A thermodynamic theory of sorption in glassy polymers

Sorption of vapors by glassy polymers is known to have properties that are different from those typical for sorption by rubbery polymers or liquids. Despite an importance for many practical applications, this difference is still not fully understood. We developed a thermodynamic theory for description of penetrant activity in the glassy state. The presented approach considers the glass transition in terms of a second order phase transition and utilizes heat capacity changes for calculation of thermodynamic activity. The formalism developed here explains a deviation of penetrant activity in the glassy state from equilibrium values. Moreover, the sorption-desorption hysteresis can be explained in thermodynamic terms as well. We demonstrate that not only the heat capacity change of the polymer, but also the heat capacity change of the penetrant should be considered to understand the difference between three cases: sorption by equilibrium liquid, absorption by glass and desorption from glass. The theory presented here is tested on experimental data on water sorption by maltodextrin and bovine submaxillary gland mucin. A very good fitting of the experimental data for both sorption and desorption cases is demonstrated. The values of heat capacity increments of the components obtained by fitting the penetrant activity data can be used to get new insights into the non-equilibrium properties of glassy materials