Polyelectrolyte Assemblies: Fundamentals And Applications

Charged assemblies bearing opposite or complementary charges span natural (proteins, enzymes, DNA) to synthetic materials (surfactants, synthetic polyelectrolytes). Assembly is facilitated by electrostatic attraction and entropic release of counterions, and most often occurs in aqueous media. Notably decades ago, Michaels described synthetic polyelectrolyte complexes as brittle when dry but “leathery or rubberlike” when wet, which points to the strong effect of water on the mobility of a charged assembly. Here, the molecular origin of the glass transition is quantified for several charged macromolecular systems is investigated using calorimetry and molecular modeling as a function of water content. A general relationship is revealed as it holds for two completely different types of charged systems (pH- and salt-sensitive) and for both polyelectrolyte complexes and polyelectrolyte multilayers, which are made by different paths. This suggests that water facilitates the relaxation of charged assemblies by reducing attractions between oppositely charged intrinsic ion pairs. We further demonstrate the dual role of water and temperature in the dynamics of polyelectrolyte complexes by showing time-temperature and time-water superpositioning in a single polyelectrolyte complex system for the first time. With regard to applications, charged polymers can assemble with functional nanomaterials such as reduced graphene oxide or MXene sheets. These 2D nanomaterials are conductive, facilitating their application as sensors or electrodes for energy storage. Here, the nature of polyelectrolyte-2D nanomaterial assemblies is investigated as thin films. A reversibly stretchable MXene/polyelectrolyte strain sensor and humidity sensor is demonstrated, and the origin of this response is discussed. Also, mechanically robust graphene-based electrodes are presented as assemblies with nanoscale Kevlar

Molecular dynamics simulations of strained and defective carbon nanotubes

Carbon nanotubes are tubular molecules of pure carbon with typical diameters of 1 nm – 100 nm and lengths from 100 nm up to several cm. The nanotubes have outstanding electronical and mechanical properties which has resulted in remarkable scientific interest and in proporsals of various applications. For example, their ability to be either metals or semiconductors enables the usage of nanotubes as components of electronic devices, while excellent mechanical characteristics motivate the use of nanotubes as reinforcement agents in composite structures and in nanoelectromechanical devices. This thesis aims to contribute to the understanding of the mechanical properties of carbon nanotubes and it contains two parts. The first part concentrates on initially defect-free but strained nanotubes and on the deformations and defects induced by the strain. The employed methods are empirical and tight binding molecular dynamics simulations. As results the criteria for uniform and discontinuous buckling deformations are reported. In addition, defect formation and strain relaxation are discussed and the stability of various strained and deformed structures is assessed. The second part of the thesis evaluates defects as a means to improve the bulk mechanical properties of a nanotube sample. Defects, and irradiation as a method of inducing them, are proposed to improve mechanical load transfer between a nanotube and its surroundings. These proposals are verified by analytics and molecular dynamics simulations based on classical empirical potential. The load transfer between nanotubes is found to improve significantly in the presence of defects. This concept is extended to bundles of nanotubes where the improved tube-tube load transfer is predicted to increase shear and stiffen the bundle at moderate irradiation doses. The load transfer has great significance for reinforcement of polymer composites in which the nanotube bundles may act as reinforcement fibers. Furthermore, the mechanical degradation of individual tubes as a result of the defects is also assessed. Point defects have little effect on the axial stiffness of an individual tube but the tensile strength may decrease to a fraction of the strength for a perfect tube. Although individual tubes deteriorate in strength because of the defects, the results indicate that the overall mechanical properties of a nanotube sample can be significantly improved by imperfections in the structure of the tubes.reviewe

Interactions between polyelectrolytes mediated by ordering and orientation of multivalent non-spherical ions in salt solutions

