Role of chain architecture and ion size on morphology, dynamics, and viscoelasticity of ionomers

Ionomers, polymers with a small fraction of ions in their backbone, form ionic nanodomains and can function as physical crosslinks in polymer networks. The intensity of the physical crosslinks which in turn controls their morphology, dynamics, and rheological response, can be controlled by the charge density and relative size of the ions on the chains. In this regard, the relative position of cation and anion in the form of bridged and pendant ionomers can be used as a design parameter to control the viscoelasticity of these networks. These two families of ionomers differ in ion attachment to their backbone, with bridged featuring both ion types bound to the backbone, while the pendant structure resembles organic cationic ionomers with backbone-bound cations and free anions. In this study, coarse-grained molecular dynamics simulations will be used to study their morphology, dynamics, and rheology. Our results show that bridged ionomers exhibit a percolated ionic network, while pendant ionomers have distinct aggregates. Dynamical heterogeneities are evident in both structures but more pronounced in bridged ionomers at higher temperatures. A direct relationship between ionic bond lifetime and structural relaxation time is established. This linkage indicates the collective ion dynamics in bridged ionomers, whereas pendant ones show initial ion-pair dissociation before escaping their local environment. Similarly, bridged ionomers show higher shear viscosity. Ion size affects their morphology and dynamics by influencing chain spacing and ionic interaction strength. This, in turn, can influence the viscoelastic and mechanical properties of the ionomer. Morphological and rheological results will also be presented for ionomers with different cation sizes.