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Name: Shira
Status: Teacher
Grade: 9-12
Location: CT
Country: USA
Date: February 2009



Question:
I am teaching the students about polymers/plastics and want them to understand cross-linking by making the polyvinyl alcohol and borate 'slime'. How does the hydrogen bond to the carbon backbone AND the oxygen from the borate? Also, I have found conflicting information about their comparative flexiblities and strengths. Can you tell me their comparative flexibilities and strengths and why?


Replies:
How does the hydrogen bond to the carbon backbone AND the oxygen from the borate?

I'm not sure what you're asking here. Carbon atoms can bond to four other atoms (for example, methane is CH4, one carbon bonded to four hydrogens). In polymers, carbons can bond to each other forming a long chain. In such a polymer, each carbon is bonded to two other carbons to form the backgone, but that leaves two more open bonds. The properties of the polymer depend on what the other two bonds are. In the case of PVA, one of the carbon's free bonds is to a hydrogen atom, and the other bond is to a hydroxyl group, OH. The OH group makes the PVA soluble in water because the OH can bond with water or other polar groups, such as the borate that is used in this experiment.

If you're asking more generally why the boron is preferred over water, then we get into thermodynamics. Molecules attempt to find the lowest-energy, accessible configuration. In the case of borate and PVA, the Boron complexes with the PVA at the alcohol group (OH) on the PVA producing a lower energy configuration. Since lower-energy is preferred, this is the preferred configuration.

Can you tell me their comparative flexibilities and strengths and why?

I think one of the problems you might be facing is that "comparative flexibility" and "strength" are not well-defined, scientific terms. I will hazard a guess here and suggest you may be trying to understand why the slime responds to forces they way it does -- for example, why when you push softly, it deforms and stretches, but if you pull it hard, it tears. This is called 'non-newtonian' behavior, and it is remarkably complicated. The PVA molecules are chemically bonded (cross-linked) and tangled up on each other, and the nature and intensity of energy it takes to untangle them 'smoothly' rather than just ripping them is very hard to calculate. So the "strength" of the material is not only dependent on how much PVA and borax you use, but also on how hard and how quickly you push on the slime. In other words, any 'table of strength' values would have to consider a long list of variables -- and the variability you are seeing could be due to all the possible different conditions and compositions.

There are a ton of resources online that describe how the complexation (the crosslinking) occurs and also how and why this slime is non-newtonian. Perhaps if you check out a few different ones, you can get a better feel for what's going on. A web search with terms borate, PVA, and complex will give you a long list.

I hope this helps,
Burr Zimmerman



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