Detection of layer‐by‐layer self‐assembly multilayer films by low‐temperature plasma mass spectrometry

The detection of layer‐by‐layer self‐assembly multilayer films was carried out using low‐temperature plasma (LTP) mass spectrometry (MS) under ambient conditions. These multilayer films have been prepared on quartz plates through the alternate assembling of oppositely charged 4‐aminothiophenol (4‐ATP) capped Au particles and thioglycolic acid (TGA) capped Ag particles. An LTP probe was used for direct desorption and ionization of chemical components on the films. Without the complicated sample preparation, the structure information of 4‐ATP and TGA on films was studied by LTP‐MS. Characteristic ions of 4‐ATP (M) and TGA (F), including [M]^+?, [M‐NH₂]⁺, [M‐HCN‐H]⁺, and [F + H]⁺, [F‐H]⁺, [F‐OH]⁺, [F‐COOH]⁺ were recorded by LTP‐MS on the films. However, [M‐CS‐H]⁺ and [F‐SH]⁺ could not be observed on the film, which were detected in the neat sample. In addition, the semi‐quantitative analysis of chemical components on monolayer film was carried out, and the amounts of 4‐ATP and TGA on monolayer surface were 45 ng/mm² and 54 ng/mm², respectively. This resulted the ionization efficiencies of 72% for 4‐ATP and 54% for TGA. In order to evaluate the reliability of present LTP‐MS, the correlations between this approach and some traditional methods, such as UV–vis spectroscopy, atomic force microscope and X‐ray photoelectron spectroscopy were studied, which resulted the correlation coefficients of higher than 0.9776. The results indicated that this technique can be used for analyzing the films without any pretreatment, which possesses great potential in the studies of self‐assembly multilayer films. Copyright © 2013 John Wiley & Sons, Ltd.     

Preparation and resistivity study of silver nanocomposite films using spin‐assisted layer‐by‐layer assembly

Nanocomposite films [Ag/(PAH‐PSS)_nPAH]_m were fabricated on a silicon substrate using a time‐ and cost‐efficient spin‐assisted layer‐by‐layer (SA‐LbL) self‐assembly technique. A virtually monolayer‐like layer of self‐assembled silver nanoparticles was formed when deposition time increased to 30 min. It was found that polymer multilayers could effectively decrease the resistivity of silver nanoparticle monolayer, which was far higher than that of bulk silver metal; however, the resistivity of Ag/(PAH‐PSS)_nPAH multilayer films increased along with the increasing of the number of polymer bilayers. XPS investigations showed that silver nanoparticles were partially oxidized, which might be the major cause of the high resistivity of silver nanoparticle monolayer. Copyright © 2008 John Wiley & Sons, Ltd.     

Mechanical property and corrosion resistance of zirconia/polydopamine nanocomposite multilayer films fabricated via a novel non‐electrostatic layer‐by‐layer assembly technique

Zirconia/polydopamine (ZrO₂/PDA) nanocomposite multilayer films were constructed on Si substrate via a novel nonelectrostatic layer‐by‐layer (NELBL) assembly technique. The building block of this technique is the newly reported dopamine molecule, which can be attached to almost all material surfaces and undergo oxidation‐polymerization to form PDA layers; more importantly, the outer hydroxyl groups of the PDA layer can chelated with certain inorganic oxide nanoparticles to generate oxide films. Thus, ZrO₂/PDA nanocomposite multilayer films were fabricated by sequential NELBL deposition of PDA and ZrO₂ nanoparticles. The formation of the ZrO₂/PDA nanocomposite multilayer films was monitored by the water contact angle (WCA) and ellipsometric thickness measurements, while the microstructure of the fabricated films was analyzed by means of atomic force microscope (AFM), field emission scanning electron microscope (FESEM), X‐ray photoelectron spectrum (XPS), and X‐ray diffraction (XRD) analysis. The mechanical and anticorrosion behaviors of the annealed ZrO₂/PDA nanocomposite multilayers were found to be greatly enhanced as compared with that of the annealed homogeneous ZrO₂ film. The better mechanical and anticorrosion behaviors of the annealed ZrO₂/PDA nanocomposite multilayers than the annealed homogeneous ZrO₂ film may be closely related to their special microstructure. Namely, the organic–inorganic hybrid microstructure of the annealed ZrO₂/PDA nanocomposite multilayers may largely account for the increased nanohardness and corrosion resistance. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and magnetic properties of organic multilayer films based on the layer‐by‐layer assembly

