Interpretation of light depolarization data in terms of polymer crystallinity

Theoretical principles of light depolarization technique (LDT) have been analysed. It has been shown that the traditional way of interpretation assuming a linear relationship between depolarization ratio, average optical retardation of a single birefringent plate, D, and volume fraction of crystalline material (crystallinity) is incorrect. An exact non-linear relationship between depolarization ratio and the product DE (E is average number of plates in the light path) has been derived. The parameter DE has been shown to be proportional not to the crystallinity alone but to the product of crystallinity and average crystal thickness, (x). A method of decoupling light depolarization data, based on measurements at different sample orientations and different wavelengths, has been outlined.