There are good reasons for this all based on molecular composition of the chains. Polymers" are actually mostly amorphous which means they will have both.Īnd you might ask, is there any relationship between Tm and Tg? Great question! And the answer is "yes!" Here's a plot of a few common polymers showing how Tg and Tm (in degrees Kelvin) both increase together. Portion undergoes the glass transition only, and the crystalline Same sample of a polymer can have both a glass transition temperatureĪnd a melting temperature. Usually makes up 40-70% of the polymer sample. In ordered crystals, but are just strewn around in any old fashion, evenīut even crystalline polymers will have some amorphous portions. Polymers that is, polymers whose chains are not arranged The glass transition is a different kind of transition which happens to amorphous Melting happens when the polymer chains fall out of theirĬrystal structures, and become a disordered liquid. We have to make something clear at this point. Rubber elastomersĪre used above their Tg's, that is, in the rubbery state, Hard plastics like polystyrene and poly(methyl methacrylate),Īre used below their glass transition temperatures that is in
Some polymers are used above their glass transition temperatures, and some are used below. When the polymer is cooled below this temperature, it becomes hard and brittle, like glass. There is a certain temperature(different for each polymer) called the glass transition temperature, or Tg for short. The glass transition is pretty much what it sounds like. This transition is something that only happens to polymers, and is one of the things that make polymers unique. 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.