End Group Effects on Monolayers of Functionally-Terminated Poly(dimethylsiloxanes) at the Air-Water Interface

Abstract

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.

Surface Behavior of Boronic Acid-Terminated Silicones

“…The isotherms for the hydride-terminated PDMS are very similar to those reported for methyl-terminated PDMS, 2,6,7,9 while those for the silicone boronic acids show similar transitions to other α,ω-difunctionalized PDMS. 9 In all isotherms, irrespective of the end group (H versus boronic acid), there is a plateau corresponding to a first-order phase transition starting at pressures between 8 and 10 mN m −1 , with a slight increase for the low-molecular-weight compounds as previously observed. 9 Only the molecular areas for transitions and the length of the plateaus vary with chain length.…”

“…9 This transition must correspond to an additional conformational change for Langmuir XXXX, XXX, XXX−XXX molecules that are terminated with polar tethering groups. Without polar end functionalization, the molecules are free to slide over one another as thicker (non-monolayer) organic films are formed, while the surface pressure remains constant.…”

“…7 Further compression leads to a phase transition. In early work, Lenk et al 9 proposed the formation of helices during this phase transition for both functionalized and non-functionalized PDMS. Later work on PDMS by Kim et al 7 suggested a multilayering rather than helix formation on the basis of vibrational sum frequency spectroscopy measurements.…”

“…13 The strength of this type of anchoring has been shown to impact the surface pressure at small molecular areas and the surface pressure at film collapse. 9,12 Boronic acids are known to provide a pH-sensitive binding site for diols, specifically the 1,2-and 1,3-diols that are common in saccharides. The binding affinity for different diol-containing compounds varies significantly (over orders of magnitude), which enables differentiation of sugars.…”

Adsorption properties of oligo(fluorooxetane)-b-poly(ethylene oxide)-b-oligo(fluorooxetane) triblock copolymers at the air–water interface: Comparison of hydroxyl and acetate end groups

“…In p-A monolayer measurements on hydrocarbon end-capped poly(ethylene oxide), Muller et al came to the same conclusion that the effects of different hydrocarbon length end caps are manifested in the high p region only [3]. In a film balance study reported by Koberstein et al, poly(dimethylsiloxanes) with a variety of end group functionalities, including methyl and hydroxyl, were examined [30]. The results of Koberstein et al showed collapse pressures for hydroxyl and methyl end caps that were markedly different ($10 mN/m for methyl and $25 mN/m for hydroxyl).…”