Cross-Linked Polymers

Polymers  are chemicals made up of  repeating chemical units, called monomers.

Polymerization is the process by which monomers attach to each other to form a chain made up of thousands of monomer units.

*Styrofoam is the Dow Chemical’s brand name for foamed polystyrene.

Cross-linking

Single threads of fiber have more uses when twisted together to form rope or cords as well as woven together to form rugs, blankets and other materials. Like threads of fiber, chains of polymers have more uses when the chains are linked together as well as linked to themselves. Chemicals are added to form what are called cross-links.

Though not as strong or rigid as the bonds between the monomers within the polymer chain, these cross-links change the macroscopic physical properties of the polymer. For example, when you stretch a rubber band then release it, the rubber band returns to its original shape. The polymers making up the rubber band have many cross-links that prevent the individual chains from sliding past each other. Instead, the cross-links allow the separate chains to move a limited distance, then when the stress of being stretched is removed, the polymer chains move back to their original position. In other words, when a stretched rubber band is releases, it returns to its original shape.

Stretching and releasing a rubber band demonstrates the transfer of mechanical energy to potential energy to kinetic energy. In other words, the Law of Conservation of Energy is demonstrated. Energy is the ability to do work. Work is the  product of force on an object (or system) times the distance the object (system) moves. Work is done on the rubber band when it is stretched. Thus, the stretched rubber band now has this energy. When the rubber band is released, the potential energy of the stretched rubber band is used to move the rubber band back to its original shape. The speed of the rubber band increases and if you are holding one end the band will do work on your fingers . In other words the rubber band springs back and hits your fingers.

Investigation:

What effect do the number of cross-links have on the mobility of the polymer chains?

1. Make a model of cross-links.

2. The higher the concentration of the borax solution in making slime, the greater the number of cross-links formed between the polymer in “Elmers Glue.

One example of cross-linking is vulcanization . In vulcanization, a series of cross-links are introduced into an elastomer to give it strength. This technique is commonly used to strengthen rubber.

Classes of Polymers

Polymer science is a broad field that includes many types of materials which incorporate long chain structure of many repeat units as discussed above. The two major polymer classes are described here.

Elastomers,or rubbery materials, have a loose cross-linked structure. This type of chain structure causes elastomers to possess memory. Typically, about 1 in 100 molecules are cross-linked on average. When the average number of cross-links rises to about 1 in 30 the material becomes more rigid and brittle. Natural and synthetic rubbers are both common examples of elastomers. Plastics are polymers which, under appropriate conditions of temperature and pressure, can be molded or shaped (such as blowing to form a film). In contrast to elastomers, plastics have a greater stiffness and lack reversible elasticity. All plastics are polymers but not all polymers are plastics. Cellulose is an example of a polymeric material which must be substantially modified before processing with the usual methods used for plastics. Some plastics, such as nylon and cellulose acetate, are formed into fibers (which are regarded by some as a separate class of polymers in spite of a considerable overlap with plastics). As we shall see in the section on liquid crystals, some of the main chain polymer liquid crystals also are the constituents of important fibers. Every day plastics such as polyethylene and poly(vinyl chloride) have replaced traditional materials like paper and copper for a wide variety of applications. The section on Polymer Applications will go into greater detail about the special properties of the many types of polymers.

Rigor 

1. Find out the difference in configuration and conformation of the geometric structure of a polymer.

2. What are copolymers and of what use are they?

3. What does branched chain polomer mean?