Charge-Density-Dominated Phase Behavior and Viscoelasticity of Polyelectrolyte Complex Coacervates

Abstract

The composition dependence of the phase behavior and viscoelasticity of polyelectrolyte complex coacervates was investigated in a series of model acrylamide copolymers with varying charge density and nonionic comonomer hydrophobicity. The polymers, which were prepared by postpolymerization functionalization of poly­(N-acryloxy succinimide), were prepared with charged tertiary amine or carboxylic acid side chains on between 60 and 100% of the monomers, with the balance composed of either hydrophilic (acrylamide) or hydrophobic (butyl acrylamide) functional groups. Characterization by optical turbidity and small-amplitude oscillatory shear rheology revealed that as the charge density of the polymers decreased, the transition salt concentration and concentration of polymer in the coacervates also decreased, while the relaxation dynamics sped up. Interestingly, these properties depended strongly on the charge density but were relatively insensitive to the hydrophobicity of the nonionic comonomer. This counterintuitive result, which we attribute to the relatively weak potential of mean force for the interaction of short hydrated alkyl chains, suggests that choosing hydrophobic groups with relatively strong interactions will be critical for designing coacervates with targeted material properties.

Patterning Polyelectrolyte Complexes with Alternating Monomer Sequence Distributions

“…This suggests that by reducing the effective charge density and increasing polymer hydrophobicity, chains can rearrange on slower timescales into monodisperse size distributions in the final state (Figure 2B). This is consistent with reported observations in the other bulk coacervate systems 14,16.…”

Recent progress in the science of complex coacervation

“…Surface properties such as the interfacial tension between the surfactant and coacervate phase have been measured using atomic force microscopy 38,54 and surface forces apparatus. 55,56 The structure and dynamics of bulk coacervates have been characterized using rheology 37,47,53,[57][58][59][60][61][62][63][64][65] along with X-ray and neutron scattering. 62,66,67 Finally, the thermodynamics of bulk coacervation has been characterized using isothermal titration calorimetry (ITC).…”

“…46,144 Controlled polymerization also has allowed for the synthesis of random copolymers to facilitate the introduction of multiple functionalities. 64 Lastly, the expanding breadth of synthetic approaches for preparing charged polymers has allowed for the design of elegant experiments to test specific molecular features, such as hydrophobicity 53 or polarity. 40 Another key challenge for both experiments and for theoretical descriptions is the need to understand how the specific chemical identity of both the polymers and salt ions affect coacervation.…”

“…For all of these questions, the challenge is one of trying to understand changes in the structure of water. To date, the majority of approaches rely on indirect phenomenological measurements such as changes in the phase behavior of coacervates, 40,64,139 differences in the amount of water introduced into a sample by different ions, 53 or measurements of "non-freezing," surface-bound water. 89 Experimentally, new techniques such as terahertz dielectric spectroscopy have the potential to help directly access these structural changes, 149,150 and these questions look to be key to future materials design questionseven beyond coacervation.…”

“…45,140,275 A combination of experimental results using polypeptides of controlled chirality and molecular dynamics simulations demonstrated that a continuous sequence of approximately eight amino acids of the same chirality was needed to drive the formation of a stable β -sheet structure, tipping the stability of the system from liquid complex coacervates to solid precipitates. Systematic variation of hydrophobicity in both polypeptides 139 and synthetic polymers 40,53,64 has mostly demonstrated that hydrophobic polymers form denser coacervate phases, 40,53,139 though in some cases these differences were negligible. 64 In a separate study, Hyman, Alberti, and Pappu used mutagenesis studies of the FUS family of IDPs to demonstrate a hierarchy of interactions driving phase separation.…”