No abstract

The friction of dry self-assembled monolayers, chemically attached to a solid surface and comprising a well-defined interface for sliding, is compared to the case of two solids separated by an ultrathin confined liquid. The monolayers were condensed octadecyltriethoxysilane (OTE). The liquid was squalane (C,,H,,), a film 2.0 nm thick confined between parallel plates of mica. The method of measurement was a surface forces apparatus, modified for oscillatory shear. The principal observations were the same in both cases: (1) Predominantly elastic behavior in the linear response state was followed by a discontinuous transition to a mostly dissipative state at larger deformations. The elastic energy stored at the transition was low, of the order of 0.1 kT per molecule. This transition was exactly repeatable in repetitive cycles of oscillation and reversible with pronounced hysteresis. (2) The dissipative stress in the sliding state was almost independent of peak sliding velocity when this was changed over several decades. Significant (although smaller) elastic stress also persisted, which decreased with increasing deflection amplitude but was almost independent of oscillation frequency. (3) The adhesive energy in the sliding state was significantly reduced from that measured at rest. This similarity of friction in the two systems, dry and wet sliding, leads us to speculate that, similar to plastic deformation of solids, sliding in the confined liquid films is the result of slippage along an interface.

The linear frequency‐dependent shear rheology and force–distance profiles of molecularly‐thin fluids of very different structure were contrasted: a globular molecule octamethylcyclotetrasiloxane (OMCTS), branched alkanes (3‐methylundecane and squalane), and a polymer brush in near‐theta solution (polystyrene‐polyvinylpyridine). In each case the data suggest a prolongation of the longest relaxation time (τ1) with increasing compression. At frequencies ω > 1/τ1 the shear response was “solid‐like”, but at ω < 1/τ1 it was “liquid‐like”. OMCTS under mild compression exhibited seeming power‐law viscoelastic behavior with G′(ω) = G″(ω) over a wide frequency range. Of the branched‐molecule fluids, 3‐methylundecane exhibited oscillatory force–distance profiles; this confirms prior computer simulations. But squalane (6 pendant methyl groups in an alkane chain 24 carbons long) showed one sole broad attractive minimum. Polymer brushes in a near‐theta solvent exhibited changes qualitatively similar to those OMCTS, in particular, a smooth progression of longest relaxation time, generating a transition from “liquid‐like” to “solid‐like” shear rheology with decreasing film thickness. The common trend of shear response in these systems, in spite of important differences in molecular structure and force–distance profiles, is emphasized.

The synthesis, structure, and reactivity of undec-10-ene-1-thiol monolayers assembled on planar and nanocrystalline (curved) Au is presented. Cyclic voltammetry and infrared spectroscopy are used to probe the structural changes in the monolayers (on planar Au) upon irradiation with γ-rays. Oligomerization of the monolayers during the γ-ray exposures is indicated by the observed decrease in the intensities of infrared bands associated with the olefin functionality. From infrared spectra obtained during γ-ray exposures of the undec-10-ene-1-thiol monolayers on planar Au, it is proposed that the oligomerization reaction is controlled by the distance the tethered olefin groups can move. That is to say the reaction is stress limited. Dropcast films of undec-10-ene-1-thiol/Au nanoclusters (1.3 and 3.4 nm diameter Au crystals) do not exhibit decreases in the olefin infrared bands after large γ-ray exposures. This decrease in reactivity for the olefin monolayers supported on the Au nanocrystals is suggested to be the result of interdigitation of the alkane chains from neighboring alkanethiolate Au clusters that exist in the dropcast films.

SYNOPSISThe accessibility of starch in polyethylene starch blends was investigated by computer simulation, percolation theory, and acid hydrolysis experiments. The object of this work was to model the bilateral invasion of microbes in polyethylene-starch blends as a function of starch concentration ( p ) , and thickness of the material. It was found that computer simulations in three dimensions were in agreement with both percolation theory and the acid digestion experiments. In computer simulation the accessibility is highly dependent on the percolation threshold concentration ( p , ) , which is 31.17%. Similarly, the accessibility of starch is highly dependent on an apparent percolation threshold near 30% by volume or approximately 40% by weight of starch. At p < p c a small amount of starch is removed from the surfaces only, but at p > p c connected pathways existing throughout the bulk of the material facilitate large amounts of starch extraction. The sharpness of the transition at pe increases with the ratio of sample thickness to starch particle size. The results of this work have application to conduction and reacting systems where one component is dispersed in a matrix of the other.