SOFT MATTER IN A TIGHT SPOT - NANORHEOLOGY OF CONFINED LIQUIDS AND BLOCK-COPOLYMERS

Abstract

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 (tau(1)) with increasing compression. At frequencies omega > 1/tau(1) the shear response was &apos;&apos;solid-like&apos;&apos;, but at omega < 1/tau(1) it was &apos;&apos;liquid-like&apos;&apos;. OMCTS under mild compression exhibited seeming power-law viscoelastic behavior with G&apos;(omega) approximate to G &apos;&apos;(omega) 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 &apos;&apos;liquid-like&apos;&apos; to &apos;&apos;solid-like&apos;&apos; 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.