Making Slime and Glue in the Classroom

Fig. 3 - 'Gunge'

Polymers are long molecules made up of repeating units called monomers that are joined together by covalent bonds. Natural, or "biopolymers" include proteins, DNA and cellulose, while rayon, polystyrene, Teflon and PVC are among the many synthetic polymers currently available. The predominant element in most of these polymers is carbon.

The physical properties of all polymers are dependent on the length of the molecule and the presence or absence of side chains and cross linkages between polymer chains. As molecule length increases, so, too, do melting and boiling points, impact resistance and viscosity. In addition, as branching and cross linking increase, the flexibility and durability of the polymer is also increased.

Making Slime – Materials and Teaching Method

Slime, Silly Putty and similar preparations owe their properties to the cross links made between molecules of the polymer poly vinyl acetate (PVA) and borax particles. PVA is readily available as a craft or wood glue, and consists of repeating vinyl acetate monomers (see figure 1). Borax, a common ingredient in laundry detergents, can be obtained from supermarkets in the form of sodium tetraborate (see figure 2).

Fig.1 - PVA Glue

The following materials and equipment are required per group of around four students:

•                30 ml PVA glue

•                7 ml borax solution (this can be prepared by dissolving a tablespoon of sodium tetraborate in warm water)

•                ziplock bag

•                30 ml water

•                food colouring

Students should be instructed to copy down the following directions. A teacher-led explanation could follow this.

•                Combine the PVA glue with 30 ml water in a ziplock bag.

•                Add a drop of food colouring, close the bag and mix the ingredients around.

•                Add 7 ml borax solution.

Fig. 2 - Borax Soap

•                Close the bag again and squash it around. As the borax and glue combine they form "cross linked" polymers and the mixture begins to form a semi-solid (see figure 3).

The following questions could be written on the board after students write up the experiment and their observations:

1.             How does adding the borax to the PVA glue change the properties of the mixture?

2.             Does your slime bounce when you drop it on the floor?

3.             Pull the slime apart slowly and compare the results with when you pull it apart quickly.

4.             Viscosity is a measure of a substance’s ability to "spread out." What do you think will happen to the viscosity of your slime if more borax is added to it?

5.             Adding acid to the slime will remove some of the borax cross links. Explain how this will affect the properties of the slime.

Note that slime is regarded as a "Non Newtonian" fluid because its viscosity does not obey predictable laws. It can display both elastic and inelastic properties as a result of the borax cross links between the PVA chains

Making Casein Glue – Materials and Teaching Method

Many glues are made from either natural or synthetic polymers. One of the properties these polymers have in common is that they can form hydrogen bonds with other surfaces such as wood and paper.

Casein glue is made from the milk protein, casein, which consists of multiple amino acid molecules linked together.In this procedure, vinegar is added to allow the casein molecules to clump together, while the sodium bicarbonate acts to neutralise the vinegar and allow the casein to become sticky.

The following materials and equipment are required per group of around four students:

Fig.4 - Casein Protein

•                100 ml milk

•                10 ml white vinegar

•                ½ teaspoon sodium bicarbonate (baking soda)

•                beaker

•                hotplate

•                thermometer

•                "chux" wiping cloth

•                plastic cup

Students should be instructed to copy down the following directions, which could be followed by a teacher-led explanation.

•                Heat 100 ml milk to about 55°C in a beaker. Use a thermometer to monitor the temperature.

•                Stir in 10ml white vinegar to the milk.

•                Remove from heat and stir until the solid curds are separated from the yellowish whey.

•                Filter the mixture through the dishcloth.

•                Squeeze out any remaining liquid from the curds (see figure 4) and place the curds in a plastic cup.

•                Add two tablespoons of water and 1/2 teaspoon of baking soda and stir to form a thick, opaque liquid. Keep adding baking soda until no more bubbles appear.

Students can test their glue on wood, paper and small pieces of fabric. The following questions could be written on the board after students write up the experiment and their observations.

1.             What type of molecule is casein?

2.             Why are the vinegar and sodium carbonate added in this procedure?

3.             What type of bonds form between the casein glue and the surfaces it sticks to?

Polymers Follow-Up Activities

Casein plastic, another derivative of the milk protein, casein, can be made using the method described above, but without adding the sodium carbonate to the milk curds. The curds will form a plastic if allowed to harden overnight. Different plastics such as polystyrene, bakelite and polythene can be classified as "thermosetting" or "thermosoftening" by touching them with a red hot piece of wire or other type of metal. Thermosoftening plastics will melt, whereas thermosetting plastics will burn or char.

References

"Designer Polymers." BBC. 2010.

"What Are Polymers?" Wisedude, 2006