Why does a polyimide coating prevent water absorbtion in silica fibers?

[Topher925]

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?

 

[Achy47]

Thank you for your question. The phenomenon you are describing is called "capillary action" or "capillary pressure". This is the ability of a liquid to flow against gravity in a narrow space due to surface tension and adhesion forces between the liquid and the surrounding material. In the case of the polyimide coating on the silica fiber, the high hydrophilicity of the polyimide allows it to absorb and retain water, creating a barrier that prevents water from penetrating into the hydrophilic silica. The capillary pressure of the water within the polyimide is indeed greater than that of the silica, which helps to maintain this barrier. This is a commonly used method in many industries, including the optical fiber industry, to protect materials from water damage.