Polymer Structure

Engineering polymers include natural materials such as rubber and synthetic materials such as plastics and elastomers. Polymers are very useful materials because their structures can be altered and tailored to produce materials 1) with a range of mechanical properties 2) in a wide spectrum of colors and 3) with different transparent properties.

Mers

A polymer is composed of many simple molecules that are repeating structural units called monomers. A single polymer molecule may consist of hundreds to a million monomers and may have a linear, branched, or network structure. Covalent bonds hold the atoms in the polymer molecules together and secondary bonds then hold groups of polymer chains together to form the polymeric material. Copolymers are polymers composed of two or more different types of monomers.

Polymer Chains (Thermoplastics and Thermosets)

A polymer is an organic material and the backbone of every organic material is a chain of carbon atoms. The carbon atom has four electrons in the outer shell. Each of these valence electrons can form a covalent bond to another carbon atom or to a foreign atom. The key to the polymer structure is that two carbon atoms can have up to three common bonds and still bond with other atoms. The elements found most frequently in polymers and their valence numbers are: H, F, Cl, Bf, and I with 1 valence electron; O and S with 2 valence electrons; n with 3 valence electrons and C and Si with 4 valence electrons.

Multiple combining of multiple ethylene mer units leads to polymerization by the opening of double bonds.

The ability for molecules to form long chains is a vital to producing polymers. Consider the material polyethylene, which is made from ethane gas, C2H6. Ethane gas has a two carbon atoms in the chain and each of the two carbon atoms share two valence electrons with the other. If two molecules of ethane are brought together, one of the carbon bonds in each molecule can be broken and the two molecules can be joined with a carbon to carbon bond. After the two mers are joined, there are still two free valence electrons at each end of the chain for joining other mers or polymer chains. The process can continue liking more mers and polymers together until it is stopped by the addition of anther chemical (a terminator), that fills the available bond at each end of the molecule. This is called a linear polymer and is building block for thermoplastic polymers.

The polymer chain is often shown in two dimensions, but it should be noted that they have a three dimensional structure. Each bond is at 109° to the next and, therefore, the carbon backbone extends through space like a twisted chain of TinkerToys. When stress is applied, these chains stretch and the elongation of polymers can be thousands of times greater than it is in crystalline structures.

The length of the polymer chain is very important. As the number of carbon atoms in the chain is increased to beyond several hundred, the material will pass through the liquid state and become a waxy solid. When the number of carbon atoms in the chain is over 1,000, the solid material polyethylene, with its characteristics of strength, flexibility and toughness, is obtained. The change in state occurs because as the length of the molecules increases, the total binding forces between molecules also increases.

It should also be noted that the molecules are not generally straight but are a tangled mass. Thermoplastic materials, such as polyethylene, can be pictured as a mass of intertwined worms randomly thrown into a pail. The binding forces are the result of van der Waals forces between molecules and mechanical entanglement between the chains. When thermoplastics are heated, there is more molecular movement and the bonds between molecules can be easily broken. This is why thermoplastic materials can be remelted.

Liner backbone structures are like bundles of long hairs tangled together. Branch backbone networks grow in paths that are similar to the way tree branches grow from trees. Network backbone structures have chains that connect to each other the way a road would connect a town.

There is another group of polymers in which a single large network, instead of many molecules is formed during polymerization. Since polymerization is initially accomplished by heating the raw materials and brining them together, this group is called thermosetting polymers or plastics. For this type of network structure to form, the mers must have more than two places for boning to occur; otherwise, only a linear structure is possible. These chains form jointed structures and rings, and may fold back and forth to take on a partially crystalline structure.

Since these materials are essentially comprised of one giant molecule, there is no movement between molecules once the mass has set. Thermosetting polymers are more rigid and generally have higher strength than thermoplastic polymers. Also, since there is no opportunity for motion between molecules in a thermosetting polymer, they will not become plastic when heated.