CRYSTALLINITY

A polymer’s tendency to have order and form crystallites derives from the regularity of the chains, presence (or absence) and arrangement of bulky groups, and the presence of secondary forces, such as hydrogen bonding. For example, isotactic polystyrene with phenyl groups arranged on one side of the polymer backbone is highly crystalline, while the atactic form (with a random arrangement of phenyl groups) is highly amorphous.

Polyamides are also highly crystalline due to strong hydrogen bonding. High-density polyethylene exhibits no hydrogen bonding, but its linear structure makes it highly crystalline. Low-density polyethylene, on the other hand, has branches and a lower crystallinity. It does not pack as easily as the high-density polymer.

The mechanical and thermal behaviors depend partly on the degree of crystallinity. For example, highly disordered (dominantly amorphous) polymers make good elastomeric materials, while highly crystalline polymers, such as polyamides, have the rigidity needed for fibers. Crystallinity of polymers correlates with their melting points.