Two polymers containing pyridine rings were prepared by free‐radical polymerization and confirmed by Fourier transform infrared and ¹H NMR spectra. The preparation of four multilayer films that were obtained by self‐assembly of the polymer and the transition metal neutralized polyelectrolyte on PE substrate was described. UV–vis spectra and atomic force microscopy images were applied to characterize these films and indicate the uniform assembling process. The driving force for building up the multilayer films was identified by infrared spectroscopy to be the coordination interaction. The magnetic behavior was examined as a function of magnetic field strength at 30 kOe and as a function of temperature (5–300 K). All films display strong soft ferromagnetic properties and higher than those of the bulk materials. The magnetic results show that the layer‐by‐layer self‐assembling approach is beneficial to the ordered alignment of adjacent paramagnetic spins and induces better magnetic phenomena. Copyright © 2015 John Wiley & Sons, Ltd.     

Layer‐by‐layer assembly of gelatin

Electrostatic assembly of one species can be realized using gelatin as a polyampholyte. Under suitable conditions where the electrostatic attraction and repulsion were both significant and in balance, linear growth of multilayers driven by electrostatic interactions was sustained over many successive assembly steps, and the maximum amount of adsorption of each layer was reached when the solution pH was around the isoelectric point. The rearrangement of the adsorbed chains after drying was confirmed by contact angle analysis. In addition with only one species involved, the assembled thin films should be chemically uniform rather than layered. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1252–1257, 2008     

Layer‐by‐layer assembly of readily detachable chitosan and poly(acrylic acid) polyelectrolyte multilayer films

Free‐standing layer‐by‐layer (LbL) assembled thin films have recently found utility in a broad range of applications. Previously reported free‐standing LbL films have generally required covalent modifications to improve aqueous stability and render these films suitable for biomedical applications. Here, we engineered chitosan and poly(acrylic acid) containing polyelectrolyte multilayer films, which are readily detached from hydrophilic silicon in aqueous conditions. These films demonstrate remarkable stability over 28 days in simulated in vivo conditions (pH 7.4, phosphate buffered saline at 37 °C) without the incorporation of any covalent crosslinking modifications. These films exhibit moduli (27–420 kPa) resembling that of many biological tissues including tendon, show high visible light transmittance of greater than 50%, and prevent fibronectin adsorption. The properties of this new detachable LbL film architecture indicate its promise for use in a variety of applications, particularly in medicine and biotechnology. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 127–131     

Preparation and characterization of layer‐by‐layer self‐assembly membrane based on sulfonated polyetheretherketone and polyurethane for high‐temperature proton exchange membrane

A concept of preparing high‐temperature proton exchange membranes with layer‐by‐layer (LBL) self‐assembly technique was proposed and the sulfonated polyetheretherketone (SPEEK) and polyurethane (PU) with 200 LBL deposition cycles denoting (SPEEK/PU)₂₀₀ membrane was prepared in this research. Owing to the strong electrostatic interaction between ? group in SPEEK and ? C? N⁺ group in PU, (SPEEK/PU)₂₀₀ membrane with LBL self‐assembly structure showed a favorable structural stability. The phosphoric acid (PA)‐doped (SPEEK/PU)₂₀₀ membrane showed a higher proton conductivity relative to PA doped SPEEK/PU membrane by solution casting method (SPEEK/PU)₂₀₀/40%PA membrane possessed a proton conductivity value of 2.90 × 10^?2 S/cm at 150 °C under anhydrous conditions. The LBL self‐assembly structure provided a possibility to reduce the negative effect from polymer skeleton blocking charge carrier species even immobilizing protons. Moreover, the (SPEEK/PU)₂₀₀ membrane presented the particularly noteworthy mechanical property even with PA doping. The tensile stress values at break were 72.8 and 24.1 MPa, respectively, for (SPEEK/PU)₂₀₀ and (SPEEK/PU)₂₀₀/40%PA membrane at room temperature, which were obviously higher than the reported values of 15.9 and 2.81 MPa for SPEEK/PU and SPEEK/PU/60%PA membrane. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 3446–3454     

