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Abstract

Complex coacervation occurs between oppositely charged species, resulting in the solute-rich phase (coacervate phase) and solute-lean phase (supernatant phase). Recently, scientists have turned their attention to this phenomenon due to its unique structural and rheological properties. For example, coacervates are promising candidates for many practical applications such as drug delivery and rheology modifiers as well as model systems to understand the liquid-liquid phase separation in biology, which is essential for the formation of membraneless organelles within living cells and prebiotic evolution. However, a deeper understanding of the phase behavior of coacervates, especially how different design parameters influence the properties of the resulting coacervate phase and the role of electrostatic interactions in coacervation, is crucial to use coacervate systems to explain the biological process and synthesize coacervate-based materials for various engineering applications. This dissertation aims to contribute to the study of coacervation by exploring the effects of charge fraction (f), monomer sequence, chain stiffness, and coacervate composition on the phase behavior of coacervates using coarse-grained simulations. First, the dependence of coacervate phase density on the charge fraction of linear polyelectrolyte (PE) chains is determined by simulations and experiments. Although the scaling dependence obtained from experiments deviates from theory, simulation results indicate the conditions for experiments to reproduce theoretical predictions. Simulation also demonstrates that the salt partitioning between two phases is dependent on the PE chain chemistry, which is in line with experimental results and usually not discussed in theory. Then, we explore the phase behavior of coacervates formed by random statistical copolyelectrolytes whose sequence is determined by the first-order Markov process. Both simulation and theory show that the higher blockiness of PE chains leads to a denser coacervate phase. Beyond theoretical predictions, simulation also reveals a transition for the coacervate phase from a homogeneous state to an inhomogeneous state when the chain blockiness further increases. In addition, the influence of chain stiffness on coacervation is investigated by modeling the coacervates containing semiflexible chains. Coarse-grained simulations demonstrate the first-order phase transition for the coacervate phase from the isotropic state to the nematic state as the chain stiffness increases. By comparing with the semidilute solution of neutral semiflexible polymers, the role of electrostatics in facilitating the isotropic-to-nematic transition in the coacervate phase is qualitatively discussed. We also show that the chains at the interface between the coacervate and supernatant phases prefer parallel orientation to the interface. Finally, the structural and dynamic properties of hybrid coacervates between linear PEs and oppositely charged spherical particles are obtained by simulations. Simulation results verify the theoretical scaling relationships. Diffusion coefficients of the charged particles are measured for different chain lengths to show the distinct dynamic properties of this hybrid system. For small particles with weak charges, the particle dynamics remain Rouse-like as the PE chains crossover from Rouse to entanglement dynamics. However, as the charges per particle increase, particle dynamics become complicated and exhibit non-Rouse dynamics. To conclude, the implications of this work to the broader community and the future research directions are proposed.

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