Table-salt enabled interface-confined synthesis of covalent organic framework (COF) nanosheets

Two-dimensional covalent organic frameworks (COFs) are gaining tremendous interest for their potential applications in a diversity of fields. However, synthesis of COF nanosheets (CONs) usually suffers from tedious exfoliation processes and low yields. Herein, we present an exfoliation-free and scalable strategy to prepare few-layered CONs based on interface-confined synthesis, in which cheap and recyclable table salt (NaCl) is used as the sacrificial substrate. Salt particles are introduced into the reaction system, creating billions of solid–liquid interfaces. Oligomers formed upon the reaction between monomers are immediately adsorbed on salt surfaces, and the following polymerization leading to crystalline CONs is exclusively confined to salt surfaces. Salts can be easily removed by water washing, producing CONs with the thickness down to a few nanometers and lateral sizes up to hundreds of micrometers depending on the size of salt particles and the concentration of monomers. Four different kinds of CONs, both imine-linked and boron-containing, are synthesized from this generic method. As a demonstration, we prepare highly permeable and selective membranes using resultant CONs as building blocks. Thanks to the defect-free stacking of CONs with thin thicknesses and large lateral sizes on porous substrates, the membranes precisely separate similarly sized dyes while allowing ultrafast water permeation. This interface-confined strategy opens a new platform for the controllable and scalable synthesis of COF nanosheets and is essential for the burgeoning real-world applications of COFs in various fields.

We present an exfoliation-free and scalable strategy to prepare few-layered CONs by the interface-confined synthesis. The resultant CONs are assembled into selective layers for molecular separations.     

Self‐assembly of metal–organic framework thin films containing metalloporphyrin and their photocatalytic activity under visible light

With [5,10,15,20‐tetra(4‐carboxyphenyl)porphyrin]Mn(III) and sterically controlled 2,2¢‐dimethyl‐4,4¢‐pyridine as the main raw materials, metal–organic framework thin films containing metalloporphyrin (MnPor‐MOF) with catalytically active sites were built up on functionalized quartz glass surfaces using a layer‐by‐layer self‐assembly method. Retaining active catalytic sites and having a porous reticular structure, the MnPor‐MOF films possessed remarkable photocatalytic activity for oxidative degradation of methylene blue in the presence of hydrogen peroxide under visible‐light irradiation. Most meaningfully, the MnPor‐MOF films were highly stable and simply and conveniently reusable, and are thus a potentially new organic material for photocatalytic wastewater treatment. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermally curable organic/inorganic hybrid polymers as gate dielectrics for organic thin‐film transistors

New low‐temperature curable organic/inorganic hybrid polymers were designed and synthesized as gate dielectrics for organic thin‐film transistors (OTFTs). Allyl alcohols were introduced to polyhedral oligomeric silsesquioxane (POSS) via hydrosilyation to produce an alcohol‐functionalized POSS derivative (POSS‐OH). POSS‐OH was then reacted with hexamethoxymethylmelamine at carrying molar ratios at 80 °C in the presence of a catalytic amount of p‐toluenesulfonic acid to give highly cross‐linked network polymers (POSS‐MM). The prepared thin films were smooth and hard after the thermal cross‐linking reaction and had very low leakage currents (<10^?8 A/cm²) with no significant absorption over the visible spectral range. Pentacene‐based OTFTs using the synthesized insulators as gate dielectric layers had higher hole mobilities (up to 0.36 cm²/Vs) than a device using thermally cross‐linked poly(vinyl phenol) and melamine as the gate dielectric layer (0.18 cm²/Vs). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3260–3268     

