Polymers are chains of molecules formed by linking small molecules together. Copolymers are obtained by joining two different types of molecules called monomers. They can be classified by the arrangement of monomers, shape of structures formed by polymer chains, and properties they impart to finished products. Block copolymers have a simple structure but exhibit phase separation, forming small and rigid structures used in shoe soles and automobile tires. Graft copolymers have a main chain with branches consisting of a different monomer, imparting unique properties to the finished product.
In chemistry, a polymer is a chain of molecules formed by linking many small molecules together. Polymers come in several varieties, one type being the copolymer. A copolymer is obtained when two different types of molecules are joined together in the same polymer chain.
Each of the two types of molecules is called a monomer and the arrangement of the monomers leads to different categories and subcategories of copolymers. For example, an alternating copolymer consists of monomers A and B that alternate regularly, so the copolymer chain is: ABABABAB. A random copolymer is one with random sequences of A and B, such as BAAABABAABB. Periodic copolymers are even more complicated, as they consist of monomers A and B arranged in a repeating sequence. For example, if the A and B sequence described in the second example above were to repeat without changing, even though the sequence itself appears random, this would be a periodic copolymer.
Block copolymers have perhaps the simplest structure, but also the strangest attributes. These copolymers are made up of “blocks” where all of one type of monomer is grouped together, with all of the other type grouped together. It is as if two ordinary polymers are joined at the ends. In these compounds, the two monomers behave in a way scientists call phase separation.
Phase separation is the same thing that happens when oil and water, two incompatible substances, separate in a container. However, the compounds in block copolymers are atomically bonded together, so instead of separating visually like oil and water, they separate on a microscopic scale. This separation into the blocks forms very small but very rigid structures. These substances are strong enough that some types of block copolymers are used in shoe soles and automobile tires. It is also interesting to note that block copolymers can consist of not just two monomers, but in some cases up to five or more.
Another way to classify copolymers is by the shape of the structures formed by the polymer chains. Straight copolymers are just a single chain, while branched copolymers have one main chain with other chains branching laterally at intervals. A common type of branched copolymer is the graft copolymer, where the backbone consists of monomer A only and the branches consist of monomer B only.
In this case, both monomers can impart properties to the finished graft copolymer. An example would be impact resistant polystyrene. It has a polystyrene main chain which gives the material its strength. Chains of the polybutadiene compound are grafted onto the supporting structure of the polystyrene, which gives the material a resilience that ordinary polystyrene lacks.
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