Crosslinking Polymers
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| Student in 2010 with Elmer's white glue and laundry starch |
Chemistry is probably the most fun, especially for boys, when chemicals are mixed together to make gooey substances to play with. Sam has mixed together Guar gum and polyvinyl alcohol, which are long molecules joined together in long chains which slip and slide over each other very easily, so they are very slick and runny. But then, if you add sodium tetraborate it causes little bridges or crosslinks to connect the long molecules together, making a stretchy web of molecules.....or slime! These little crosslinks are fairly weak, so they break easily when the slime is stretched, so the slime flattens out on a flat surface. We added a bit of green food coloring just to make it visually interesting.
But you don't need chemicals such as these, which are a little harder to obtain, to have fun with making polymers; all you need is glue and liquid laundry starch. I prepare plastic cups in advance to make the experiment go smoother. First put 4 tablespoon of water into a clear plastic disposable cup and mark where the level of the water comes to with a permanent marker on the outside of the cup. Add 4 more tablespoon of water and mark the level again. Pour out the water. Now you are ready for your students. Have your students look carefully at the texture and consistency of white Elmer's glue and/or Elmer's Blue Glue, and liquid laundry starch before you start mixing them. My boys described the white glue was thick and sticky, the blue glue was thick and gooey and the starch was thin, slippery and white.
Now add laundry starch up to the second line.
When laundry starch is added to the glues the consistency changes immediately.
As you knead the laundry starch into the glues, the glue-starch becomes a ball that you can take from the cup. It is now bouncy, stretchy and somewhat elastic.
The different glues might be made of different molecules, my boys supposed. I reminded them of what they had learned about polymers (long chains of molecules hooked together), and illustrated what had just happened in the experiment by using 3 chains of paper clips laying side by side. 
I showed them that the paper clip strands slide by each other easily, and this is how the glue alone acts. I then hooked two from one chain to two from another chain, making cross-links. 
I showed them how the chains cannot easily slide back in forth now, illustrating the changes that occur when the laundry starch is added to the glue.
I showed them that the paper clip strands slide by each other easily, and this is how the glue alone acts. I then hooked two from one chain to two from another chain, making cross-links. I showed them how the chains cannot easily slide back in forth now, illustrating the changes that occur when the laundry starch is added to the glue. More fun with polymers at Science Kids at Home and at Macrogalleria.
