|MadSci Network: Chemistry|
Dear AmeriCorp Student, I am a chemist and a chemical engineer and have spent over ten years in research, and have never asked myself the question posed to you by your students. I do not KNOW for certain how those toys work, but I have a good idea. It is not so obvious as a chemical reaction or magic, but you can demonstrate the same principle to your student with a few easy to get items. Get a few good, sturdy normal rubber bands that do not stretch too far. Find a drinking glass with smooth outer walls and stretch a few rubber bands around the glass. Two or three should do. Get some dry ice at a grocery or convenience store. Put the glass with the rubber bands stretched around the outside of the glass in an inexpensive drink cooler or ice chest. Place the dry ice around the rubber bands. Wait long enough for the glass to get as cool as the dry ice. Quickly remove the glass from the ice chest (using goggles and insulated rubber gloves, of course)and remove the rubber bands from the glass (If they don't come off easily, you may have to sprey the glass with a spray cooking oil first). You will see that the rubber bands do not behave like rubber bands. They do not shrink back to size when they are very cold. They retain the circumference they had when wrapped around the glass. When the rubber bands warm up again, they do act normally. You can repeat this experiment again and again until the rubber bands break. The reason that the rubber bands and the shrinky dinks behave this way is caused by the molecular structure forced on them when above the rubberization temperature. At room temperature, rubber bands are above their rubberization temperature. They stretch and return to their original shape right away. However, when you stretch a rubber band then cool it to below the rubberization temperature, its molecules are locked in their "stretched" or forced structure. The more complex and branched a polymer molecule is, the higher the rubberization temperature (on average. Rubber bands have very linear structure so they have a low rubberization temperature. As for the shrinky dinks, their structure is different enough from rubber bands that room temperature is below their rubberization temperature. When they were made by a machine known as an extruder in sheet form, the sheet of rubbery polymer, while still hot, was pulled in the forward and cross direction. While still in tension, the sheet of stretched polymer was cooled, retaining the stretched shape. When put into a warm oven, the stretched rubbery material returns to its unstretched size. Sometimes the polymer also contains trapped gas because it is foamed to add tension in another direction (Thickness). I hope that helps and that the experiment works. If dry ice is not cold enough for rubber bands, you may have to resort to liquid nitrogen if you can find some. Sometimes research labs and hospitals will have some and let you have a little. I hope this helps.
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