Crystallinity, Crystal Structure and Measurement
topic
PET crystallises in triclinic unit cell (a=4.56, b=5.94, c=10.75 Å, α=98.5°, β=118°, γ=112°). Crystallinity measured by: WAXD (wide-angle X-ray diffraction, characteristic peaks at 2θ=17.5°, 22.5°, 25.6°), density gradient column (ρc=1.455 g/cm³, ρa=1.335 g/cm³), DSC (heat of fusion ΔHf, crystallinity = ΔHf/ΔHf° where ΔHf°=135 J/g). As-spun POY: 0–5%; FDY after drawing: 35–50%.
Role
Crystallinity measurement is fundamental to understanding polyester structure and predicting mechanical properties, dye diffusion rates, and thermal behaviour. Multiple measurement methods and their correlations are standard in fibre characterisation curricula.