The formation of Langmuir monolayers from poly(dimethylsiloxane) oligomers (molecular weights of 900-4000) terminated with methyl, hydroxyl, epoxide, carboxyl, and amine groups is described. The isotherms (except for oligomers with molecular weights below 1500) show the characteristic transitions commonly observed for methyl-terminated PDMS. In addition, the functionally-terminated materials show a transition associated with orientation of the PDMS chains normal to the surface to form a closepacked monolayer. For oligomers with molecular weights below 1000 this transition overlaps the standard configurational transitions, and the overall shape of the isotherm is determined by the oligomer molecular weight, the functional group, and the nature of the subphase. Most of the materials have cross-sectional areas at collapse of about 100 A* 12/chain, consistent with a structure where the molecules form helices oriented perpendicular to the surface of the subphase. Shorter amine-terminated materials have areas as low as 50-60 A2/chain, consistent with the formation of extended cis-trans caterpillar structures oriented normal to the subphase surface.

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 , -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.