Polymers are molecules that are really, really big! Long polymer molecules consist of many monomer units joined together. If we imagine a polymer as a train, a monomer would be a boxcar and a typical polymer might have 10,000 boxcars. That seems big, but what can we compare it to? Most polymers are less than 0.1 micron long, although a few may be almost 100 microns long. The width of a human hair is about 100 microns, so the length of the longest polymer molecule might compare to the width of a human hair. Suddenly that doesn’t seem so big anymore! And yet, the polymer is made up of 10,000 monomers. Let’s look at the properties of two polymers, in paper and plastic, to see what we can learn about them.
Materials
• Newspaper
• Plastic trash bag
• Scissors
Experiment
- Cut a newspaper sheet vertically to obtain a 3-inch wide strip. See how nicely you can tear this strip of paper along its long axis.
- Cut another sheet of newspaper horizontally to obtain a second strip that is about 3" wide. Try to tear this strip of paper along its long axis.
- Compare how easy (or hard) it was to tear these two strips of newspaper.
- Cut a 2" × 6"-piece of plastic from a trash bag.
- Pull the piece of plastic horizontally along its long axis until you can see the plastic get lighter in color, but don’t break it.
- Once you can’t pull the plastic horizontally any more (without breaking), carefully pull it along the short axis (vertically), again without tearing.
- Pull the piece of plastic again in the horizontal direction.
What’s happening?
You may have noticed when ripping the vertical newspaper strip that it is pretty easy to tear it in a straight line. Ripping newspaper that has been cut horizontally, however, gives a crooked tear that quickly runs off the edge. Newspaper consists of cellulose polymer fibers aligned in the vertical direction, so when you tear the paper vertically you’re separating molecules from neighboring molecules.
Fig. 1. Microscopic view of cellulose fibers in paper.
When you rip the paper horizontally, the tear zig-zags across the paper to find a path around individual fiber molecules. We are not strong enough to break individual polymer molecules, but we can separate neighboring molecules that have only weak forces between them. See Figures 1 and 2 for a microscopic view and model, respectively. (Microscope view: https://www.azom.com/news.aspx?newsID=59119)
Figure 2. When making newsprint, the paper is pressed under heavy rollers that cause the fibers to align vertically (with the grain), as shown in the model. When you rip the paper vertically (red line), you only have to separate neighboring molecules or fibers. If you attempt to rip the paper across horizontally (blue line), the tear zig-zags around individual molecules.
The long polymer molecules in a plastic trash bag are all mixed up, like a big pile of spaghetti. As you pull horizontally on the plastic the molecules begin to line up (Figure 3). When you then start to pull the plastic in the other direction, it becomes easier to do so, because the fibers are not as bunched up and neighboring molecules don’t hold on to each other as tightly.
Figure 3. What happens to polymer molecules when you stretch a plastic bag? Different colors represent different polymer chains.
References
https://www.youtube.com/watch?v=kUYe1hh4Hu8
https://www.teachengineering.org/activities/view/nyu_plastic_activity1
To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts
