Changing the properties of polymers and plastics

One way is of course to produce a different polymer or plastic with a different chemical structure, but there are also other possibilities. This series of activities allows students to explore some of them:

• The use of plasticisers 
• Cross-linking polymer chains 
• The effect of changing the length of the polymer chains
• The effect of the presence of branched chains

Equipment required

Set 1: Plasticised and unplasticised PVC

• Plasticised PVC – squeezy toy and/or PVC cling film
• Unplasticised PVC – piece of uPVC pipe or guttering.

Set 2: Cross-linking (or vulcanising) rubber

• Rubber – microscope slide with a thick coating of Copydex glue (set up at least the night before)
• Vulcanised rubber – thick rubber band

The glue can be peeled off the microscope slide by students when they are ready to use it. One sample will not last long enough for all groups to use it so have several in reserve.

Set 3: Changing the length of polymer chains

• Candle wax or candle
• Polyethene (eg carrier bag)
• Sample of medium chain length hydrocarbon (eg kerosene) in a sealed tube.

Set 4: Branching chains in polyethene

• High density polyethene – a ‘rustly’ supermarket carrier bag
• Low density polyethene – a ‘quiet’ department store carrier bag.
• All the above items should be labelled.

Additional items:

• Assorted worksheets and helpsheets as chosen by the teacher for each group
• Some groups may need glue and scissors.

Differentiating the activity

Students should describe the properties of the samples in each set and try to explain the difference they observe. The activity can be left very open-ended and students given no help, or the support documents listed below can be used:

• List of possible properties for students to choose from (List of properties)
• Table to complete (Table)
• Explanations for use in completing the table (Explanations)
• Cut and stick diagrams (Diagrams)
• Information sheet (Information sheet).

Each item in the list is provided as a separate document so teachers can choose how much help to give to each class or group of students. The most able students could be given the samples and perhaps the diagrams.

They can discuss in groups why there are differences between the polymers in each group of substances. If they get stuck or wish to check their answers when they have thought of an explanation they could be given the information sheet.

The least able students could be given the properties list and the table to begin with, then the diagrams and explanations when they have decided on the properties. Giving them only one step at a time will help to prevent them from becoming overwhelmed.

Possible answers to questions

The depth at which these questions can be answered could vary widely depending on where students are in their learning journey.

Plasticisers

A plasticiser is a small molecule that can be put in between the large molecules of a polymer.

The plasticiser acts a little bit like oil in a car engine – it helps the polymer molecules slide over each other easily.

Without the plasticiser the molecules attract each other and tend to stick together.

This gives the substance a rigid structure which might make it useful in construction.

The plasticiser changes the properties of the substance and allows it to be used for a wider range of things.

Branching chains

Polymers are usually made by heating the monomer and passing it over a catalyst.

The sort of catalyst used can affect how much the polymer chain branches.

Polymer branches are like the branches of a tree – they are made of the same stuff as the main chain but go off in a different direction.

If the polymer chains have no branches, they pack in close to each other and the material has a high density.

The chains can pass over each other easily so the material is very stretchy. If the polymer chains have a lot of branches then the chains cannot pack together as closely and the material has a lower density.

The branches catch on each other, preventing the chains from sliding easily past one another. This makes the plastic less stretchy.

Cross-linking

Rubber and some other polymers can be cross-linked. A chemical reaction takes place that connects the chains to each other permanently.

This makes the whole structure more rigid and less elastic. It also makes the material a lot stronger and harder.

Vulcanised rubber is cross-linked using sulfur. Some of the structures inside your body are held together in the same way because proteins can be cross-linked with sulfur too.

The length of the polymer chains

Long chains get tangled up in each other and stick together far more than short chains.

This means that polymers made of long chain molecules have higher melting points than those made of short chains.

Shorter molecules can pass over each other more easily so materials made of these molecules are softer and more ‘squishy’.