Crossover to surface flow in supercooled unentangled polymer films

Abstract

We study the driven flow of an unentangled glassy polymer film with a free upper surface and supported below by a substrate using nonequilibrium molecular dynamics simulations based on a bead-spring model. Above the glass transition temperature T [subscript g] , simple Poiseuille laminar flow is observed with the film mobility defined as the flow current density per unit pressure gradient scaling as h³ with the film thickness h . Below T [subscript g] , the film mobility becomes independent of h , signifying surface transport. This is in full agreement with recent experiments on the time evolution of capillary waves in polystyrene films supported by silica. A mobile layer is found responsible for the surface transport, as previously conjectured. Our result also shows that it has a velocity profile decaying exponentially into the bulk.