Effects of Ion Group Distribution on the Structure and Dynamics of Amorphous Ionomers

Ionic clusters control the structure, dynamics, and transport in ionic polymers. These ionic groups associate into random clusters in melts, where the distribution and morphology of the clusters impact the transport in these materials. Using fully atomistic molecular dynamics simulations we study melts of polystyrene sulfonate with 10% sulfonation with random, precise and blocky distributions of of the SiO₃^- sulfonate groups along the backbone. We find that for random distribution, the clusters in average are elongated, whereas the clusters are more globular when the sulfonate groups are equally spaced along the backbone. Elongated and globular coexists for blocks of sulfonate groups. The average ionic cluster size is comparable for the random and precise distributions and larger for the blocky distribution. We show that the low wavevector peak in the structure factor S(q) is much more well-defined for the precise and block cases compared to the random one. Interestingly, over the time scales of 1000 ns studied, the mobility of the block chains is higher than the random and precise, even though the ionic clusters are larger. The Na+ counterions are condensed and move at the same rate as the sulfonate groups.