Effect of Polymer Chemistry on the Linear Viscoelasticity of Complex Coacervates

Abstract

Complex coacervates can form through the electrostatic complexation of oppositely charged polymers. The material properties of the resulting coacervates can change based on the polymer chemistry and the complex interplay between electrostatic interactions and water structure, controlled by salt. We examined the effect of varying the polymer backbone chemistry using methacryloyl- and acryloyl-based complex coacervates over a range of polymer chain lengths and salt conditions. We simultaneously quantified the coacervate phase behavior and the linear viscoelasticity of the resulting coacervates to understand the interplay between polymer chain length, backbone chemistry, polymer concentration, and salt concentration. Time-salt superposition analysis was used to facilitate a broader characterization and comparison of the stress relaxation behavior between different coacervate samples. Samples with mismatched polymer chain lengths highlighted the ways in which the shortest polymer chain can dominate the resulting coacervate properties. A comparison between coacervates formed from methacryloyl vs acryloyl polymers demonstrated that the presence of a backbone methyl group affects the phase behavior, and thus the rheology in such a way that coacervates formed from methacryloyl polymers have a similar phase behavior to those of acryloyl polymers with ∼10× longer polymer chains.

Charge Density and Hydrophobicity-Dominated Regimes in the Phase Behavior of Complex Coacervates

“…When all of the repeat units of the polymer are made more hydrophobic, either by modifying the backbone or the charged sidechains, the salt resistance of the coacervates typically increases. 22,23 When hydrophobic groups are incorporated as nonionic comonomers, however, different trends have been observed. In short polypeptide systems, for example, switching the nonionic, hydrophobic residues from glycine, to alanine, and to leucine induces measurable shifts in the apparent salt resistance of the complexes, on the order of 15-50 mM depending on the peptides' charge densities.…”

“…Because the total hydrophobic content of the polymers increases as the charge density decreases, however, the interaction parameter should increase as the charge density decreases. 23 As a result, the magnitude of this effect should be strongest for the longest alkyl chains, in which the marginal increase in hydrophobic content for each charged monomer that is replaced by a hydrophobic monomer is greatest. Interestingly, however, for polymers at the same charge density, the salt resistance is almost identical for polymers with short alkyl chains (C0-C4 at 70% charge and above), even though these polymers should have increasingly unfavorable interactions with the surrounding water that should systematically increase the critical salt concentration.…”

“…Quantification of the full phase diagram, and particularly the partitioning of salt between the coacervate and supernatant phases, may provide insight into whether changing the hydrophobic sidechain lengths further affects the phase behavior by changing the effective dielectric constant of each phase. 23 Scattering measurements may additionally provide insight into whether the hydrophobic content drives a difference in the chain collapse and/or compaction of the polymer chains before or after coacervation. And experiments in which the molecular weight and backbone chemistry of the polymers are varied may help reveal why other polymer systems appear to be more sensitive to small changes in the hydrophobic sidechain size than the polyacrylamides investigated in this work.…”

DNA dynamics in complex coacervate droplets and micelles

“…In general, increasing the chain length results in more ionic bonds per polyelectrolyte chain and therefore the relaxation of the complex coacervate becomes slower. 18,36,37 Yet, more complex effects can occur when the oppositely charged polyelectrolytes have different lengths and their lengths are varied independently from each other: in some cases, only one of the two polyelectrolyte species determines the dynamic response. 18 At the moment it is difficult to identify the origin of this asymmetric response because there are only a few studies that have quantitatively determined the chain length effects on the dynamics in complex coacervates with mismatched chain lengths.…”

Hybrid Block Copolymer Synthesis by Merging Photoiniferter and Organocatalytic Ring‐Opening Polymerizations

“…The selection of photoiniferter polymerization was made because it is conducted at ambient temperature as the ROP, facilitating the development of combined polymerization systems without external heating. It also showed great potential for generating intricate block copolymer structures, [14] ultra-high-molecular-weight polymers, [15] and surface modified materials. [16] By incorporating a nucleophilic moiety capable of ROP into the photoiniferter, it is possible to use light as an abundant, contactless, and tunable energy source to facilitate the simultaneous synthesis of disparate block copolymers under mild conditions with temporal control over the vinyl monomer-derived block length.…”