Poly(ethylene Oxide)-block-polystyrene Research Articles

Micellar Nanoreactors Enabled Site-Selective Decoration of Pt Nanoparticles Functionalized Mesoporous SiO2 /WO3-x Composites for Improved CO Sensing.

Site-selective and partial decoration of supported metal nanoparticles (NPs) with transition metal oxides (e.g., FeOx ) can remarkably improve its catalytic performance and maintain the functions of the carrier. However, it is challenging to selectively deposit transition metal oxides on the metal NPs embedded in the mesopores of supporting matrix through conventional deposition method. Herein, a restricted in situ site-selective modification strategy utilizing poly(ethylene oxide)-block-polystyrene (PEO-b-PS) micellar nanoreactors is proposed to overcome such an obstacle. The PEO shell of PEO-b-PS micelles interacts with the hydrolyzed tungsten salts and silica precursors, while the hydrophobic organoplatinum complex and ferrocene are confined in the hydrophobic PS core. The thermal treatment leads to mesoporous SiO2 /WO3-x framework, and meanwhile FeOx nanolayers are in situ partially deposited on the supported Pt NPs due to the strong metal-support interaction between FeOx and Pt. The selective modification of Pt NPs with FeOx makes the Pt NPs present an electron-deficient state, which promotes the mobility of CO and activates the oxidation of CO. Therefore, mesoporous SiO2 /WO3-x -FeOx /Pt based gas sensors show a high sensitivity (31±2 in 50ppm of CO), excellent selectivity, and fast response time (3.6s to 25ppm) to CO gas at low operating temperature (66°C, 74% relative humidity).

Noble Metal Nanoparticles Decorated Metal Oxide Semiconducting Nanowire Arrays Interwoven into 3D Mesoporous Superstructures for Low-Temperature Gas Sensing.

Mesoporous materials have been extensively studied for various applications due to their high specific surface areas and well-interconnected uniform nanopores. Great attention has been paid to synthesizing stable functional mesoporous metal oxides for catalysis, energy storage and conversion, chemical sensing, and so forth. Heteroatom doping and surface modification of metal oxides are typical routes to improve their performance. However, it still remains challenging to directly and conveniently synthesize mesoporous metal oxides with both a specific functionalized surface and heteroatom-doped framework. Here, we report a one-step multicomponent coassembly to synthesize Pt nanoparticle-decorated Si-doped WO3 nanowires interwoven into 3D mesoporous superstructures (Pt/Si-WO3 NWIMSs) by using amphiphilic poly(ethylene oxide)-block-polystyrene (PEO-b-PS), Keggin polyoxometalates (H4SiW12O40) and hydrophobic (1,5-cyclooctadiene)dimethylplatinum(II) as the as structure-directing agent, tungsten precursor and platinum source, respectively. The Pt/Si-WO3 NWIMSs exhibit a unique mesoporous structure consisting of 3D interwoven Si-doped WO3 nanowires with surfaces homogeneously decorated by Pt nanoparticles. Because of the highly porous structure, excellent transport of carriers in nanowires, and rich WO3/Pt active interfaces, the semiconductor gas sensors based on Pt/Si-WO3 NWIMSs show excellent sensing properties toward ethanol at low temperature (100 °C) with high sensitivity (S = 93 vs 50 ppm), low detection limit (0.5 ppm), fast response–recovery speed (17–7 s), excellent selectivity, and long-term stability.

Rationally Designed Dual‐Mesoporous Transition Metal Oxides/Noble Metal Nanocomposites for Fabrication of Gas Sensors in Real‐Time Detection of 3‐Hydroxy‐2‐Butanone Biomarker

AbstractTransition metal oxides/noble metal (TMOs/NM) nanocomposites are one kind of important material for semiconductor gas sensors. The controllable construction of a highly connected mesoporous structure and easily accessible active sites is essential for gas sensing performance but remains a great challenge. Herein, a soluble mercapto phenolic resin polymer mediated co‐assembly approach is proposed for the construction of ordered dual mesoporous structure and the simultaneous loading of highly dispersed noble metal nanoparticles. The home‐made soluble mercapto phenolic resin polymer enabled the co‐assembly of transition metal precursors, noble metal precursors, and poly(ethylene oxide)‐block‐polystyrene (PEO‐b‐PS) micelles, resulting in a straightforward synthesis of ordered dual‐mesoporous TMOs/NM nanocomposites (e.g., WO3/Au, TiO2/Au, NbOx/AuPd). As proof of the concept, the synthesized dual‐mesoporous WO3/Au materials are applied for sensing of 3‐hydroxy‐2‐butanone, a biomarker of food‐borne pathogenic bacteria Listeria monocytogenes. The sensors exhibit high sensitivity (Ra/Rg = 18.8 to 2.5 ppm) and high selectivity based on their noble metal sensitization and superior mesopore connectivity for gas diffusion. Furthermore, the synthesized gas sensors are integrated into a wireless sensing module connected to a smartphone, providing a rapid and convenient real‐time detection of 3‐hydroxy‐2‐butanone.

