END GROUP EFFECTS ON MONOLAYERS OF FUNCTIONALLY-TERMINATED POLY(DIMETHYLSILOXANES) AT THE AIR-WATER-INTERFACE

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 close-packed 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 angstrom2/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 angstrom2/chain, consistent with the formation of extended cis-trans caterpillar structures oriented normal to the subphase surface.