Polymer Monolayers Research Articles

Layered Ultrathin Proton Conductive Film Based on Polymer Nanosheet Assembly

As described in this paper, a layered ultrathin proton conductive film was prepared using a poly(N-dodecylacrylamide-co-2-acrylamido-2-methylpropanesulfonic acid) (p(DDA/AMPS)) polymer nanosheet. The surface pressure-area isotherm revealed that p(DDA/AMPS) formed a stable monolayer at the air-water interface. The polymer monolayer was transferred onto a solid substrate using the Langmuir-Blodgett technique. X-ray diffraction measurements of a 30-layer film of p(DDA/AMPS) showed clear Kiessig fringes and one Bragg peak, which indicate that the multilayer film took a uniform layered structure. The monolayer thickness was determined as 1.85 nm from the Bragg peak. The proton conductivity of p(DDA/AMPS) polymer nanosheet through the layer plane direction was studied with changing temperature and relative humidity. The proton conductivity of the multilayer film was on the order of 1 × 10⁻⁵ S/cm at 100% RH with 20 °C and increased to 1 × 10⁻² S/cm at 70 °C. The proton conductivity of the multilayer film showed a more than 10 times higher value than that of the spin-coated film because of the formation of a uniform and consecutive hydrophilic nanochannel through the hydrophilic region in the multilayer film.

Monolayer studies on poly(isobutylcyanoacrylate)-ampicillin association.

Surface pressure-area isotherms (pi-A) of poly(isobutylcyanoacrylate) monolayers with or without glucose and dextran as polymerization adjuvants used in the preparation of nanoparticles have been derived from measurements made at the air-water interface with the subphase pH at 2.7, 5.5 or 8.8. The isotherms were characteristic of the expanded type of polymer monolayer curves, yielding unusually low limiting area values compared with those of other known poly(acrylate) derivatives. This behaviour may be explained by the folding of polar moieties of the polymer groups in the water subphase. Ampicillin incorporated during preparation of nanoparticles had a negligible effect on the general behaviour of adjuvant-free monolayers, but a significant one on the adjuvant-loaded polymer monolayer system which showed an increase in surface area throughout the compression cycle, as compared with the surface area of the adjuvant-free polymer system although the collapse pressure was practically the same at 67 mN m-1.

The Effects of Interface Structure and Polymerization on the Friction of Model Self-Assembled Monolayers

The friction between two model atomic force microscope tips and two hydrocarbon monolayers has been examined using molecular dynamics simulations. An amorphous hydrocarbon tip and a flat diamond tip were both employed. One monolayer was composed of linear alkane chains and the other was composed of chains that were polymerized in a regular pattern near the tip–monolayer interface. When friction is decomposed into the forces on individual chains pushing and resisting sliding, the monolayer composed of linear alkane chains exhibited strong pushing forces immediately after clearing tip features at the sliding interface. When this monolayer is paired with the amorphous tip, the strong pushing forces resulted in low friction compared to a monolayer composed of polymerized chains. When the diamond tip is employed, commensurate meshing with the chains of the linear-alkane monolayer resulted in chains resisting tip motion for longer durations. The consequence of this is higher friction compared to the polymerized monolayer, despite the linear-alkane monolayer’s more symmetric chain response at resisting-to-pushing transitions.

Interfacial shear rheology of highly confined glassy polymers

We present results on interfacial shear rheology measurements on Langmuir monolayers of two different polymers, poly(vinyl acetate) and poly(methyl methacrylate) as a function of surface concentration and temperature. While for the high glass transition poly(methyl methacrylate) polymer we find a systematic transition from a viscous dominated regime to an elastic dominated regime as surface concentration is increased, monolayers of the low glass transition polymer, poly(vinyl acetate), remain viscous even at very high surface concentrations. We further interpret the results in terms of the soft glassy rheology model of Sollich et al. [P. Sollich, F. C. Lequeux, P. Hebraud and M. E. Cates, Phys. Rev. Lett., 1997, 78, 2020–2023] and provide evidence of possible reduction in glass transition temperatures in both poly(methyl methacrylate) and poly(vinyl acetate) monolayers due to finite size effects.

Fabrication of Mixed Monolayer of Biodegradable Polymers and Organo-modified Montmorillonite

We investigated formation of ultrathin hybrid films composed of a biodegradable polymers (ex. poly(ethylene succinate), poly(L-lactide), and poly(e-caprolactone)) and organo-modified montmorillonite nanosheet at an air/water interface. When a chloroform solution of the biodegradable polymer was spread onto a surface of an pure water in a Langmuir trough, a mixed monolayer of biodegradable polymer was formed by hybridization with the clay nanosheets at the air/water interface. These results indicated that some ammonium cations spread onto the clay suspension were dissolved into the aqueous subphase before the hybridization with the clay nanosheets. The hybrid monolayers were transferred onto a mica substrate by Langmuir-Blodgett method to form a mixed monolayer. Interestingly, the densities of dimethyl dioctadecyl (DMDO) ammonium determined by the thermogravimetric analysis were constant in the hybrid multilayers prepared from the clay suspensions at the same concentration, regardless of the concentrations of the amphiphilic ammonium salt solutions. Out-of plane and in-plane XRD profiles of the films showed that the cations of DMDO would lie down on the clay layer in the hybrid film.

