Interfacial Properties of Fully Condensed Functional Silsesquioxane: A Langmuir Monolayer Study

We have studied interfacial properties of fully condensed octakis[{2-(3,4-epoxycyclohexyl)etyl}dimethylsilyloxy]octasilsesquioxane (OE-POSS). The surface pressure–area isotherm obtained in a Langmuir trough together with Brewster angle microscopy indicated formation of a stable, homogeneous liquid-like monolayer. For low molecular areas, the monolayer undergoes a stepwise collapse showing unusual multilayer morphology. A collapse mechanism is proposed to be a consequence of progressive decrease in the distance between silicon–oxygen cages after reorganization of side groups. Hysteresis experiment showed incomplete respreading of small aggregates and successive decrease in molecular areas. Relaxation measurements indicated a slight impact of desorption and molecular rearrangements on the monolayer stability. Interfacial rheology was investigated for compressional deformation of the air/water interface. Both components of surface dilational modulus (elastic, E ′, and viscous, E ″) were determined by the oscillating barriers method. With increasing frequency, E ′ decreased while E ″ increased almost linearly. The loss angle tangent indicated elastic response of the monolayer in the range of low deformation rates and viscous behavior when subjected to fast deformation. The rheological behavior of the monolayer is also strongly affected by the collapse mechanism that begins at 20 mN/m.