Electropolymerization of layer‐by‐layer precursor polymer films

The layer‐by‐layer (LbL) self‐assembly has been used to fabricate polymer thin films on any solid substrates. The multilayer polymer thin films are constructed by alternating adsorption of anionic and cationic polymers. Polyelectrolyte multilayer ultrathin films containing anionic poly[2‐(thiophen‐3‐yl)ethyl methacrylate‐co‐methacrylic acid]; P(TEM‐co‐MA) and cationic poly[4‐(9H‐carbazol‐9‐yl)‐N‐butyl‐4‐vinyl pyridium bromide]; P4VPCBZ, were fabricated. The growth of multilayer ultrathin films was followed by UV–Vis absorption spectrophotometer and surface plasmon resonance spectroscopy (SPR). The deposition of P(TEM‐co‐MA)/P4VPCBZ as multilayer self‐assembled ultrathin films regularly grow which showed linear growth of absorbance and thickness with increasing the number of layer pair. Cross‐linking of the layers was verified by cyclic voltammetry (CV), UV–Vis spectrophotometry and electrochemical surface plasmon resonance (EC‐SPR) spectroscopy with good electro‐copolymerizability. This was verified by spectroelectrochemistry. The SPR angular‐reflectivity measurement resulted in shifts to a higher reflectivity according to the change in the dielectric constant of the electropolymerized film. Copyright © 2011 John Wiley & Sons, Ltd.     

The fabrication of layer‐by‐layer mode LaCoO3 film by pulsed laser deposition

The relationship between strain and growth conditions in LaCoO₃ thin film was obtained to control the magnetic‐electric characteristics. The LaCoO₃ thin films on the SrTiO₃ substrates have been achieved by the pulsed laser deposition method, and the reflection high‐energy electron diffraction method (RHEED) was applied to monitor the growth process in situ; the layer‐by‐layer growth mode was discovered. The X‐ray diffraction and atomic force microscopy were applied to the phase analysis, and the layer thickness and the layer‐by‐layer growth mode were uncovered. Compared with the 100‐nm LaCoO₃ thin films, the strain in the layer‐by‐layer ultra thin film was more controllable. The enhanced magnetic properties of the layer‐by‐layer mode ultra‐thin films could be tested in future work.     

Functional membranes prepared by layer‐by‐layer assembly and its metal ions adsorption property

PVDF/(PEI‐C/PAA)_n functional membranes were prepared by layer‐by‐layer (LbL) assembly, and their heavy metal ions adsorption capability was investigated. The changes in the chemical compositions of membrane surfaces were determined by X‐ray photoelectron spectroscopy (XPS). XPS results show that the surface of the PVDF membrane can be alternatively functionalized by PEI‐C and PAA. The membrane surface hydrophilicity was evaluated through water contact angle measurement. Contact angle results show that the surface hydrophilicity of the membrane surface depends on the outermost deposited layer. Morphological changes of membrane surfaces were observed by scanning electron microscopy (SEM). The water fluxes for these membranes were elevated after modification. The performances of the PVDF/(PEI‐C/PAA)_n membranes on the adsorption of copper ions (Cu²⁺) from aqueous solutions were investigated by inductively coupled plasma (ICP). The results indicate that the PVDF/(PEI‐C/PAA)_n functional membranes show high copper ions adsorption ability. Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular layer‐by‐layer deposition of highly crosslinked polyamide films

A molecular layer‐by‐layer (mLbL) deposition process is demonstrated to synthesize conformal coatings of crosslinked polyamide. This process controls the rapid reaction of trimesoyl chloride and m‐phenylene diamine, unlike interfacial polymerization techniques which produce rough films and poorly defined network structure. Layer‐by‐layer polyamide films appear structurally similar to interfacially polymerized films with a linear film growth rate of ≈0.9 nm per cycle. Films made by mLbL deposition show a 70‐fold decrease in surface roughness as compared to a commercial, interfacially polymerized polyamide. Surface chemistry could be controlled based on which reaction step was performed last, leading to amine or carboxylic acid rich surfaces. With the ability to control chemical structure throughout the crosslinked network, this technique provides new routes to build polyamide films and improve analysis techniques for commercial applications such as reverse osmosis membranes. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Electrorheological properties of carbon nanotube/polyelectrolyte self‐assembled polystyrene particles by layer‐by‐layer assembly

Sulfonated polystyrene (PS) particles were prepared by the sulfonation of PS microspheres with H₂SO₄. Then, composite particles were synthesized by layer‐by‐layer (LbL) self‐assembly with funtionalized multiwall carbon nanotubes (fMCNTs) and polyelectrolytes on sulfonated PS particles. The amount of fMCNTs on PS particles was adjusted by controlling the number of fMCNT layers by LbL self‐assembly. Composite particles were characterized by ζ‐potential analysis, scanning electron microscopy, and thermal analysis. The electrorheological (ER) properties of composite particles in insulating oil was investigated with varying the number of fMCNT layers under controlled electric fields. It was observed that the number of fMCNT layers was a critical factor to determine the ER properties of composite particles. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1058–1065, 2008     

