Computer simulation of microphase separation in ionic copolymers

The formation of lamella microphases in symmetric neutral-ionic block copolymers has been investigated by constant volume-constant temperature (NVT) molecular-dynamics computer simulations using a generic coarse-grain model. Computations of counterion diffusion, pressure tensor, and the anisotropy of the structure factor are used to characterize the order-disorder transition (ODT). There is strong counterion condensation on the ionic blocks at temperatures well above the ODT; this creates a slight imbalance in the volume composition of the two blocks and results in a perforated lamella structure in the microphase. Below the ODT counterion diffusion is decoupled from the chain motions but is strongly anisotropic due to the microphase morphology. The high counterion diffusional mobility is discussed in terms of the relatively low value of the glass transition for the ionic blocks.