SPREADING OF HIGH-MOLECULAR-WEIGHT POLYMER MELTS ON HIGH-ENERGY SURFACES

The spreading behavior of high molecular weight poly(dimethylsiloxane) (PDMS) drops on silica surfaces has been investigated through optical microscopy and ellipsometry. For molecular weights higher than the entanglement molecular weight, both the drop profile and the macroscopic spreading kinetics differ from the well-known low molecular weight behavior: a "bump" appears on the side of the macroscopic drop at thicknesses comparable to the radius of gyration of the chains while the macroscopic spreading slows down and finally stops, departing from the usually observed Tanner's law. The existence of the bump is highly sensitive to the nature of the monomer-substrate interactions. The lateral extension of the bump evolves with time in a diffusive manner, and the molecular weight dependence of the corresponding diffusion coefficient appears compatible with a reptation process for the transport of the chains along the surface. These results will be discussed and compared to recent theoretical predictions which take into account two characteristic features of the hydrodynamics of entangled polymer melts, namely, a finite slippage of the chains at the solid wall and an enhanced friction of the monomers in contact with the surface due to solid-monomer interactions.