Morphology Effects on Diffusion of Selective Penetrants in Block Copolymers from Constrained Random Walk Model and Molecular Dynamics Simulations

Block copolymers, in which ions dissolve in and diffuse through one microphase while the other provides mechanical strength, are used in transport applications. Their transport properties are sensitive to the morphology; the dependence of diffusion or conductivity on morphology was previously predicted based on the fraction of orientations of the morphology that allow conduction versus homopolymer diffusion or conductivity. However, factors such as segregation strength, volume fraction, and molecular weight also vary across studies and affect the results. Here, we predict diffusion based on numerical results of random walks confined by surfaces. Coarse-grained molecular dynamics (MD) simulations are then used to investigate influence of degree of segregation, where ions are mapped onto selective penetrants. Through a sample with randomly oriented grains, the diffusion would be ~33% for cylinders and ~67% for lamellae versus that of homopolymer. However, the results for the inner portion of gyroid are intermediate between these values and depend on monomer volume fraction, showing that the gyroid phase may not perform better than lamellae for transport applications, in contrast to what has been expected.