Yield strength of glued Langmuir-Blodgett films determined by friction force microscopy

We used friction force microscopy measurements to determine the yield strength of several structurally similar Langmuir-Blodgett (LB) bilayer films deposited on a hydrophobic substrate. Film failure was initiated by increasing the load applied by the probe of the atomic force microscope in the course of continuous scanning at nominally the same location on the sample. This film failure was readily detected in friction versus load curves, as well as by imaging of trenches created due to removal of the film. The depths of the trenches formed in the course of these yield strength experiments were consistent with complete removal of these bilayer films, as evidenced by comparisons to film thicknesses measured by ellipsometry. The structure of the LB bilayer was modified by replacing a tetra-chain amphiphile bearing four quaternary ammonium groups with a polymeric surfactant resulting in little change in the yield strength. On the other hand, the addition of a polyanionic gluing layer at the central interface of the bilayers almost doubled the yield strength of the films. To uncover any possible structural effects created by changes in the terminal functionality, the hydrocarbon top layer of the bilayer was replaced with a perfluorinated capping layer. In spite of the changes in frictional properties, the yield strength of this film also appeared to be dominated by the presence of the glued interface.