Dependence of Surface Free Energy of Polystyrene Thin Films on Temperature for Grafting Treatment

End-grafted polymer monolayers play an important role in colloidal stabilization, adhesion, and biocompatibility of artificial organs in medicine. There are two main methods to form end-grafting polymer layer:grafting-to and grafting-from methods. Grafting-to method has merit that is easy to conduct and easy to set a homogeneous degree of polymerization. We formed end-grafted polystyrene monolayers by a grafting-to method;a vinyl terminated polystyrene reacted with a H-terminated Si wafer and the polystyrene grafted to it. To clarify surface properties of the grafted polystyrene layer on the grafting density, we controlled it by a reaction temperature;The density could be controlled from 0.09 to 0.20 chains/nm2. At a lower grafting density region, the layer has high surface energy, high layer density, thin thickness, and smooth surface morphology. While at a higher grafting density region, the layer has low surface energy, low layer density, thick thickness, and smooth surface morphology.

Fluid to soft-glass transition in a quasi-2D system: thermodynamic and rheological evidences for a Langmuir monolayer

We report an experimental study that points out the existence of a fluid to soft-glass transition in Langmuir polymer monolayers of poly(methyl methacrylate) (PMMA), for which the water/air interface behaves as a poor-solvent. The temperature dependence of surface pressure vs. surface area equilibrium isotherms shows a glass-like transition temperature at T(g,2D)≈ 298 K, significantly lower than the value for bulk PMMA (T(g,bulk)≈ 378 K). The plot of the film thickness h vs. temperature shows a sharp change of slope at about the same temperature, 298 K, which is a typical hallmark of a glass transition in thin polymer films [J. L. Keddie, R. A. L. Jones, R. A. Cory, Europhys. Lett., 1996, 27, 59-64]. Furthermore, slightly above T(g,2D), the temperature dependence of the dilational viscosity does not follow an Arrhenius law, but instead can be described by a Vogel-Fulcher-Tamman equation with parameters that are typical of a fragile glass. Not only the qualitative behavior of three distinct equilibrium and dynamic properties, but also the quantitative agreement of the values of T(g) obtained, are a strong evidence of the existence of a fluid to soft-glass transition in this quasi-2D system.

Accurate modeling approach for the structural comparison between monolayer polymer tubes and single-walled nanotubes

In a recent computational study, we found highly structured ground states for coarse-grained polymers adsorbed to ultrathin nanowires in a certain model parameter region. Those tubelike configurations show, even at a first glance, exciting morphological similarities to known atomistic nanotubes such as single-walled carbon nanotubes. In order to explain those similarities in a systematic way, we performed additional detailed and extensive simulations of coarsegrained polymer models with various parameter settings. We show this here and explain why standard geometrical models for atomistic nanotubes are not suited to interpret the results of those studies. In fact, the general structural behavior of polymer nanotubes, as well as specific previous observations, can only be explained by applying recently developed polyhedral tube models.

Functional biohybrid materials synthesized via surface-initiated MADIX/RAFT polymerization from renewable natural wood fiber: Grafting of polymer as non leaching preservative

Crude wood fibers represent a wide class of renewable resources. The surface modification of such materials via covalent grafting of polymer offers new surface properties with non-leaching coating. The grafting of the polymer chains was achieved by surface-initiated controlled radical polymerization through a grafted xanthate chain transfer agent. Macromolecular design via interchange of xanthate (MADIX) technique was chosen to graft poly(vinyl acetate), polystyrene, poly( n-butyl acrylate) and poly(4-vinylbenzyl chloride)-polystyrene amphiphilic cationic copolymers. Water contact angle measurements highlighted the hydrophobization of the wood fiber surface with a nanoscaled polymer monolayer indicating the appropriate coverage of the fiber. X-ray photoelectron spectroscopy showed the successful grafting of the polymer after drastic washing procedure. The quaternization of the grafted polystyrene- co-poly(4-vinyl benzyl chloride) copolymers with tertiary amine allows the introduction of biocide quaternary ammonium functions while preserving the hydrophobic character of the modified wood fiber when introducing a long alkyl chain in the statistical copolymer. Finally, the cationic copolymer was subjected to Coniophora Puteana to evaluate its propensity to limit the fungi expansion.

