Probing the Inhomogeneous Charge Distribution on Annealed Polyelectrolyte Star Polymers in Dilute Aqueous Solutions

Tailoring the Architecture of Molecular Bottlebrushes via Click Grafting-Onto Strategy

Molecular bottlebrush (MBB) refer to a synthetic macromolecule, in which a mass of polymeric side chains (SCs) are covalently connected to a macromolecular backbone densely, representing an important type of unimolecular nanomaterial. The chemical composition, size, shape, and surface property of MBB can be precisely tailored by varying the backbones and SCs as well as the grafting density (Gdst ). Meanwhile, the topological structure of backbones and SCs can also significantly affect the chemical and physical properties of MBBs. For the past few years, by combining the structure features of MBB, the polymers with diverse architectures using MBB as building block are synthesized, including linear, branched, and cyclic MBB etc. These promising architectural features will bring MBBs with diverse architectures and lots of applications in advanced materials. For this reason, this work is interested in giving a briefly summary of the recent progress on tailor of well-defined MBBs with diverse architectures using grafting-onto strategy combined with controlled polymerization technique.

Elucidating the Effect of Amine Charge State on Poly(β-amino ester) Degradation Using Permanently Charged Analogs

With synthetic ease and tunable degradation lifetimes, poly(β-amino ester)s (PBAEs) have found use in increasingly diverse applications, from gene therapy to thermosets. Protonatable amines in each repeating unit impart pH-dependent solution behavior and lifetimes, with acidic conditions favoring solubility, yet slowing hydrolysis. Due in part to these interconnected phenomena governing pH-dependent PBAE degradation, predictive degradation models, which would enable user-defined lifetimes, remain elusive. To separate the effects of charge state and solution pH on PBAE degradation, we synthesized poly(β-quaternary ammonium ester)s (PBQAEs), which differ from their parent PBAEs only by an additional methyl group, generating polymers with pH-independent cationic charge. Like PBAEs, PBQAE hydrolysis accelerates with increasing pH, although at a given pH, PBAE degradation outpaces PBQAE degradation. This difference is more pronounced in basic solutions, suggesting that deprotonated PBAE amines accelerate hydrolysis, providing an additional tuning parameter to PBAE lifetime and informing the degradation of PBAEs and other pH-responsive polymers.

Proteins Adsorbing onto Surface-Modified Nanoparticles: Effect of Surface Curvature, pH, and the Interplay of Polymers and Proteins Acid-Base Equilibrium

Protein adsorption onto nanomaterials is a process of vital significance and it is commonly controlled by functionalizing their surface with polymers. The efficiency of this strategy depends on the design parameters of the nanoconstruct. Although significant amount of work has been carried out on planar surfaces modified with different types of polymers, studies investigating the role of surface curvature are not as abundant. Here, we present a comprehensive and systematic study of the protein adsorption process, analyzing the effect of curvature and morphology, the grafting of polymer mixtures, the type of monomer (neutral, acidic, basic), the proteins in solution, and the conditions of the solution. The theoretical approach we employed is based on a molecular theory that allows to explicitly consider the acid-base reactions of the amino acids in the proteins and the monomers on the surface. The calculations showed that surface curvature modulates the molecular organization in space, but key variables are the bulk pH and salt concentration (in the millimolar range). When grafting the NP with acidic or basic polymers, the surface coating could disfavor or promote adsorption, depending on the solution's conditions. When NPs are in contact with protein mixtures in solution, a nontrivial competitive adsorption process is observed. The calculations reflect the balance between molecular organization and chemical state of polymers and proteins, and how it is modulated by the curvature of the underlying surface.

pH-Dependent complexation and polyelectrolyte chain conformation of polyzwitterion-polycation coacervates in salted water

The phase behavior and chain conformational structure of biphasic polyzwitterion-polyelectrolyte coacervates in salted aqueous solution are investigated with a model weak cationic polyelectrolyte, poly(2-vinylpyridine) (P2VP), whose charge fraction can be effectively tuned by pH. It is observed that increasing the pH leads to the increase of the yielding volume fraction and the water content of dense coacervates formed between net neutral polybetaine and cationic P2VP in contrast to the decrease of critical salt concentration for the onset of coacervation, where the P2VP charge fraction is reduced correspondingly. Surprisingly, a single-molecule fluorescence spectroscopic study suggests that P2VP chains upon coacervation seem to adopt a swollen or an even more expanded conformational structure at higher pH. As the hydrophobicity of P2VP chains is accompanied by a reduced charge fraction by increasing the pH, a strong pH-dependent phase and conformational behaviors suggest the shift of entropic and enthalpic contribution to the underlying thermodynamic energy landscape and chain structural dynamics of polyelectrolyte coacervation involving weak polyelectrolytes in aqueous solution.

