Purpose
Illustrate the effect of cross-linking on a polymer.

Materials

• Guar gum
• Borax, sodium tetraborate decahydrate, Na2B4O7 · H2O
• Red food coloring
• Glass stirring rod
• Empty glass juice bottle (~300 mL)
• 100 mL graduated cylinder
• 100 mL beaker
• Electronic balance

Safety

• Read the MSDS sheets for all chemicals before using them
• Wear chemical safety glasses
• Remove the original label from the juice bottle and clearly label the container to ensure it is not mistaken as drinkable juice

Procedure

1. Prepare 25 mL of a saturated solution of sodium borate by mixing borax with water until no more dissolves. (solubility ~1 g / 16 mL H2O)
2. Add 100 mL of distilled water into the empty glass juice container.
3. Add 3 drops of red food coloring.
4. Add 0.6 g of guar gum and stir until dissolved.
5. While swirling the liquid in the bottle, slowly add 5 mL of the saturated borax solution prepared in step 1.
6. Cap the juice bottle.

Results

• The addition of sodium borate to an aqueous suspension of guar gum produces a cross-linked gel.
• The resulting cross-linked polymer forms a red gel that looks like juice.

Follow-up Teaching Notes
The juice bottle can be opened and quickly shaken forward and back causing the gel to partially leave the bottle walls and then quickly snap back.

Connections
polymers, cross-linking

Extension

• Borax solution and Elmer’s glue is an example of cross linking where varying the amount of borax solution produces the effect of slime or silly putty.
• Nylon demonstration kit (available from ScholAR distributors) is another great example of a polymer synthesis.

Disposal/Clean-up
The natural polymer can be disposed of in the garbage.

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