8/9/2019 DDCI5

1/15

Structures andproperties of polymers

Part 2

8/9/2019 DDCI5

2/15

Condensation polymers

Polymers formed by condensation reactionthats used to make esters

Need at least two suitable functionalgroups per monomer

Terylene- a polyester - is a typicalcondensation polymer

8/9/2019 DDCI5

3/15

Condensation polymers

Nylon + Terylene linear polymers idealfor making fibres

Nylon polyamide formed fromcondensation of a dicarboxylic acid and adiamine

Water is eliminated when nylon is formed

8/9/2019 DDCI5

4/15

How polymers are effectedby temperature changes

Heats solids made of small molecules melt to formliquid an eventually boil

Polymers not so simple

E.g. rubber cooled in liquid nitrogen becomes brittleand 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 moveit does so breaking

8/9/2019 DDCI5

5/15

How polymers are effectedby temperature changes

If you heat the glassy material, polymerchains reach a temp at which they move

relative to each other. This is the glasstransistion temperature (T

g)

When polymer is warmer than this, we seethe typical plastic properties we expect-

8/9/2019 DDCI5

6/15

How polymers are effectedby temperature changes

On further heating we reach the meltingtemperature (Tm)

The crystalline regions break down andpolymer becomes a viscous fluid

These processes are reversible forthermoplastics

8/9/2019 DDCI5

7/15

How polymers are effectedby temperature changes

8/9/2019 DDCI5

8/15

How polymers are effectedby temperature changes

Todays polymers are designed to have Tgand Tm values which are suitable for the

manufacturers needs

8/9/2019 DDCI5

9/15

Matching polymer propertiesto needs

Different polymers with different usesneed polymers with different Tg

Two important ways of changing Tg is byusing copolymerisation and plasticisers

8/9/2019 DDCI5

10/15

Matching polymer propertiesto needs

Different polymers with different usesneed polymers with different Tg

Two important ways of changing Tg is byusing copolymerisation and plasticisers

8/9/2019 DDCI5

11/15

Matching polymer propertiesto needs

Pure poly(chloroethene)- PVC has a Tg of about80 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 belowered.

One way of doing this is to copolymerise thechloroethene with a small amount of ethenyl

ethanoate

8/9/2019 DDCI5

12/15

Matching polymer propertiesto needs

Introduces different side groups into thepolymer chain

Chains pack together less well attractive forces

are weaker Polymer is more flexible because the chains can

move over one another more easily

8/9/2019 DDCI5

13/15

Matching polymer propertiesto needs

Another way is to use a molecularlubricant a plasticiser

Allows the PVC chains to slide over eachother more easily

Diagram shows a plasticiser in placebeween two polymers

8/9/2019 DDCI5

14/15

Matching polymer propertiesto needs

Plasticiser have to be chosen verycarefully so they are compatible with thepolymer

Di-(2-ethylhexyl)hexandioate is commonlyused as a plasticiser for PVC

Important that the plasticisers added tocling film dont dissolve in fatty food asthey may be harmful to health

8/9/2019 DDCI5

15/15