Bonding in polymers

Two levels of bonding in a polymer

A polymer is a very large molecule, usually drawn as a long chain. The atoms along each polymer chain are joined by strong covalent bonds. These are the same kind of strong bonds found within simple molecular substances.

However, a solid polymer contains many separate polymer molecules lying alongside and around one another. Intermolecular forces attract one chain to neighbouring chains. These attractions are weaker than the covalent bonds within a chain, but a long polymer molecule can experience attractions at many points along its length. Together, these numerous attractions can have a substantial effect on the solid.

It is therefore important to identify which interaction is being discussed:

• within a chain: strong covalent bonds between atoms;
• between chains: weaker intermolecular forces between polymer molecules.

How the arrangement produces bulk properties

A material's properties arise from the collective bonding and arrangement of enormous numbers of particles. An individual carbon atom is not itself hard, flexible or stiff; these are bulk properties of a sample of material.

The covalently bonded backbone gives each polymer molecule strength because separating atoms along a chain would require strong covalent bonds to be broken. The way neighbouring long chains attract one another then affects how easily those chains can move past each other.

Where the overall intermolecular attractions between chains are stronger, the chains are held together more firmly. The solid tends to be harder and stiffer, and greater force is required to pull or move its molecules apart. Where the attractions allow chains to move past one another more readily, the material tends to be softer and more flexible. The exact combination of chain structure and intermolecular forces means that different polymers can have very different strengths, hardnesses and degrees of flexibility.

Melting a polymer

Heating transfers energy to the polymer molecules. For the solid to melt, the chains must gain enough energy to move past one another, so intermolecular forces between the chains must be overcome. Stronger overall intermolecular forces require more energy to overcome and therefore give a higher melting point.

Melting does not normally break the strong covalent bonds along the polymer chains. If those covalent bonds were broken, the polymer molecules themselves would be chemically changed rather than merely melted.

Connecting structure to a material choice

Chemists relate microscopic structure to visible properties when choosing a polymer for a product. A flexible polymer may suit an object that must bend, whereas a harder or stiffer polymer may suit a component that must retain its shape. A polymer used at a raised temperature must also have intermolecular attractions strong enough for the material not to soften or melt too readily. The useful property comes from the bonds, forces and their arrangement—not from a single atom possessing that property.

Bonding to recall

• A polymer molecule is a long chain of atoms joined by strong covalent bonds.
• Separate polymer chains attract one another through weaker intermolecular forces.
• Long chains have intermolecular attractions at many points, so the combined attraction can be substantial.

Structure–property links

• Stronger overall attractions between chains → less chain movement → generally greater hardness and stiffness.
• Easier movement of chains → generally greater flexibility and softness.
• Strong covalent bonds give strength along each chain; attractions between chains affect how firmly the bulk solid holds together.
• Bulk properties arise from bonding and arrangement. Individual atoms are not hard, flexible or stiff.

Melting

Stronger intermolecular forces → more energy needed to separate the chains → higher melting point.

On melting, intermolecular forces are overcome; covalent bonds within the polymer chains are not normally broken.