TIME-OF-FLIGHT SECONDARY-ION MASS-SPECTROMETRIC MEASUREMENTS OF MOLECULAR-WEIGHT DISTRIBUTIONS FOR FUNCTIONALLY TERMINATED OLIGOMERS AND TRANSFERRED LANGMUIR-BLODGETT-KUHN MONOLAYERS

We report the first direct measurements of the molecular weight distri bution in transferred condensed monolayers of amphiphilic oligomers de termined by time-of-flight secondary ion mass spectrometry (TOFSIMS). Two alpha,omega-functional oligomers of poly(dimethylsiloxane) are inv estigated, comprising pentylamine and propylcarboxy end groups. Measur ements are performed directly on condensed monolayer films transferred to silver-coated substrates using the Langmuir-Blodgett-Kuhn (LBK) te chnique. These results are compared to the original distributions meas ured on submonolayer thin films of the original oligomers prepared by spin coating onto silver-coated substrates and to molecular weight det erminations provided by end group titration and size exclusion chromat ography analyses. Different families of ions are found for the two dif ferent thin film preparation methods. A number of tentative assignment s are proposed for these masses, based upon consideration of the effec ts of the LBK film deposition process and the influence of interaction s between the end groups and the substrate. The molecular weight distr ibutions for LBK films of both oligomers is found to be narrower and s hifted to higher molecular weights than are those for the correspondin g spin-coated films. The changes in molecular weight distribution obse rved are attributed to dissolution of lower molecular weight species i nto the aqueous subphase during the LBK film deposition process.