Quantitative analysis of an organic thin film by XPS,AFM and FT‐IR

We report the characterization of Firpic (iridium(III)bis[4,6‐di‐fluorophenyl]‐pyridinato‐N,C²,]picolinate) organic thin film prepared by vacuum deposition to provide a systematic route to organic film quantification. To analyze the characteristics of thin Firpic films on a Si substrate, various techniques such as XPS, Fourier transform infra‐red (FT‐IR) spectrometer, and atomic force microscopy (AFM) are utilized. The Firpic films remain stable without surface morphological or compositional change during deposition and after exposure to X‐ray irradiation or atmospheric environment, for which qualities these films are believed to be an ideal platform as a pure organic thin film. The monotonic increases in FT‐IR and XPS intensities with film thickness are matching well with each other. In particular, from the XPS intensity analysis, the relative atomic sensitivity factors of the present system, electron attenuation length, and molecular density in the organic thin film can be evaluated. Copyright © 2011 John Wiley & Sons, Ltd.     

Patterning of spontaneous rolling thin polymer films for versatile microcapillaries

We investigate the spontaneous rolling of polydimethylsiloxane (PDMS) thin films and demonstrate the fabrication of capillaries with topographical and chemical patterns on the inner wall. Thin films of PDMS are either coated by a layer of hard material or have their surface hardened by plasma oxidation. They are then driven out of equilibrium by selective solvent swelling in vapor phase resulting in a tubular rolled‐up system. The inner diameter of those is measured as a function of layer thickness for different solvents and capping types. Those results are shown to be in good agreement with Timoshenko theory. Before rolling, the future inner surface can be characterized and functionalized. We demonstrate topographical and chemical patterning, respectively by embossing and microcontact printing. These methods are very simple and can easily produce cylindrical capillaries with inner diameter between 20 and some hundreds of microns with fully functionalized inner surface, overcoming many difficulties encountered in conventional soft lithography techniques. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 721–728     

New colorless substrates based on polynorbornene‐chlorinated polyimide copolymers and their application for flexible displays

Novel polynorbornene (PNB)‐polyimide (PI) copolymers were synthesized based on poly(N‐phenyl‐exo‐norbornene‐5,6‐dicarboximide) (PPhNI) and chlorinated PI (BPDA/TCDB). Polynorbornene copolymers (PNCs) with diverse compositions of anhydride were synthesized via ring opening metathesis polymerization (ROMP) of N‐phenyl‐exo‐norbornene‐5,6‐dicarboximide (PhNI) and exo‐7‐oxanorbornene‐5,6‐dicarboxylic anhydride(exo‐NA), followed by copolymerization through a reaction with aromatic dianhydride (3,3′,4,4′‐biphenyltetra‐carboxylic dianhydride, BPDA) and tetrachlorinated diamine (2,2′,5,5′‐tetrachlorobenzidine, TCDB). The copolymer (PNIC) films exhibited good optical transparency with a transmittance of around 70% at 400 nm and a good thermal stability with a glass transition temperature at 276–300 °C. These flexible films also resisted most organic solvents and chemicals, such as methanol, acetone, tetrahydrofuran, N‐methylpyrrolidone, ethyl acetate, hydrochloric acid, sodium hydroxide, and hydrogen peroxide, etc. Indium tin oxide (ITO) coated thin films were prepared at various substrate deposition temperatures with a radio frequency (r.f.) planar magnetron sputtering system. The ITO thin films that were deposited onto the PNIC copolymer substrates had good electrical and optical properties. An organic light‐emitting device (OLED) was fabricated using the PNIC copolymer substrate with a structure of PNIC08/ ITO (anode)/hole‐transporting layer (HTL)/emitting & electron‐transporting layer (EM&ETL)/aluminum (cathode). The flexible OLED fabricated on the ITO‐grown PNIC substrate exhibited a performance that was comparable to corresponding ITO‐grown glass substrates. Therefore, the ITO‐grown PNIC substrate could possibly be a promising candidate as a substrate for flexible displays. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1806–1814, 2010     

Layer‐by‐layer ionomer films made from poly(styrene‐co‐styrenesulfonic acid) and poly(methyl methacrylate‐co‐4‐vinylpyridine) in tetrahydrofuran

