Structures and properties of polymers : Structures and properties of polymers Part 2
Condensation polymers : 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
Condensation polymers : 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
How polymers are effected by temperature changes : 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
How polymers are effected by temperature changes : 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-
How polymers are effected by temperature changes : 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
How polymers are effected by temperature changes : How polymers are effected by temperature changes
How polymers are effected by temperature changes : 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
Matching polymer properties to needs : 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
Matching polymer properties to needs : 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
Matching polymer properties to needs : 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
Matching polymer properties to needs : 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
Matching polymer properties to needs : 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
Matching polymer properties to needs : 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