The Effect of Crystallinity on Polymer Density

For simplicity, let us assume that only crystalline and amorphous phases are present. With a given polymer, the properties of each phase remain the same but the relative amounts of the phases can vary, and this can strongly influence the bulk properties, particularly the mechanical properties. This is illustrated for polyethylene below, the most crystallizable polymer due to its simple repeat unit structure (—CH2—CH2—). 

Since polymer chains are packed together more efficiently and tightly in crystalline areas than in amorphous areas, crystallites will have a higher density. Thus, low-density (0.92 g/cm ) polyethylene is estimated to be about 43% crystalline, while high density (0.97 g/cm ) polyethylene is about 76% crystalline. Density is, in fact, a convenient measure of the degree of crystallinity. Because the volumes of the crystalline and amorphous phases are additive, density and degree of crystallinity are related by 

TABLE 4.1 Tlie Influence of Crystallinity on Some of the Properties of Polyethylene  

Commercial Product Low Density Medium Density High Density 

Branching, equivalent CH3 groups/lOOO carbon atoms 15 30 5-15 1-5 

---