Thin films were fabricated layer‐by‐layer (LbL) via ionic bonds formed between a cationic ionomer and an anionic ionomer, which were produced via proton transfer from poly(styrene‐co‐styrenesulfonic acid) to poly(methyl methacrylate‐co‐4‐vinylpyridine) in an organic solvent, tetrahydrofuran. Ionic contents of the ionomers were very low down to 5.6 mol %, much lower than usual polyelectrolytes. The build up of the LbL films was demonstrated by UV/vis spectroscopy: the absorbance of the phenyl rings in styrene residues increased with the number of depositions (thus the number of layers). Transmission electron microscopy observation of strained thin films showed unique deformation mode, involving many bands that developed both in the parallel and perpendicular directions to the stress axis. This is quite different from the deformation modes seen for ionomer blend films and for coextruded polystyrene/poly(methyl methacrylate) multilayer tapes. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 101–105, 2012     

Light‐Induced Solubility Modulation of Polyfluorene To Enhance the Performance of OLEDs

Liquid‐phase processing is a key prerequisite for the cost‐efficient fabrication of organic electronic devices. We report an approach for light‐induced modulation of the solubility of π‐conjugated polymers (polyfluorene) with side chains functionalized with hydroxycinnamic acid. Irradiation with light cleaves the solubilizing side chains and renders the thin films of the polyfluorene insoluble. In a proof of concept device, polyfluorenes were applied as emissive layers in OLEDs. Photoirradiation of the emission layer leads to an increase in OLED performance combined with a modulation of the solubility of the thin film. These results offer the possibility for further development in terms of manipulating the solubility and emissive parameters of an important class of functional materials.     

Orientation control in nanoparticle filled block copolymer cold zone annealed films

Nanoparticles provide an attractive route to modifying polymer thin film properties, yet controlling the dispersion and morphology of functionalized nanoparticle filled films is often difficult. Block copolymers can provide an ideal template for directed assembly of nanoparticles under controlled nanoparticle‐polymer interactions. Previously we observed that neat films of cylinder forming poly(styrene‐b‐methyl methacrylate) PS‐b‐PMMA block copolymer (c‐BCP) orient vertically with dynamic sharp thermal cold zone annealing (CZA‐S) over wide range of CZA‐S speed (0.1–10) μm/s. Here, we introduce a low concentration (1–5 wt %) of nanoparticles of phenolic group functionalized CdS (fCdS‐NP), to PMMA cylinder forming polystyrene‐b‐poly (methyl methacrylate) block copolymer (c‐BCP) films. Addition of the fCdS‐NP induces a vertical to horizontal orientation transition at low CZA‐S speed, V = 5 μm/s. The orientation flip studies were analyzed using AFM and GISAXS. These results confirm generality of our previously observed orientation transition in c‐BCP under low speed CZA‐S with other nanoparticles (gold [Au‐NP], fulleropyrrolidine [NCPF‐NP]) in the same concentration range, but reveal new aspects not previously examined: (1) A novel observation of significant vertical order recovery from 5–10% vertical cylindrical fraction at V = 5 μm/s to 46–63% vertical cylindrical fraction occurring at high CZA‐S speed, V = 10 μm/s for the fCdS nanoparticle filled films. (2) We rule out the possibility that a nanoparticle wetting layer on the substrate is responsible for the vertical to horizontal flipping transition. (3) We demonstrate that the orientation flipping results can be achieved in a nanoparticle block copolymer system where the nanoparticles are apparently better‐dispersed within only one (matrix PS) domain unlike our previous nanoparticle system studied. We consider facile processing conditions to fabricate functionalized nanoparticles filled PS‐PMMA block copolymer films with controlled anisotropy, a useful strategy in the design of next generation electronic and photonic materials. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 604–614     

