Chemisorption of poly(methylhydrogensiloxane) on oxide surfaces: a quantitative investigation using static SIMS

Monolayers of poly(methylhydrogensiloxane) (PMHS) were prepared on oxide powder surfaces consisting mostly of SiO2. The polymer is covalently bonded via SiOSi bonds formed by reaction of SiH groups with surface SiOH groups. The large powder surface area allows the determination of the average number of surface bonds by classical chemical titration. Depending on the coverage of PMHS the average number of surface bonds ranges from 15 to 25 for a linear polymer chain with an average length of 30 repeat units. Static SIMS spectra of the samples show peak patterns similar to those obtained from PMHS prepared on silver targets. However, the fragment ion intensity distributions strongly depend on the average number of surface bonds per polymer chain. These intensity distributions are quantitatively analyzed using a simple statistical fragmentation model. The model assumes that fragments having the structure of the polymer backbone with different lengths are released from nonbonded polymer sections only. The probability of bond cleavage in the backbone is assumed to be constant. The model predicts the experimentally obtained intensity distributions very well. The average numbers of surface bonds calculated from fragment ion intensities are in very good agreement with the independent data obtained by chemical titration. Some information about the population distribution of surface bonds can also be obtained. This example shows that appropriate physical models of the secondary ion emission process in static SIMS can be used to obtain very detailed quantitative information about the molecular structure of polymer surfaces.