Crystallinity

Crystallinity is the degree to which a solid exhibits long-range, periodic atomic or molecular order. In crystalline materials, atoms or molecules are arranged in a repeating lattice over large distances; in amorphous materials, order is limited to short ranges. Many materials show a spectrum between these extremes, with semi-crystalline materials containing both crystalline and amorphous regions. In polymers, crystallinity specifically refers to the fraction of chains organized into ordered, tightly packed regions (crystalline lamellae) within a largely disordered matrix.

Quantifying crystallinity typically yields a crystallinity degree (Xc) between 0 and 1 (or 0% and 100%). Common measurement methods include differential scanning calorimetry (DSC), which estimates Xc from the heat of fusion relative to the heat of fusion for a fully crystalline reference; X-ray diffraction (XRD), which separates sharp crystalline peaks from broad amorphous halos; and density or solid-state NMR techniques. In metals and ceramics, crystallinity is often referred to as crystalline fraction and can influence properties such as strength, toughness, and diffusion.

Factors that influence crystallinity include cooling rate, annealing, molecular weight and tacticity (in polymers), impurities, and processing conditions. Slower cooling or post-processing annealing generally increases crystallinity by allowing molecules to rearrange into ordered structures, while rapid cooling can trap an amorphous state.

Effects of crystallinity are widespread: higher crystallinity typically raises melting temperature, stiffness, and chemical resistance, while reducing impact strength and solubility. It also affects optical properties and barrier performance in polymers. Common semi-crystalline polymers include polyethylene and polypropylene; highly crystalline materials often exhibit distinct spherulitic morphologies and higher rigidity.