Activity 3.3 - Cross-linking of polymers

Learning Aims:
  • Understanding of the process of cross-linking of polymers and a consequent change of mechanical properties of polymer.
Materials:
  • borax,
  • two beakers (one smaller and one bigger),
  • white glue,
  • balances,
  • hot water,
  • glass bar,
  • dyes,
  • spoon,
  • cloth
Suggestions for use:

Procedure:

  1. Dissolve ca 4 g of borax in 100 mL of hot water in a beaker and well stir up to complete dissolution of borax.
  2. Dissolve 10 mL of white glue in 5-6 mL of hot water in the second beaker and add few drops of selected dye. Well mix.
  3. Add 2 spoons of borax solution to mixture of water and glue in the second beaker and stir.
  4. A clod is formed. Remove it, dry it and compact it in palms for few minutes.
  5. Play few minutes with formed polymer, follow its mechanical properties.

Discussion:

Synthetics polymers are substances which consist of long chains of big molecules – macromolecules. Their structure is similar to chain. The macromolecules are composed of a large number of smaller periodically repetitive units like segments of chain.

Linear polymer

White glue is a mixture of polymer and water. Molecules of the polymer are stacked like small pieces of spaghetti. Tangled molecules give to the white glue sticky character but it is not liquid. If we put the white glue on air, the water from the glue is evaporating from the polymer and polymer chains are stacked to the surfaces and connect them together.

If we add a Borax, which contains borate ions, the ions can form a connection between the chainsof the polymer molecules and three-dimensional network is formed. As a consequence, the properties are changed and the Gluep is more solid than the original white glue (like in the case if you overcook the spaghetti).Of course, Gluep is also less sticky and elastic than the original white glue.

You know that white glue is linear non-cross-linked polymer. What happened after addition of borax? The cross-linked polymer has been formed with three-dimensional polymer network. Chains has been prolonged.
Possible questions:
  • How the properties of formed polymer changed?

The Gluep is more solid than the original white glue (like in the case if you overcook the spaghetti).Of course, Gluep is also less sticky and elastic than the original white glue.

  • Why is white glue the glue? What is the mechanismus of glue effect?

White glue is a mixture of polymer and water. Molecules of the polymer are stacked like small pieces of spaghetti. Tangled molecules give to the white glue sticky character but it is not liquid. If we put the white glue on air, the water from the glue is evaporating from the polymer and polymer chains are stacked to the surfaces and connect them together.