We explored molecular films of polyionic materials built by electrostatic deposition on a charged surface. Formation of self-assembled monolayers is monitored for poly(styrenesulfonate) (PSS) adsorbed on charged surfaces of amine-terminated self-assembled monolayer (SAM) and poly(allylamine) (PAA) on a PSS monolayer. In both cases, polyions are adsorbed on oppositely-charged surfaces. Observations of PSS monolayers at various stages of electrostatic deposition reveal inhomogeneous self-assembly at the earliest stages of deposition. During the first several minutes of deposition, negatively-charged PSS macromolecules tend to adsorb on selected defect sites of positively-charged SAM (scratches, microparticles, and edges) and form islands composed of PSS coils. At this stage, electrostatic adsorption of PSS chains is predominant and equilibration of the surface structure is not achieved by the slow surface diffusion mechanism. Only longer deposition times (>10 min) result in an equilibration of polymer layers and formation of a homogeneous thin PSS layer composed of highly flattened macromolecular chains. The monolayer thickness is between 1.0 and 1.5 nm, with a microroughness of about 0.2 nm. Self-assembly of a second PAA layer on top of a PSS monolayer follows similar tendencies, resulting in the formation of homogeneous PAA/PSS bilayers with an overall thickness of 1.7−2.5 nm.

Scanning probe microscope observations of monolayers of a classic boundary lubricant, stearic acid (STA), reveal long-range dynamics of wear and reconstruction of monomolecular films under the shear forces caused by the sliding tip. The STA monolayer in a fluid state displays a flow of material from the worn area and its redistribution resulting in multilayer formation within the range of 80 µm. Surface diffusion of mobile organic material is responsible for the observed long-range effects of the local shear stresses produced within the contact area. Solid and fluid monolayers have very different velocity dependencies of the friction forces. For solid monolayers, we observe a monotonic increase of the friction forces with velocity rising from 0.02 to 1000 µm/s. In contrast, for the fluid STA monolayers the friction forces behave nonmonotonically with a maximum value around 0.2 µm/s. We observe significant compression of the STA monolayers under the tip reaching 35% of initial thickness before the fatal damaging. The observed compression can be related to the collective tilting of the molecules under normal loads due to a formation of gauche conformers in alkyl chains. Estimated Young's modulus is in the range of 0.2-0.7 GPa for very small deformations (<3% of thickness compression). However, the elastic modulus decreases sharply to tens of MPa at higher deformations.

Scanning probe microscopy observations of monolayers of a classical boundary lubricant, stearic acid (STA), reveal long-range dynamics of wear and reconstruction of monomolecular films under the shear forces caused by the sliding tip. In the range of sliding velocities studied, the friction forces behave non-monotonically with a maximum value around 0.2 #m/s. The STA monolayer in a fluid state displays a flow of material from the worn area and its redistribution to form local heterogeneous multilayers. The range of influence of the material removal on the monolayer structure around the worn area is up to 80 #m, or more than an order of magnitude larger than the actual worn area. Surface diffusion of mobile organic material is responsible for the observed long-range spreading of local shear stresses produced in the area of wear. It demonstrates the non-local nature of the stress-induced reconstruction in the fluid monolayers, in contrast with solid monolayers, which show very localized wearing processes.