The synthesis and properties of CaCu3Ti4O12 thin films

The CaCu₃Ti₄O₁₂ (CCTO) thin films were synthesized via a metal‐organic solution containing stoichiometric amounts of the metal cations at 700 °C for 1 h. The stable metal‐organic solution was prepared by dissolving calcium nitrate, copper nitrate, and tetrabuty titanate in grain alcohol. The phases, microstructures, and electric properties of CCTO thin films were characterized by X‐ray diffraction, scanning electron microscopy, atomic force microscope, and electric measurements. The results show that the CCTO thin films have homogeneous microstructure, smooth surface, low leakage current, and high values of dielectric constant. The low leakage current can be attributed to the small surface roughness. The high value of dielectric constant can be attributed to the internal barrier layer capacitor mechanism and metal‐insulator‐semiconductor junction of CCTO thin films. Copyright © 2013 John Wiley & Sons, Ltd.     

Crystallinity of adsorbed EVA nanofilms: relevance to confinement and interfacial interactions

We used infrared spectroscopy to estimate the crystalline component concentration in thin films of ethylene‐co‐vinyl acetate (EVA) copolymers spin‐coated on gold and functionalized substrates. Bulk calibration methods are elaborated that allow the establishment of quantitative relationships between infrared response and the degree of crystallinity via simple equations. These equations are applied and transposed to the study of thin films in such a way as to avoid preferential orientation effects induced by adsorption. Therefore, for calibration we select groups that vibrate in the same direction and belong to the crystalline phase. Our results show a decrease of the crystalline amount in EVA9, 14, 24 and 28 thin films, as determined on the basis of infrared spectroscopy experiments. The low crystallinity degree calculated for EVA thin films is in good agreement with the reduced crystalline morphology observed by atomic force microscopy. Observed amorphization is considered to be induced by confinement (gold substrates) and interfacial interactions (functionalized substrates), which lead to a disordered chain conformation that makes crystallization less favourable. Copyright © 2003 John Wiley & Sons, Ltd.     

Organic vapor sensing properties and characterization of α-naphthylmethacrylate LB thin films

Determination of organic vapor sensing properties of α-Naphthylmethacrylate (α-NMA) monomer based Langmuir-Blodgett (LB) thin films was aimed in this study. LB thin film fabrication was performed on quartz glass and quartz crystal substrates in order to investigate the characterization and organic vapor properties of α-NMA materials by using UV-Visible, Atomic Force Microscopy (AFM) and Quartz Crystal Microbalance (QCM) techniques. π-A isotherm graph was taken and a suitable surface pressure value were primarily determined as 13?mN m^?1 for successful α-NMA LB thin film fabrication. Transfer ratio value was found to be ≥ 0.93 for quartz glass and quartz crystal substrates. The typical frequency shift per layer was obtained as 16.93?Hz/layer and the deposited mass onto a quartz crystal was calculated as 271.30?ng/layer (1.02?ng mm^?2). The sensing responses of α-NMA LB films against dichloromethane, chloroform, toluene and m-xylene were measured by QCM system. Dichloromethane created the maximum shift in the resonance frequency than other organic vapors used in this study. Results exhibited that α-NMA LB thin films were potential candidates for organic vapor sensing applications, especially high sensitive detection of dichloromethane at room temperature.     

Functionalized Nanoporous Thin Films From Photocleavable Block Copolymers

A polystyrene‐block‐poly(ethylene oxide) block copolymer bearing a photocleavable junction between the blocks is used to form nanoporous thin films with carboxylic acid functions homogeneously distributed on the pore walls. The presence of the carboxylic acid groups is evidenced by fluorescence spectroscopy after their reaction with a diazomethane functionalized fluorescent dye. In addition, the initial light‐responsive thin film, acting as a photoresist, can be easily patterned to selectively generate porosity in predetermined areas. In that way, fluorescent patterns can be obtained as evidenced by fluorescent microscopy.     

