Quantification of grafted poly(ethylene glycol)‐silanes on silicon by time‐of‐flight secondary ion mass spectrometry

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Abstract
Silicon grafted monodisperse poly(ethylene glycol) (PEG) silanes with various PEG chain lengths and mixtures of these were systematically analyzed with static time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). The mass spectra show differences in the various relative signal intensities, an observation that was used to elucidate important aspects of the grafting process. The relationship between PEG‐silane fragment ion abundances and Si+ ion abundances were used to (i) qualitatively describe layer thicknesses of grafted mixtures of PEG‐silanes on silicon, (ii) construct a calibration curve from which PEG chain length (or molecular mass) can be determined and (iii) quantitatively determine surface mixture compositions of grafted monodisperse PEG‐silanes of different chain lengths (3, 7 and 11 PEG units). The results suggest that discrimination does take place in the adsorption process. The PEG‐silane with the shorter PEG chain is discriminated for mixtures containing PEG3‐silane, whereas the PEG‐silane with the longer PEG chain is discriminated in PEG7/PEG11‐silane mixtures. The origin of this difference in adsorption behavior is not well understood. Aspects of the grafting process and the TOF‐SIMS analyses are discussed. Copyright © 2002 John Wiley & Sons, Ltd.