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