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 (T g ) 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 (T m ) 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. Today’s polymers are designed to have T g and T m values which are suitable for the manufacturers needs

9. Matching polymer properties to needs Different polymers with different uses need polymers with different T g Two important ways of changing T g is by using copolymerisation and plasticisers

10. Matching polymer properties to needs Different polymers with different uses need polymers with different T g Two important ways of changing T g is by using copolymerisation and plasticisers

11. Matching polymer properties to needs Pure poly(chloroethene)- PVC has a T g 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 T g 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