Home School Life Journal

Home School Life Journal ................................................................................................................painting by Katie Bergenholtz

Crosslinking Polymers

Sam, 2008
James, 2008

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.

Sam, 2008
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.

James, 2010

Have your students put either glue in the cup up to the level of the line. 
Now add laundry starch up to the second line.
When laundry starch is added to the glues the consistency changes immediately.


Quentin (2010) with Elmer's white glue and laundry starch


Quentin (2008), Elmer's blue glue and laundry starch
James (2008) with Elmer's blue glue and laundry starch
 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.

Sam (2010) with Elmer's white glue and laundry starch


Alex (2010) with Elmer's white glue and laundry starch

Quentin (2008) with Elmer's blue glue and laundry starch
Katie (2008) with Elmer's white glue homemade Gak
 The blue glue-starch ball becomes more hard and lumpy than the white glue-starch ball.
Quentin (2008) with Elmer's blue glue homemade slime
Sam (2008) with Elmer's white glue homemade Gak
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.
More fun with polymers at Paul's Treehouse , here at Science Kids at Home and here at Macrogalleria.






5 comments:

  1. Very fun!

    I've been meaning for to say for a while too, that I love your fall header pictures :)

    ReplyDelete
  2. We made Gak with Borax once but then someone commented that it was dangerous so I have been meaning to make it with liquid starch but I never have yet.

    ReplyDelete
  3. You do such a good job of explaining how chemistry works. I bet your kids are learning so much about this.

    And I always find it interesting how so many common "chemicals" are worrying to people if you actually know the problems with eating them.

    ReplyDelete
  4. The children are always asking for more gak - it is a lot of fun. We used gumdrops and toothpicks to demonstrate the polymer chains, but I like the paper clips - they move more like the gak :)

    ReplyDelete
  5. This is AWESOME! Eli just asked me for some Chemistry again...I just picked the books up from the library today.

    ReplyDelete

Thank you so much for taking the time to comment. It means so much.