Polymer Classification: Crystallinity

In long-chain polymer solids, the polymer structure affects the proportion of crystalline domains, which are sites of ordered, close-packed polymer chains, to amorphous domains, which are regions of disordered, randomly oriented polymer chains.

Due to the large size of polymer chains, ordered close-packing is not observed across entire molecules. So, it is challenging to achieve highly crystalline polymers, and amorphous domains are always present in polymers.

Often, branching or large substituents in the polymer chain increase the proportion of amorphous domains, resulting in noncrystalline, amorphous polymers.

Crystalline domains provide toughness, whereas amorphous domains lend flexibility.

However, both domains impart hardness and brittleness below the glass transition temperature. Above that, amorphous domains become increasingly flexible.

On further heating to the crystalline melting temperature, or melt transition temperature, all crystalline regions become amorphous and fluid, so crystalline polymers melt at a precise temperature.

Amorphous polymers gradually transform to a liquid state, with no sharply defined melt transition temperature.