Polymers for special uses

Step 1 - Activities

Test some super-absorbent polymers

Description of the activity
The experience aims to show the behaviour of 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. Finally, a simple analysis of the results is proposed.

Materials

• 3 transparent plastic cups half-filled with tap water
• a graduated cylinder or graduated transparent cup
• a kitchen balance (division: 1 g)
• gummy bears
• disposable nappy
• sheet gelatin
• water polymer crystals (available at plant stores)

Procedure

Gummy bears

1. Obtain two different colored gummy bears. Weight one, then place it in the plastic cup with tap water.
2. Wait for at least 1h, then place the wet bear close to the dry bear. What do you observe? What about softness, transparency, shape and size?
3. Weight the swollen bear to evaluate the quantity of water that has been absorbed.
4. Repeat step 2 and 3 since the equilibrium has been reached, meaning that no more change is detected.
5. Finally, try to calculate the percent of swelling (in terms of percentage increase of weight).

Sheet gelatin

1. Obtain two sheets of gelatin for food. Weight one, then place it in the plastic cup with tap water.
2. Wait for half hour, then place the wet sheet close to the dry sheet. What do you observe? What about softness, transparency, shape and size?
3. Weight the swollen sheet to evaluate the quantity of water that has been absorbed.
4. Repeat step 2 and 3 since the equilibrium has been reached.
5. Finally, try to calculate the percent of swelling (in terms of percentage increase of weight).

Water crystals

1. Obtain 10 water crystals. Take one as reference and weight the others before placing them in the plastic cup with tap water.
2. Wait for at least 2h then place the wet crystals close to the dry crystal. What do you observe? What about softness, transparency, shape and size?
3. Weight the swollen crystals to evaluate the quantity of water that has been absorbed.
4. Repeat step 2 and 3 since the equilibrium has been reached.
5. Finally, try to calculate the percent of swelling (in terms of percentage increase of weight).

Disposable nappy

1. Take a piece of the inside material of a disposable nappy (about 1g). Place it in a transparent graduated cup (better if a graduated cylinder is available) and shape it on the bottom of the cup.
2. Register the volume of the dry material
3. Then add one tablespoon of water and stir gently.
4. Repeat step 3 since all the material has became transparent.
5. Register the final volume of the swollen plastic and try to calculate the percent of swelling (in terms of percentage increase of volume).

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.
Collagen, sodium polyacrylate and polyacrylamide are polymers different in terms of chain structure, number and strength of links in the crosslinked structure. 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.
Transparency is a common characteristic of the four kinds of hydrogels, indeed it depends on inside water.
The percent of swelling can be calculated by the following formula (if weights are registered):



or by the following (if volumes are registered)