system
Given the following post, respond as if you're commenting on Reddit:
human
On a molecular level, what is happening to molecules/atoms when a sticky substance is torn in half? For example: modeling clay. What creates the "tension" in these substances?
gpt
Tearing something in half is a physical change, not a chemical change. While that means that chemical bonds aren’t necessarily broken or formed during this process, weaker interactions caused by intermolecular forces between molecules (as well as the surface energy penalty to create two new surfaces) hold the substance together and must be overcome for a tear/fracture to occur. Something being “sticky” just means that the intermolecular forces between your hand and that substance are relatively large, but it’s really the IMFs that are present between different molecules of the substance that affect how easily something tears.

As it turns out, modeling clay is little bit more complicated. What we think of as modeling clay is more generally known as polymer clay, which is composed of PVC (polyvinyl chloride). PVC is the stuff that those white pipes are made of, so by itself, it’s pretty tough. A plasticizer is added to decrease the attraction between the polymer chains just enough so that you’re left with a flexible substance that you describe as having “tension.” Ultimately, the degree of “tension” or plasticity comes from the intermolecular forces between the polymer and plasticizer, as well as how much cross-linking is present in the polymer. The combination of PVC and plasticizer gives modeling clay some interesting rheological properties: when we mold the clay, we apply stress and it flows like a viscous liquid. But when we release the clay, it behaves as a solid.