MaterialsInMotion

When my group and I were working on our project, one person came to the realization that one of our previous assumptions was wrong: the polyvinyl acetate in the glue wasn’t the polymer being cross-linked by the borax—it was the polyvinyl alcohol.

The process starts with dissolving the borax in water, which causes the borax to separate into borate and other molecules that do not affect the chain-linking of PVA. The borate is the key product that allows the PVA to be cross-linked and explains why we can’t directly add the borax into the glue and corn starch, which was an aspect of the experiment that I was confused about.

So how do the two molecules actually bond? Each borate molecule has four negative OH tails bonded to a boron in a X-shape, and each repeating unit of PVA has a negative OH tail as well. The borate attracts two PVA chains on opposite ends, and each of the borate’s OH tails bonds with a hydrogen from a repeating unit of PVA. The resulting water molecule gets kicked off, and the borate forms a dynamic covalent bond with the remaining oxygen from the PVA repeating unit, meaning that the bond is strong can be reformed after being broken.

To my understanding, though, Elmer’s liquid white glue is mostly made of polyvinyl acetate. If polyvinyl alcohol is the polymer being cross-linked, what role does polyvinyl acetate play? Is it just there, floating around the ball? Or is it being cross-linked as well by one of the products formed when the borax was dissolved in the water?