hohman MM, Brenner MP, Rutledge GC. Polymer 2001;42:9955–67
animation clip that shows electrospinning: http://nano.mtu.edu/Electrospinning_start.html
SU8 is a viscous polymer used for patterning at the nanoscale. When exposed to an electron beam, SU-8's long molecular chains cross-link. The part of the material that is not cross-linked can the be easily washed away with a developing chemical.
Polymer Solar Cell
Polymer solar cells are a type of organic solar cell: they produce electricity from sunlight. A relatively novel technology, they are being researched by universities, national laboratories and several companies around the world.
The following discussion is regarding the physics of polymer solar cell. Organic photovoltaics are comprised of electron donor and electron acceptor materials rather than semiconductor p-n junctions. The molecules forming the electron donor region of organic PV cells, where exciton electron-hole pairs are generated, are generally conjugated polymers possessing delocalized π electrons that result from carbon p orbital hybridization. These π electrons can be excited by light in or near the visible part of the spectrum from the molecule's highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), denoted by a π -π* transition. The energy gap between these orbitals determines which wavelengths of light can be absorbed.
Unlike in an inorganic crystalline PV material, with its band structure and delocalized electrons, excitons in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4eV. This strong binding occurs because electronic wavefunctions in organic molecules are more localized, and electrostatic attraction can thus keep the electron and hole together as an exciton. The electron and hole can be dissociated by providing an interface across which the chemical potential of electrons decreases. The material that absorbed the photon is the donor, and the material acquiring the electron is called the acceptor. The polymer chain is the donor and the fullerene is the acceptor. After dissociation has taken place, the electron and hole may still be joined as a geminate pair and an electric field is then required to separate them.
After exciton dissociation, the electron and hole must be collected at contacts. However, charge carrier mobility now begins to play a major role: if mobility is not sufficiently high, the carriers will not reach the contacts, and will instead recombine at trap sites or remain in the device as undesirable space charges that oppose the drift of new carriers. The latter problem can occur if electron and hole mobilities are highly imbalanced, such that one species is much more mobile than the other. In that case, space-charge limited photocurrent (SCLP) hampers device performance.
As an example of the processes involved in device operation, organic photovoltaics can be fabricated with an active polymer and a fullerene-based electron acceptor. The illumination of this system by visible light leads to electron transfer from the polymer chain to a fullerene molecule. As a result, the formation of a photoinduced quasiparticle, or polaron, occurs on the polymer chain and the fullerene becomes an ion-radical C60-. Polarons are highly mobile along the length of the polymer chain and can diffuse away. Both the polaron and ion-radical possess spin S= ½, so the charge photoinduction and separation processes can be controlled by the Electron Paramagnetic Resonance method.
PDMS Microfludic Chip Fabrication
PDMS revolutionized the field of micro-patterning. In order to pattern micro-structures on a surface, the method traditionally used in the past was photolithography. The image on the left depicts this process which is costly and time consuming. It requires careful spin-coating of photoresist on a wafer, exposure to a powerful UV source and etching of silicon dioxide with the notoriously dangerous Hf acid. In addition, the process has to take place into an expensive clean room,since even the tiniest speck of dust on the surface of the wafer, compromises this delicate experiment.
PDMS (PolyDimethylSiloxane), can be bought at a low price in bulk quantities and is delivered into two pre-polymeric components: the elastomer and the curing agent. Upon use, the two are mixed at a 10:1 weight ratio. Once the mix is de-gassed to eliminate air bubbles, it is ready for use. A silicon micro-patterned wafer, manufactured through a photolithography method, can be used as a master. PDMS is poured on top, and once the polymer cures it can be peeled off the surface of the master and features the desired patterns. Here lies the beauty of soft lithography: the silicon master can be reused several times, which eliminates the need of going back into the clean room for a long long time.