Charge transport and contact resistance in coplanar devices based on colloidal polyaniline dispersion

The charge transport properties of thin films prepared from colloidal dispersion of polyaniline stabilized by poly(N‐vinylpyrrolidone) (PANI/PVP) have been investigated. The electrical characterization of coplanar device comprising of gold electrodes and PANI/PVP film deposited by spin coating served to gain insights into the contact and bulk resistance. The films prepared from PANI/PVP colloidal dispersion show high stability over a large temperature range. Temperature dependent measurements in the range from 90 to 350 K reveal that the charge transport can be described by a three‐dimensional variable‐range hopping mechanism as the dominant mode in the films. The stability of the films cast from dispersion within a large temperature range opens the possibility of the application as a polymer semiconductor layer in sensors and charge‐transport interlayer in organic solar cells. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1710–1716     

Edge‐Selectively Functionalized Graphene Nanoplatelets

Graphene, as a single layer of graphite, is currently the focal point of research into condensed matter owing to its promising properties, such as exceptional mechanical strength, high thermal conductivity, large specific surface area, and ultrahigh electron‐transport properties. Therefore, various physical and chemical synthetic procedures to prepare graphene and/or graphene nanoplatelets have been rapidly developed. Specifically, the synthesis of edge‐selectively functionalized graphene (EFG) has been recently reported by using simple and scalable approaches, such as “direct” Friedel‐Crafts acylation reactions in a mild acidic medium and a mechanochemical ball‐milling process. In these approaches, chemical functionalization predominantly take place at the edges of the graphitic layers via the covalent attachment of targeted organic “molecular wedges”. In addition, the distortion of the crystalline structures in the basal plane, which is beneficial for preserving the unique properties of the graphitic framework, can be minimized. In addition, the efficient exfoliation of graphene can be achieved, owing to the strong repulsive forces from the covalently linked wedges and strong shear forces during the reaction. Furthermore, EFG shows promising potential in many useful applications, such as highly conductive large‐area films, metal‐free electrocatalysts for the oxygen‐reduction reaction (ORR), and as additives in composite materials with enhanced properties. Herein, we summarize the recent progress and general aspects of EFG, including synthesis, reaction mechanism, properties, and applications.     

Phase separation of polymer‐functionalized SWNTs within a PMMA/polystyrene blend

Phase separation of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends was used as a means to segregate PS‐ or PMMA‐functionalized single‐walled carbon nanotubes (SWNTs) in thin films. Dilute solutions (5 wt % in THF) of 1:1 PS/PMMA blends containing the functionalized nanotubes were spin cast and annealed at 180 °C for 12 h. Two different polymer molecular weights were used (M_n = 8000 or M_n = 22,000), and were of approximately equivalent molecular weight to those attached to the surface of the nanotubes. Nanotube functionalization was accomplished using the Cu(I)‐catalyzed [3 + 2] Huisgen cycloaddition, in which alkyne‐decorated nanotubes were coupled with azide‐terminated polymers, resulting in polymer‐SWNT conjugates that were soluble in THF. Characterization of the annealed films by scanning Raman spectroscopy, which utilized the unique Raman fingerprint of carbon nanotubes, enabled accurate mapping of the functionalized SWNTs within the films relative to the two phase‐separated polymers. It was found that nanotube localization within the phase‐separated polymer films was influenced by the type of polymer attached to the nanotube surface, as well as its molecular weight. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 450–458, 2009     

