The more crystalline a substance is, the harder it gets; however, it also becomes more brittle along with increasing crystallinity.

The shorter the main molecular chain and the lower the branch, the higher the crystallinity!

Increasing density or crystallinity, respectively, produces higher tensile strength, stiffness as well as higher chemical resistance; however, it creates a reduction of impact strength.

Therefore, plastics require a certain crystallinity, since it is responsible for the stability of the plastic. However, in order to produce a certain elasticity of the plastic, amorphous regions are also required.