How crystalline is my cellulose specimen? Probing the limits of X-ray diffraction

Cellulose serves as a skeleton for many of the useful products upon which we rely on each day. When we want to learn about a skeleton, it makes sense to think about X-ray methods. The same can be said when it comes to learning about the crystallinity of cellulose. Over the past six decades, the Segal X-ray diffraction (XRD) method has been popular for judging the percent crystallinity of powder samples. However, XRD patterns for ideal cellulose crystals can be easily simulated, and limitations of the Segal and other methods become obvious. Calculated patterns for model 100% crystalline powder particles are predicted to be less crystalline by the Segal method. Except for the Rietveld method, current approaches do not account for particle orientation or different shapes of crystallites. The Rietveld method has so many variables that it can easily overfit the data. The take-away message is that routine XRD examination is important for showing sample characteristics, but fractional crystallinity values are affected by constraints related to simplifications required for the analysis.