Effect of sample moisture content on XRD-estimated cellulose crystallinity index and crystallite size

Abstract

Although X-ray diffraction (XRD) has been the most widely used technique to investigate crystallinity index (CrI) and crystallite size (L200) of cellulose materials, there are not many studies that have taken into account the role of sample moisture on these measurements. The present investigation focuses on a variety of celluloses and cellulose containing materials—from loblolly pine wood to tunicin, and evaluated moisture-induced changes in CrI and L200. It was observed that upon introduction of a small amount of water (5%) into P2O5 dried samples, for most samples, both absolute intensity of (200) reflection and its full width at half maximum declined. Moreover, (200) peak position (2θ max) increased when the samples became moist. Although the extent of such changes were material dependent, in general, a greater degree of change was associated with lower sample CrI. For CrI, maximum and minimum increases occurred for oven dried NaOH treated red pine holopulp and tunicin, respectively. For L200, maximum and minimum increases were for wood and tunicin, respectively. Moreover, 2θ max position for (200) reflection increased most for the wood and oven dried NaOH treated red pine holopulp (acid chlorite delignified milled-wood) and least for tunicin. The nonparametric statistical test “sign test” further supported these results. Observations from longer duration drying experiments, post moistening, indicated that the changes to the XRD parameters were reversible to some degree. Based on the findings it is concluded that for most cellulose materials with Segal CrI < 90% the moisture content has a significant bearing on the XRD-estimated CrI and L200 data. Consequently, it is essential that when such materials are compared, their diffractograms should be obtained under similar levels of sample moisture content.