5.7: Polymer properties by design


Polymer properties by design

  • The glass transition temperature (Tg) is the temperature at which the polymer chains can move relative to each other - when the polymer is warmer than this, it becomes flexible.
  • When the melting temperature (Tm) is reached, the cyrstalline areas break down and the polymer becomes a viscous fluid - these processes are reversible for thermoplastics.
  • Longer chains make stronger polymers, but a critical length has to be reached beofre the strength increases. 
  • Increasing chain lenght increases the tensile strength of a polymer as the longer chains are more tangled together, making them less flexible. Also, longer chains have more points of contact with chains of neighbouring polymer molecules, therefore there are more intermolecular bonds to hold the chains together.
  • Two important ways of changing Tg involve copolymerisation and plasticisers. 
  • Crystalline: areas in a polymer where the chains are closely packed in a regular way. 
  • The more crystalline the polymer, the stronger the and less flexible it becomes - the polymer chains pack closely together, maximising intermolecular bonds. 
  • When a polymer is stretched (cold-drawn) a neck forms. In the neck the polymer chains are lined up to form a more crystalline region. This process carries on until all the polymer, except the ends which are being pulled, has alligned chains. 
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