Rubbers are made up of long hydrocarbon polymer chains. Polymers are long chains made up of many repeating smaller units which are called monomers. A single polymer molecule can be thousands of atoms long.
Polymers are formed in a process called polymerisation in which the monomers are joined together either by addition polymerisation or condensation polymerisation. Condensation polymerisation is a reaction that produces water as a by-product. This type of polymerisation occurs when the monomers contain a hydroxyl group. Most rubbers are formed by addition polymerisation. This process simply involves the “addition” of single monomers to each other without creating any by-products. Addition polymers are formed when double bonds within the monomers are broken. One bond within the double bond is broken causing the double bond to “open up”. This open bond allows the monomer to join to another monomer which has also “opened up”. This process can continue until an infinitely long chain is formed. The number of repeating monomer units in the polymer is known as the degree of polymerisation. As the degree of polymerisation increases, the density of the material increases as well as the melting and boiling points.
Both the length of the polymer chain and the type of monomer in the chain can affect the properties of the final material. Polymers can be made with a single type of monomer or with a mixture of two or more different monomers. A polymer which is produced from the polymerisation of two different monomers is referred to as a copolymer. Similarly, a polymer which is created from three different monomers is called a terpolymer.
Rubber polymers are normally made up of hydrocarbon monomers which contain only hydrogen and carbon atoms. Hydrogen and carbon atoms are bonded together by covalent bonds, where atoms share a pair of electrons. Covalent bonds allow atoms to have a full outer electron shell without changing their net charge. Due to the number of electrons in their outer electron shells, carbon is only stable if it has four covalent bonds and hydrogen will be stable when it has one covalent bond.
The four bonds made by carbon are equidistant from each other in three dimensions – they are all 109 degrees apart. This means that the long polymer chains with carbon backbones will not be flat but will have natural angles and kinks in them. When the rubber is stretched the bond angles will be distorted and the molecules will be “straightened out”. However, as soon as the force is removed the molecules will “bounce” back to their original shape. This is what gives rubber its high elasticity.
The polymer molecules are bonded together with covalent bonds, these molecules are then attracted to each other by intermolecular bonds. The level of attraction between the molecules depends on how close the molecules are to each other, the closer that they are the stronger the attraction will be. Rubber polymers have an amorphous, random shape meaning that they do not fit together perfectly and cannot be stacked tightly on top of each other. This keeps the polymer chains further apart and weakens the intermolecular bonds between them.
The weak intermolecular bonds between the polymer chains make rubber behave differently to other engineering materials such as metals or plastics. The special quality which makes rubbers “rubbery” is their high elasticity. These elastic polymers are known as elastomers. Elastomers are viscoelastic materials, meaning that they exhibit both elastic and viscous behaviours and they generally have a high elongation to break. The weak intermolecular bonds in an elastomer allow the polymer chains to slide over each other. In the raw state, rubbers behave almost as if they were melted and are easily moulded and shaped. However, the weak bonds between the rubber polymer molecules lead to rubbers being less strong than other materials which contain strong bonds between the molecules. Elastomers generally have a low Youngs modulus and high failure strain compared to other materials.