Soft Matter in a Tight Spot: Nanorheology of Confined Liquids and Block Copolymers

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.