Application of Thermochemical Method to Determine the Crystallinity Degree of Cellulose Materials

Currently, to characterize the crystallinity of cellulose, such an estimated parameter as the crystallinity index is used, measured by various methods and techniques. The main purpose of this article was to develop a thermochemical method for determining the real degree of crystallinity (X) of cellulose based on the measurement of the enthalpy of wetting. Various cellulose samples, such as MCC, pure cotton cellulose, bleached wood pulps, mercerized celluloses, and viscose rayon fibers, were used. For these samples, the exothermic wetting enthalpy (ΔHw), the maximum amount of sorbed moisture (Ao), as well as the X-ray index of crystallinity (CrI) were studied. The dependence of ΔHw on Ao was linear and can be expressed by the equation: ΔHw = k Ao, where the coefficient k = −336 (J/g). After substituting the theoretical value Ao,a = 0.5 (g/g) into this equation, the numerical value of maximum wetting enthalpy ΔHw,a = −168 (J/g) for completely amorphous cellulose was obtained. As a result, the equation for calculating the real crystallinity degree (X) expressed in mass fractions was derived: X = 1 − (ΔHw/ΔHw,a). Analysis of the obtained results showed that only the X parameter can characterize the real content of crystallites in cellulose samples, instead of the approximate CrI parameter.