Effects of Solvent Evaporation Rate and Poly(acrylic acid) on Formation of Poly(ethylene oxide)-<i>block</i>-polystyrene Micelles from Emulsion

In this work, a solution of poly­(ethylene oxide)-<i>block</i>-polystyrene (PEO-<i>b</i>-PS) block copolymer in an organic solvent was dispersed in water to form an emulsion in the presence of poly­(acrylic acid) (PAA), which upon solvent evaporation produced micelles, and the “emulsion and solvent evaporation” process was studied. It was found that PAA interacted with the PEO corona of the micelles to reduce the curvature, transforming the aggregates from cylinders into vesicles when 1,2-dichloroethane was the solvent. However, when a more volatile solvent, dichloromethane, was used instead, cylindrical micelles were obtained. Even from 1,2-dichloroethane, cylinders were the predominant species when the initial solution concentration was higher (i.e., shorter evaporation process) or when PAA with a much higher molecular weight was used. On the basis of these observations, the interplay between the solvent evaporation rate and the mass transport and chain reorganization at the interface is discussed. In addition, some intermediate structures were observed, which provided insight into the assembly process

Facile Synthesis of AuPt Alloy Nanoparticles in Polyelectrolyte Multilayers with Enhanced Catalytic Activity for Reduction of 4‑Nitrophenol

In this work, bimetallic AuPt alloy nanoparticles were synthesized <i>in situ</i> in polyelectrolyte multilayers (PEMs) via an ion-exchange and coreduction process, in which the PEM support also served to suppress the Au–Pt phase separation, and thus enabled formation of AuPt alloy nanoparticles over a wide composition range. The PEM supported AuPt alloy nanoparticles exhibited higher catalytic activity than Au and Pt monometallic ones for the reduction of 4-nitrophenol by NaBH₄, showing synergistic effects between Au and Pt. This work provides a facile approach to <i>in situ</i> synthesis of polymer supported bimetallic nanoparticles of tailored composition for optimum performance in catalysis and other applications

Analysis of Nanodomain Composition in High-Impact Polypropylene by Atomic Force Microscopy-Infrared

In this paper, compositions of nanodomains in a commercial high-impact polypropylene (HIPP) were investigated by an atomic force microscopy-infrared (AFM-IR) technique. An AFM-IR quantitative analysis method was established for the first time, which was then employed to analyze the polyethylene content in the nanoscopic domains of the rubber particles dispersed in the polypropylene matrix. It was found that the polyethylene content in the matrix was close to zero and was high in the rubbery intermediate layers, both as expected. However, the major component of the rigid cores of the rubber particles was found to be polypropylene rather than polyethylene, contrary to what was previously believed. The finding provides new insight into the complicated structure of HIPPs, and the AFM-IR quantitative method reported here offers a useful tool for assessing compositions of nanoscopic domains in complex polymeric systems

Interfacial Interactions between Poly(3-hexylthiophene) and Substrates

Interfacial interactions between poly­(3-hexylthiophene) (P3HT) and substrate surface have been investigated. P3HT nanowhiskers of single molecule thickness were prepared from chloroform solution, and their adsorption on substrates of various surface chemistries was investigated using atomic force microscopy (AFM) and Raman spectroscopy. P3HT monolayer nanowhiskers with edge-on molecular orientation were found to adsorb readily onto a SiO₂ substrate, and the amount of adsorption was significantly higher on a SiO₂ surface modified with a perfluorohexyl monolayer; no P3HT adsorption was observed on a hexyl monolayer. These results suggest that electron-withdrawing groups rather than surface energy govern the interfacial interactions. On a highly oriented pyrolytic graphite (HOPG) surface, P3HT molecules adsorbed in face-on orientation, and edge-on monolayer nanowhiskers were absent on the surface. Raman spectroscopy data revealed strong charge-transfer interactions between face-on P3HT molecules and the HOPG surface

Multilayered Core–Shell Structure in an Impact Polypropylene Copolymer Investigated by Atomic Force Microscopy–Infrared

The balanced mechanical properties of impact polypropylene copolymer (IPC) are largely attributed to the core–shell structure of its dispersed rubber particles, yet experimental observation of the outer shell interface between the rubber phase and the polypropylene (PP) matrix is challenging. In this article, atomic force microscopy-infrared (AFM-IR) was employed to study a commercial IPC to determine its phase structure. Quantitative analysis of the nanodomain composition in situ by AFM-IR in combination with the chain structure of the copolymers obtained ex situ by fractionation and NMR revealed a core surrounded by a rubber layer, comprising the ethylene–propylene segmented copolymer (EsP) and ethylene–propylene random copolymer (EPR), respectively, which suggests the existence of an outer shell for the particle composed of the ethylene–propylene block copolymer (EbP). The EbP fraction in the IPC was then replaced by an ethylene-deuterated propylene diblock copolymer (EbDP), which was then melt-blended with all other fractions to reconstruct the IPC. Both AFM-IR spectroscopic analysis and imaging of the nanodomains in the reconstructed IPC showed that the EbDP molecules are located at the interface between the rubber phase and the PP matrix, forming an outer shell for the particle. The results provide direct and unambiguous experimental evidence for the multilayered particle structure in the IPC. Mechanical test results further demonstrated that the outer shell for the rubber particle was beneficial to the tensile and impact properties of the alloy

