Effect of mixing order on structure and formation of polyelectrolyte complexes

Polyelectrolyte complexation can be thermodynamically favorable under appropriate solution conditions, yet the kinetics of complex formation often dictate whether systems reach equilibrium or remain arrested in nonequilibrium states. Specifically, we propose that initial asymmetries in concentration, chain conformation, and counterion environment, arising from the order of component addition, can direct the system along distinct kinetic pathways, leading to variations in the yield of polyelectrolyte complex obtained. While such kinetic effects were evident in both strong and weak polyelectrolyte interactions, we expect that the latter gives a greater capacity to relax toward equilibrium, whereas stronger polyelectrolyte complexes (PEC) remain arrested in nonequilibrium states due to hindered molecular rearrangement.

We investigated the role of mixing order in the formation of Polyelectrolyte Complexes (PECs) with varying interaction strengths using two model systems: poly(diallyldimethylammonium) (PDADMA) with poly(4-styrenesulfonate) (PSS) as a strong-interaction pair and with poly(acrylate) (PA) as a weak-interaction pair. These systems were chosen for their established literature studies, enabling us to define a focused study on kinetic pathways and equilibrium versus non-equilibrium effects on mixing states. To this end, a systematic set of experiments were conducted, involving varying charge ratio, molecular weight, ionic strength, and mixing sequence. Additionally, coarse-grained molecular dynamics (MD) simulations were performed to visualize and analyze the early-stage complexation of the PDADMA/PSS system, as well as the nature of polyelectrolyte complex formed under different initial conditions.

Our results demonstrate that mixing order of PEC components has a significant impact on the kinetics of PEC formation and its kinetic trapping behavior, particularly in non-stoichiometric (i.e., charge-imbalanced) and strongly interacting systems. Experiments reveal that greater complex yield consistently arises when the limiting component is introduced into a solution containing the excess component, pointing to a strong kinetic component in the assembly pathway. Simulations reveal that structural arrest can occur during early-stage complexation, depending on polymer conformation and counterion condensation at the time of mixing. These findings highlight the importance of kinetic control in PEC assembly and offer molecular-level insight into nonequilibrium complexation behavior.