Applicability of light depolarization technique to crystallization studies

Abstract Critical analysis of light depolarization technique (LDT) widely used in polymer crystallization kinetics is presented. It is demonstrated, that intensity of light depolarized on a system of birefringent crystals cannot be treated as a measure of volume fraction of crystalline phase (degree of crystallinity). Intensity of light depolarized in a polycrystalline system is a non-linear function of the product of the degree of crystallinity and function of average crystal dimensions. Closed-form expression for depolarization ratio is derived and the range of conditions where linear approximation is acceptable is discussed. In spite of interpretational weaknesses, further development of light depolarization technique seems to be justified by potentially short response time. LDT may appear useful for studying rapid crystallization processes (above 1 kHz sampling frequency), which cannot be followed by measurements of density, X-ray diffraction or calorimetry. However, the LDT data either have to be combined with independent measurements of crystal thickness, or treated as a ‘crystallization characteristic’ per se, quantitatively inconsistent with calorimetric, volumetric or X-ray diffraction data.