Solvent evaporation and interdiffusion in polymer films

Abstract

Solvent evaporation from homopolymer and heteropolymer films along with theinterdiffusion of solvent into these films are studied by molecular dynamics simulations.Due to the high viscosity of polymer melts, in many cases polymer films are made by firstdissolving the polymer in a low viscosity solvent, spreading the solution on a substrate andsubsequently evaporating the solvent. Here we study the last part of this process, namelythe evaporation of solvent from a polymer film. As the solvent evaporates, the polymerdensity at the film/vapour interface is found to increase sharply, creating a polymer densitygradient which acts as a barrier for further solvent evaporation. For both homopolymerand heteropolymer films, the rate of solvent evaporation is found to decreaseexponentially as a function of time. For multiblock co-polymer films the resultingdomain structure is found to be strongly affected by the relative stiffness of the twoblocks. The reverse process, namely the interdiffusion of solvent into a polymerfilm, is also studied. For homopolymer films the weight gain by the film scales ast1/2, which is expected for Fickian diffusion. The diffusivityD(c) determined from the one-dimensional Fick’s diffusion equation agreeswell with that calculated from the corrected diffusion constant using theDarken equation. Far above the polymer glass transition temperature,D(c) is nearly independent of concentration. However, as the temperature decreasesD(c) is found to depend strongly on the state of the polymer and is related to the shape of thesolvent concentration profile. Finally, the swelling of a multiblock copolymer film in whichthe stiffer block is below its glass transition temperature is also studied. While the solventswells only the softer block of the copolymer, the weight gain by the film remains Fickian.