Abstract
We discuss the spreading of a semi-dilute solution of neutral, flexible chains on a solid surface, assuming that a) the solvent is good and is non-volatile (dry spreading) and b) the polymer does not adsorb on either interface (solid/liquid and liquid/air). The polymer solute has two main effects : a) it modifies the spreading coefficient of the liquid and b) it introduces a new contribution to the disjoining pressure, which, as pointed out by de Gennes [7], is present only when the polymer cannot exchange with a reservoir. We show here that this disjoining pressure stabilizes the film for the q = 0 mode of undulation, but does not contribute to the stability of the film against undulations at q ≠ 0 : each half wavelength of one undulation plays the role of a reservoir for the other half. This marginal stability is suggestive of a phase transition. We analyse the free energy of a completely spread droplet, incorporating both Van der Waals forces and polymer contributions, and construct a phase diagram involving three different states (a) bulk droplet of solution, (b) solution film containing polymer and (c) film with no polymer. The droplet cannot spread only above a certain polymer concentration øw. When the Van der Waals forces are weak, the balloon should be surrounded by a film of pure solvent