Multivalent ions in solutions with polyelectrolytes (PE) induce electrostatic correlations that can drastically change ion distributions around the PEs and their mutual interactions. Using coarse-grained molecular dynamics simulations, we show how in addition to valency, ion shape and concentration can be harnessed as tools to control like-charged PE-PE interactions. We demonstrate a correlation between the orientational ordering of aspherical ions and how they mediate the effective PE-PE attraction induced by multivalency. The interaction type, strength, and range can thus be externally controlled in ionic solutions. Our results can be used as generic guidelines to tune the self-assembly of like-charged polyelectrolytes by variation of the characteristics of the ions

Theoretical and computational analysis of the electrophoretic polymer mobility inversion induced by charge correlations

Electrophoretic (EP) mobility reversal is commonly observed for strongly charged macromolecules in multivalent salt solutions. This curious effect takes place, e.g., when a charged polymer, such as DNA, adsorbs excess counterions so that the counterion-dressed surface charge reverses its sign, leading to the inversion of the polymer drift driven by an external electric field. In order to characterize this seemingly counterintuitive phenomenon that cannot be captured by electrostatic mean-field theories, we adapt here a previously developed strong-coupling-dressed Poisson-Boltzmann approach to the cylindrical geometry of the polyelectrolyte-salt system. Within the framework of this formalism, we derive an analytical polymer mobility formula dressed by charge correlations. In qualitative agreement with polymer transport experiments, this mobility formula predicts that the increment of the monovalent salt, the decrease of the multivalent counterion valency, and the increase of the dielectric permittivity of the background solvent, suppress charge correlations and increase the multivalent bulk counterion concentration required for EP mobility reversal. These results are corroborated by coarse-grained molecular dynamics simulations showing how multivalent counterions induce mobility inversion at dilute concentrations and suppress the inversion effect at large concentrations. This re-entrant behavior, previously observed in the aggregation of like-charged polymer solutions, calls for verification by polymer transport experiments.Comment: 14 pages, 9 figure

A Coarse-grained Model for Aqueous Two-phase Systems: Application to Ferrofluids

Aqueous two-phase systems (ATPSs), that is, phase-separating solutions of water soluble but mutually immiscible molecular species, offer fascinating prospects for selective partitioning, purification, and extraction. Here, we formulate a general Brownian dynamics based coarse-grained simulation model for a polymeric ATPS comprising two water soluble but mutually immiscible polymer species. A third solute species, representing, e.g., nanoparticles (NPs), additional macromolecular species, or impurities can readily be incorporated into the model. We demonstrate that the model captures satisfactorily the phase separation, partitioning, and interfacial properties of a model ATPS composed of a polymer mixture of dextran and polyethylene glycol (PEG) in which magnetic NPs selectively partition into one of the two polymeric phases. The NP partitioning is characterized both via the computational model and experimentally under different conditions. The simulation model captures the trends observed in the experiments and quantitatively links the partitioning behavior to the component species interactions. Finally, the response of the simulation model to external magnetic field, with the magnetic NPs as the additional partitioned component, shows that the ATPS interface fluctuations can be controlled by the magnetic field at length scales much smaller than those probed experimentally to date

Molecular crowding facilitates assembly of spidroin-like proteins through phase separation

Gaining insights into the processes that transform dispersed biopolymers into well-ordered structures, such as soluble spidroin-proteins to spider silk threads, is essential for attempts to understand their biological function and to mimic their unique properties. One of these processes is liquid-liquid phase separation, which can act as an intermediate step for molecular assembly. We have shown that a self-coacervation step that occurs at a very high protein concentration (> 200 gl(-1)) is crucial for the fiber assembly of an engineered triblock silk-like molecule. In this study, we demonstrate that the addition of a crowding agent lowers the concentration at which coacervation occurs by almost two orders of magnitude. Coacervates induced by addition of a crowding agent are functional in terms of fiber formation, and the crowding agent appears to affect the process solely by increasing the effective concentration of the protein. Furthermore, induction at lower concentrations allows us to study the thermodynamics of the system, which provides insights into the coacervation mechanism. We suggest that this approach will be valuable for studies of biological coacervating systems in general.Peer reviewe