Although the fundamental property of bulk polymers is the degree of polymerization, the physical structure of the chain is also an important factor that determines the macroscopic properties.
The terms configuration and conformation are used to describe the geometric structure of a polymer and are often confused. Configuration refers to the order that is determined by chemical bonds. The configuration of a polymer cannot be altered unless chemical bonds are broken and reformed. Conformation refers to order that arises from the rotation of molecules about the single bonds. These two structures are studied below.
The two types of polymer configurations are cis and trans. These structures can not be changed by physical means (e.g. rotation). The cis configuration arises when substituent groups are on the same side of a carbon-carbon double bond. Trans refers to the substituents on opposite sides of the double bond.
Stereoregularity is the term used to describe the configuration of polymer chains. Three distinct structures can be obtained. Isotactic is an arrangement where all substituents are on the same side of the polymer chain. A syndiotactic polymer chain is composed of alternating groups and atactic is a random combination of the groups. The following diagram shows two of the three stereoisomers of polymer chain.
If two atoms are joined by a single bond then rotation about that bond is possible since, unlike a double bond, it does not require breaking the bond.
The ability of an atom to rotate this way relative to the atoms which it joins is known as an adjustment of the torsional angle. If the two atoms have other atoms or groups attached to them then configurations which vary in torsional angle are known as conformations. Since different conformations represent varying distances between the atoms or groups rotating about the bond, and these distances determine the amount and type of interaction between adjacent atoms or groups, different conformation may represent different potential energies of the molecule. There several possible generalized conformations: Anti (Trans), Eclipsed (Cis), and Gauche (+ or -). The following animation illustrates the differences between them.
Like the polymer growth simulation, the conformation lattice simulation takes a statistical approach to the study of polymers. Probabilities of the different conformations are assigned which produces a polymer chain with many possible shapes. Click the icon to enter the virtual laboratory.
The geometric arrangement of the bonds is not the only way the structure of a polymer can vary. A branched polymer is formed when there are "side chains" attached to a main chain. A simple example of a branched polymer is shown in the following diagram.
There are, however, many ways a branched polymer can be arranged. One of these types is called "star-branching". Star branching results when a polymerization starts with a single monomer and has branches radially outward from this point. Polymers with a high degree of branching are called dendrimers Often in these molecules, branches themselves have branches. This tends to give the molecule an overall spherical shape in three dimensions.
A separate kind of chain structure arises when more that one type of monomer is involved in the synthesis reaction. These polymers that incorporate more than one kind of monomer into their chain are called copolymers. There are three important types of copolymers. A random copolymer contains a random arrangement of the multiple monomers. A block copolymer contains blocks of monomers of the same type. Finally, a graft copolymer contains a main chain polymer consisting of one type of monomer with branches made up of other monomers. The following diagram displays the different types of copolymers.
|Block Copolymer||Graft Copolymer||Random Copolymer|
An example of a common copolymer is Nylon. Nylon is an alternating copolymer with 2 monomers, a 6 carbon diacid and a 6 carbon diamine. The following picture shows one monomer of the diacid combined with one monomer of the diamine:
In addition to the bonds which hold monomers together in a polymer chain, many polymers form bonds between neighboring chains. These bonds can be formed directly between the neighboring chains, or two chains may bond to a third common molecule. Though not as strong or rigid as the bonds within the chain, these cross-links have an important effect on the polymer. Polymers with a high enough degree of cross-linking have "memory." When the polymer is stretched, the cross-links prevent the individual chains from sliding past each other. The chains may straighten out, but once the stress is removed they return to their original position and the object returns to its original shape.
One example of cross-linking is vulcanization . In vulcanization, a series of cross-links are introduced into an elastomer to give it strength. This technique is commonly used to strengthen rubber.
Polymer science is a broad field that includes many types of materials which incorporate long chain structure of many repeat units as discussed above. The two major polymer classes are described here.
Elastomers,or rubbery materials, have a loose cross-linked structure. This type of chain structure causes elastomers to possess memory. Typically, about 1 in 100 molecules are cross-linked on average. When the average number of cross-links rises to about 1 in 30 the material becomes more rigid and brittle. Natural and synthetic rubbers are both common examples of elastomers. Plastics are polymers which, under appropriate conditions of temperature and pressure, can be molded or shaped (such as blowing to form a film). In contrast to elastomers, plastics have a greater stiffness and lack reversible elasticity. All plastics are polymers but not all polymers are plastics. Cellulose is an example of a polymeric material which must be substantially modified before processing with the usual methods used for plastics. Some plastics, such as nylon and cellulose acetate, are formed into fibers (which are regarded by some as a separate class of polymers in spite of a considerable overlap with plastics). As we shall see in the section on liquid crystals, some of the main chain polymer liquid crystals also are the constituents of important fibers. Every day plastics such as polyethylene and poly(vinyl chloride) have replaced traditional materials like paper and copper for a wide variety of applications. The section on Polymer Applications will go into greater detail about the special properties of the many types of polymers.