PDMS allows for microfluidic devices that are cheap, easy to make and therefore disposable. In addition, the surface properties of the natively hydrophobic polymer can easily be manipulated: exposure to an oxygen plasma renders the PDMS surface hydrophilic, while covalent grafting of PEG silane on PDMS renders it non-adhesive to protein. It comes as no surprise that PDMS has several biomedical applications. For example PDMS channels are used for cell sorting, where different cells are separated by electrosmosis.
Gelatin is a special protein in the culinary universe. As Harold McGee writes, "Gelatin is the easiest, most forgiving protein any cook deals with. Heat it up with water and its molecules let go of each other and become dispersed among the water molecules; cool it and they rebond to each other; heat it again and they disperse again." This is the exact opposite of most other proteins in the culinary world, which become unfolded and tangled during this process. That is why eggs solidify, meat becomes stiff, and milk curdles. Gelatin can form a solid with as little as 1% by weight in a stock. At this concentration, the long chains can overlap and form a continuous network. When the gelatin cools below 40 C, the individual polymers try to return to the triple helix form that they have in collagen.
This reversibility is interesting from a soft matter perspective. For instance, what physical features of gelatin allow this reversibility? How does the issue of reversibility related to polymers in general?
A major manufacturer of gelatin is Rousselot, which provides some interesting data about their product on their website:
- "For temperature above 35°C, gelatine gives a solution exhibiting a viscosity ranging from 1.5 and 5 mPa.s. This is measured by the time a 6.67% gelatine solution takes to flow through a standardized viscosimetric pipette at a temperature of 60°C."
- "In the gelatine world, gel strength is traditionally referred as Bloom. It is the force, expressed in grams, necessary to depress by 4 mm the surface of a gelatine gel with a standard plunger (AOAC). The gel has a concentration of 6.67% and has been kept 17 hours at 10°C." As a physicist, I think that a better definition should be possible. The 4 mm, "standard" plunger, and 17 hour time period seem somewhat arbitrary. How is gel strength measured in the rheological community in general?
- "A gel or a solution of gelatine is a polydisperse macromolecule made of thousands of amino-acid chains that are either free or linked to each other. Each amino-acid chain has a molecular weight between 10 000 and several hundred thousands Daltons (Mw)."
Gelatin is so popular since its melting point is close to the human body temperature. This is similar to the melting point of fat in dairy products and this effect can be enhanced through other polymers (some of which are described below). Mixtures of polymers is a topic that we haven't seem to have covered in class yet.
Polymers from seaweed
- Under the broad, but increasingly imprecise, label of "molecular gastronomy," chefs around the world are experimenting with hydrocolloids. Physically, this means nothing more than a suspension of particles in water. Gastronomically, the challenge of creating a stable suspension leads to much of the food that we eat. For instance, Grant Achatz, the chef of Alinea in Chicago, utilizes polymers like agar-agar and gelatin in his cuisine. This allowed him created a veil of Guinsess beer over a confit of beef short ribs, among other novel creations.
Xantham, locust bean, and guam gum
Xantham is a polymer often used for its shear-thinning properties. This is ideal for batters, like tempura. It is also used to prevent crystallization in order to prolong the lifetime of bread and maintain the texture of ice cream. Locust bean gum has similar uses. Guar gum can be used substituted for xantham gum and has four times the water binding capacity of the locust bean gum.
Note: much of the information about hydrocolloids was adapted from khymos.org, a fantastic collection of information about the science of cooking.
One hot area of polymer research is the properties of spider silk. Spider silk is extremely strong, elastic and durable. Scientists are currently trying to create synthetics polymers with similar properties and also better understand the natural material. Spider silk starts as a protein solution that dries and lengthens into a string as the spider spins its web. Small crystal structures form as the silk dries, giving strength. (Source: http://web.mit.edu/newsoffice/2006/spider.html)
Polyurethane foams are used everyday in products such as furniture and bed padding, insulators in refrigerators and freezers to say a few. It is also used in less common products such as flame-retarding insulation layers and artificial limbs.
