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Structures and properties of polymers

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  1. Structures and properties of polymers Part 2

  2. Condensation polymers • Polymers formed by condensation reaction that’s used to make esters • Need at least two suitable functional groups per monomer • Terylene- a polyester - is a typical condensation polymer

  3. 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

  4. 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 can’t change shape if it has to move it does so breaking

  5. 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-

  6. 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

  7. How polymers are effected by temperature changes

  8. How polymers are effected by temperature changes • Today’s polymers are designed to have Tg and Tm values which are suitable for the manufacturers needs

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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 don’t dissolve in fatty food as they may be harmful to health