Slime

Gooey Slime seems the opposite of pattern. Yet there is an underlying pattern called a polymer. Moreover, the polymers are crosslinked, forming yet a larger pattern.

Slime is in the ocean, in our bodies, in our yards. Our bodies make some kinds. Kids make a similar kind out of glue. Slime seems the opposite of pattern, but up close there is a fascinating pattern that makes slime what it is: slimy. Let’s examine the kind of slime kids make out of glue, such as Elmer’s glue. 

The glue contains molecules from which we can make polymers. A polymer, in turn, is a pattern made up of repeating molecules, or “monomers” that are connected by relatively strong covalent chemical bonds. 

In slime, the repeating unit is poly vinyl alcohol (PVOH). Modeling this with Lego makes features of this PVOH more tangible.

The red Lego blocks show Oxygen atoms, the brown blocks show the Carbon atoms. The small white block shows the Hydrogen atoms. 

Two repeating units of PVOH (separated to show how to build with Lego)

Two connected units of PVOH 

Linking lots of these PVOH units makes a chemical pattern called a polymer. Can you identify the repeating unit in the image below? A polymer is made of up of many, many of these units. (Yes, it is actually called a “repeating unit”).  It is a good exercise to give students a picture like this and have them try to identify and circle the repeating unit. 

Lots of these polymers, dissolved in water, float past each other, rather fluid. (The polymers can bend, a limitation of the Lego models, which don’t bend). 

Slime is made of lots of these polymers, but with with an additional feature, called crosslinking, to give it the sticky, slimy qualities. The PVOH molecules in slime are weakly crosslinked by another chemical (discussed below).  PVOH can be crosslinked because this polymer has regions of positive and negative charge. That is, PVOH molecules have “polarity.” In our Lego model, the little white rectangle (Hydrogen atom) has a net positive charge and the red rectangle underneath (Oxygen atom) has a net negative charge. The  reason the Oxygen has a net negative charge is because the Oxygen pulls on the electrons (negatively charged) it shares with Hydrogen. 

You could say the oxygen “hogs” the electrons. So each repeating unit in polymer has a negatively charged region and a positively charged region. Water (H20) itself is like this, with the oxygen hogging the electrons from the hydrogen.

In the case of water and PVOH, the Hydrogen atom has a net positive charge because its electrons are hogged by Oxygen.

Because the PVOH molecules are polar, as is water, it dissolves in water. 

 

Another consequence of PVOH being polar is that some other polar molecules can crosslink the PVOH loosely, giving it a pliability, a reformable yet elastic quality, characteristic of slime.

The crosslinking makes a pattern of patterns. The weak crosslinking can break and reform over time, patterns of change, when we move it slowly (move it quickly and it hardens up).

We can show the weak crosslinking using magnets. In a very general and rough way the crosslinking attraction between PVOH molecules is weak and breakable, like the attraction between magnets, and is due to opposites attracting, plus and minus charges. This weak crosslinking is what makes slime pliable, sticky, and, well, slimy.

It’s easy to fit magnets in large Lego, or Mega Bloks.

Tape magnets inside a block, or let them hold themselves together by magnetic attraction.

The white blocks (hydrogen) of the PVOH stick to the red block (oxygen) of the crosslinking chemical. We can model crosslinking crudely with magnets inside the white block and inside the red block. They will stick until we pull a little, as in Slime. 

Here’s my daughter playing with Slime she made with my sister Monica (a Slime expert). This is the kind of Slime we described above, with blue dye added. Below, Monica’s Slime students show off theirs.

Other Polymers kids use in making Slime, like Alginate gel, also depends upon crosslinking. Sodium alginate is a polymer which can be extracted from brown seaweed and kelps.  (Photo of my four year old holding Alginate Slime gel that we made). 
Alginate is one of the structural polymers that help to build the cell walls of these plants. It has some unusual properties and a wide variety of uses. When sodium alginate is put into a solution of calcium ions, the calcium ions replace the sodium ions in the polymer. Each calcium ion can attach to two of the polymer strands. This is called cross-linking. Like the glue Slime, the alginate Slime, is made up of cross-linked polymers.
We will devote one Beautiful Discovery Box to explore the patterns underlying Slime.