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.
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.
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.