The meaning of crystallinity when judged by x‐rays

Abstract

AbstractThis discussion of the variability of polymer x‐ray diffraction patterns tries to show that there is no simple definition of crystallinity when judged by x‐ray diffraction, and that x‐ray crystallinity may differ sharply from judgments of order by other techniques. It is recommended that a clear distinction be made between a measurement obtained from a diffraction pattern and the model which is used to interpret that measurement. Many precise and valuable measurements can be made of the varying aspects of the pattern. These measurements are perfectly valid and, if done in a consistent and verifiable manner, allow one to compare and relate samples having different treatment histories or different physical properties. These relationships are useful and meaningful as such. Difficulties arise from the interpretation of these measurements. The interpretation involves an unfortunate combination of semantics, morphology, and mental models. It is shown that there is no unequivocable interpretation that completely satisfies everyone in this fast‐changing area of study. The cause of this dissatisfaction stems from the simple fact that we do not really know the exact arrangement of the polymer chain from one of its ends to the other. Thus, all models and all interpretations have no meaning beyond being useful concepts. Difficulty comes when we make the mistake of assuming that our models are true pictures of the solid state of polymers. It is hoped that, in time, enough detailed measurements will be obtained so that generalities can be drawn and so that some of the dilemmas of interpretation can be resolved.