Dynamics of polymer melts confined between adsorbing surfaces spaced at molecular dimensions

Abstract

New physics emerges when polyphenylmethylsiloxane (PPMS) adsorbed between mica surfaces spaced at molecular dimensions undergoes oscillatory shear. The polymers experience a dynamic liquid-like to rubber-like transition at the onset of long range surface forces $(\sim$4.5 R$\sb{\rm G}).$ The long time relaxations of the polymers are effectively frustrated at the start of the strong repulsive short range surface forces (hard-wall $\sim$2.8 R$\sb{\rm G}).$ The static surface forces and dynamic regimes scale with the size of the polymer molecules. This reflects the universal ranges of polymer melts perturbed by the adsorbing surfaces.

The dynamics manifest a nonequilibrium freezing of certain degrees of freedom of the molecules as the film thickness decreases. In the rubber-like regime (2.8-4.5 R$\sb{\rm G}),$ the linear relaxation modulus resembles that of an entangled polymer melt. Surprisingly, the molecular weights of the PPMS are less than the entanglement molecular weight. The dynamics are not reptation in nature; they reflect the long range topological constraints imposed by the adsorbing surfaces. In the short range hard-wall regime $(<$2.8 R$\sb{\rm G}),$ the impeded long time relaxations indicate the interdigitation of the two polymer layers tethered to the adsorbing surfaces.

The structure and correlation in the rubber-like and hard-wall regimes break up when the strain amplitudes are high in the non-linear viscoelastic regime. The short time local relaxations are dominated by the enhanced slip between the segments. The long time global relaxations are accelerated by the strain amplitudes after the rupture of the structure. When viewed at a constant frequency scale, there exist characteristic strains where the dynamic correlation changes. These characteristic strains increase with frequencies; it takes higher strains to break the short time and local correlations.