Hierarchical Branched Mesoporous TiO2-SnO2 Nanocomposites with Well-Defined n-n Heterojunctions for Highly Efficient Ethanol Sensing.

The direct assembly of functional nanoparticles into a highly crystalline mesoporous semiconductor with oriented configurations is challenging but of significance. Herein, an evaporation induced oriented co‐assembly strategy is reported to incorporate SnO2 nanocrystals (NCs) into a 3D branched mesoporous TiO2 framework by using poly(ethylene oxide)‐block‐polystyrene (PEO‐b‐PS) as the template, SnO2 NCs as the direct tin source, and titanium butoxide (TBOT) as the titania precursor. Owing to the combined properties of ultrasmall particle size (3–5 nm), excellent dispersibility and presence of abundant hydroxyl groups, SnO2 NCs can easily interact with PEO block of the template through hydrogen bonding and co‐assemble with hydrolyzed TBOT to form a novel hierarchical branched mesoporous structure (SHMT). After calcination, the obtained composites exhibit a unique 3D flower‐like structure, which consists of numerous mesoporous rutile TiO2 branches with uniform cylindrical mesopores (≈9 nm). More importantly, the SnO2 NCs are homogeneously distributed in the mesoporous TiO2 matrix, forming numerous n–n heterojunctions. Due to the unique textual structures, the SHMT‐based gas sensors show excellent gas sensing performance with fast response/recovery dynamics, high sensitivity, and selectivity toward ethanol.

Ordered mesoporous CoO/CeO2 heterostructures with highly crystallized walls and enhanced peroxidase-like bioactivity

With increasing requirements of replacing the high-cost and unstable natural enzymes, the rational design of highly efficient inorganic mimetic enzymes (nanozymes), as one of the most important bioactive materials, has attracted more and more attention in recent years. In our work, using the solvent evaporation induced co-assembly strategy with high-molecular weight poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer as the template and two metal salts as the co-precursors in one-pot system, we synthesized ordered mesoporous heteroatom-doped CeO2 with high surface area, bimodal pore sizes and highly crystallized walls. The composite materials were then constructed as the mimetic enzymes to investigate their peroxidase-like catalytic performance and detect H2O2 and glucose as bioactive materials in diverse systems including complex biological samples. It was demonstrated that mesoporous 5%CoO/CeO2 catalyst was the best peroxidase mimic, following the typical Michaelis–Menten kinetics and exhibiting the excellent peroxidase-like catalytic performance toward 3,3′,5,5′-tetramethylbenzidine (TMB) with comparable Km and Vm values to horseradish peroxidase (HRP) enzyme. A highly sensitive and selective colorimetric assay was developed to detect H2O2 and glucose with detection limits of 0.88 and 19.6μM, respectively. The promising catalytic behavior of mesoporous 5%CoO/CeO2 as the peroxidase-like bioactive catalyst could be ascribed to its interconnected large mesopores, rich exposed surface defects and oxygen vacancies introduced by the heterojunction interface of ultrasmall CeO2 and CoO nanocrystals within pore walls.

Surface Engineering of Spherical Metal Nanoparticles with Polymers toward Selective Asymmetric Synthesis of Nanobowls and Janus-Type Dimers.

New synthetic methods capable of controlling structural and compositional complexities of asymmetric nanoparticles (NPs) are very challenging but highly desired. A simple and general synthetic approach to designing sophisticated asymmetric NPs by anisotropically patterning the surface of isotropic metallic NPs with amphiphilic block copolymers (BCPs) is reported. The selective galvanic replacement and seed-mediated growth of a second metal can be achieved on the exposed surface of metal NPs, resulting in the formation of nanobowls and Janus-type metal-metal dimers, respectively. Using Ag and Au NPs tethered with amphiphilic block copolymers of poly(ethylene oxide)-block-polystyrene (PEO-b-PS), anisotropic surface patterning of metallic NPs (e.g., Ag and Au) is shown to be driven by thermodynamical phase segregation of BCP ligands on isotropic metal NPs. Two proof-of-concept experiments are given on, i) synthesis of Au nanobowls by a selective galvanic replacement reaction on Janus-type patched Ag/polymer NPs; and ii) preparation of Au-Pd heterodimers and Au-Au homodimers by a seed-mediated growth on Janus-type patched Au/polymer NPs. The method shows remarkable versatility; and it can be easily handled in aqueous solution. This synthetic strategy stands out as the new methodology to design and synthesis asymmetric metal NPs with sophisticated topologies.

