Isotropic-to-Nematic Transition in Salt-Free Polyelectrolyte Coacervates from Coarse-Grained Simulations

Abstract

Recent interest in complex coacervation between oppositely charged polyelectrolytes (PEs) has been fueled by its relevance to biology in the context of membraneless organelle formation within living cells. For PEs with limited flexibility (such as double-stranded DNA), theoretical treatments and recent experiments have reported the emergence of liquid crystalline order (LCO) within the resulting coacervate phases. In this work, we study the underlying physics of this phenomenon using coarse-grained molecular dynamics simulations of symmetric semiflexible–semiflexible and asymmetric semiflexible–flexible coacervates. By comparing coacervates with the corresponding semidilute solutions of neutral polymers, we demonstrate that the presence of Coulomb interactions in coacervates facilitates orientational ordering, in agreement with theoretical predictions. Quantitative comparisons between our simulations and theory indicate that, for asymmetric nematic coacervates, the strong orientational ordering of stiff polyanions induces a weak ordering of the flexible polycationsan effect that was not anticipated by available theoretical studies. Simulations reveal that, for nematic coacervates, the preferred orientation of the PE chains at the liquid–liquid coacervate–supernatant interface is parallel, and the alignment of semiflexible PEs is homogeneous. The results presented here provide new molecular-level insights into the intra-coacervate LCO and will help motivate further experimental and theoretical activities in this area.

The Role of Polymer Chain Stiffness and Guest Nanoparticle Loading in Improving the Glass Transition Temperature of Polymer Nanocomposites

“…From kθ = 20 onwards, 𝑆 2 continues to increase at a slower rate (smaller slope than that in part II) until it reaches a maximum value of 0.88 at kθ = 60, indicating more order in the semicrystalline state. The variation of 𝑆 2 with polymer chain stiffness observed here is in agreement with similar works presented in the literature [2,77].…”

“…The glass transition temperature ( T g ) and melting temperature ( T m ) are two important properties of polymers related to the order of polymer chains in any polymer system [ 1 ]. The degree of ordering of chains in the bulk mainly depends on stiffness which can be adjusted by tuning the bending modulus ( k θ ) in the simulations incorporated via the bending potential given in Equation (3) [ 2 , 3 , 4 ]. A finitely long polymer chain behaves like a flexible polymer at smaller values of k θ and it turns into a rod-like structure at high enough values of k θ .…”

Salt-Added Solutions of Markov Polyampholytes: Diagram of States, Antipolyelectrolyte Effect, and Self-Coacervate Dynamics

“…This fluctuation correction is negligible, and the mean-field theory (MFT) is valid when F E F vol ≃ ξ E − 3 w ϕ 3 ≃ w 3 / 2 ϕ 3 w ϕ 3 ≃ w 1 / 2 ≪ 1 According to eq , this requirement essentially implies that the chains should be sufficiently stiff to ensure the rigorous applicability of the MFT, p = l / d ≫ 1 (see Sections 13 and 26 of ref ). This requirement is consistent with the applicability range of the mean-field theory to semidilute solutions of neutral polymers. , It always holds for most real polymers because l > d is fulfilled even for the flexible chains. , However, we also note that p should not be very high as the globule/self-coacervate is assumed isotropic; i.e., the possibility of nematic order (albeit it may be promoted by Coulomb interactions − ) is not considered herein. The electrostatic contribution to the persistent length, which should be positive at high charge blockiness but negative for alternating or random PAs, is also neglected.…”

The Molecular Picture of the Local Environment in a Stable Model Coacervate

“…Despite this significant shift in the stability of PECCs (without requiring additives or chemical reactions), the underlying molecular picture leading to this stability has remained not well understood. Although the above-mentioned computational and theoretical studies have been instrumental in delineating the phase equilibrium and rheological properties of coacervates, the large (typically, prohibitively large) costs of explicit solvation and related system size requirements limited the molecular dynamics (MD) simulation studies to modeling polyelectrolytes in implicit solvents (52)(53)(54)(55)(56)(57). Consequently, a molecular picture of changes in the local solvation environment (including explicit water and ions), dynamics, and structure stabilizing the complex coacervates have remained crucially needed, as it would allow the development of novel tools to control and tune the stability and properties of PECCs.…”