(110)‐Oriented ZIF‐8 Thin Films on ITO with Controllable Thickness

(110)‐oriented zeolitic imidazolate framework (ZIF)‐8 thin films with controllable thickness are successfully deposited on indium tin oxide (ITO) electrodes at room temperature. The method applied uses 3‐aminopropyltriethoxysilane (APTES) in the form of self‐assembled monolayers (SAMs), followed by a subsequent adoption of the layer‐by‐layer (LBL) method. The crystallographic preferential orientation (CPO) index shows that the ZIF‐8 thin films are (110)‐oriented. A possible mechanism for the growth of the (110)‐oriented ZIF‐8 thin films on 3‐aminopropyltriethoxysilane modified ITO is proposed. The observed cross‐sectional scanning electron microscopy (SEM) images and photoluminescent (PL) spectra of the ZIF‐8 thin films indicate that the thickness of the ZIF‐8 layers is proportional to the number of growth cycles. The extension of such a SAM method for the fabrication of ZIF‐8 thin films as described herein should be applicable in other ZIF materials, and the as‐prepared ZIF‐8 thin films on ITO may be explored for photoelectrochemical applications.     

Synthesis and phase‐separation behavior of α,ω‐difunctionalized diblock copolymers

A series of diblock copolymers of n‐pentyl methacrylate and methyl methacrylate (PPMA/PMMA BCP) with one or two terminal functional groups was prepared by sequential anionic polymerization of PMA and MMA using an allyl‐functionalized initiator and/or and end‐capping with allyl bromide. Allyl functional groups were successfully converted into OH groups by hydroboration. The morphology in bulk was examined by temperature‐dependent small‐angle X‐ray measurements (T‐SAXS) and transmission electron microscopy (TEM) showing that functional groups induced a weak change in d‐spacings L₀ as well as in the thermal expansion behavior. T‐SAXS proved that the lamellar morphologies were stable over multiple heating/cooling cycles without order‐disorder transition (ODT) until 300 °C. While non‐functionalized BCP formed parallel lamellae morphologies, additional OH‐termination at the PMMA block forced in very thin films (ratio between film thickness and lamellar d‐spacing below 1) the generation of perpendicular lamellae morphology through the whole film thickness, as shown by Grazing‐incidence small‐angle X‐ray scattering experiments (GISAXS) measurements. Functionalized BCP were successfully used in thin films as templates for silica nanoparticles in an in‐situ sol–gel process. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Ultra‐Fast Degradation of Chemical Warfare Agents Using MOF–Nanofiber Kebabs

The threat associated with chemical warfare agents (CWAs) motivates the development of new materials to provide enhanced protection with a reduced burden. Metal–organic frame‐works (MOFs) have recently been shown as highly effective catalysts for detoxifying CWAs, but challenges still remain for integrating MOFs into functional filter media and/or protective garments. Herein, we report a series of MOF–nanofiber kebab structures for fast degradation of CWAs. We found TiO₂ coatings deposited via atomic layer deposition (ALD) onto polyamide‐6 nanofibers enable the formation of conformal Zr‐based MOF thin films including UiO‐66, UiO‐66‐NH₂, and UiO‐67. Cross‐sectional TEM images show that these MOF crystals nucleate and grow directly on and around the nanofibers, with strong attachment to the substrates. These MOF‐functionalized nanofibers exhibit excellent reactivity for detoxifying CWAs. The half‐lives of a CWA simulant compound and nerve agent soman (GD) are as short as 7.3 min and 2.3 min, respectively. These results therefore provide the earliest report of MOF–nanofiber textile composites capable of ultra‐fast degradation of CWAs.     

Imaging the domain structure of organic semiconductor films

State‐of‐the‐art solution‐processed organic field‐effect transistors typically use polycrystalline organic semiconductor thin films as the active layer. Although it is widely regarded that boundaries between polycrystalline domains are a likely source of charge trapping limiting charge carrier mobility, little is known about the detailed domain structure of such films. Furthermore, variations in local order particularly in conjugated polymer films are likely to further impede charge transport. In recent years a number of techniques have been exploited that are able to provide information regarding local domain orientation and molecular order in polycrystalline organic thin films. These techniques have provided new information regarding the nature of domain structure providing an opportunity to directly evaluate the influence of domain structure on device operation. This article aims to provide a timely review of the experimental approaches used to date and provide a perspective for future work. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011