Interaction between colloids with grafted diblock polyampholytes

pH-Dependent Polyelectrolyte Bridging of Charged Nanoparticles

Systems comprised of polyelectrolytes and charged nanoparticles are of great technological interest, being common components in formulations among other uses. The colloidal stability of formulations is an important issue, and thus a lot of effort has been made to study the interactions of individual components in these systems. Here, the complexation and adsorption of an annealed (pH-dependent) polyelectrolyte to two spherical nanoparticles has been studied using coarse-grained Monte Carlo simulations. This has been done mainly by varying the solution pH and separation distance (concentration) between the nanoparticles. The polyelectrolyte charge distribution is seen to vary with nanoparticle separation distance, and its ability to bridge both nanoparticles changes with pH. The flexible polyelectrolyte creates compact, multilink bridges at short nanoparticle separation distances and evolves to a stretched single-link bridge at longer distances, where a larger fraction of the polyelectrolyte wraps around the nanoparticles. The annealed polyelectrolyte is also compared with a quenched polyelectrolyte of similar fixed fractional charge. Here, a difference is found in the adsorption ability at low pH/ionization due to the ability of the annealed polyelectrolytes to concentrate charges in the vicinity of the nanoparticle. At intermediate polyelectrolyte charge fractions and with increasing nanoparticle separation distances, the annealed system is able to link nanoparticles at larger distances as compared to the quenched, in good agreement with experimental observations. The results in this work contribute to the understanding of the effect of annealed polyelectrolytes and pH variations in the phase behavior of polyelectrolyte-nanoparticle systems, potentially aiding in the design and optimization of pH-responsive systems.

Formation of Polymer Brushes with Diblock Copolymers on a Planar Surface

We use molecular dynamics simulations method to investigate the behavior characteristics of AB diblock copolymers that are adsorbed on a planar surface. Adsorption density has been distinguished, depending on the adsorption manner of A-block on the (100) surface and formation of brushes. It is examined in detail that conformational behavior of the brushes affects the adsorption density. In addition, we make a comparison of linear brush with length ratio of the A-block to the chain, in the cases of the fixed length of chain and the fixed length of A-block, respectively. The result shows that the adsorption density is strongly affected by the length ratio of the A-block to the chain. And our findings can be used as a guide for fabrication and preparation of actual synthetic polymer brushes on a solid surface by the approach of physical adsorption.

Conformational transitions of weak polyacids grafted to nanoparticles

The charge distribution on polyelectrolytes is a key factor, which controls their conformation and interactions. In weak polyelectrolytes, this distribution is determined by a number of factors, including the solvent conditions and local environment. In this work, we investigate charge distributions of chains end-grafted on a spherical nanoparticle in a salt solution, using grand canonical titration Monte Carlo simulations of a coarse-grained polymer model. In this approach, the ionization state of each polymer bead fluctuates based on the dissociation constant, pH of the solution, and interactions with other particles in the system. We determine charge and polymer conformations as functions of the pH and solvent quality. We compare the results to a fixed charge model and also investigate the role of grafting density and the effect of curvature on the film morphologies.

Wrinkle-assisted linear assembly of hard-core/soft-shell particles: impact of the soft shell on the local structure

This article addresses wrinkle assisted assembly of core-shell particles with hard cores and soft poly-(N-isopropylacrylamide) shells. As core materials we chose silica as well as silver nanoparticles. The assembled structures show that the soft shells act as a separator between the inorganic cores. Anisotropic alignment is found on two length scales, macroscopically guided through the wrinkle structure and locally due to deformation of the polymer shell leading to smaller inter-core separations as compared to assembly on flat substrates without confinement. The structures were analysed by means of scanning electron microscopy. Radial distribution functions are shown, clearly highlighting the impact of confinement on nearest neighbour distances and symmetry. The observed ordering is directly compared to Monte-Carlo simulations for hard-core/soft-shell particles, showing that the observed symmetries are a consequence of the soft interaction potential and differ qualitatively from a hard-sphere situation. For the silver-poly-(N-isopropylacrylamide) particles, we show UV-vis absorbance measurements revealing optical anisotropy of the generated structures due to plasmon coupling. Furthermore, the high degree of order of the assembled structures on macroscopic areas is demonstrated by laser diffraction effects.

