Preparation and Properties of a Cross-Linking Polymer

Borax (sodium borate, Na₂B₄O₇10H₂O) can be used to cross-link chains of polyvinyl alcohol polymer in water.  The resulting network of cross-linked chains produces a gel that traps water molecules.  This gel has fascinating properties when handled, squeezed, stirred, poured, or stretched.  It can be used to demonstrate acid-base reactions and equilibrium.

    Materials

    Making "Slime"

• 4% Polyvinyl alcohol, PVA, 240 mL
• 4% Borax, 100 mL (4.0 g household Borax diluted to 100 mL)
• 12 plastic or paper cups
• 12 plastic sticks or tongue depressors
• Food coloring
• 24 Zip-closure plastic bags
• 12 graduated cylinders, 50 mL
• 12 watch glasses or Petri disher

    Making "Gluep"

• Elmer's (or any other white) glue, 200 mL
• Food coloring or tempera colors
• 12 sticks (craft sticks)
• 12 plastic or paper cups, 6 or 10 oz.
• 24 Zip-closure plastic bags
• 12 teaspoon measures
• 12 Tablespoon measures

    Advance Preparation

    For "Slime"

    For "Gluep"

    Pre-Laboratory Discussion

Show students the dry PVA powder and the 4% Borax solution.  Demonstrate the solution's viscosity by pouring from one beaker to another.  Explain how PVA and borax solutions were made.  Show the structural formula of polymer chains and borate ion.  In water, hydrolyzes to form a borate-boric acid buffer system:

B(OH)₃  +  2 H₂O <==>  B(OH)₄^-  +  H₃O⁺

The B(OH)₄^- ion is believed to cross-link the polymer chains as shown:

             Additional Background