Influence of graphene oxide incorporation and chemical cross‐linking on structure and mechanical properties of layer‐by‐layer assembled poly(Vinyl alcohol)‐Laponite free‐standing films

Functional fillers in multilayered films provide opportunity in tailoring the mechanical properties through chemical cross‐linking. In this study, Laponite‐graphene oxide co‐dispersion was used to incorporate graphene oxide (GO) easily into polyvinyl alcohol (PVA)/Laponite layer‐by‐layer (LBL) films. The LBL films were found to be uniform and the layer thickness increased linearly with number of depositions. The process was extended to a large number of depositions to investigate the macroscopic mechanical properties of the free‐standing films. The LBL films showed remarkable improvements in mechanical properties as compared to neat PVA film. The GO‐incorporated LBL films displayed higher enhancements in the tensile strength, ductility, and toughness as compared to that of PVA/Laponite LBL films, upon chemical cross‐linking. This suggests the advantageous effects of GO incorporation. Interestingly, cross‐linking of LBL films for longer time period (>1 h) and higher temperature (～80 °C) was not found to be much beneficial. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2377–2387     

Layer‐by‐Layer Assembly of Polyelectrolyte and Nanoparticles,Monitored by Capillary Electrophoresis

Layer‐by‐layer (LBL) assembly is a versatile nanofabrication technique, and investigation of its kinetics is essential for understanding the assembly mechanism and optimizing the assembly procedure. In this work, the LBL assembly of polyelectrolyte and nanoparticles were monitored in situ by capillary electrophoresis (CE) for the first time. The assembly of poly(diallyldimethylammonium chloride) (PDDA), and gold nanoparticles (AuNPs) on capillary walls causes surface‐charge neutralization and resaturation, and thus yields synchronous changes in the electroosmotic flow (EOF). The EOF data show that formation of multilayers follows first‐order adsorption kinetics. On the basis of the fit results, influencing factors, including number of layers, concentration of materials, flow rate, and size of AuNPs, were investigated. The stability and robustness of the assembled coatings were also characterized by CE. It was found that degradation of PDDA layers follows first‐order chemical kinetics, while desorption of AuNPs takes place in a disorderly manner. The substrate strongly affects assembly of the underlying layer, while this effect is rapidly screened with increasing number of layers. Furthermore, we demonstrate that the EOF measuring step does not disturb LBL assembly, and the proposed method is reliable and rugged. This work not only studies in detail the LBL adsorption/desorption process of polyelectrolyte and nanoparticles, but also offers an alternative tool for monitoring multilayer buildup. It may also reveal the potential of CE in fields other than analytical separation.     

Nanoparticulate Hollow TiO2 Fibers as Light Scatterers in Dye‐Sensitized Solar Cells: Layer‐by‐Layer Self‐Assembly Parameters and Mechanism

Hollow structures show both light scattering and light trapping, which makes them promising for dye‐sensitized solar cell (DSSC) applications. In this work, nanoparticulate hollow TiO₂ fibers are prepared by layer‐by‐layer (LbL) self‐assembly deposition of TiO₂ nanoparticles on natural cellulose fibers as template, followed by thermal removal of the template. The effect of LbL parameters such as the type and molecular weight of polyelectrolyte, number of dip cycles, and the TiO₂ dispersion (amorphous or crystalline sol) are investigated. LbL deposition with weak polyelectrolytes (polyethylenimine, PEI) gives greater nanoparticle deposition yield compared to strong polyelectrolytes (poly(diallyldimethylammonium chloride), PDDA). Decreasing the molecular weight of the polyelectrolyte results in more deposition of nanoparticles in each dip cycle with narrower pore size distribution. Fibers prepared by the deposition of crystalline TiO₂ nanoparticles show higher surface area and higher pore volume than amorphous nanoparticles. Scattering coefficients and backscattering properties of fibers are investigated and compared with those of commercial P25 nanoparticles. Composite P25–fiber films are electrophoretically deposited and employed as the photoanode in DSSC. Photoelectrochemical measurements showed an increase of around 50 % in conversion efficiency. By employing the intensity‐modulated photovoltage and photocurrent spectroscopy methods, it is shown that the performance improvement due to addition of fibers is mostly due to the increase in light‐harvesting efficiency. The high surface area due to the nanoparticulate structure and strong light harvesting due to the hollow structure make these fibers promising scatterers in DSSCs.     

