6.6 - Ceramics, Polymers & Composites

What holds a polymer together

A polymer is a material made of very large molecules, each built from many small repeating units joined into a long chain by covalent bonds.

Bonds along and between the chains

• Along a chain, atoms are held together by strong covalent bonds. These do not break when a polymer melts.
• Between chains, there are only weak intermolecular forces.
• Because the chains are so long, the large number of intermolecular forces adds up to a strong overall attraction. This is why almost all polymers are solids at room temperature.
• The total attraction is still weaker than the bonding in giant covalent or ionic structures, so polymers have lower melting points.

How the chains lie sets the properties

• Chains close together - More intermolecular forces between them, so the material is stiffer, harder, stronger, and has a higher melting point.
• Chains further apart - Chains slide over one another easily, so the material is flexible, softer, and melts at a lower temperature.
• Longer chains - Greater surface contact, leading to stronger total intermolecular forces.

Softening and cross-links

• Polymers soften over a range of temperatures before melting as chains gradually pull apart.
• Some polymers have cross-links - covalent bonds joining one chain directly to another:
  • Without cross-links, separate chains slide past each other when heated, so the polymer softens and can be remoulded.
  • With cross-links, chains cannot slide, making the polymer strong, hard, stiff, and resistant to melting when heated.