Morphological Response of a Spherical Polyelectrolyte Brush to Solvent Quality and Electrostatic Interaction Strength

Responses of assembled structures of block polyelectrolytes to electrostatic interaction strength

In this paper, the responses of assembled behaviors of block polyelectrolytes (PEs) to the strength of electrostatic interactions are studied through molecular dynamic simulations. The results show that the assembled structures closely depend on the electrostatic strength. It should be noted that PE coacervation can outweigh the nucleation of hydrophobic blocks and invert the micelle structures at strong electrostatic strengths, leading to the formation of inverted micelles of PE cores and hydrophobic coronas. In the poor solvent condition for neutral block, diverse anisotropic micelles are presented; candy-like conventional micelles of hydrophobic cores and PE patches coexist with inverted candy-like micelles of PE cores and hydrophobic patches and with Janus micelles of semi-neutral aggregate and semi-PE cluster in the presence of divalent and trivalent counterions. The formation of conventional or inverted micelle is largely determined by the type of micellar fusion, which results from the nucleation competition between electrostatic correlation and hydrophobic interaction. The merge of micelles mediated by hydrophobic attraction leads to conventional hydrophobic cores, and the fusion induced by electrostatic correlations results in PE cores micelles. At strong electrostatic strengths, the PE chains exhibit rich conformations at trivalent counterions, ranging from a fully collapsed state to a rod-like state, and parallel alignment of PE chains is found.

Self-assembled morphologies of polyelectrolyte-grafted nanoparticles directed by oppositely charged polymer matrices

Self-assembled structure of polymer grafted nanoparticles is an interesting and growing subject in the field of hybrid electronics and high energy density materials. In light of this, the self-assembled morphologies of polyelectrolyte (PE) sparsely grafted nanoparticles tuned by oppositely charged matrix chains are studied using molecular dynamics simulations. Our focus is to elucidate the effect of matrix chain polymerization on modulating the stretching properties of tethered PE layers, on the self-assembled structuring of nanoparticles. Through varying the matrix chain length and stiffness as well as electrostatic interaction strength, rich phase behaviors of PE coated nanoparticles are predicted, including spherical micelle-like structures being preferred with short matrix chains and percolating network morphologies favored with long matrix chains, which is more pronounced with an enhanced matrix chain rigidness. To pinpoint the mechanisms of self-assembled structure formation, the thickness of grafted layers, the gyration radius of tethered chains, and pair correlation functions between nanoparticles are analyzed carefully. Additionally, electrostatic correlations, manifested as the bridging via matrix chains, are examined by identifying three states of matrix PE chains. Our simulation results may be useful for designing smart polymer nanocomposites based on PE coated nanoparticles.

Self-assembled structure of polymer grafted nanoparticles is an interesting and growing subject in the field of hybrid electronics and high energy density materials.

Viscosity of polyelectrolyte-grafted nanoparticle solutions

The effect of charges and hydrogen bonding on viscosity in solutions containing polyelectrolyte-grafted nanoparticles (PENP) has been investigated using molecular dynamics (MD) simulations. The electrostatic interaction between the charged monomers on the grafted chains, which increases with the degree of ionization, causes the grafted polymers to stretch and increases the hydrodynamic size of the nanoparticles. The viscosity of the solution is partially governed by the balance between the entanglement of grafted chains and the electrostatic repulsion. Moreover, the charge-assisted hydrogen bonds between the monomers of different particles further enhance the viscosity of the solution. For shorter grafted chains, a majority of hydrogen bonds are formed within the same particle and thus show no significant enhancement in viscosity. The addition of polymer chains with hydrogen bonding sites has been shown to bridge multiple nanoparticles, creating a network structure, that increases viscosity. The chain stiffness has been shown to have a direct correlation with bridging and thus the viscosity of the solution.

