Polymers

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  • Polymers
    • A polymer is a long molecule made up from lots of small molecules. The small molecules that add together are called monomers.
    • Addition polymerisation
      • If the monomers contain a double bond they can add together to mke a polymer.
      • No other product is formed.
      • Because the same basic unit is repeated over and over again in the polymer chain, the polmer structurecan be represented more simply by drawing the repeating unit.
    • Properties
      • The properties of a polymer depend on six factors:
        • Chain length- the longer the chain the stronger the polymer. This is because: longer chains become more entangled and have stronger intermolecular bonds.
        • Side groups on the polmer chain: the more polar the side groups, the stronger the bonds between them.
        • Branching: straight, unbranched polymer chains can pack closer together, allowing stronger bonds between the chains.
        • Chain flexability: the more rigid the chain the stronger the polymer. Hydrocarbon chains are very flexible, benzene rings make the polymer more rigid.
        • Cross-linking: more extensive cross-linking makes the polymer harder to melt.
        • Stereoregularity: the more regular the orientation of the side groups, the closer the packing and the stronger the polymer.
    • Thermoplastics
      • These are polmers without cross-links between the chains.
      • The intermolecular bonds between the chains are much weaker than the covalent cross-links in a thermoset.
      • The attractive forces in thermoplastics can be overcome by warming.
      • The chains can slide over each other and the polymer can be deformed.
      • On cooling, the weak bonds between the polymer chains reform and the polmer holds its new shape.
    • Thermosets
      • These polymers have extensive cross-linking between the polymer chains.
      • The bonds between the chains are much stronger than in thermoplastics.
      • The covalent bonds cannot be broken by warming.
      • The chains cannot move relative to each other and the polmer cannot change shape

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