Abstract
AbstractThe 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.
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