Lifelong Learning Programme

This project has been funded with support from the European Commission.
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Chemistry is all around us
Copyright 2015
This project has been funded with
support from the European Commission

Educational Packages

Materials for Special Uses

Polymers for special uses

Step 2 - Activities

Test some super-absorbent polymers

Description of the activity
The experience aims to test four kinds of super-absorbent materials that belong to our everyday life. Gummy bears and sheet gelatin contain collagen, a super-absorbent natural polymer (extracted from leaf), while disposable nappies and water crystals are based on super-absorbent synthetic polymers. The behavior of the materials in presence of water as a function of temperature and addition of table salt is studied and justified.

Materials
  • 3 transparent plastic cups half-filled with cold (between 10 and 25°C) water
  • 3 transparent plastic cups half-filled with hot (about 70°C) water
  • 2 empty transparent plastic cups
  • 1 tablespoon
  • sodium chloride (table salt)
  • gummy bears
  • disposable nappy
  • sheet gelatin
  • water polymer crystals (available at plant stores)

Procedure

a. Test Gummy bears
Gummy bears and temperature
  1. Obtain three different colored gummy bears. Place one in cold water, the second in hot water and take the last as your reference.
  2. Wait for at least 1h, then place the two wet bears close to the dry bear. What did it happen to bear tested with cold water? What did it happen to bear tested with hot water?
  3. Try to justify what observed.
Gummy bears and table salt
  1. Take the bear tested in cold water and place it in the plastic cups half-filled with cold water again.
  2. Add one tablespoon of table salt and stir gently for few minutes.
  3. What’s happened? Try to justify what observed.

b. Test sheet gelatin
Sheet gelatin and temperature
  1. Obtain three sheets of gelatin for food. Place one in cold water, the second in hot water and take the last as your reference.
  2. Wait for half hour, then place the two wet sheets close to the dry sheet. What did it happen to gelatin tested with cold water? What did it happen to gelatin tested with hot water?
  3. Try to justify what observed.
Sheet gelatin and table salt
  1. Take the sheet gelatin tested in cold water and place it in the plastic cups half-filled with cold water again.
  2. Add one tablespoon of table salt and stir gently for few minutes.
  3. What’s happened? Try to justify what observed.

c. Test water crystals
Water crystals and temperature
  1. Obtain three water crystals. Place one in cold water, the second in hot water and take the last as your reference.
  2. Wait for at least 2h (better if 3 or 4); when needed replace hot water. Then place the two wet crystals close to the dry crystal. What did it happen to crystal tested with cold water? What did it happen to crystal tested with hot water?
  3. Try to justify what observed.
Water crystals and table salt
  1. Take the water crystal tested in cold water and place it in the plastic cups half-filled with cold water again.
  2. Add one tablespoon of table salt and stir gently for few minutes.
  3. What’s happened? Try to justify what observed.

d. Test disposable nappies
Disposable nappies and temperature
  1. Take three pieces of the inside material of a disposable nappy (each as big as to fill about ¼ of the plastic cup). 2. Place the first piece in a empty plastic cup.
  2. Then add one tablespoon of cold water (between 10 and 25°C) and stir gently. Repeat this action three times.
  3. Place the second piece in a empty plastic cup.
  4. Then add one tablespoon of hot water (about 70°C) and stir gently. Repeat this action three times.
  5. What did it happen by using cold water? What did it happen by using hot water? Try to justify what observed. Disposable nappies and table salt
  6. Take the cup containing the gel swollen with cold water (by following points 2 and 3) and add one tablespoon of table salt by stirring gently.
  7. What’s happened? Try to justify what observed.

Conclusions and teaching notes
Gummy bears contain collagen together with other ingredients such as sugars, fruit juices, waxes.
Sheet gelatin is made of pure collagen.
Inside material of disposable nappies is a synthetic polymer called sodium polyacrylate.
Water crystals are made of a synthetic polymer called polyacrylamide.
They all are characterized by capacity of absorbing a great quantity of water, but, depending of their structure, the rate of absorption and the percent of absorbed water is different. Accordingly, also the appearance of the gels is different: i.e. swollen pure collagen (gelatin) is very soft while swollen water crystals are soft but less than gelatin.
The behavior is different also when the polymers are tested with hot water. Indeed materials containing collagen likely dissolve in water at 60-70°C, because weak links, responsible of crosslinked structure, are broken by giving moderate energy, thereby preventing gel formation. Crosslinked polyacrylamide and sodium polyacrylate claim stronger links that cannot be broken at 70°C.
Table salt has an interesting role in water absorption when disposable nappies are tested: in this case the addition of sodium chloride to swollen material allows the water to escape, thus causing the collapsing of gel. The phenomenon can be justified by a sort of competition between sodium chloride and water: there is higher affinity between sodium chloride and polymer than between water and polymer.