Why do most plastics contract when heated?

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Most things expand when they heat up and contract when cooled, which is why we do things like leave gaps concrete sidewalks for expansion and run a lid you can’t open under hot tap water to loosen it.

My general understanding is higher energy = more molecules moving around and bumping into each other and causing expansion, and removing energy/ heat results in the opposite. I know there are some exceptions like water which expands when frozen since the molecules line up in an orderly(ish) crystalline formation when frozen, so is something similar happening to plastic, except without actually requiring a state change?

In: Chemistry

Plastics are created in such a way that they exist in a relatively unfavorable configuration, long polymer chains oriented nicely by rapid cooling, the heat disrupts this nice orientation.

“Contract” and “shrink” aren’t the same thing.

Plastics, like almost everything else, tend to expand when heated. BUT…plastics also have a pretty low melting point and soften easily under heating so they get more liquid when heated than most things we interact with and the have surface tension and try to pull back into a minimum surface shape (a ball). Since most plastics are in thin sheets, that makes them “roll up” a lot when heated and it looks like they’re shrinking. Their total volume isn’t actually going down (it’s going up) but they’re reconfiguring from a thin sheet to a thick blob.

If you heat aluminum foil to melting it will do the same thing…it’s just that we very rarely encounter metals near their melting point but it happens all the time with plastic.

Most plastic stuff that will shrink a lot is vacuum formed. Heat is used to make it pliable and then it’s basically stretched over a mold. When you heat it again it wants to go to it’s natural unstretched state