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

Abstract

We report on results of molecular dynamics simulations for supported polymer films with explicit solvent. The simulation represents the polymers by bead-spring chains and the solvent particles by monomers. The interaction between polymer and solvent favors mixing. We find that the solvent acts as a plasticizer. The glass transition temperature T(g) is reduced relative to the pure polymer film. Near T(g) we explore equilibrium properties as a function of temperature and solvent concentration. We find that the structure and dynamics of the films are spatially heterogeneous. The solvent density is enriched at the supporting wall and at the free surface where the film is in equilibrium with solvent vapor. At both interfaces the solvent dynamics is fast, but smoothly crosses over to bulk dynamics when moving from the interfaces toward the center of the film. A smooth gradient from enhanced dynamics at the interfaces to bulk behavior in the film center is also found for the monomers. We show that the same formula used to parametrize the spatial gradient of the dynamics in the pure polymer film may also be applied here. Furthermore, we determine the concentration dependence of the relaxation time of the solvent in the center of film and compare this dependence to models proposed in literature.

Interfacial mobility scale determines the scale of collective motion and relaxation rate in polymer films

“…Figure 1c shows that, relative to the bulk, T g of polymer films on a rough surface increases with decreasing film thickness, while for the smooth surfaces T g decreases with decreasing film thickness. This is consistent with many previous studies 5,7,19,[30][31][32] . For the rough substrate, we also study a variety of substrate interaction strengths and substrate rigidities.…”

“…As a baseline to understand the changes of interfacial scales and cooperative dynamics in films, we need to know the changes in the overall relaxation dynamics as we vary film thickness and substrate properties. Such changes have been considered in previous works 5,7,19,[30][31][32] , where it has been shown that T g for a smooth substrate is typically reduced, while T g for a rough substrate increases. To provide a reference for the dynamical behaviour of our films, we evaluate the mean dynamics as measured by the coherent intermediate scattering function 33 .…”

“…However, an attractive smooth surface with a modest interaction strength may also enhance the dynamics 7,9,10,[17][18][19] , demonstrating that the strength of polymer-substrate interactions or surface roughness (or surface patterning) can have significant effects. For supported films, the enhancement or slowing down of dynamics induced by two different interfaces complicates the interpretation of the thickness dependence of T g [3][4][5]9,[11][12][13][14]19 . Moreover, some experiments suggest an interfacial scale associated with these dynamical changes [20][21][22] .…”

“…The observed changes in T g of polymer films are usually interpreted in terms of the local mobility changes near their interfaces. Many studies have reported that a repulsive or neutral wall (such as a free surface) decreases T g [3][4][5][6] , while an attractive substrate typically increases T g 1,4,[7][8][9][10][11][12][13][14][15][16] . However, an attractive smooth surface with a modest interaction strength may also enhance the dynamics 7,9,10,[17][18][19] , demonstrating that the strength of polymer-substrate interactions or surface roughness (or surface patterning) can have significant effects.…”

A unifying framework to quantify the effects of substrate interactions, stiffness, and roughness on the dynamics of thin supported polymer films

“…Much of the effort aimed at understanding the property changes in thin polymer films has centered on measurements related to the stiffness of these films 3,4 and changes of molecular mobility, as quantified by the glass-transition temperature T g . 5,6 Many experimental studies, 5,[7][8][9][10][11][12][13][14] as well as computational [15][16][17][18][19][20] studies, have reported large property changes in thin films. These changes have been mainly attributed to a combination of substrate interaction and geometrical confinement.…”