A Micelle Fusion-Aggregation Assembly Approach to Mesoporous Carbon Materials with Rich Active Sites for Ultrasensitive Ammonia Sensing.

Nanostructured carbon materials have received considerable attention due to their special physicochemical properties. Herein, ordered mesoporous carbons (OMCs) with two-dimension (2D) hexagonal mesostructure and unique buckled large mesopores have successfully been synthesized via a micelle fusion-aggregation assembly method by using poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymers as a template and resorcinol-based phenolic resin as a carbon precursor. The obtained ordered mesoporous carbons possess unique fiber-like morphology, specific surface area of 571-880 m(2)/g, pore volume of 0.54 cm(3)/g and large mesopores (up to 36.3 nm) and high density of active sites (i.e., carboxylic groups) of 0.188/nm(2). Gas sensor based on the ordered mesoporous carbons exhibits an excellent performance in sensing NH3 at a low temperature with fast response (<2 min), ultralow limit of detection (<1 ppm), and good selectivity, due to the large pore sizes, high surface area and rich active sites in the carbon pore walls.

Controlled Synthesis of Ordered Mesoporous Carbon-Cobalt Oxide Nanocomposites with Large Mesopores and Graphitic Walls

Ordered mesoporous carbon (OMC)-metal oxide composites have attracted great interest due to their combination of high surface area, uniform pores, good conductivity of mesoporous carbon, and excellent photo-, electro- and chemical sensing properties of metal oxides. Herein, OMC-metal oxide composites with large mesopores and monodispersed CoOx nanoparticles were synthesized via a controllable multicomponent cooperative coassembly of ultrahigh-molecular-weight poly(ethylene oxide)-block-polystyrene (PEO-b-PS) copolymers, resol (soluble phenoic resin carbon precursor), and cobalt nitrate (cobalt oxide precursor). The obtained nanocomposites possess a face-centered cubic (fcc) mesoporous structure, large pore size (13.4–16.0 nm), high surface area (394–483 m2/g), large pore volume (0.41–0.48 cm3/g), and uniform CoOx nanoparticles with tunable diameters (6.4–16.7 nm). The long chain length of amphiphilic PEO-b-PS template molecules contributes to large mesopores and thick pore walls that allow a controllable ...

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

In this work, a solution of poly(ethylene oxide)-block-polystyrene (PEO-b-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 in...

Adsorption kinetics and stability of poly(ethylene oxide)-block-polystyrene micelles on polystyrene surface

Herein, adsorption kinetics of poly(ethylene oxide)-block-polystyrene (PEO-b-PS) micelles from aqueous solution on PS thin films was investigated by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). The stability of adsorbed micelles against water washing was enhanced by the strong physical interaction between the substrate-identical core blocks of micelles and the PS substrate. The adsorption kinetics was investigated and analyzed by a model considering both the effects of diffusion and micelles reorganization on the PS surface. The micelles adsorption process was well captured by this model which demonstrated that the micelles reorganized on the surface after adsorption. The fitting results exhibited that the micelles with longer core blocks had less tendency to reorganize on the surface while reorganization took more part in the adsorption process of the micelles with shorter core blocks. In the stability studies, micelles with shorter PS blocks could partially desorb from the PS substrate while micelles with longer PS blocks were totally hindered from desorption. Desorption process of micelles was evaluated by the hydrodynamics-induced rolling model. The results indicated that hydrodynamic force-induced partial slip of the micelles on the PS surface might be responsible for micelle desorption. Because of the hydrophilicity of adsorbed micelle layer, adsorption amount of BSA on the modified surface was greatly reduced comparing with that of the bare PS surface.

Structural evolution of low-molecular-weight poly(ethylene oxide)-block-polystyrene diblock copolymer thin film.

The structural evolution of low-molecular-weight poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer thin film with various initial film thicknesses on silicon substrate under thermal annealing was investigated by atomic force microscopy, optical microscopy, and contact angle measurement. At film thickness below half of the interlamellar spacing of the diblock copolymer (6.2 nm), the entire silicon is covered by a polymer brush with PEO blocks anchored on the Si substrate due to the substrate-induced effect. When the film is thicker than 6.2 nm, a dense polymer brush which is equal to half of an interlamellar layer was formed on the silicon, while the excess material dewet this layer to form droplets. The droplet surface was rich with PS block and the PEO block crystallized inside the bigger droplet to form spherulite.

