Crystallinity

Abstract

Polymers, other that thermosets or some fully amorphous materials such as PMMA, may be considered to be a composite of fully crystalline material and an amorphous matrix (see Crystalline Polymers, Amorphous Polymers). The maximum crystallinity that can be readily achieved depends critically on the molecular structure of the polymer with symmetrical chain molecules that allow regular close packing giving the highest crystallinities. The modulus and yield stress of the crystalline material is greater than that of the amorphous component so the mechanical properties of a semi-crystalline polymer will vary depending on the ratio of crystalline to amorphous material. For example in PEEK the modulus will typically vary from 3.5 to 4.6 GPa as the crystallinity increases from 16% to 39% and the yield stress changes from 72 to 106 MPa over the same crystallinity range. Along with the increase in modulus and yield stress there is a reduction of extension to break, typically for PEEK this will reduce from 250% to 150% as the crystallinity increases over the range 15 to 35%. The modulus of a semi-crystalline polymer may be estimated from the Tsai-Halpin equations for composites.