Self-pouring polymer

Although chemistry textbooks typically depict substances as consisting of particles that are spherical in shape, the reality is somewhat more complicated. From proteins to plastics, we can better explain the properties of many materials that we encounter every day in terms of particles resembling chains of beads. You can demonstrate this idea with a liquid that you can cut and siphons out of a beaker.

Use the experiment when teaching polymers and their properties to learners aged 11–16. You could also use it when teaching hydrogen bonding at post-16.

Kit

• 4 g high molecular weight poly(ethylene glycol), also known as poly(ethane-1,2-diol) and polyox – see tips
• 25 cm³ propan-2-ol
• Stirring rod
• 100 cm³ beaker
• 1 L beaker
• 2 x 600 cm³ beakers
• Food dye
• Scissors
• Clamp and stand

Preparation

Add 3–4 g of poly(ethylene glycol), PEG, to a 100 cm³ beaker. Add 25 cm³ of propan-2-ol and stir rapidly to disperse the powder. Add 10 drops of a food dye to the mixture at this stage to help you see the effect more clearly.

Add approximately 400 cm³ of water to a 1 L beaker and tip the dispersed mixture as quickly as possible into the water. Continue mixing with a stirring rod until the solution thickens and begins to climb the rod. Leave it for a few hours to fully hydrate before use.

You can keep the solution in a sealed bottle for several months. If the viscosity drops on standing, you can revive it by sprinkling some extra PEG powder onto the surface of the solution, shaking and leaving it for 24 hours to dissolve.

In front of the class

Use your prepared PEG solution to demonstrate the polymer’s unusual properties.

Self-siphoning liquid

Fill one of the 600 cm³ beakers with your PEG solution. Place the clamp stand in your demonstrating space and put the second 600 cm³ beaker (dry and empty) next to the base of the clamp stand. Clamp the PEG solution beaker with its spout above the empty beaker Tip the upper beaker gently until the liquid starts to pour out of the spout into the second beaker. Keep the angle of the upper beaker constant and carefully elevate it by moving the clamp’s height to lengthen the stream.

Even as the level of liquid in the upper beaker drops beneath the level of its spout, the liquid will continue pouring out until it is nearly empty.

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Illustrate polymers' unusual properties with a self-siphoning solution

Source: © Declan Fleming created with Chemix (https://chemix.org)

Use sharp scissors, keep the blade moving and cut close to the rim for best results

Source: © Declan Fleming created with Chemix (https://chemix.org)

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Cuttable liquid

Demonstrate the unusual cohesive forces in the liquid by cutting the solution. To do this, place an empty 600 cm³ beaker in your demonstrating space, hold the beaker with your PEG solution above it and pour the liquid into the empty beaker. Pour over the edge, not the spout, of the beaker. Once you have established a good flow of liquid, return the upper beaker nearly to vertical (less than 30° of tilt). Using sharp scissors in your other hand, cut through the liquid approximately 1 cm from the rim of the upper beaker and keep the blade moving as you cut for best results. You will see the part of the liquid just above the cut spring back into the upper beaker.

Teaching goal

Poly(ethylene glycol) is used widely in drug delivery, wound dressings, artefact conservation, creams, pastes, cosmetics and more recently mRNA vaccines. The effects seen here result from the solution’s viscoelastic properties – it acts a bit like a liquid and a bit like a solid. The material is viscous, a property of liquids that you can experience in the resistance when you try to stir it. It is also elastic (resists deformation), a property more associated with solids.

The poly(ethylene glycol) particles are more like long chains than small sphere-like molecules. Chemists make the chains by condensation addition polymerisation of ethane-1,2-diol (better known as ethylene glycol) as shown.

n HO–CH₂ –CH₂ –OH → HO–(CH₂ –CH₂ –O)_n –H + n H₂ O

Every third atom in the chain is an oxygen, which can accept hydrogen bonds from water, and this helps the two substances to mix.

These chains become entangled as you apply a force – like the weight of the liquid pulling down on the stream of fluid – causing them to display more elastic properties. You can model this with a pile of rope or Christmas bead decorations: if you pull quickly, the material will knot up (modelling elastic behaviour), but if you pull slowly, the chains will disentangle and separate (modelling viscous behaviour).