Amine-functionalized nanoporous thin films from a poly(ethylene oxide)-block-polystyrene diblock copolymer bearing a photocleavable o-nitrobenzyl carbamate junction

The synthesis of a poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer bearing an o-nitrobenzyl carbamate junction is reported via a one-pot atom transfer radical polymerization (ATRP)–copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC). The accordingly obtained block copolymer was self-assembled into thin films showing, after solvent annealing, ordered PEO cylinders oriented normal to the film surface and embedded in a PS matrix. In the next step, the easy photocleavage of the o-nitrobenzyl carbamate junction and the subsequent removal of the PEO nanophase were demonstrated. This led to amine-functionalized nanoporous PS thin films as evidenced by the successful coupling of a fluorescent dye bearing a carboxylic acid group. The possibility to obtain fluorescent nanopores was then further exploited to pattern the films using a photolithographic mask.

Poly(ethylene oxide) Crystal Orientation Changes in an Inverse Hexagonal Cylindrical Phase Morphology Constructed by a Poly(ethylene oxide)-block-polystyrene Diblock Copolymer

A poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer with number-average molecular weights of 7.7k g/mol for the PS block and 21.4k g/mol for the PEO block was used to study the PEO crystal orientation changes at different crystallization temperatures (Tx) via small- and wide-angle X-ray scattering techniques. For this diblock copolymer, an inverse hexagonal cylinder (IHC) phase morphology was identified with PS cylinders hexagonally packed within the PEO matrix. In this IHC morphology, the PEO blocks were tethered on the convex interfaces of the PS domains, and the crystallization of PEO blocks was outside of the cylinders. The crystal orientation of the PEO blocks (the c-axis of the PEO crystals) after crystallization among the PS cylinders was, for the first time, found to change with respect to the long cylinder axis, â, depending solely on Tx. At very low Tx's, when the samples were quenched into liquid nitrogen, the crystals possessed a random orientation. When −30 °C ≤ Tx ≤ 5 °C, P...

Onsets of Tethered Chain Overcrowding and Highly Stretched Brush Regime via Crystalline−Amorphous Diblock Copolymers

Lamellar single crystals grown in dilute solutions can be used as templates for tethered chain analysis. Two series of diblock copolymers, poly(ethylene oxide)-block-polystyrene (PEO-b-PS) and poly(l-lactic acid)-block-polystyrene (PLLA-b-PS), were used as model templates to generate tethered PS blocks on the single-crystal basal surfaces. Controlled and tunable reduced tethering density, σ, defined by σπRg2 (where σ is the tethered chain density and is equal to the reciprocal of the covered area of the chain and Rg is the radius of gyration of this tethered chain in its end-free state at the same conditions), could be achieved in a broad range (up to 24) by changing the molecular weights (MW's) of the crystalline and amorphous blocks and by varying the crystallization temperature (Tx) of different PEO-b-PS and PLLA-b-PS solutions. For PEO and PLLA homopolymers crystallized in dilute solutions, the lamellar crystal thicknesses (dCRYST) were observed to be proportional to the reciprocal undercooling ΔT (where ΔT = Td − Tx and Td is the equilibrium dissolution temperature of the crystals). The σ of the tethered PS chains on the crystal surface increased with decreasing ΔT because at a fixed MW of the PEO or PLLA block, an increase in the dCRYST was evidence of a decrease in the number of folds. When we plotted the relationships between 1/dCRYST and Tx for these two series of diblock copolymers, sudden and discontinuous changes of the slopes in some of these were observed at σ = 3.7 (σ*). This was as a result of the drastic interaction change of the neighboring PS tethered chains. An average reduced surface free energy of the tethered PS chains (ΓPS) was defined and used as a parameter to characterize the PS tethered chain interactions. The relationship between ΓPS and σ showed a discontinuous transition at σ*, which had a close similarity to the hard-sphere-like interaction model. This could be identified as the onset of the tethered PS chain overcrowding in solution. This transition indicates that the extra entropic surface free energy created by the repulsion of tethered PS chains started to affect the nucleation barrier of the PEO or PLLA block crystallization. On the basis of the scaling laws, the onset of highly stretched brush regime could be identified at σ = 14.3 (σ**). In the ΓPS vs σ plot, the transition appears to be continuous. Thus, a crossover regime in the tethered PS chains exists between σ* = 3.7 and σ** = 14.3. It is defined as the regime where the interaction of the tethered PS chains undergoes changes from being noninteracting toward penetration to, finally, chain stretching normal to the surface.

