Polymers are the dream material of the future. Averil Macdonald explains
Students love to hear predictions about what their future will be like and many are inspired by ideas about saving the environment. Here are some excellent ways to get them thinking about the vast range of properties that can now be incorporated in polymeric materials. Polymers are more than plastic bags and toys. They can be designed to dissolve in water, conduct electricity, change colour or shape with the application of an electric field or heat... the list is endless.
The properties of polymers stem from the fact that they are long-chain molecules. A nice demonstration to get key stage 4 students talking about them is to cut a liquid polymer with scissors. A solution of 4g polyethylene oxide in 25ml ethanol diluted with 250ml water will be ideal.
Add a little food dye to make the liquid more visible. As the liquid is poured from one beaker to another, tilt the upper beaker at about 45 degrees and cut sharply about 20mm below the lip of the beaker. This polymer is a viscoelastic liquid and so will contract back into the upper beaker from the cut. This takes a little practice but the amazing experience of cutting a liquid is worth it.
Solids that flow
Other polymers also have odd properties. Potty putty has the ability to flow when left on a table but becomes hard and bouncy when hit or dropped.
This depends on the speed at which the long chain molecules are able to flow past each other. Let them move slowly under a weak force, such as gravity or gentle stretching, and they will move a long way. Hit, bounce or suddenly pull them and the reaction is different - the polymer molecules resist the force and fail to move. Potty putty is, therefore, a solid that can flow - as is the Earth's mantle just below the crust.
lRelatedactivity: potty putty can be used to demonstrate the consistency of the mantle below the Earth's surface for KS4 Sc4 (ideas and evidence in science), or just for fun at KS3.
Add a small amount of saturated sodium tetraborate solution (half a small pipette) to a teaspoon of PVA glue and stir until it comes away from the sides of the container. If necessary, add a tiny amount more of the solution. Knead it to ensure full mixing. This takes patience. Warning! - if you add too much sodium tetraborate solution the mix will become too hard. See www.chemsoc. orgnet works learnnet jeseiindex2.htm
There is little doubt that the world our students will grow old in will be filled with these designer materials. But plastics are also the source of vast amounts of waste that won't go away. Here are some ideas to get students talking. Did you know that every day more than eight million used disposable nappies are dumped in landfill sites and that some parts will take up to 200 years to degrade? They may be disposable but they are not biodegradable. It is possible for disposable nappies to be made from corn starch but this costs much more than the ingredients used at the moment (a polymer called sodium polyacrylate provides the fluid absorption).
* Related investigation for KS3 (investigating scientific questions): which brand of disposable nappy gives best value for money?
Tests might include the rate at which water is poured into the nappy and possible temperature effects. They may also consider likely explanations for differences such as different mass of absorbent polymer (sodium polyacrylate). Granules of sodium polyacrylate can be extracted from the lining by removing the upper lining fibres. Prolonged exposure of skin to the granules should be avoided.
lRelated discussion for KS4 (SC4), AS-level perspectives in science: if companies cannot use cheap biodegradable materials, should disposable nappies be outlawed? Simply preventing people using these nappies is one option, but parents may have a different viewpoint.
Polymers can be reused, but not as a mix. The different types can be separated using electrostatics. If you rub a plastic balloon on your jumper, it becomes electrostatically charged. This happens to all polymers.
The science of tribology shows that when rubbed on a metal, for example, some polymers become positively charged, others become negatively charged, forming a triboelectric series akin to the reactivity series in chemistry.
Rub a polymer with one lower in the series and it will become positively charged, rub it with one higher in the series and it becomes negatively charged. This can be used to discriminate between polymers in a waste recycling plant. Ford is investing heavily in this technology to allow its cars to be recycled and larger proportions of cars are now being made of polymers. See "Rubbish" on www.nelsonthornes.com secondarysciencerisupdatesPhysics.html
lRelated activities for KS4 (Sc3). Look at plastic containers in the home and note which ones have the recycle symbol on it. Note which polymer each is made of.
New legislation will require the majority of plastic components to be recyclable. For example, mobile phones will be made of shape-memory polymers which change shape at higher temperatures so they spring apart instead of having to be disassembled by hand. Owners will have to be warned not to leave their phones in the sun or they may return to find pieces of a jigsaw waiting for them.
lRelated demonstration for KS4 (Sc3). The ability to remember a shape can be demonstrated using a vending machine cup (not expanded polystyrene).
Hold it in a clamp and play a hot air gun into it. It will wither into a disc. The cup was originally cut as a disc from a sheet of plastic then heated and stretched over a mould, and reverts to its original shape when heated. Yogurt pots make attractive patterns when these shrivel up.
After the oil is gone
Many students know that polymers are often made from petroleum, which won't last for ever. Trials are under way to modify the chloroplast of a cress plant so that when photosynthesis occurs, polyhydroxybuterate is produced instead of starch. The cress cell is then dissolved and the polymer can be used to make a range of items. This work is in its early stages but it promises to create an endless supply of totally biodegradable polymer.
* Related discussion for KS4 (Sc4) or AS in perspectives on science: should we solve the problem of dwindling resources by creating plants to grow biodegradable polymers, thus replacing the use of petroleum for making plastics?
On a more high-tech note, soon there will be video wristwatches so if pupils seem to be preoccupied with the time it may be because they are watching Tomb Raider. Light-emitting polymers will be made into flexible TV screens only 3mm thick which may be hung on a wall. Handbags can have these polymers embedded in their linings so that they light up when opened and we can easily find what's hidden in their depths. There will also be plastic electronics on the labels in your clothes so that your T-shirt can talk to your washing machine to tell it how best to wash it and shops can detect you going past and invite you in.
* Related discussion for KS4 (Sc4) or AS in perspectives on science: is imprinting electronics into our clothes a possible invasion of privacy?
Plastics will become the dominant material as we progress into the 21st century. There will be a vast range of careers in designer polymers.
Discussion may convince students that there is a bright future for them if they study science.
Averil Macdonald is a physics lecturer at Reading University