Effects of surface hydration on the deposition of silane monolayers on silica optical fiber

A polymer coating applied to a silica optical fiber prevents mechanical damage to the fiber associated with handling. The adhesion of this coating is critical to the long-term mechanical reliability of the fiber. Typically, silane coupling agents that bond to the polymer and the silica fiber surface are mixed into the polymer before it is applied to the fiber. Self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) are a good model system for studying the reaction of silane coupling agents to the fiber surface. X-ray photoelectron spectroscopy (XPS) of OTS monolayers deposited on both silicon wafers, with an oxide layer, and silica glass fiber has demonstrated that incomplete monolayers form on dehydrated silica, but with partial rehydration a complete, well-ordered monolayer can be formed. These results are consistent with our previous work on flat, fused silica substrates in which the film coverage and degree of ordering were obtained from infrared spectroscopy of the monolayers. There is greater OTS coverage on a dehydrated fiber than on a dehydrated flat wafer. These results show that improved adhesion using coupling agents in fiber coatings may be obtained by exposure for several minutes of the freshly drawn fiber to water before coating. The work also shows the value of using XPS to study the surface of an optical fiber. The degree of OTS coverage and film density on a fiber can be obtained by comparing XPS measurements made after preparing SAMs in the same way on a fiber and a flat substrate.