Ordered Poly(p‐phenylene)/Layered Double Hydroxide Ultrathin Films with Blue Luminescence by Layer‐by‐Layer Assembly

Lavender layers : A poly(p‐phenylene) anionic derivate and exfoliated Mg‐Al layered double hydroxide monolayers were assembled into ultrathin films with well‐defined blue fluorescence (see picture; the numbers indicate the number of bilayers), long‐range order, and high photostability. These films work as multiple quantum‐well structures for valence electrons.

     

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     

Self‐assembly of metallo‐supramolecular block copolymers in thin films

The self‐assembly of a metallo‐supramolecular PS‐[Ru]‐PEO block copolymer, where ‐[Ru]‐ is a bis‐2,2′:6′,2″‐terpyridine‐ruthenium(II) complex, in thin films was investigated. Metallo‐supramolecular copolymers exhibit a different behavior as compared to their covalent counterparts. The presence of the charged complex at the junction of the two blocks has a strong impact on the self‐assembly, effecting the orientation of the cylinders and ordering process. Poly(ethylene oxide) cylinders oriented normal to the film surface are obtained directly regardless of the experimental conditions over a wide range of thicknesses. Exposure to polar solvent vapors can be used to improve the lateral ordering of the cylindrical microdomains. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4719–4724, 2008     

Odd‐even effect during layer‐by‐layer assembly of polyelectrolytes inspired by marine mussel

Biomimetic polyelectrolyte of Dopa modified poly(acrylic acid) (PAADopa) was synthesized taking advantage of Dopa, the major unit of marine adhesive proteins. Zinc crosslinked PAADopa (PAADopa‐Zn) were formed at acidic pH for more compact structure and assembled with the positively charged polyethylenimine (PEI) to build robust polyelectrolyte multilayers at high salt concentration. Effects of pH, crosslinking degree, and salt concentration on polymer structure, film building process, and morphology were investigated, respectively. An “odd‐even” effect was observed by quartz crystal microbalance with dissipation and AFM in the presence of zinc ion, which becomes more obvious with an optimum crosslinking degree (Zn/Dopa = 2.0) under high salt concentration (0.6 M NaCl). It indicates the different swollen properties of PEI chain and PAADopa‐Zn complexes during the layer‐by‐layer building process under optimum crosslinking degree of PAADopa‐Zn at high salt concentration. Such odd‐even phenomenon of the biocompatible system is of critical importance for understanding the mechanism of layer formation and film structures. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 245–255     

Stimuli‐responsive micelles of amphiphilic AMPS‐b‐AAL copolymers in layer‐by‐layer films

Aqueous reversible addition‐fragmentation chain transfer polymerization was used to synthesize poly(N‐[3‐(dimethylamino)propyl]acrylamide) (PDMAPA) cationic homopolymers and micelle‐forming, pH‐responsive, amphiphilic diblock copolymers of poly(sodium 2‐acrylamido‐2‐methyl‐1‐propanesulfonate‐block‐N‐acryloyl‐L ‐alanine) (P(AMPS‐b‐AAL)). At low pH, the AAL blocks are protonated rendering them hydrophobic, whereas the AMPS blocks remain anionically charged because of the pendant sulfonate groups. Self‐assembly results in core–shell micelles consisting of hydrophobic cores of AAL and negatively charged shells of AMPS. Using solutions of these micelles with anionic coronas and of the cationic homopolymer PDMAPA, layer‐by‐layer (LbL) films were assembled at low pH, maintaining the micelle structures. Several block copolymers with varying AMPS and AAL block lengths were synthesized and used in the formation of LbL films. The thickness and morphology of the films were examined using ellipsometry and atomic force microscopy. The stimuli‐responsive behavior can be triggered by submersion of the film in water at neutral pH to disrupt the micelles. This behavior was monitored by observing the decrease in film thickness and alteration of the film morphology. The micelles were also loaded with a model hydrophobic compound, pyrene, and incorporated into LbL films. The release of pyrene from the films was monitored by fluorescence spectroscopy at varying pH values (1, 3, 5, and 7). As the pH of the solution increases, the rate of release increases. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011