A Surface with Superoleophilic-to-Superoleophobic Wettability Gradient

A strategy combining polyelectrolyte multilayer (PEM) deposition and counterion exchange was developed to fabricate wettability gradient surfaces on rough aluminum with wetting characters continuously varied from superoleophilic to superoleophobic. Counterion exchange kinetics was adopted as a means to tailor the surface chemical composition spatially, with the gradient ultimately reflecting position-dependent immersion time during the dipping of substrate in salt solution. Wettability depended on the identity and concentration of the counterion in the outermost PEM layer. Gradients could be erased and rewritten through the exchange of counterions, and the gradient’s wetting character was evaluated by measuring both water and oil contact angles. The surface chemical composition gradient was further investigated by X-ray photoelectron spectroscopy

Synthesis of Hollow Ag–Au Bimetallic Nanoparticles in Polyelectrolyte Multilayers

Ag nanoparticles of ∼20 nm size and rather uniform size distribution were synthesized in polyelectrolyte multilayers (PEMs) via an ion-exchange/reduction process in two stages (seeding and growth), which were used as sacrificial templates to fabricate Ag–Au bimetallic hollow nanoparticles via galvanic replacement reaction. The reaction process was monitored by UV–vis spectroscopy. The morphology and structure of the nanoparticles were characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy, which confirmed the formation of hollow Ag–Au bimetallic nanoparticles. UV–vis absorbance spectroscopy and TEM results indicated that both size and optical properties of the Ag nanoparticles in the PEM can be controlled by manipulating ion content in the PEM and the number of the ion-exchange/reduction cycle, whereas that of Ag–Au bimetallic nanoparticles were dependent on size of the Ag templates and the replacement reaction kinetics. The hollow Ag–Au bimetallic nanoparticles exhibited a significant red shift in the surface plasmon resonance to the near-infrared region. The strategy enables facile preparation of hollow bimetallic nanoparticles in situ in polymer matrixes

A Surface Exhibiting Superoleophobicity Both in Air and in Seawater

Superoleophobic surfaces have attracted increasing interest in recent years due to their potential application in various fields. In this paper, we report a surface that exhibits superoleophobicity both in air and in seawater. A polyelectrolyte multilayer (PEM) is assembled on an aluminum substrate with a micro/nano hierarchical surface structure, and the counterion in the PEM is exchanged with perfluorooctanoate (PFO), making the surface superhydrophobic and superoleophobic in air. When submerged in artificial seawater, the surface exhibits underwater superoleophobicity, with a 1,2-dichloroethane contact angle of 163°. X-ray photoelectron spectroscopic analysis and controlled experiments reveal that, upon exposure to seawater, the PEM spontaneously exchanges the PFO counterion with the chloride and sulfate ions in the seawater, making the surface hydrophilic and hence oil-repelling underwater. When withdrawn from seawater, superoleophobicity in air is restored by treating the surface in a PFO solution shortly to reinstall the PFO counterion. The switching between the two wetting states (superoleophobicity in air and underwater) is completely reversible. This simple and versatile approach can be readily extended to other substrates, making it a promising method for introduction of dual superoleophobicity to surfaces used in many fields

Effect of Divalent Counterions on Polyelectrolyte Multilayer Properties

When exposed to divalent counterion solutions, polyelectrolyte multilayer (PEM) films of poly­(diallyl­dimethyl­ammonium chloride) and sodium poly­(styrene­sulfonate) (NaPSS) prepared in the presence of monovalent salt, or equilibrated with such a salt, are physically cross-linked by divalent counterion incorporation, altering PEM properties significantly. The rapid cross-linking was monitored by the quartz crystal microbalance with dissipation (QCM-D) method, which finds PEM deswelling and rigidification after exposures to a low concentration of Cu­(NO₃)₂; at higher concentration, deswelling is countered by increased PEM uptake of the salt, which disrupts polyelectrolyte–polyelectrolyte ion pairs. Divalent ion incorporation into PEMs has the character of ion exchange, and incorporated divalent ions are quickly and completely removed when presented with monovalent salt solution but not with water. While counterion cross-linking extends across the bulk of the PEM, the fraction of exchanged counterions remains low. Entropically driven binding of divalent ions to NaPSS in solution was studied for Cu­(NO₃)₂ and other divalent nitrate salts by isothermal titration microcalorimetry and dynamic light scattering to support the QCM-D conclusions

Ion Dispositions in Polyelectrolyte Multilayer Films

Polyelectrolyte multilayers (PEMs) fabricated through layer-by-layer (LbL) assembly from sodium chloride-containing solutions of poly­(diallyldimethylammonium chloride) (PDDA) and poly­(styrene sulfonate) (PSS) were examined by quartz crystal microbalance (QCM), QCM with dissipation (QCM-D), UV–vis spectroscopy, and X-ray photoelectron spectroscopy (XPS) to determine the dispositions of polyelectrolytes and counterions across the PEM thickness. The key experiment was dry film QCM, which by quantifying the incremental mass depositions during LbL assembly uncovered excess polyelectrolyte charge and excess polyelectrolyte charge density as functions of deposition number. Counterion dispositions depended strongly on salt concentration, and trends in the two PEM charge parameters established three salt concentration regimes: zero to near zero salt ([NaCl] ≲ 0.1 M), low salt (0.1 M ≲ [NaCl] ≲ 0.75 M), and high salt ([NaCl] ≳ 1.5 M]). The first two are associated with linear LbL growth while the latter is associated with exponential LbL growth. At zero salt, no counterions are present in the PEM bulk (middle), while at low salt, an excess of PDDA charge across the bulk coincides with an excess of counteranions. Differently, at high salt, deposited PSS permeates the PEM bulk, conveying an excess of countercations. At all salt concentrations, the PEM surface charge alternates according to the capping polyelectrolyte’s identity. Accumulations of small ions in the PEM bulk can be ascribed to property asymmetries between the two deposited polyelectrolytes, but the roles played by different chain properties remain incompletely understood