CHEMISORPTION OF POLY(METHYLHYDROGENSILOXANE) ON OXIDE SURFACES - A QUANTITATIVE INVESTIGATION USING STATIC SIMS

Monolayers of poly(methylhydrogenisiloxane) (PMHS) were prepared on ox ide powder surfaces consisting mostly of SiO2. The polymer is covalent ly bonded via Si-O-Si bonds formed by reaction of Si-H groups with sur face Si-OH groups. The large powder surface area allows the determinat ion of the average number of surface bonds by classical chemical titra tion. 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 pea k patterns similar to those obtained from PMHS prepared on silver targ ets. However, the fragment ion intensity distributions strongly depend on the average number of surface bonds per polymer chain. These inten sity distributions are quantitatively analyzed using a simple statisti cal fragmentation model. The model assumes that fragments having the s tructure of the polymer backbone with different lengths are released f rom nonbonded polymer sections only. The probability of bond cleavage in the backbone is assumed to be constant. The model predicts the expe rimentally obtained intensity distributions very well. The average num bers of surface bonds calculated from fragment ion intensities are in very good agreement with the independent data obtained by chemical tit ration. Some information about the population distribution of surface bonds can also be obtained. This example shows that appropriate physic al models of the secondary ion emission process in static SIMS can be used to obtain very detailed quantitative information about the molecu lar structure of polymer surfaces.