Multiresponsive Transitions of PDMAEMA Brushes for Tunable Surface Patterning

Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) is an attractive polymer for switchable surface coatings based on its multiresponsiveness toward environmental triggers (temperature, pH-value, ionic strength). In this in situ study, we present the complex and tunable thermoresponsiveness of PDMAEMA Guiselin brushes (9 nm, dry thickness), which were prepared via an efficient grafting-to approach. Combining in situ atomic force microscopy (AFM) visualizing the surface topography (x-y plane) and spectroscopic ellipsometry monitoring the swelling behavior of the polymer film (layer thickness, z-direction) offers for the first time a three-dimensional insight into thermoresponsive transitions on the nanoscale. While PDMAEMA films exhibit LCST behavior in the presence of monovalent counterions, it can easily be switched toward an UCST thermoresponsiveness via the addition of small quantities of multivalent ions. In both cases, the transition temperature as well as the sharpness and reversibility of the transition can be tuned via a second external trigger, the ionic strength. Whereas homogeneous surfaces were observed both below and above the LCST in monovalent salt solutions, the UCST transition was characterized by the in situ formation of a nanostructured surface of pinned PDMAEMA micelles with entrapped multivalent counterions. Moreover, it was demonstrated for the first time that the characteristic dimensions of the nanopattern (the diameter and height of the pinned micelles) could be tuned in situ by the pH- and induced UCST thermoresponsiveness of PDMAEMA. This approach therefore provides a novel bottom-up strategy to create and control polymeric nanostructures in an aqueous environment.

Superlubricity of Zwitterionic Bottlebrush Polymers in the Presence of Multivalent Ions

In this study, we report lubrication properties of physisorbed zwitterionic bottlebrush polymers in the presence of multivalent ions using the surface force apparatus. Unlike polyelectrolyte brushes, the lubrication properties of which diminish drastically in the presence of multivalent ions at concentrations as low as 0.1 mM, zwitterionic bottlebrush polymers exhibit friction coefficients as low as ∼10^-3 at such concentrations of multivalent ions up to intermediate normal loads. This lubrication ability persists until surface wear occurs at high normal loads. The surface wear is demonstrated to be triggered by the multivalent ions bridging the polymer chains and dehydrating the zwitterionic moieties. Finally, the analysis of the polymer film stability suggests that the partial desorption of polymers in the presence of the ions does not affect the lubrication performance. Therefore, even in the physisorbed state, zwitterionic brushes perform significantly better than covalently grafted polyelectrolyte brushes in the presence of multivalent ions.

Self-assembly of polyelectrolyte diblock copolymers within mixtures of monovalent and multivalent counterions

In this study, the self-assembly behavior of polyelectrolyte (PE) diblock copolymers in solutions containing mixtures of monovalent and multivalent counterions was investigated using molecular dynamics simulation. The properties of the assembled micelles and counterion condensations at different charge fractions of multivalent ions have been discussed. The bridging effect of multivalent ions induces the electrostatic correlations of the PE chains, leading to the fusion of large micelles and the formation of bulky aggregates. Notably, lamellar and well-organized face-centered cubic (FCC) arrangements of the assembled micelles were observed in the mixture of monovalent and trivalent ions. At large fractions of multivalent ions, cylindrical and lamellar precipitates composed of the assembled micelles were formed owing to the inter-connecting coronas. The mixtures of monovalent and multivalent counterions allow the regulation of the electrostatic interactions and tuning of the properties in assembled micelles.

