32.6: Summary

You should now be familiar with the following concepts:

• Above T_g polymers are rubbery, whereas below T_g they are glassy
• Rubbery behaviour arises from the polymer's ability to change its conformation at high temperatures
• Glassy behaviour arises from the polymer's lack of ability to change its conformation at low temperatures
• Many factors affect the T_g of a polymer. Some factors are to do with the chemistry of the polymer:
  • chain length
  • chain flexibility
  • side groups
  • branching
  • cross-linking
  • the presence of plasticisers

  Other factors are to do with the method of measuring T_g:

  • strain rate
  • cooling or heating rate
• You should also understand the theory of kinetics, which explains how glasses form and noted that there are many different experimental methods for measuring the glass transition temperature in polymers.
• Finally you should have observed how the behaviour of a rubber ball varies when it is bounced above, below and near its T_g and seen how the behaviour of silly putty varies at different strain rates.

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