Various models and equations of water vapor sorption by hydrophilic polymers were considered. It was shown that these models often do not correspond to the sorption mechanism. These models can be mathematically adapted to the experimental isotherm regardless of the real sorption mechanism. Even when sorption occurs according to the same mechanism, various authors use different models and equations. This study is based on the volume absorption mechanism and the Van Krevelen method of group contributions. As a result, a universal physicochemical equation was proposed, which makes it possible to adequately describe the sorption isotherms of amorphous hydrophilic polymers knowing only the chemical formulas of repeating units of these polymers. To calculate the sorption isotherms for semicrystalline samples, it is necessary to use an additional parameter, namely the degree of amorphicity (Y). The adequacy of the derived equation was verified for samples of cellulose and other natural polysaccharides, as well as for samples of synthetic hydrophilic polymers such as polyvinyl alcohol, polyamide-6, and polycaprolactone having various Y-values. The verification showed that the experimental isotherms are almost identical to the isotherms calculated by the universal equation.
In this research, the structural characteristics, specific surface area, sorption of water vapor, and wetting enthalpy of various polysaccharides (cellulose, hemicelluloses, starch, pectin, chitin, and chitosan) have been studied. It was confirmed that crystallites are inaccessible for water, and therefore water molecules can interact only with polar groups in noncrystalline (amorphous) domains of biopolymers. The isotherms of water vapor sorption for various polysaccharides had sigmoid shapes, which can be explained by the absorption of water molecules in heterogeneous amorphous domains having clusters with different packing densities. The method of contributions of polar groups to sorption of water molecules was used, which allowed to derivate a simple calculating equation to describe the shape of sorption isotherms. The wetting of biopolymers with water was accompanied by a high exothermic thermal effect, in direct proportion to the amorphicity degree. The sorption values and wetting enthalpies of amorphous domains of biopolymers were calculated, which allowed to find the hydrophilicity index and compare the hydrophilicity of the various polysaccharides.
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.
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