In order to prevent water from being absorbed into a silica based optical fiber, a very thin coating of polyimide (polyamide) is often applied to the outer cladding. This works very well to prevent water from penetrating into the hydrophilic silica. But polyimide is very hydrophilic as well, even more so than silica I believe. So how is it that something absorbs water very well can act as a water barrier? My only guess is that capillary pressure of the water inside the polyimide is much greater than that of the silica due to the polyimide being more hydrophilic. So, at the interface between the two materials there will be a concentration gradient in the direction normal and away from the surface of the silica. If this is what happens, is there a name for this phenomenon?