“Chemically Shielded” Poly(ethylene oxide) Single Crystal Growth and Construction of Channel-Wire Arrays with Chemical and Geometric Recognitions on a Submicrometer Scale

A series of poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymers were used to generate nucleation sites for the crystal growth of a homo-PEO fraction in solution. The number-average molecular weights of the PEO blocks (MnPEO) were similar, and the number-average molecular weights of the PS blocks (MnPS) ranged from 4.6K to 17K g/mol. In PEO-b-PS/(chlorobenzene/octane) solutions, square-shaped single crystals bounded by four {120} planes were isothermally grown and observed with transmission electron and atomic force microscopy. A “sandwich” lamellar structure, constructed by a PEO single crystal layer covered by two tethered PS block layers on the top and bottom crystal basal surfaces, was found. These diblock copolymer single crystals were used as seeds to grow homo-PEO (MnPEO = 56K g/mol) single crystals in amyl acetate. When the MnPS in the block copolymer was 4.6K g/mol, the edges and corners of the {120} bounded PEO-b-PS single crystals served as nucleation sites to initiate the further growth of the homo-PEO single crystal. As the MnPS of the block copolymers increased, the homo-PEO crystal growth was increasingly hampered along the {120} edges of the PEO-b-PS single crystals. When the MnPS of the block copolymer was 17K g/mol, only the four corners of the PEO-b-PS single crystal could still act as nucleation sites. The four edges were chemically “shielded” by the tethered PS blocks. This indicates that increasing the MnPS led to a higher reduced tethering density of the PS blocks on both the basal surfaces of the PEO-b-PS single crystals. The repulsion generated among the tethered PS blocks caused the PS blocks located near and at the edges to advance along the [120] direction. Interestingly, this local environment only accepted the PEO-b-PS molecules but rejected the homo-PEO molecules from further growth. As a direct result of this study, novel channel-wire arrays on a submicrometer length scale having chemical and geometric recognitions could be fabricated via alternating crystal growth of PEO-b-PS and homo-PEO. This fabrication provided robustly controlled arrays with spacing down to 50 nm.

Initial-Stage Growth Controlled Crystal Orientations in Nanoconfined Lamellae of a Self-Assembled Crystalline−Amorphous Diblock Copolymer

Crystallization temperature (Tc)-dependent crystal orientations within nanoconfined lamellae have been studied in a self-assembled poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer. The copolymer possesses number-average molecular weights of M̄nPEO = 8.7K and M̄nPS = 9.2K, and it forms a microphase-separated lamellar structure in the melt. It has been found that the PEO crystal (the c axis) orientations are determined by only varying the Tc.1 Based on real-time simultaneous two-dimensional (2D) small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) experiments, the formation of crystal orientation is found in an early stage of PEO crystallization confined between the PS lamellae. To understand whether the crystal orientation is determined during the primary nucleation or crystal growth step, specifically designed self-seeded crystallization experiments are carried out using 2D SAXS and WAXS techniques. Experimental results suggest that primary nuclei (self-seeds) do not possess specific orientation with respect to the PS lamellar surface normal, disregarding the history of crystal orientation in the samples generated before the self-seeding process. It is found that the initial stage of crystal growth determines the final crystal orientation in the nanoconfined lamellae. Studies of the correlation lengths (apparent crystallite sizes) along both [120] directions of the PEO crystals with various crystal orientations indicate that the PEO crystals formed in nanoconfined lamellae undergo a change from a one-dimensional to a two-dimensional growth with increasing Tc.

Crystallization Temperature-Dependent Crystal Orientations within Nanoscale Confined Lamellae of a Self-Assembled Crystalline−Amorphous Diblock Copolymer

For a lamella-forming poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer (MnPEO = 8.7K and MnPS = 9.2K), the glass transition temperature of the PS blocks is 62 °C, and the melting temperature of the PEO crystals is around 51 °C when the sample is crystallized below 40 °C. The PEO blocks thus crystallize in a one-dimensionally confined lamellar space of 8.8 nm, as studied recently by one-dimensional small-angle X-ray scattering (SAXS) and transmission electron microscopy. In this report, the crystal orientation (the c-axis of the PEO crystals) within nanoscale confined lamellae has been investigated using combined two-dimensional SAXS and wide-angle X-ray scattering experiments. The c-axis orientation in the PEO crystals is observed for the first time to change from random to perpendicular, then to inclined, and finally to parallel to the lamellar surface normal, depending only on the crystallization temperature (Tc). Detailed crystallographic analyses indicate that the c-axis orientati...