Swelling of Crosslinked Polymers in Liquids and Vapors. Rational Explanation of Thermodynamic Paradoxes

Schroeder's paradox, stronger swelling of gelatin in liquid water compared to swelling in saturated water vapor, did not receive any acceptable thermodynamic explanation for more than 100 years. By an automatic registration of the growing size of a single polymeric sphere (of less than 1 mm in diameter) placed first into saturated vapor atmosphere and then into corresponding liquid, we showed that the above effect is a general phenomenon characteristic of all crosslinked polymers and all types of aqueous or organic media. Explanation of the paradox is presented in terms of higher activity and chemical potential of the bulk liquid compared to saturated vapor over it, due to the unidirectional attraction of the surface molecules toward the condensed liquid phase. Also, it was found that saturated vapor can exist in a prolonged equilibrium with polymer samples which incorporate any amount of the sorbate within the broad range from the vapor-equilibrated to liquid-equilibrated extremes. This paradox is explained by the dependence of the system of inter-chain interactions within the network on the pre-history of the sample.