Polymers - Crystallinity vs. Amorphousness

[CrimpJiggler]

I'm trying to get an understanding of what properties these two forms give a compound. I read that crystalline polymers are very strong but also very brittle whereas amorphous polymers are very tough meaning they can be bent and twisted but won't break. I then read that an example of a highly crystalline polymer is Kevlar and an example of a highly amorphous polymer is polycarbonate. They use polycarbonate to make shatterproof glass and its so tough they even use it to make bulletproof windows. Kevlar fibres are used to make slash/stab proof and even bullet proof clothing. If I'm not mistaken, Kevlar is a common material used in bulletproof vests.

I don't really understand it. They said crystalline polymers are strong but brittle. Kevlar is used to make clothing so it obviously isn't brittle. They said amorphous polymers are tough. From what I read, polycarbonate has a high (higher than room temperature) glass transition temperature so that tough, shatter proof glass is polycarbonate in its glassy state. I'm guessing then that the reason its so shatter proof is because its an extremely strong material so it takes extreme amounts of energy to cause it to break. That makes sense, its just Kevlar that I'm confused about. If crystalline polymers are brittle, why is Kevlar so flexible?

 

[Borek]

Your thinking is correct - and statements that you read are overgeneralized rules of thumb. They do point in the right direction, but they don't tell whole story, as you have nicely shown comparing properties of Kevlar and polycarbonate.