Publications

Abstract not provided.

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 effects of mixed functionality and degree of curing on the stress-strain behavior of highly cross-linked polymer networks are studied using molecular dynamics simulations. The networks are made dynamically in a manner similar to epoxy network formation, and the average functionality of the cross-linker, f av, is systematically varied from 3 to 6 by mixing cross-linkers with functionalities f = 3, 4, and 6. Stress-strain curves are determined for each system from tensile pull simulations. The range of strain of the plateau region (R P) in the stress-strain curve, failure strain (ε f), and failure stress (σ f) for fully cured networks are found to have a power law dependence on f av as ∼f av α. For R P and ε f, α is determined to be -1.22(3) and -1.26(4), respectively. The failure strain is equal to the strain needed to make taut the maximum of the minimal paths through the network connecting the two solid surfaces. The failure stress, however, shows two distinct regions. For f av α ≤ 4, σ f increases with increase in f av and α = 1.22(5). In this f av regime, the work to failure is constant. For f av α ≥ 4, the systems fail interfacially, σ f becomes a constant, and work to failure decreases with f av. These mechanical properties are also found to depend on the degree of curing. With decrease in percentage of curing, failure stress decreases and failure strain increases. The mode of failure changes from interfacial to bulk. © 2004 American Chemical Society.

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.

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.