Molecular dynamics simulations of concentrated polymer solutions in thin film geometry. II. Solvent evaporation near the glass transition

Abstract

We perform molecular dynamics simulations of a coarse-grained model of a polymer-solvent mixture to study solvent evaporation from supported and freestanding polymer films near the bulk glass transition temperature T(g). We find that the evaporation process is characterized by three time (t) regimes: An early regime where the initially large surplus of solvent at the film-vapor interface evaporates and the film thickness h varies little with t, an intermediate regime where h decreases strongly, and a final regime where h slowly converges toward the asymptotic value of the dry film. In the intermediate regime the decrease of h goes along with an increase of the monomer density at the retracting interface. This polymer-rich "crust" is a nonequilibrium effect caused by the fast evaporation rate in our simulation. The interfacial excess of polymer gradually vanishes as the film approaches the dry state. In the intermediate and final time regimes it is possible to describe the simulation data for h(t) and the solvent density profile phi(L)(y,t) by the numerical solution of a one-dimensional diffusion model depending only on the y direction perpendicular to the interface. The key parameter of this model is the mutual diffusion coefficient D(L) of the solvent in the film. Above T(g) we find that a constant D(L) allows to describe the simulation data, whereas near T(g) agreement between simulation and modeling can only be obtained if the diffusion coefficient depends on y through two factors: A factor describing the slowing down of the dynamics with decreasing solvent concentration phi(L)(y,t) and a factor parametrizing the smooth gradient toward enhanced dynamics as the film-vapor interface is approached.

Molecular dynamics simulations of concentrated polymer solutions in thin film geometry. I. Equilibrium properties near the glass transition

“…8͒, will be utilized in part II of this work which is devoted to the study of solvent evaporation from polymer films. 19 …”

“…͓We focus on S ͑y , S ͒ because the modeling by Eq. ͑8͒ will be utilized in part II; 19 the same modeling could also describe S ͑y , S ͒ at the supporting wall and P ͑y , S ͒ at both interfaces.͔ This parametrization reads…”

“…This parametrization will be an important ingredient in the analysis of the solvent evaporation in part II. 19 The final section ͑Sec. IV͒ summarizes our main results.…”

Effect of polydispersity on the bulk‐heterojunction morphology of P3HT:PCBM solar cells

“…Finally, molecular dynamics (MD) simulations using the leap‐frog algorithm are performed to simulate the solvent evaporation process. General studies on the evaporation process of thin film can be found in literature . Recent efforts by Negi et al and Carrillo et al incorporate the effects of air/film and substrate/film interactions.…”

Semi-infinite boundary conditions for the simulation of interfaces: The Ar/CO2(s) model revisited

“…As a result, most studies of adsorbed fluids resort to a plain truncation of fluid-fluid forces, without any attempt to correct for the missing interactions. [8][9][10][11][12][13][14][15][16][17][18] Already a while ago, Mansfield and Theodorou pointed out that truncating r −6 fluid-fluid interactions in systems where a z −3 fluid-wall potential is employed would have the effect of considerably favoring adhesive over cohesive interactions, and hence, very much affect the work of adhesion of adsorbed polymer films. 19,20 In order to remedy this problem, they developed a method for the calculation of tail corrections of adsorbed fluids.…”