Response of a Permanently Charged Polyelectrolyte Brush to External Ions: The Aspects of Structure and Dynamics

Highly Efficient Conversion of Salinity Difference to Electricity in Nanofluidic Channels Boosted by Variable Thickness Polyelectrolyte Coating

The inherent limits of the current produced by imposing salinity gradients along a nanofluidic channel having "hard" boundary walls heavily constrain the resulting energy harvesting efficacy, acting as major hindrances against the practicability of harnessing high power density from the mixing of water having different salinities. In this work, the infusion of variable-thickness polyelectrolyte layer of a conical shape is projected to augment salinity gradient power generation in nanochannels. Such a progressive thickening of a charged interfacial layer on account of axially declining ion concentration facilitates the shedding of enhanced numbers of mobile ions, bearing a net charge of equal and opposite to the surface-bound ions, into the mainstream current flow. We show that the proposed design can convert energy at a higher efficiency as compared to both solid-state and available polyelectrolyte layer (PEL)-covered nanochannels. The same is true for the maximum power density at moderate and high concentration ratios including natural salt gradient conditions for which more than 50% increase is achievable. The maximum values achieved for efficiency and power density read 50.3% and 6.6 kW/m2, respectively. Our results provide fundamental insights on strategizing variable-thickness polyelectrolyte layer grafting on the nanochannel interfaces, toward realizing high-performance osmotic power generators by altering the local ionic clouds alongside the grafted layers and enhancing the ionic mobility by inducing a driving potential gradient concomitantly. These findings open up a new strategy of efficient conversion of the power of the salinity difference of seawater and river water into electricity in a nanofluidic framework, surpassing the previously established limits of blue energy harvesting technologies.

Smart Copolymer Surface Derived from Geminized Cationic Amphiphilic Polymers for Reversibly Switchable Bactericidal and Self-Cleaning Abilities

Bacterial adhesion and colonization on material surfaces pose a serious problem for healthcare-associated devices. Cationic amphiphilic polymer brushes are usually used as surface coatings in antibacterial materials to endow an interface with excellent bactericidal efficiency, but they are easily contaminated, which puts a great limitation on their application. Herein, novel antibacterial copolymer brush surfaces containing geminized cationic amphiphilic polymers (pAGC₈) and thermoresponsive poly(N-isopropylacrylamide) polymers (pNIPAm) have been synthesized. Surface functionalization of polymer brushes was investigated by X-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy, and water contact angle measurements. A proportion of AGC₈ and NIPAm units in copolymer brushes has been adjusted to obtain a high-efficiency bactericidal surface with minimal interference to its self-cleaning property. The killing and releasing efficiency of the optimized surface simultaneously reached up to above 80% for both Staphylococcus aureus and Escherichia coli bacteria, and the bactericidal and self-cleaning abilities are still excellent even after three kill-release cycles. Such a novel copolymer brush system provides innovative guidance for the development of high-efficiency antibacterial materials in biomedical application.

Structural and electrostatic properties between pH-responsive polyelectrolyte brushes studied by augmented strong stretching theory

In this paper, we study electrostatic and structural properties between pH-responsive polyelectrolyte brushes by using a strong stretching theory accounting for excluded volume interactions, the density of polyelectrolyte chargeable sites and the Born energy difference between the inside and outside of the brush layer.In a free energy framework, we obtain self-consistent field equations to determine electrostatic properties between two pH-responsive polyelectrolyte brushes. We elucidate that in the region between two pH-responsive polyelectrolyte brushes, electrostatic potential at the centerline and osmotic pressure increase not only with excluded volume interaction, but also with density of chargeable sites on a polyelectrolyte molecule. Importantly, we clarify that when two pH-responsive polyelectrolyte brushes approach each other, the brush thickness becomes short and that a large excluded volume interaction and a large density of chargeable sites yield the enhanced contract of polyelectrolyte brushes. In addition, we also demonstrate how the influence of such quantities as pH, the number of Kuhn monomers, the density of charged sites, the lateral separation between adjacent polyelectrolyte brushes, Kuhn length on the electrostatic and structural properties between the two polyelectrolyte brushes is affected by the exclusion volume interaction. Finally, we investigate the influence of Born energy difference on the thickness of polyelectrolyte brushes and the osmotic pressure between two pH-responsive polyelectrolyte brushes.

Hyperbranched anion exchangers prepared from polyethylene polyamine modified polymeric substrates for ion chromatography

High hydrophilic anion stationary phases play a crucial role in the separation behavior of ion chromatography. Herein, we report novel polymeric anion exchangers grafted with polyethylene polyamines, including ethylenediamine, diethylenetriamine, triethylenetetramine and tetraethylene pentaamine, via a facile epoxy-amine polymerization method. The anion exchangers were characterized by scanning electron microscopy, thermogravimetry, Fourier transform infrared spectrometry and elemental analysis. The chromatographic performance of the stationary phases was evaluated with the separation of common inorganic anions, organic weak acids and highly polarizable anions. Seven common anions (F^-, Cl^-, NO₂^-, Br^-, NO₃^-, SO₄^2- and HPO₄^2-) can be separated within 18 min by using hydroxide eluent in isocratic mode. By adopting different polyethylene polyamines as hyperbranched units, the four types of new stationary phases displayed high efficiencies and good reproducibility. The columns exhibit large exchange capacities at 76.5-184.8 μmol•column^-1 (4.6 × 150 mm, i.d.) with efficiency up to 20293 plate m^-1 (Cl^-). The RSDs of the retention time were less than 0.27% and the RSDs of the efficiency were less than 1.95% by consecutive injections after working for two months. The self-fabricated column was successfully applied to determine the chloride content in exhaled breath condensate.