Brushes of flexible, semiflexible, and rodlike diblock polyampholytes: Molecular dynamics simulation and scaling analysis

Planar brushes of flexible, semiflexible, and rodlike diblock polyampholytes are studied using molecular dynamics simulations in a wide range of the grafting density. Simulations show linear dependence of the average thickness on the grafting density in all cases regardless of different flexibility of anchored chains and the brushes' different equilibrium conformations. Slopes of fitted lines to the average thickness of the brushes of semiflexible and rodlike polyampholytes versus the grafting density are approximately the same and differ considerably from that of the brushes of flexible chains. The average thickness of the brush of flexible diblock polyampholytes as a function of the grafting density is also obtained using a simple scaling analysis, which is in good agreement with our simulations.

Chemical and structural changes in a pH-responsive mixed polyelectrolyte brush studied by infrared ellipsometry

This work provides direct chemical and structural insight into pH-dependent changes of an ultrathin (d=12 nm) mixed polyelectrolyte brush. In-situ infrared spectroscopic ellipsometry was used for the first time to study the gradual pH-responsive behavior of the brush, constituted of weak anionic and cationic polyelectrolytes, poly(acrylic acid) (PAA) and poly(2-vinylpyridine) (P2VP), respectively. The pH-dependent infrared fingerprints in the mid-infrared spectral range were analyzed as a function of chemical and structural changes in the mixed brush caused by pH changes. Thereby, the IR spectra were directly correlated to different chemical states of the brush, giving previously not accessible new information on the ionization of the thin film. In contrast to other techniques (e.g., classical attenuated total reflection IR spectroscopy) we used almost plane Si-substrates for the IR ellipsometric approach with application of a single reflection mode. The optical path through Si is of minimal length, which makes a large spectral range accessible. For the most pronounced bands of the carboxyl group at 1718 cm(-1) and the carboxylate ion at 1565 cm(-1), the band amplitudes were correlated with the degree of ionization of the carboxylic groups. Interpretation of the pH-dependent changes in the spectral signature reveals gradual changes of the chemical structures of the mixed brush between three distinct switchable states: strongly ionized PAA at pH 10, strongly ionized P2VP at pH 2, and mainly nonionized functional groups in a "dry" PAA-P2VP polyelectrolyte complex in the range from pH=4 to pH=7. At intermediate pH, the IR spectra confirm the previously made hypothesis of the formation of a polyelectrolyte complex between P2VP and PAA in the mixed brush. From IR spectra it is also concluded that the polyelectrolyte complex is formed as a result of a small fraction of ionized functional groups.

Molecular dynamics simulation of semiflexible polyampholyte brushes--the effect of charged monomers sequence

Planar brushes formed by end-grafted semiflexible polyampholyte chains, each chain containing an equal number of positively and negatively charged monomers, are studied using molecular dynamics simulations. Keeping the length of the chains fixed, the dependences of the average brush thickness and equilibrium statistics of the brush conformations on the grafting density and the salt concentration are obtained with various sequences of charged monomers. When similarly charged monomers of the chains are arranged in longer blocks, the average brush thickness is smaller and the dependence of brush properties on the grafting density and the salt concentration is stronger. With such long blocks of similarly charged monomers, the anchored chains bond to each other in the vicinity of the grafting surface at low grafting densities and buckle toward the grafting surface at high grafting densities.

Interplay between liquid crystalline and isotropic gels in self-assembled neurofilament networks

Neurofilaments (NFs) are a major constituent of nerve cell axons that assemble from three subunit proteins of low (NF-L), medium (NF-M), and high (NF-H) molecular weight into a 10 nm diameter rod with radiating sidearms to form a bottle-brush-like structure. Here, we reassemble NFs in vitro from varying weight ratios of the subunit proteins, purified from bovine spinal cord, to form homopolymers of NF-L or filaments composed of NF-L and NF-M (NF-LM), NF-L and NF-H (NF-LH), or all three subunits (NF-LMH). At high protein concentrations, NFs align to form a nematic liquid crystalline gel with a well-defined spacing determined with synchrotron small angle x-ray scattering. Near physiological conditions (86 mM monovalent salt and pH 6.8), NF-LM networks with a high NF-M grafting density favor nematic ordering whereas filaments composed of NF-LH transition to an isotropic gel at low protein concentrations as a function of increasing mole fraction of NF-H subunits. The interfilament distance decreases with NF-M grafting density, opposite the trend seen with NF-LH networks. This suggests a competition between the more attractive NF-M sidearms, forming a compact aligned nematic gel, and the repulsive NF-H sidearms, favoring a more expansive isotropic gel, at 86 mM monovalent salt. These interactions are highly salt dependent and the nematic gel phase is stabilized with increasing monovalent salt.