Polyurethane foams find widespread use because of their versatility and also manufacturing efficiency. The manufacturing efficiency arises because the polymerization reactions consume all raw materials and leave no byproducts. In addition, selective catalysts promote high production rates and provide control over the physical properties of the final product. Economic efficiency occurs because processing is carried out at low pressure. Thus, capital costs are relatively low, and improving existing processes or developing new products does not require extensive capital reinvestments. Versatility arises because additives, such as fillers, plasticizers, dyes, and so forth, produce products with a wide range of mechanical properties, surface textures, and colors.
Foams are organized into three major categories: high-density flexible foams, low-density flexible foams, and low-density rigid foams. All are produced by the same strategy. An exothermic reaction between polyisocyanates and polyols generates the polymer. The heat liberated during the reacting then initiates the formation of gas bubbles and the polymerization process stabilizes foam growth. To control shape, the reacting mixture is either injected into a mold or formed as a slab on a conveyor belt and cut to size after curing.
Work done at MIT has shown that shape memory polymers are attractive to biomedical applications where in devices are implanted in the body, or internal wounds are endoscopically sealed with sutures.
Shape-memory polymers possess the ability to memorize a permanent shape that can substantially differ from their initial temporary shape. Large bulky devices could thus potentially be introduced into the body in a compressed temporary shape by means of minimally invasive surgery and then be expanded on demand to their permanent shape to fit as required. In the same way, a complex mechanical deformation could be performed automatically instead of manually by the surgeon. The transition from the temporary to the permanent shape could be initiated by an external stimulus such as a temperature increase above the switching transition temperature Ttrans of the polymer
In metallic alloys, the shape-memory effect is due to a martensitic phase transition. In contrast, the polymers designed to exhibit a thermally induced shape-memory effect require two components on the molecular level: cross-links to determine the permanent shape and switching segments with Ttrans to fix the temporary shape. Above Ttrans, the permanent shape can be deformed by application of an external stress. After cooling below Ttrans and subsequent release of the external stress, the temporary shape is obtained. The sample recovers its permanent shape upon heating to T > Ttrans. Cross-links can be either covalent bonds or physical interactions. Recently, we have reported on shape-memory polymers, which are covalently cross-linked polymer networks containing hydrolyzable switching segments. Emphasis in the present work was put on the development of a group of polymers that contain physical cross-links. These thermoplastics are easily processed from solution or melt and are substantially tougher than polymer networks. In particular, they are degradable, showing linear mass loss during hydrolytic degradation.
A challenge in endoscopic surgery is the tying of a knot with instruments and sutures to close an incision or open lumen. It is especially difficult to manipulate the suture so that the wound lips are pressed together under the right stress. When the knot is fixed with a force that is too strong, necrosis of the surrounding tissue can occur. If the force is too weak, scar tissue, which has poorer mechanical properties, forms and may lead to the formation of hernias. A possible solution is the design of a smart surgical suture, whose temporary shape would be obtained by elongating the fiber with controlled stress. This suture could be applied loosely in its temporary shape; when the temperature was raised above Ttrans, the suture would shrink and tighten the knot, applying the optimum force.
REF: "Biodegradable, Elastic Shape-Memory Polymers for Potential Biomedical Applications" Science [0036-8075] Lendlein yr:2002 vol:296 iss:5573 pg:1673
Silly Putty is a mixture of inorganic silicone polymers created by accident during World War II through rubber replacement research. The original Silly Putty is composed of 65% dimethyl siloxane (hydroxy-terminated polymers with boric acid), 17% silica (crystalline quartz), 9% Thixatrol ST (castor oil derivative), 4% polydimethylsiloxane, 1% decamethyl cyclopentasiloxane, 1% glycerine, and 1% titanium dioxide. It has interesting propoerties as a non-Newtonian fluid including bouncing, breaking when hit sharply and flowing over long periods of time. While it has been used in physical therapy and an adhesive in space as well as a toy, the material did not have many practical uses.