Karen A. Beitler
'Poly' means many and 'mer' means 'units'. A polymer is made up of many 'mer' units linked together in long chains. Polymers can be very large molecules and possess a variety of properties depending on the extent to which they have been processed and what they are made from. Most polymers are former hydrocarbons or derivatives of hydrocarbons.
Natural polymers such as silk, cotton, starches (spaghetti is a polymer!) and even sand are large molecules made from monomers of carbon, hydrogen and oxygen. Carbon has four bonding sites and bonds to itself with single, double and triple bonds. Carbon(C) atomic number 6, Hydrogen, (H), atomic number 1, and Oxygen (O), atomic number 8, form two of the four macromolecules that make up all of life: carbohydrates (sugars) and lipids (fats). When Nitrogen (N) atomic number 7 is added then protein and nucleic acids are formed. Carbohydrates are the sugars, starch and the cellulose in a plant; they are made up of long chains of the monomer glucose. Carbohydrates provide energy for the cell. Lipids are long hydrocarbon chains they provide protection and selection for the cell. Proteins are long chains of polypeptides. Polypeptides are made up of long chains of amino acids and other elements. These macromolecules are the building blocks of all living things and are cycled throughout the Earth. Seventy eight percent of our atmosphere is nitrogen, 21% oxygen and most of the 1 % left is carbon, water vapor and other gases. The waters of the Earth are hydrogen, and oxygen with a large supply of carbon and our land is made up of decaying matter and minerals.
The simplest molecule in the decay of living thing is a hydrocarbon. This molecule is a chain of hydrogen and carbon atoms and is classified by the type of bond it contains: single, double or triple. Hydrocarbons are organic molecules, polymers that occur in nature. They make up fossil fuels, are in all plants and animals and form pigments that make the green vegetable green and orange vegetables orange. The largest percentage of natural rubber is a hydrocarbon polymer that has become one of the most manipulated structures to date.
Synthetic polymers are often derived from organic polymers. Synthetic polymers are difficult to avoid in today's world. As technology advances polymers become more complex and much more diverse. Polymer products have revolutionized our lives. Most man-made polymers are some form of hydrocarbon or hydrocarbon derivative. The term plastic refers to polymers that are synthesized from fossil fuels. All plastics are polymers but not all polymers are plastic. Some artificial polymers can stick very well to materials such as metals and are used coatings. Polymers can be molded, stretched, shaped, reformulated and made into just about everything we use everyday. A polymer alone or in combination with other materials has a global market reach in everyday items.
Polymers in everyday life
Open a door or even a window and you will see all kinds of polymers. Inside every living thing is a very complex polymer called DNA that carries the blueprint to recreate that living organism. Plants contain cellulose, an excellent fiber built out of polysaccharides that make up cotton, plant stems, wood and paper. Organic polymers like cellulose were used to make some of the first the first synthetic polymers. Starch is another organic polymer found in corn, wheat and potatoes that is also made of the monomer glucose and used as the basis of biologically degradable polymer mixtures. Starch is used to make environmentally friendly 'packaging peanuts" which dissolve in water. Starch can be broken down by another polymer, enzymes, into glucose so the cell's mitochondria can use it for energy. Enzymes have found a place in coating synthetic polymers to reduce infection when placed inside the body. This field of research is new and upcoming and holds promise for the making of artificial organs.
What makes these three polymers different is how the glucose molecules are put together in the polymer chain. The difference between starch & cellulose is where the OH molecule is located on the glucose ring. Alpha glucose, will make starch for energy, beta-glucose will make cellulose for strength. Enzymes are proteins made by protein synthesis inside of cells, starch & cellulose are made by dehydrations synthesis where a water molecule is lost and the glucose molecules are joined, a condensation reaction. If all of the water were removed from that bowl of spaghetti, what would be left would be hard, brittle noodles, starch, with different properties but still the same molecule. By studying the structures of these polymer molecules and how they are formed in nature, man has learned to make synthetic polymers that we find around us everyday.
Polymer Make up
All compounds are made up of two or more different types of atoms. For example H
2
(hydrogen gas) is a molecule but H
2
O is a compound because both hydrogen and oxygen are present. H
2
O is also a molecule because how hydrogen and oxygen bond. Polymers are long chains of molecules. Polymers are very large molecules and can be branched or cross-linked. There are two major types of polymers; natural, as in found in nature, and synthetic, formulated by man. Natural polymers are complex carbohydrates such as cellulose, glycogen, cotton, wool, silk and starch. Natural polymers have enzymes to break them down. Enzymes lower the activation energy needed for natural substances to breakdown, thereby increasing decomposition time. And there are established pathways that recycle each of the components. Natural polymers were the inspiration for man-made polymers.
The first polymers were made by a condensation reaction called dehydration synthesis. In this condensation reaction, losing a water molecule combines "mers" and chain is formed. In the making of polymers a series of reactions called condensation reactions take place and monomers or monomer chains are added to one another to form longer chains. Other names for these reactions are 'step-growth polymerization' or 'condensation polymerization'. If there were two monomers labeled 'A' and 'B' then these reactions can form A-A and B-B, polymers as well as A-B polymers. A small molecule is usually liberated.
This type of reaction is used as a basis for the making of many important polymers. Only the number of monomers that are available to bond limits this process. When other elements were added, the range of polymerization expanded and the properties of the polymers became more diverse. These 'copolymers' were compressed to become tougher, harder, and more elastic, and have greater strength. Still these did not compare with natural polymers where one identical monomer unit was added at a time to the ends of polymer chains and the process of breaking down and making new compounds has evolved over eons of time. New techniques were developed in the making of synthetic polymers and a four-step process is now used in the synthesis of most polymers.
