The Glass Transition

Plastic object left in the outside cracks or breaks more easily during the winter that it would in the summer time. The phenomenon is known as the glass transition. This transition is something that only happens to polymers, and is one of the things that make polymers unique. 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, namely, in the glassy state. Their Tg is well above room temperature, both at about 100°C. Rubber elastomers like polyisoprene and polyisobutylene, are used above their Tg, namely, in the rubbery state, where they are soft and flexible.
But 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. However, 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. And what should be noted is that the amorphous portion undergoes the glass transition only, and the crystalline portion undergoes melting only.
Some polymers have high Tg, and others have low Tg. What makes different polymers have different Tg? The answer is: How easily the chains move. A polymer chain that can move around fairly easily will have a very low Tg, while one that doesn't move so well will have a high one. This is because the more easily a polymer can move, the less heat it takes for the chains to commence wiggling and break out of the rigid glassy state and into the soft rubbery state.