Structures and properties of polymers

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Structures and properties of polymers

• Part 2

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Condensation polymers

• Polymers formed by condensation reaction thats
  used to make esters
• Need at least two suitable functional groups per
  monomer
• Terylene- a polyester - is a typical condensation
  polymer

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Condensation polymers

• Nylon Terylene linear polymers ideal for
  making fibres
• Nylon polyamide formed from condensation of a
  dicarboxylic acid and a diamine
• Water is eliminated when nylon is formed

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How polymers are effected by temperature changes

• Heats solids made of small molecules melt to
  form liquid an eventually boil
• Polymers not so simple
• E.g. rubber cooled in liquid nitrogen becomes
  brittle and can be smashed
• It becomes GLASSY
• poly(propene) becomes brittle at about -10 C
• Structure of many polymers mixture of ordered
  areas (crystalline) and random (amorphous)
• In glassy state the amorphous regions become
  frozen so cant cant change shape if it has to
  move it does so breaking

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How polymers are effected by temperature changes

• If you heat the glassy material, polymer chains
  reach a temp at which they move relative to each
  other. This is the glass transistion temperature
  (Tg)
• When polymer is warmer than this, we see the
  typical plastic properties we expect-

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How polymers are effected by temperature changes

• On further heating we reach the melting
  temperature (Tm)
• The crystalline regions break down and polymer
  becomes a viscous fluid
• These processes are reversible for thermoplastics

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How polymers are effected by temperature changes
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How polymers are effected by temperature changes

• Todays polymers are designed to have Tg and Tm
  values which are suitable for the manufacturers
  needs

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Matching polymer properties to needs

• Different polymers with different uses need
  polymers with different Tg
• Two important ways of changing Tg is by using
  copolymerisation and plasticisers

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Matching polymer properties to needs

• Different polymers with different uses need
  polymers with different Tg
• Two important ways of changing Tg is by using
  copolymerisation and plasticisers

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Matching polymer properties to needs

• Pure poly(chloroethene)- PVC has a Tg of about 80
  C rigid and quite brittle at room temp
• Used to make drain pipes
• Sometimes called unplasticised PVC or uPVC
• To make it more flexible the Tg needs to be
  lowered.
• One way of doing this is to copolymerise the
  chloroethene with a small amount of ethenyl
  ethanoate

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Matching polymer properties to needs

• Introduces different side groups into the polymer
  chain
• Chains pack together less well attractive
  forces are weaker
• Polymer is more flexible because the chains can
  move over one another more easily

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Matching polymer properties to needs

• Another way is to use a molecular lubricant a
  plasticiser
• Allows the PVC chains to slide over each other
  more easily
• Diagram shows a plasticiser in place beween two
  polymers

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Matching polymer properties to needs

• Plasticiser have to be chosen very carefully so
  they are compatible with the polymer
• Di-(2-ethylhexyl)hexandioate is commonly used as
  a plasticiser for PVC
• Important that the plasticisers added to cling
  film dont dissolve in fatty food as they may be
  harmful to health

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