We've all seen a "rubber" ball bounce. The term rubber is a generic term that refers
to the properties of the ball and not its specific make-up So what makes a ball
Check out the video of a racquetball. We recognize it as a rubber ball because it bounces. (For more information about this topic see the page on elastomers.
|What do you think would happen to the ball if it gets cold? I don't mean Mississippi winter cold (which isn't really cold at all). Or even Wisconsin winter cold (which is pretty cold). I mean really, really cold! This video shows the racquetball being placed into liquid nitrogen, which is really, really cold! How cold is it? Liquid nitrogen boils at minus 320 degrees Fahrenheit. After a few minutes in liquid nitrogen the racquetball is really, really cold !|
|Imagine the racquetball now that it is really, really cold - what do you expect to happen if we bounce the racquetball now? Look at the video and see what happens when we drop the cold racquetball. (Don't cheat and look at the video first - think about what you expect to happen to a really cold object.) Did it do what you expected?|
|Now we are going to let the racquetball warm up a little bit. If we bounce it again, what will happen? Check out the video.|
|Now we'll let the racquetball warm up slowly and drop the ball several times as it gets warmer. Look at the last two videos to see what happens as the ball warms up.|
|How do you explain what you have just seen? The ball is a polymer that becomes a glass at low temperatures. At this temperature the ball bounces as a rigid object, just like a glass marble. At room temperature, the ball behaves as we expect a "rubbery" substance to behave (in polymer terms it's an elastomer). At some temperature in between these two states, the ball is at its glass transition temperature, or Tg , and doesn't bounce at all. Check out this video clip again. The ball is at (or near) its Tg and it hardly bounces at all.|
If you've been observant, you should have noticed a different sound as the ball bounced at the various temperatures. (Yes, observation involves more than just the things that you see!) Go back and check out the bouncing ball at various temperatures and listen carefully for the difference in the sound of the bouncing ball.
Now you're ready to observe the properties of many substances that are made of polymers. Pay close attention to all of those "plastic" or "rubbery" materials the next time there is a cold spell where you live.