PET is hygroscopic: When exposed to the atmosphere it will readily absorb water vapor from the surrounding air. The migration of water vapor into the PET granules occurs over time and will eventually slow and stop when the moisture content reaches a state of equilibrium with that of the surrounding air. At that equilibrium point, the water vapor within the PET granule will be evenly distributed throughout the pellet’s interior.
The water molecules are attracted to the polymer chains, forming a strong intermolecular bond. That bond is what makes drying PET difficult. Even a small amount of moisture in PET will cause a chemical reaction (hydrolysis) when the polymer is heated above its melt temperature.