Self-assembly in amphiphilic spherical brushes

The structure of amphiphilic spherical brushes, consisting of the nano-SiO₂ core, the hyperbranched polyamidoamine subshell, and a grafted layer of long hydrophobically modified polyacrylamide (HMPAM) chains, in aqueous solution was analyzed and described in the framework of the original mean-field approach. The scaling estimations of the hydrodynamic radius of such polymer brushes as a function of the number of grafted macromolecules allow concluding that the HMPAM shells are in a globular state and that the region of the stretched chains adjacent to the grafting surface is a minor part of the grafted macromolecules and does not have a significant impact on the self-assembly of the HMPAM shell caused by the complex hydrophobic-hydrophilic composition of their monomer units. In mean-field theory, the amphiphilic nature of HMPAM was taken into account by attaching the hydrophobic side group H to some fraction of monomer units of the hydrophilic P backbone. The strong attraction of H groups causes the aggregation of macromolecules, whereas the affinity of hydrophilic P groups to solvent forces the aggregates to increase their surface. Due to such effective surface activity, in poor solvent, the grafted amphiphilic macromolecules could form a spherical compacted structure around the nanoparticle or self-assemble into a "hedgehog" structure with several "spines" having hydrophobic core and hydrophilic shell. State diagrams, obtained theoretically, reveal that the "hedgehog" structure is preferable for a wide range of energetic parameters.

Visual Detection of Thiocyanate Based on Fabry-Perot Etalons with a Responsive Polymer Brush as the Transducer

The detection of thiocyanate (SCN^-) is particularly important in industrial effluents and biological fluids because of the toxic nature of SCN^-. Herein, a metal-insulator-metal (MIM) resonator for visual detection of SCN^- is presented based on a poly[(2-(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) brush. The MIM resonator exhibits obvious color change as the concentration of SCN^- changes, which can be easily distinguished by the naked eyes. In addition, the as-prepared MIM resonator also shows the advantages of good anti-interference, excellent reusability, and fast response rate. Combining the above advantages, the proposed MIM resonator may provide a broad perspective for a wide variety of visible-light applications.

Interactions of star-like polyelectrolyte micelles with hydrophobic counterions

Hydrophobicity of a counterion has a profound effect on the interaction with polyelectrolytes similar to that of multivalency. Specifically, understanding this interaction in weak polyelectrolyte micelles might assist in developing nanocarriers for pH-controlled encapsulation and release. We used star-like weak polyelectrolyte micelles of polystyrene-block-poly(2-vinyl pyridine) (PS-P2VP) with fixed aggregation number as a model polyelectrolyte, and cobalt bis(1,2-dicarbollide) (COSAN) as a model hydrophobic anion. We used NMR to assess the mobility of the polymer segments in the presence of varying amounts of COSAN, and at varying protonation degrees of the polyelectrolyte. Same experiments with indifferent electrolyte (NaCl) were used as a control. Furthermore, we used coarse-grained simulations to obtain a detailed picture of the effect of hydrophobic counterions on the conformation of the micelles. A small amount of hydrophobic counterions causes morphological changes within the micelles, whereas a bigger amount causes precipitation. This was confirmed both in simulations and in experiments. Furthermore, adsorption of the counterions induces ionization of the collapsed segments of the polyelectrolyte. Although the COSAN/P2VP system is rather specific, the generic model used in the coarse-grained simulations shows that the observed behavior is a consequence of synergy of hydrophobic and electrostatic attraction between polyelectrolytes and hydrophobic counterions. Our study provides general insights into the molecular mechanisms of these interactions.

Simulating Surface Patterning of Nanoparticles by Polymers via Dissipative Particle Dynamics Method

Patchy particles are often referred to colloidal particles with physically or chemically patterned surfaces. We investigated the patterning of nanoparticle grafted by polymers, mainly consisting of patchy structures with different numbers of patches ( N_patch) and core-shell structure using the dissipative particle dynamics (DPD) method in good or poor solvents based on the experiment research. Poor solvent, large nanoparticle, proper grafting density and medium polymer length contribute to the formation of patchy structure. We introduce the effective volume fraction as an indicator to distinguish the patchy structure from core-shell structure. The reversible transition between core-shell (in a good solvent) and patchy structure (in a poor solvent) and the dependency relationship between the nanoparticle diameter and grafting density in experiment are verified. Our results pave the way for preparing the colloids with well-defined patches. The anisotropic patchy particles can self-assemble into elaborate superstructures, which are potential blocking materials for drug delivery, sensors, and electronics.