Question

When looking at the glass transition temperature, what happens when above and below it? How does this change between thermoplastic vs. thermosetting? Amorphous vs. Crystalline? Linear vs. nonlinear? When does glass formation happen vs. crystallization?

When looking at the melting temperature, what happens when above and below it? How does this change between thermoplastic vs. thermosetting? Amorphous vs. Crystalline? Linear vs. nonlinear? When does solidification happen vs. crystallization?

Answer 1

Have you ever left a plastic bucket or some other plastic object outside during the winter, and found that it cracks or breaks more easily than it would in the summer time? What you experienced was the phenomenon known as the glass transition. This transition is something that only happens to polymers, and is one of the things that make polymers unique. The glass transition is pretty much what it sounds like. There is a certain temperature(different for each polymer) called the glass transition temperature, or Tg for short. When the polymer is cooled below this temperature, it becomes hard and brittle, like glass. Some polymers are used above their glass transition temperatures, and some are used below. Hard plastics like polystyrene and poly(methyl methacrylate), are used below their glass transition temperatures; that is in their glassy state. Their Tg's are well above room temperature, both at around 100 ^oC. Rubber elastomers like polyisoprene and polyisobutylene, are used above their Tg's, that is, in the rubbery state, where they are soft and flexible.

Amorphous and Crystalline Polymers

We have to make something clear at this point. The glass transition is not the same thing as melting. Melting is a transition which occurs in crystalline polymers. Melting happens when the polymer chains fall out of their crystal structures, and become a disordered liquid. The glass transition is a transition which happens to amorphous polymers; that is, polymers whose chains are not arranged in ordered crystals, but are just strewn around in any old fashion, even though they are in the solid state.

But even crystalline polymers will have a some amorphous portion. This portion usually makes up 40-70% of the polymer sample. This is why the same sample of a polymer can have both a glass transition temperature and a melting temperature. But you should know that the amorphous portion undergoes the glass transition only, and the crystalline portion undergoes melting only.

The Glass Transition vs melting

It's tempting to think of the glass transition as a kind of melting of the polymer. But this is an inaccurate way of looking at things. There are a lot of important differences between the glass transition and melting. Like I said earlier, melting is something that happens to a crystalline polymer, while the glass transition happens only to polymers in the amorphous state. A given polymer will often have both amorphous and crystalline domains within it, so the same sample can often show a melting point and a Tg. But the chains that melt are not the chains that undergo the glass transition