Time-of-Flight Secondary Ion Mass Spectrometric Measurements of Molecular Weight Distributions for Functionally Terminated Oligomers and Transferred Langmuir-Blodgett-Kuhn Monolayers

Abstract

We report the first direct measurements of the molecular weight distribution in transferred condensed monolayers of amphiphilic oligomers determined by time-of-flight secondary ion mass spectrometry (TOFSIMS). Two α,ω-functional oligomers of poly(dimethylsiloxane) are investigated, comprising pentylamine and propylcarboxy end groups. Measurements are performed directly on condensed monolayer films transferred to silver-coated substrates using the Langmuir-Blodgett-Kuhn (LBK) technique. These results are compared to the original distributions measured on submonolayer thin films of the original oligomers prepared by spin coating onto silver-coated substrates and to molecular weight determinations provided by end group titration and size exclusion chromatography analyses. Different families of ions are found for the two different thin film preparation methods. A number of tentative assignments are proposed for these masses, based upon consideration of the effects of the LBK film deposition process and the influence of interactions between the end groups and the substrate. The molecular weight distributions for LBK films of both oligomers is found to be narrower and shifted to higher molecular weights than are those for the corresponding spin-coated films. The changes in molecular weight distribution observed are attributed to dissolution of lower molecular weight species into the aqueous subphase during the LBK film deposition process.