Studying the physics of charged macromolecules by single molecule fluorescence spectroscopy

It is well documented that conventional methods such as dynamic light scattering have encountered difficulties in characterizing charged macromolecules and, therefore, it is desirable that new methods and techniques are introduced. With the ultra-high sensitivity, single molecule fluorescence spectroscopy has successfully lowered the detection limit considerably and enabled measurement under extreme dilution conditions-around the concentration of 10^-9M-at which the effect of inter-chain electrostatic repulsion is suppressed. Furthermore, the excellent spatial and temporal resolution as well as the capacity of molecular recognition of these methods help in obtaining rich information of charged macromolecules. This paper summarizes the applications of single molecule fluorescence spectroscopy, especially fluorescence correlation spectroscopy and photon counting histogram, in the studies on charged macromolecules in aqueous solutions and plenty of new information has been revealed on the molecular conformation, counterion distribution, and a few important governing factors. The powerfulness and effectiveness of single molecule fluorescence spectroscopy make it promising in the investigations of charged macromolecules.

Gradient and asymmetric copolymers: the role of the copolymer composition profile in the ionization of weak polyelectrolytes

The ionization of weak polyelectrolytes can be altered by controlling the composition profile of the comonomers along the chain.

Explicit Ion Effects on the Charge and Conformation of Weak Polyelectrolytes

The titration behavior of weak polyelectrolytes is of high importance, due to their uses in new technologies including nanofiltration and drug delivery applications. A comprehensive picture of polyelectrolyte titration under relevant conditions is currently lacking, due to the complexity of systems involved in the process. One must contend with the inherent structural and solvation properties of the polymer, the presence of counterions, and local chemical equilibria enforced by background salt concentration and solution acidity. Moreover, for these cases, the systems of interest have locally high concentrations of monomers, induced by polymer connectivity or confinement, and thus deviate from ideal titration behavior. This work furthers knowledge in this limit utilizing hybrid Monte Carlo⁻Molecular Dynamics simulations to investigate the influence of salt concentration, pK a , pH, and counterion valence in determining the coil-to-globule transition of poorly solvated weak polyelectrolytes. We characterize this transition at a range of experimentally relevant salt concentrations and explicitly examine the role multivalent salts play in determining polyelectrolyte ionization behavior and conformations. These simulations serve as an essential starting point in understanding the complexation between weak polyelectrolytes and ion rejection of self-assembled copolymer membranes.

Role of Associative Charging in the Entropy-Energy Balance of Polyelectrolyte Complexes

Polyelectrolytes may be classified into two primary categories (strong and weak) depending on how their charge state responds to the local environment. Both of these find use in many applications, including drug delivery, gene therapy, layer-by-layer films, and fabrication of ion filtration membranes. The mechanism of polyelectrolyte complexation is, however, still not completely understood, though experimental investigations suggest that entropy gain due to release of counterions is the key driving force for strong polyelectrolyte complexation. Here we perform a comprehensive thermodynamic investigation through coarse-grained molecular simulations permitting us to calculate the free energy of complex formation. Importantly, our expanded-ensemble methods permit the explicit separation of energetic and entropic contributions to the free energy. Our investigations indicate that entropic contributions indeed dominate the free energy of complex formation for strong polyelectrolytes, but are less important than energetic contributions when weak electrostatic coupling or weak polyelectrolytes are present. Our results provide a new view of the free energy of polyelectrolyte complex formation driven by polymer association, which should also arise in systems with large charge spacings or bulky counterions, both of which act to weaken ion-polymer binding.

Local pH and Effective pK of a Polyelectrolyte Chain: Two Names for One Quantity?

In recent experiments, the "local pH" near polyelectrolyte chains was determined from the shift in the effective acidity constant of fluorescent pH indicators attached to the macromolecules. This indirect determination raises the question if the analyzed quantity was indeed the "local pH" and what this term actually means. In this study, we combined experiments and simulations to demonstrate that the shift in ionization constant is slightly lower than the difference between the pH and the "local pH". This offset is caused by correlations between fluctuations in chain conformation, small-ion distribution, and fluorophore ionization.

Local pH and effective pKA of weak polyelectrolytes - insights from computer simulations

In this work we study the titration behavior of weak polyelectrolytes by computer simulations. We analyze the local pH near the chains at various conditions and provide molecular-level insight which complements the recent experimental determination of this quantity. Next, we analyze the non-ideal titration behaviour of weak polyelectrolytes in solution, calculate the effective ionization constant and compare the simulation results with theoretical predictions. In contrast with the universal behaviour with respect to chain length, we find non-universality and deviations from theory with respect to polymer concentration and permittivity of the solvent. The latter we explain in terms of counterion condensation and ion correlation effects, which lead to reversal of the non-ideal titration behaviour at very low permittivities. We discuss the impact of these findings on the interpretation of experimental results.