Types of Polymers
There are two major types of polymers: ones that can be heated and remolded (thermoplastics) and those that cannot (thermosets). Charles Goodyear found that if he added sulfur and white lead and heat to natural rubber he could make cross links and the rubber would not melt when it was hot outside nor become hard when the temperature dropped; thus vulcanized rubber was born. This proved to be very useful for making tires, and taught scientists to heat natural polymers and add chemical cross-links to make them stronger and more durable. Suddenly thermosets were born. Thermoset polymers are molded and shaped before heating and after heating retain their shape. Thermosets are cured (or hardened) in a process known as vulcanization, which produces an irreversible chemical reaction. This changes the material forever, making a stable, chemical and thermal resistant durable product. While thermosets have their uses, however, they are not easily broken down and therefore pose environmental issues. What happens to the tire when it is no longer useful for a car to ride on?
In search of a more pliable material, chemists found they could heat natural polymers and mold them without adding chemicals to crosslink and change them permanently. These became known as thermoplastics, the change in these products is purely physical and with reapplication of heat, reversible. This makes recycling of thermoplastics a possibility until the material begins to degrade. While thermoplastics do service humans, they will eventually end up in landfills and are not completely recyclable.
Scientists continue to work with heat and rubber and other natural polymers to create an "in-between" type of polymer. This is known as a thermoplastic elastomer. This is a blended material that has two phases, called co-polymers - one that is pliable (ionomers) and the other made of more than one type of monomer is called a block co-polymer. Ionomers have a few ionic groups attached to them that "tie up" the polymer backbone chains, like a cross-link would. If heated these ionic clusters will break up and can be reprocessed and recycler like natural polymers. Cool them and the ionic cluster form again and the material acts like a cross link again. Elastomers are a type of thermoplastic that retain their shape after being deformed, both natural and synthetic rubber make good raw materials tires for this reason.
Block copolymers are made out of one, two, or more co-monomers. The different monomer "blocks" alternate in a series helping to bond making high-temperature resilience and low-temperature flexibility. Polyurethane, shoe soles and tire treads are made from block copolymers. The next generation Nano devices could be based on these copolymers as we search for materials that are better insulators and photoresists. Photoresists are polymeric coatings that are designed to change properties upon exposure to light and are used to print the patterns of conductors on circuit boards and the tiny transistors on microchips. Our next-generation computers may come from polymer modification chemistry. Polymer derivatives are being explored everyday; hopefully scientists will continue to look to nature for better ways to recycle these materials as well.
One process called cross-linking makes polymers stronger, flexible, and more durable and keeps natural polymers from rotting. This may be one link in the recycling process. Tanning is an example of a cross-linking process that creates a usable but totally recyclable product This is the process of making animal skin (rawhide) into leather by adding brains, oak bark or chrome salts to alter the protein structure of the skin causing the crosslinks that make the skin more pliable. Add a little friction (heat) and the skin becomes soft, resilient, and supple. Scientists are looking at this process to make better polymers for clothing and to make products more recyclable.
Addition of inorganic particles to polymers can augment the strength, conductivity, optical and catalytic activity of polymers. Owen Webster in his article Living Polymerization Methods states that "Possibly the most useful physical property of polymers is their low density versus strength." He tells of the "nonstop circumnavigation of the world by a plastic airplane on one tank of gas and by the construction of an airplane light enough to fly more than 110 km nonstop under human pedal power." As man continues to explore the diverse natural world, he learns to apply the principals and processes found in nature to make better products for humans and for the Earth. The problem may be in the products we have already created and our ability to break them back into useful elements that will become future products.
Plastics
As scientists have learned to heat and add molecules to make up the world of plastics as we know it. The numbering of polymers and efforts to recycle, reuse, reclaim plastics in our world has been due to the public outcry for curtail the volume of man-made products that alter ecosystems in the natural world. Humans dredge millions of gallons of fossil fuels from underneath the earth's surface to make their lives easier. These fossil fuels have been refined into useful human products. Fuel sources for airplanes, cars, boats and our homes have been refined in huge factories for our comfort. The byproducts of these processes are used to cover our roads (asphalt) and now just about everything we use as plastics have been developed from the last end products of this refining process.
Part of the science behind the widespread use of these copolymers is their relatively narrow molecular weight distribution and low gel content. Low gel content is a required property because gel molecules often tangle, causing processing and clarity issues. Low gel content is required for good clarity and process ability, especially for end-use applications such as film, while narrow molecular weight distribution allows for transparency. Also, low gel content also is important in multi-layer structures where gels can cause dimpling and other imperfections. Next time you squeeze that perfect portion of toothpaste on your toothbrush or cover your fries with ketchup, you may be holding an award-winning packaging product in your hands.
The polymers are also used in single or multiple layers for a number of applications such as part of the structure for toothpaste tubes and the small foil condiment pouches used at restaurants and fast-food outlets. Juice boxes and dry food packaging, among many other applications, also often contain the polymers.
Artificial (synthetic) polymers include plastics like Low-density polyethylene (LDPE) and High-density polyethylene (HDPE) used for plastic bags, food packaging, and other plastic containers. Styrofoam, plastic wrap and fabric fibers such as nylon, Rayon and Dacron (polyester) are all synthetic polymers. Also, coating materials such as Formica and Teflon, as well as hard plastics like PVC pipe and the Kevlar that is used for bulletproof vests. To show how well man made polymers have been integrated into our lives, a student will examine what she encounters in a typical morning getting ready for school and point out the polymers she comes in contact with.