Publications

Solvent evaporation from homopolymer and heteropolymer films along with the interdiffusion 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 first dissolving the polymer in a low viscosity solvent, spreading the solution on a substrate and subsequently evaporating the solvent. Here we study the last part of this process, namely the evaporation of solvent from a polymer film. As the solvent evaporates, the polymer density at the film/vapour interface is found to increase sharply, creating a polymer density gradient which acts as a barrier for further solvent evaporation. For both homopolymer and heteropolymer films, the rate of solvent evaporation is found to decrease exponentially as a function of time. For multiblock co-polymer films the resulting domain structure is found to be strongly affected by the relative stiffness of the two blocks. The reverse process, namely the interdiffusion of solvent into a polymer film, is also studied. For homopolymer films the weight gain by the film scales as t1/2, which is expected for Fickian diffusion. The diffusivity D(c) determined from the one-dimensional Fick's diffusion equation agrees well with that calculated from the corrected diffusion constant using the Darken equation. Far above the polymer glass transition temperature, D(c) is nearly independent of concentration. However, as the temperature decreases D(c) is found to depend strongly on the state of the polymer and is related to the shape of the solvent concentration profile. Finally, the swelling of a multiblock copolymer film in which the stiffer block is below its glass transition temperature is also studied. While the solvent swells only the softer block of the copolymer, the weight gain by the film remains Fickian. © 2005 IOP Publishing Ltd.

We present extensive simulations modeling the casting of multiblock polymer films by evaporation. The domain structure of the resulting film is strongly affected by varying the relative stiffness of the coblocks. The morphology changes from a bicontinuous lamellar phase when both blocks are flexible to a small-scale phase-separated phase with isolated domains as the stiffness of one of the blocks increases. As the relative stiffness of the blocks changes, the rate of evaporation, interfacial width, and morphology of the system changes. The findings can be used to tailor membrane morphology of interest to fuel-cell applications where the morphology is important for proton conduction.

The effect of polymer-polymer and solvent-polymer interactions on the behavior of the interdiffusion of a solvent in to an entangled polymer matrix was studied. The state of the polymer was changed from melt to glassy by varying polymer-polymer interaction. From simulation of equilibrated solvent-polymer solution, it was found that the glassy system with Berthelot's rule applied for the cross term is immiscible except in the dilute limit. Increasing the solvent-polymer interaction enhanced the solubility of the system without changing the nature of the diffusion process.

Abstract not provided.

The effect of cross-linker functionality and interfacial bond density on the fracture behavior of highly cross-linked polymer networks bonded to a solid surface is studied using large-scale molecular dynamics simulations. Three different cross-linker functionalities (f = 3, 4, and 6) are considered. The polymer networks are created between two solid surfaces with the number of bonds to the surfaces varying from zero to full bonding to the network. Stress?strain curves are determined for each system from tensile pull and shear deformations. At full interfacial bond density the failure mode is cohesive. The cohesive failure stress is almost identical for shear and tensile modes. The simulations directly show that cohesive failure occurs when the number of interfacial bonds is greater than in the bulk. Decreasing the number of interfacial bonds results in cohesive to adhesive transition consistent with recent experimental results. The correspondence between the stress?strain curves at different f and the sequence of molecular deformations is obtained. The failure stress decreases with smaller f while failure strain increases with smaller f.