Combined Simulation and Experimental Study of Polyampholyte Solution Properties: Effects of Charge Ratio, Hydrophobic Groups, and Polymer Concentration

Abstract

Polyampholytes are polymers with both cationic and anionic monomers that have many features in common with proteins. These molecules have many applications associated with their natural biocompatibility and their propensity to form viscoelastic fluids and gels due to their common tendency to form self-assembled structures in solution. Recently, advances in the synthesis capabilities have enabled the fine-tuning of the monomer structure and composition, such as the sequence and ratio of the charged and hydrophobic groups. We correspondingly investigate these molecular characteristics on the solution properties of a series of poly(carbonate)-based polyampholytes using both molecular dynamics simulations and small-angle neutron scattering measurements. Our simulations suggest the importance of hydrophobic groups for understanding the observed properties of our polyampholyte solutions, a well-known fact for protein solutions. In particular, our simulations indicate that the hydrophobic groups cause the polyampholyte chains to form compact micelle-like rather than diffuse clusters at low concentrations and greatly influence the concentration dependence of the correlation peak in the structure factor describing the correlation distance between the polyampholyte chains in solution. We also construct a polyampholyte solution association map where the polymer concentration and charge ratios lead to different propensities of association (or clustering) and determine a self-assembly boundary governing the transition between self-assembled clusters that resemble micelles and a transient polymer network that percolates the system.

Role of Charge Sequence in Polyampholyte Aggregation

“…As mentioned above the PA periodic structure imposes the equality 2( δc + + δc – ) = δc 0 for fluctuations more extended than the distance between three consecutive monomers (here, π/ q ★ ≈ 17 a ≈ 7σ). The wavelength 2π/ q ★ ≈ 14σ corresponds possibly to connected micelles in the strong segregation limit …”

“…The wavelength 2π/q ★ ≈ 14σ corresponds possibly to connected micelles in the strong segregation limit. 7 The unstable mode for IN at concentration c̃= 0.1 forms approximately with q ★ = 0.19a −1 at the excluded volume v = −0.58a 3 . In these conditions of concentration and excluded volume, the lowest eigenvalue −0.002 is very small in magnitude as compared to positive ones (∼10 2−3 ).…”

“…There is a significative dispersion from sequence to sequence. (7) We investigated the aggregation tendency of a PA model with uniform hydrophobicity and a charge sequence akin to that found in IDPs like IN. The charges in this model are dispersed rather than forming blocky clusters.…”

“…The theory of association was primarily developed early on, for example by Shusharina and co-workers for PE and PA micelles and for charge symmetric PA diblocks or by Castelnovo for PA diblocks at high ionic strength. Recent studies highlight the aggregation of PAs with strongly hydrophobic monomers and various charge imbalance or telechelic PAs. − Micellization of pH-sensitive diblock PAs , and the behavior of diblocks featuring a rodlike PA block providing salt-resilient aggregates, as well as triblocks, have been investigated.…”

How Dispersity from Step-Growth Polymerization Affects Polymer Dynamics from Coarse-Grained Molecular Simulations

“…Each dimer consists of two coarse-grained beads connected by a harmonic bond potential with equilibrium distance r 0 = 0.95σ, and spring constant k = 100ε/σ 2 . Beads that are not bonded to each other interact through a Lennard–Jones (LJ) interaction with size parameter σ = 1 and strength ε = 1. − Each bead is taken to have a unit mass, and all units are expressed in terms of reduced LJ units. We use 1000 dimers in a cubic box with periodic boundary conditions in all directions for our simulations.…”