polymerisation

polymerisation

1. means loads of small molecules linking together

   • plastics are forned when lots of monomers join together to make a very long molecule called a polymer
     • usually carbon based
   • under high pressure many small molecules 'join hands' (polymerise) to form long chains called polymers

2. there are lots of different types of polymers

   • polymerisation reactions involving different monomers can be used to make a wide range of polymers
   • different polymers have different physical properties
     • e.g. some are stronger
     • these physical properties make them suited for different uses
       • strong, rigid polymers such as high-density polyethene are used to make plastic milk bottles
       • light, stretchable polymers such as low-density polyethene are used for plastic bags and squeezy bottles
       • PVC is strong and durable, and it can be made either rigid or stretchy
         • rigid kind used to make window frames and piping
         • stretchy kind for synthetic leather
       • polystyrene foam is used in packaging to protect breakable things, and to make disposable coffee cups
         • trapped air in foam makes brilliant thermal insulator
       • heat-resistant polymers such as melamine resin and polypropene are used to make plastic kettles

3. polymers have replaces natural materials for some uses

   • polymers are used as alternatives to traditional materials
   • synthetic fibres like nylon and polyesters are often used to replace cotton, wool or silk fabrics
     • compared with natural alternatives they are lighter, more durable, water resistant and often cheaper
     • main disadvantage of many synthetics is that they are not breathable, can make them uncomfortable next to the skin
   • rigid PVC has largely replaced wood as material for new window frames
     • weather-resistant, strong and durable, which makes them more secure and they don't need painting every few years
     • don't look as good

4. structure and properties of polymers

   • a polymer's properties decide its uses
   • its properties depend on how the molecules are arranged
     • if the polymer chains are packed close together, the material will have a high density
     • if the polymer chains are spread out, the material will have a low density
   • also depends on how they're held together
     • forces between the different chains of the polymer hold it together as a solid mass
       • weak forces
         • chains held together by weak forces are free to slide over each other
           • means the plastic can be stretched easily, and will have a low melting point
       • strong forces
         • plastics with stronger bonds between the polyer cains have higher melting points and can't be easily stretched, as the crosslinks hold the chains firmly together

5. polymers can be modified to give them different properties

   • you can chemically modify polymers to change their properties
   • to increase their chain length
     • polymers with short chains are easy to shape and have lower melting points
     • longer chain polymers are stiffer and have higher melting points
   • made stronger by adding crosslinking agents
     • agents chemically bond the chains together, making the polymer stiffer, stronger and more heat-resistant
   • plasticisers can be added to a polymer to make it softer and easier to shape
     • plasticisers work by getting in between the polymer chains and reducing the forces between them
   • polymer can be made by more crystalline
     • crystalline polymer has straight chains with no branches
       • have higher density, are stronger and have a higher metling point