Question

What are the difference between crystalline and amorphous polymers ?

Answer 1

If we look at the abundancy of the two types, we will see that most of the polymers on Earth are amorphous polymers, which actually don't have an ordered structure on molecular level. The other type of polymers is known as semi-crystalline polymers and these have both crystalline and amorphous regions. As far as I know, fully crystalline polymers do not exist. But we can find highly crystalline (degree of crystallinity >90%) polymers.
In terms of structure, highly linear polymers are usually crystal while cross-linked polymers are amorphous
The glass transition temperature (\[T _{g}\]) is a range of temperature(s) where amorphous regions in the polymers soften. However, the crystalline regions break up and become disordered above the melting point (\[T _{m}\]). \[T _{g}\] is always smaller than \[T _{m}\] for semicrystalline polymers. Polymers are liquid above these temperatures.
General characteristics of semi-crystalline polymers: good to excellent chemical resistance, low surface friction, good to excellent wear resistance, higher shrinkage, being more opaque or translucent, thermoplasticity (depending on the degree of crystallinity)
General characteristics of amorphous polymers: average to low chemical resistance, high surface friction, low wear resistance, lower shrinkage, transparency, usually softening upon heating