STUDY OF THERMODYNAMIC STABILITY OF VARIOUS ALLOMORPHS OF CELLULOSE

Thermodynamic characteristics of cellulose samples having different crystallinity (X) and type of crystalline allomorph (CI to CIV) have been studied. Using methods of combustion calorimetry and DSC, the accurate values of standard enthalpy, ΔfHo, and free energy of formation, ΔfGo, as well as melting enthalpy of crystallites, ΔHom, were obtained. To avoid the use of uncertain indexes of crystallinity, a precise thermochemical method was proposed to determine the actual degree of crystallinity (X) of cellulose samples. It was found that with the decrease of crystallinity degree all linear dependences ΔfHo = f (X) and ΔfGo = f (X) converge at one common point, ΔfHoam = -942.4 kJ/mol and ΔfGoam = -642.6 kJ/mol, corresponding to amorphous phase cellulose at X = 0. This evidences that the amorphous phase in different cellulose samples has identical thermodynamic characteristics. On the other hand, the linear extrapolation of these dependences to the values corresponding to X = 1 gives the enthalpy and free energy of formation of different crystalline allomorphs. Based on obtained thermodynamic characteristics, the phase stability of various crystalline forms of cellulose and amorphous cellulose (CA) was estimated; and namely, the relative stability of the allomorphs decreases in the following order: CII > CIV ≥ CI > CIII > CA. These results allow to explain the structural phase transitions and reactivity of the cellulose allomorphs under certain conditions.