Back To CoursePhysical Science: Help and Review
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Nathan, a PhD chemist, has taught chemistry and physical science courses.
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.
Polymers are created through chemical reactions known as polymerizations, and the majority are produced through two basic reaction types. The first type of polymerization reaction is known as a condensation polymerization. The second type of reaction is known as chain-growth polymerization.
Condensation polymerizations, also called step-growth polymerizations, occur when two monomers react to yield a repeat unit and a smaller molecule such as water. A great example of this type of reaction is the polymerization of nylon from monomers with carboxylic acids and basic amines. The reaction (shown below) creates a link between each monomer and produces water as a by-product and is used to produce nylon fibers for clothing.
Chain growth polymerizations occur when a monomer forms a highly reactive free radical, or molecule with an unpaired electron. The free radical reacts quickly with another monomer and causes a repeat unit with another free radical. A rapid chain reaction continues the polymerization, and the polymer chain continues to grow longer. One example of a polymer made through a chain-growth polymerization is polystyrene, a polymer commonly found in disposable drinking cups.
Since many polymers are made of long, flexible chains, they become easily tangled, much like a bowl of cooked spaghetti. The disordered tangling of the polymer chains create what is known as an amorphous structure. Amorphous polymers are typically transparent and much easier to melt to make materials like kitchen cling film.
Polymer chains do not always form amorphous arrangements. Under proper conditions, such as stretching, the polymer chains can line up side by side to form orderly, crystalline arrangements. Crystalline arrangements in polymers can also be achieved through slow cooling, where individual polymer chains fold over on themselves.
Polymers can also be used to create huge 3-dimensional networks. These networks are made through the reaction of monomers with more than two possible sites for the polymerization to occur. The multiple reaction sites allow for the different chains to connect with each other to form cross-linked chains. The result of the cross-linked chains is a 3-dimensional solid that is essentially one huge molecule.
Many of the polymers that we are familiar with from our everyday lives are known as plastics. The plastics, or thermoplastics, are polymers that soften when heated and are molded into different forms. Thermoplastics are used to make everything from soda bottles to picnic cutlery.
Another application of polymers is the long strands known as fibers. Fibers include many types of synthetic yarn or rope that are made from amorphous materials such as the polyesters. Crystalline polymers can also be used to make fibers, one of the most famous being the fibers found in bullet resistant clothing.
The final industrial applications to be discussed here are rubber and foams. These materials consist of cross-linked polymer chains. Rubber is found in materials such as latex for paints or in vulcanized rubber found in tires. Foams are produced by either blowing air into the monomers during the reaction or through gases produced as reaction by-products. The soft, springy properties of foams make them ideal for bedding like polyurethane memory foam.
Polymers are not limited to the realm of manufacturing and are encountered in the natural world. One of the most abundant natural polymers, cellulose, is found within plants. Cellulose is actually a type sugar molecule (a polysaccharide) that gives plants their fibrous structure. Of course, no discussion of natural polymers is complete without mentioning deoxyribonucleic acid, or DNA. DNA consists long chains of peptides. Two parallel polypeptide chains are held together by attractive forces to give the iconic double-helix structure.
Polymers are very large molecules that are made up of thousands to millions of atoms that form a sequence of repeat units. Polymers are made through polymerization reactions where molecules called monomers bond together to form repeat units. The polymer chains can either entangle with each other to form amorphous arrangements or align to form crystalline arrangements. Polymers can also be connected through cross-linking to give 3-dimensional networks. Many modern objects use polymers in the form of thermoplastics as well as fibers. Biological polymers, such as cellulose and DNA, can be found throughout the natural world.
|Polymers||very large molecules made up of thousands to millions of atoms that form a sequence of repeat units|
|Monomers||molecules that bond together to form repeat units in a polymer chain|
|Repeat units||molecular links in the polymer chain|
|Polymerization||the chemical reaction that creates polymers|
|Condensation polymerizations/step-growth polymerizations||two monomers react to create a repeat unit and a smaller molecule|
|Chain growth polymerizations||occur when a monomer forms a highly reactive molecule with an unpaired electron|
|Free radical||a molecule with an unpaired electron|
|Amorphous||structure that forms due to the disordered tangling of polymer chains|
|Cross-linked chains||different chains that connect with each other to create a 3-dimensional solids that are essentially one huge molecule|
|Thermoplastics||polymers that soften when heated and can be molded into different forms|
|Fibers||long strands made from amorphous materials or crystalline polymers|
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Back To CoursePhysical Science: Help and Review
19 chapters | 248 lessons