system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some plastics get stronger when stretched?

I was trying to tear a plastic bag from the grocery store apart when I thought about this. I was trying to rip the "handles" off, but it just stretches to the limit of my strength, extends but doesn't tear. Why is that? I know I could tear it apart easily by applying force from another angle and whatnot, but I'm curious to why it's strengthened by the pulling.
gpt
Good question! The mechanics of plastics (which are various types of polymers) can be very different from I've to another, so don't assume it's always true, but here goes.

Plastic bags are almost always polyethylene. Depending on the processing and the size of polyethylene (PE) chains used, the properties can change. In this case, stronger bags mean a higher density PE, while weaker ones are lower density. For PE, the general pattern is mostly the same either way, though.

PE is a very "plain" polymer, which makes things fairly simple. It's just a long chain of carbon atoms, with nothing but hydrogen sticking out. So, think of it like cooked spaghetti, with tons of chains so mixed together. As you pull them apart, it's easy at first. The chains are just sliding through and past one another. Some of them start snagging though, or getting knotted together by chance. It starts getting harder to keep pulling as more and more of the chains catch, and you are either pulling the knots or hooked sections free, or breaking some of the chains as you go.

Eventually, you keep pulling to the point where the chains that are not broken yet are stretched fairly close to linear, weaving through each other still but mostly on the same axis you are pulling along. Now, you are no longer pulling apart coils and knots, but you are ripping the chains themselves. Naturally, it's harder to rip apart molecules than to pull them away from each other! At some point, chains start ripping. The more of the chains rip, the fewer you still fight against, so it has what's called a cascading failure where ripping each next chain becomes easier and easier.

One interesting detail still - as you pull, the bag starts getting thinner at some point, and becomes less transparent there. That is called 'necking'. It's where the chains are becoming more linearly aligned due to the strain of your pulling. Because the chains are more linear and less tangled, it allows them to become more densely packed together (think how tightly you can pack straight spaghetti strands together versus a clump of strands!). This tighter packing is also what makes the necking region appear less transparent, as less light can go through.