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Polymers

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1. Why a polymer such as polybutadiene can be cross-linked, however a polymer such as polyethylene cannot be cross-linked. Please make reference to what it is about the chemistry of the repeating units that allows for the formation of cross-links.

2. Car tires are made of vulcanized rubber, which is a crosslinked polymer formed through the addition of sulfur and heat to form disulfide bridges between polymer chains. Car tires are actually fairly difficult to recycle when compared to other non-crosslinked polymer products such as plastic bottles. Consider why this might be the case, and give your thoughts.

3. Shown attached is a schematic representation of the water-ethanol phase diagram.

Making reference to the phase diagram, explain in a few sentences how one could enrich a solution of 15 percent ethanol - 85 percent water to a composition of 60 percent ethanol - 40 percent water. Be qualitative in your description, not quantitative.

Furthermore, explain why this procedure could not be used to reach a completely pure composition of 100 percent ethanol.

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1. Polyethylene has only methylene CH2 repeating units which are not very reactive. On the other hand, polybutadiene has -CH=CH-CH2- repeating subunits. The double bond carbons are very reactive and are good reaction sites for reactions with other monomers. You can also have cross-linking in the polymeric chain when the conformation brings two alkene units close to each other so that they can react. The allyl carbon is also a good site of reaction where reactions like halogenation can take place forming cross-links.

2. For recycling, bonds in the polymer must be broken to ...

Solution Summary

Cross-linked polymers are examined.

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Cross-linking a Polymer

Cross-linking a Polymer

Objective
The objective of this experiment is to crosslink a polymer and observes the changes in the physical properties of the polymer as a result of the crosslinking. Changes in physical properties are also sensitive to amount of crosslinking. This experiment could be expanded to investigate various levels of crosslinking.

Materials
2-4 oz Elmer's Glue-All
1 tablespoon 20 Mule Team Borax
Water
Food coloring (optional) Choose whatever color you want for your silly putty.
¼ cup for measuring
1 cup for measuring
Wooden stick or plastic spoon for stirring
Styrofoam cups, beakers, or small bowls for holding reagents and mixing
Paper towels
1 zip lock quart size bag

Safety Guidelines
- Wear safety goggles.
- Since borax solid (20 Mule Team Borax) and solution is a cleaning product it could burn your eyes.
-

Procedure
1. Dissolve 1 tablespoon of borax in 1 cup of water. Stir it, but it may not completely dissolve.
2. In a separate beaker, bowl, or Styrofoam cup add ¼ cup of water. Squeeze in 2 to 4 oz of Elmer's glue. Stir with a clean plastic spoon or wooden stick.
3. Optional: Add one or two drops of food coloring to the glue solution, if you wish to have colored silly putty. Stir the food coloring and glue solution.
4. Pour the glue/water/food coloring solution into a quart size zip lock plastic bag.
5. Add ¼ cup of the borax/water solution to the bag.
6. Close the zip lock bag tightly. Hold it at the top to make sure it is closed securely and begin kneading the mixture in the bag by squeezing and pressing vigorously the outside of the bag with your other hand.
7. Observe that the consistency of the mixture in the bag changes as you need it.
8. When the consistency is that of silly putty, remove it from the bag and form it into a ball. Pat off excess water with a paper towel. [If you see liquid glue squirting out of the silly putty, then it has not fully reacted with the borax water. If your material is very gooey or slime-like, you probably did not add enough of the borax crosslinker.]

Questions

9. If you have a ruler to measure, drop the ball from a height of 30 centimeters. To what height does it rebound?

Questions
1. How does the glue/water solution change when the borax solution is added?
2. What would you predict would happen if you added more borax crosslinker.
3. One measure of elasticity is how far it will rebound when dropped. If you measured the distance, how far did your ball rebound?

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