Response of ionic co-polymers assemblies to solvent stimuli: A molecular Dynamics Simulation Study

Ionizable co-polymers associate in solutions where ionic cluster formation often drives the assembly. The response of ionic clusters affects the driving forces for assemblies, where larger the blocks incompatibility is, the more distinctive there response to solvent stimuli becomes. Here, using atomistic molecular dynamics simulations, we follow the response of micelles formed by a symmetric pentablock copolymer, that consists of a randomly sulfonated polystyrene (sPS) centers, tethered to poly ethylene-r-propylene block, terminated by poly (t-butyl styrene). Micelles were formed by confining the sPS blocks with sulfonation fractions f of 0- 0.55 in implicit poor solvent and measured in cyclohexane. Solvent polarity was tuned by addition of propanol to the cyclohexane, increasing the dielectric constant. We find that regardless of the ionic fraction, aggregates size decreases with addition of propanol. Concurrently while only small numbers of propanol molecules penetrate the ionic core they screen the ionic interactions resulting in slight expansion of the core. The assemblies that were initially formed by segregation of the ionic blocks are now held together by the collapse of the hydrophobic blocks.