Crystallinity by Density

Crystallinity by the density method

There is a method independent of DSC for measuring the degree of crystallinity, built on a simple observation: in the crystalline regions the chains are packed in an ordered fashion and take up less room; in the amorphous regions they lie loose and disordered, taking up more. The crystalline phase is therefore denser than the amorphous one, and the overall density of the specimen maps directly onto its crystallinity. The formula, derived from a two-phase model that assumes additivity of volumes, is X_c = [ρ_c·(ρ − ρ_a)] ÷ [ρ·(ρ_c − ρ_a)] × 100%, where ρ is the measured density of the sample, ρ_a the density of the fully amorphous phase and ρ_c that of the fully crystalline phase (both reference values for the polymer, taken from perfect crystals by X-ray diffraction and from completely amorphous material obtained by rapid quenching). The closer the sample density sits to ρ_c, the higher the crystallinity. The great strength of the method lies in the accuracy and simplicity of a density measurement. The classic technique is the density gradient column: a tube filled with a mixture of liquids whose density varies continuously from bottom to top; the polymer specimen is dropped in and floats at the exact height where the liquid density matches its own, which allows a very precise reading. Pycnometry serves the same purpose. Results usually agree well with DSC, and the two methods together give a robust characterisation. The density method assumes the sample holds no voids (pores, bubbles), which would falsify the reading. Enter the sample, amorphous and crystalline densities.