Chem 1030-010 Spring 2008
Cross-Linking a Polymer, or “How to make your own silly putty”
Objective: To cross link a common polymer to change its properties.
Introduction: We live in an age of plastics. Plastics are substances that can be molded or shaped into useful objects. These plastics are polymers that are made by joining many individual, identical units together called monomers.
Starch is a polymer of glucose molecules and proteins are polymers of amino acids. Many fibers, such as cotton (cellulose,) are also polymers.
Polymers can be made even larger by a process called “cross-linking.” One of the properties of polymers is their ability to stretch, but cross-linking creates more structure and hence, makes the polymer more rigid. Cross-linking can be done two ways, either with heat or with pressure and usually is irreversible.
Book reference: See Section 12.4 of your text book on Polymers. See also Wikipedia (http://en.wikipedia.org/wiki/Cross-link) on information regarding what cross linking is.
Procedure: For this project, you will need the following items:
WHITE Elmer’s glue
Borax (20 mule team brand is perfect.)
Water
Food coloring (for tinting the polymer only, optional)
Plastic spoons for mixing and measuring spoons
Plastic storage containers
Mix equal amounts of Elmer’s white glue with water (approximately 4 Tbs each to start,) in one of the plastic storage container. Mix well until the mixture is homogenous.
Into a second container add 1 tablespoon of borax and mix it as well as you can with 3 Tbs of water. The borax most likely will not completely dissolve in the water. Let the solution settle for a while after mixing, then decant off the clear borax solution away from any undissolved borax and add to the Elmer’s glue mix in container 1.
Stir the Elmer’s solution very well.
The cross-linking process will take place almost instantly. The polymer in the glue should clump up and become less runny. If you wish, add one or two drops of food coloring to the product at this point. Pour or scrape the cross-linked polymer out of the container and knead it for a minute or two. It should have putty like consistency.
The new polymer will become rubbery and no longer have a slimy texture. If the polymer is still too runny after kneading for a while, make a new batch and use more borax and /or make a new mixture of glue and water using slightly more glue but keeping the water volume the same.
The new polymer can be stretched, but also will break. The polymer can be stored in a Ziploc bag and is perfectly safe to play with by small children.
Be aware though that the ingredients and/or the polymer itself can stick to clothing and carpets.
Name: Date:
In the space provided, describe how you made your polymer and include any pertinent observations.
Answer the following:
Q. What happens when you stretch the polymer slowly after it is formed?
Q. What happens when you stretch the polymer quickly after it is formed?
Q. What happens if you pinch a small portion and make it into a small ball and drop it?
Attach to your report a SMALL sample (no more then 1-inch in diameter) of your cross-linked polymer in a plastic bag. A single sample may be turned in for a group, but must include the names of all people in the group to receive credit.
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