Polymers And Plastics

The aim of this exercise is to show pupils what the molecular structure of a plastic looks like and how it affects its material properties. The basic building block of plastics are long chains of molecules called Polymers.

Polymers Polymers are large molecules consisting of repeated chemical units (`mers') joined together, usually in a line, like beads on a string. Most of the plastics that make up the objects that fill our lives are made of polymers. One example is Polyvinylacetate which is a long string (poly means many) of vinyl acetate molecules that looks something like this. One vinyl acetate molecule …..And So On

PVA glue is a form of polyvinyl acetate but it is too close to being a liquid to be of use to us, because no bonds exist between the long string, they all move separately to each other and the glue flows. If you want to see it act more like a plastic try the following experiments;

PVA Polymer Experiment, Version 1 2 teaspoons PVA 2 teaspoons water Mix together 100ml hot water 2 teaspoons borax powder Dissolve borax in hot water Add three teaspoons of borax solution into all the PVA solution, stir and watch what happens.

PVA Polymer Experiment, Version 2 2 teaspoons PVA 2 teaspoons water Mix together 50ml hot water 2 teaspoons borax powder Dissolve borax in hot water Add three teaspoons of borax solution into all the PVA mixture, stir and watch what happens.

What’s Happening The borax molecules loosely bond the long springy polymer strings together, they can still move and act like springs but they are fixed together every so often. So the substance acts like a squashy blob. It is a very simple form of plastic. Borax molecules – Loosely bond the PVA molecule

With the first recipe the polymer feels wet to touch and if left will flow very gradually into a flat blob. With the second recipe the polymer is much more solid and will try to maintain its shape. It will change shape when a force is applied to it, so if it is dropped it will bounce as the polymers are first squashed then spring back into its original shape.

It’s a Non Newtonian Solid As well as being an example of a polymer the mixture is also an example of a Non-Newtonian fluid. This means it doesn’t obey Newton’s law that says adding heat to a solid or liquid should change it to the next state, a solid should become a liquid and a liquid should become a gas. With a non-Newtonian fluid it takes a force to make the change. So for instance squeezing the substance gives it the properties of a liquid, it starts to flow and the shape changes.

Adding a force allows the borax bonding forces to be broken and the long polymer molecules can move so the substance starts to act as a liquid. Take the force away and the bonds reform making the substance act as a solid. Adding a force breaks the borax bonds which means the long molecule can now move

The overall effect is that the molecules are squashed by the force then spring back once the force is removed. The material is Elastic Question : What products can you think of or find that use the springy effect of the polymers?