Effect of Counterion Binding to Swelling of Polyelectrolyte Brushes

The effect of binding strength of counterions with the polyelectrolyte chain to the swelling of polyelectrolyte brushes is studied, by investigating the swelling of both the polycation and polyanion in response to the variation of the salt concentration under the change of counterion's identity. Two polyelectrolyte brushes grafted on solid substrates are adopted: the cationic poly [2-(methacryloyloxy)ethyltrimethyl ammonium] (PMETA-X, X = F, Cl, Br, and I) and the anionic polystyrene sulfonate (M-PSS, M = Li, Na, K, and Cs). The swelling change with the salt concentration is investigated by ellipsometry, quartz crystal microbalance with dissipation, and dielectric spectroscopy. It is discovered that although the thickness of PMETA-X brushes is larger than that of M-PSS brushes of similar grafting density, the former has much less solvent incorporated than the latter. Such a difference is attributed to the weaker interaction between the PMETA⁺ chain and its halide counterions than that between the PSS^- chain and its alkali counterions, discovered by dielectric spectroscopy. This makes the original charges on the PMETA-X chain less neutralized and therefore have a higher charge density, compared with the M-PSS chain. The results demonstrate that the stronger binding of the counterions to the polyelectrolytes makes the main chains less charged, resulting in the weaker inter-chain electrostatic repulsion and less swelling of the brushes. Investigations into the effect of ion identity show the following order of binding strength: for the cationic PMETA⁺ chain, F^- < Cl^- < Br^- < I^- and for the anionic PSS^- chain, Li⁺ < Na⁺ < K⁺ < Cs⁺.

Hierarchical Surface Inspired by Geminized Cationic Amphiphilic Polymer Brushes for Super-Antibacterial and Self-Cleaning Properties

Pathogenic bacteria adhesion and formation of biofilm on the implant are the most common reasons for healthcare-associated device failure. Cationic amphiphilic polymer brushes containing covalently linked quaternary ammonium salts (QASs) are considered to be the most promising bactericidal materials, but these surfaces still suffer from incomplete bactericidal ability and serious microorganism accumulation. With this in mind, a novel kind of hierarchical surface integrating both geminized cationic amphiphilic antibacterial upper layer and zwitterionic antifouling sublayer has been developed in this study. Measurements of X-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy, water contact angle, and surface ζ-potential were performed to investigate the surface functionalization process. The thicknesses and grafting densities of the pAGC₈ upper blocks have been optimized to avert the mutual interference among different components. The optimal hierarchical surface exhibits an ultrahigh antibacterial activity and a potent self-cleaning functionality against both Staphylococcus aureus and Escherichia coli bacteria, as well as a certain protein repellence ability. Such a novel hierarchical architecture provides innovative guidance for the construction of super-antibacterial and self-cleaning brushes in many biomedical applications.

X-ray and Neutron Reflectometry of Thin Films at Liquid Interfaces

In the 1980s, Helmuth Möhwald studied lipid monolayers at the air/water interface to understand the thermodynamically characterized phases at the molecular level. In collaboration with Jens Als-Nielsen, X-ray reflectometry was used and further developed to determine the electron density profile perpendicular to the water surface. Using a slab model, parameters such as thickness and density of the individual molecular regions, as well as the roughness of the individual interfaces, were determined. Later, X-ray and neutron reflectometry helped to understand the coverage and conformation of anchored and adsorbed polymers. Nowadays, they resolve molecular properties in emerging topics such as liquid metals and ionic liquids. Much is still to be learned about buried interfaces (e.g., liquid/liquid interfaces). In this Article, a historical and theoretical background of X-ray reflectivity is given, recent developments of X-ray and neutron reflectometry for polymers at interfaces and thin layers are highlighted, and emerging research topics involving these techniques are emphasized.

Ultrahighly Charged Amphiphilic Polymer Brushes with Super-Antibacterial and Self-Cleaning Capabilities

Bacterial infection on biomaterial devices and the subsequent medical risks pose a serious problem in both human healthcare and industrial applications, resulting in a prevalence of various antimicrobial materials. Cationic amphiphilic polymer has been proposed to be a new generation of efficient antibacterial material, but the surface modified by such types of polymers still shows incomplete bactericidal ability and easily contaminated performance. With this in mind, a novel kind of geminized cationic amphiphilic polymer brush surface has been developed in this study, presenting a complete antibacterial activity, because of the synergistic biocidal effect of electrostatic and hydrophobic interactions, as well as the minimized contact area between bacteria and polymer surface. A structure self-adjustment process of polymer brush construction has been proposed, in which the mutual interference among cationic head groups can be avoided and the electrostatic repulsion and hydrophobic attraction can be balanced, in the formation of a smooth and tight surface. A self-cleaning capability of polymer surface has been performed via hydrolysis and degradation, maintaining a high antibacterial activity. Therefore, we provide a facile and possible manipulation strategy to fabricate super-antibacterial and self-cleaning surfaces in a wide range of biomedical and industrial applications.