As soon as an engineering polymer is manufactured and exposed to the atmosphere, it begins to adsorb moisture. That’s why they are called hygroscopic. Water vapor continues to migrate from the surrounding air until equilibrium is reached between the moisture content of the polymer and the surrounding air. This moisture adsorption takes place on a molecular level. The natural attraction between polymer chains and water molecules is what causes hygroscopic resins, exposed to a humid atmosphere, to take up and retain water. It is also what makes drying them difficult.