Surface‐Confined Electropolymerization of Methylene Blue on Gold Electrodes

AbstractWe report on the electrochemical behaviour and electropolymerization of self‐assembled monolayers (SAMs) of methylene blue (MB) on gold electrodes. The SAMs of MB on gold electrodes were prepared by immersing the substrates into a solution of 1.0 mM MB in absolute ethanol for different times at room temperature. Cyclic voltammetry experiments exhibited that reductive desorption of MB monolayer takes place at three different potentials on polycrystalline gold electrodes, while reductive desorption of MB monolayer consists of only one peak on single crystal Au(111) substrates. Calculated charge densities for different immersion times indicated that optimal immersion time for self‐assembly of MB is 96 h. Electropolymerization of SAMs of MB on gold electrode was achieved by applying 0.95 V for 1 s in 0.1 M borate buffer solution (pH: 9.0). It was observed that poly(MB) monolayers are highly stable in acidic media. ATR‐FTIR and UV‐vis spectra exhibited differences between monomer and polymer monolayers, which are attributed to surface‐confined electropolymerization. STM image of poly(MB) monolayer on Au(111) substrate revealed a surface that is covered by well‐ordered, collateral nanowires with an average size of 3 nm.

Interaction between a silica particle and the underside of a polymer monolayer at the air/water interface in the presence of an anionic surfactant

The properties of a monolayer of the non-ionic Pluronic F127 at the air/water interface and the effect of an anionic surfactant (sodium dodecyl sulphate, SDS) were characterized using a Langmuir film balance and the Monolayer Particle Interaction Apparatus; Pluronic F127 is a triblock copolymer of poly(ethylene oxide) (PEO groups)–poly(propylene oxide) (PO groups)–poly(ethylene oxide) (PEO groups). The surface pressure–area ( Π– A) isotherms and the forces between a silica particle and the monolayer in the water phase were measured in the absence and presence of SDS molecules. The isotherm of the monolayer showed the characteristics of a pancake-to-brush transition upon compression of the monolayer. An adhesion was measured between the particle and the F127 monolayer. We attribute it to hydrogen bonds between the PEO groups and the silanol groups of the particle. The adhesion increased upon compression of the monolayer, due to the resulting increase in the number of hydrophilic PEO groups in the water phase. The adhesion decreased upon the addition of SDS, interpreted by an adsorption of SDS to the air/water interface. The presence of SDS at the interface results in an electrostatic repulsion between the negatively charged SDS headgroups and the negatively charged silica particle. The addition of SDS also decreased the stiffness of the monolayer. This increased flexibility can be explained by (1) the more fluid nature of the SDS molecules in comparison to the polymer molecules, (2) the electrostatic repulsion between the SDS groups in the monolayer, and/or (3) binding between the hydrophobic PO groups and the hydrophobic tails of the SDS molecules.

Self-organized dendritic patterns in the polymer Langmuir–Blodgett film

We report the formation of a self-organized dendritic pattern of nanometer thickness in polymer Langmuir–Blodgett (LB) films. Poly(N-dodecylacrylamide) (pDDA)/chloroform solution was spread on a water surface to form a stable polymer monolayer. A pDDA monolayer was deposited onto a hydrophilic silicon substrate by upward deposition from a water subphase, and a second layer was then deposited by downward deposition. The substrate with the two layers was withdrawn from a clean water surface at a high speed to form the dendritic pattern, which was imaged by atomic force microscopy. The height of the pattern, 3.5nm, corresponds to the height of a bilayer pDDA LB film, suggesting that the pattern forms when the deposited outermost layer overturns by meniscus oscillation. A similar dendritic structure of narrower width and lower height was fabricated on a hydrophobic silicon substrate.

Self-Assembled Supramolecular Array of Polymeric Phthalocyanine on Gold for the Determination of Hydrogen Peroxide

This Article describes for the first time the formation of a supramolecular self-assembled monolayer of polymeric phthalocyanine (poly(CuPc)) onto a gold substrate. The latter is established through the interaction of the cyano group, belonging to the poly(CuPc), with the metal substrate. The functionalized gold substrate was characterized using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrochemical methods. Results clearly demonstrated the interaction between gold and the nitrogen atom of cyano group and showed at the same time the formation of a completely covered polymeric monolayer on the gold surface. In addition, the modified gold surface seems to exhibit a reversible redox behavior and is found to act as an electronic conductor, which allows rapid electron transfer. Electrochemical impedance spectroscopy (EIS) analyses in the presence of [Fe(CN)(6)](3-/4-) as a redox couple revealed that the modified electrode showed a much lower electron transfer resistance compared with bare gold. In addition, the modified electrode is found to catalyze the H(2)O(2) reduction very effectively, showing a catalytic current that varies linearly with the peroxide concentration in the range of 0.35 to 70 μM with a detection limit of 0.25 μM.

Equilibrium and out-of-equilibrium dynamics in a molecular layer of azopolymer floating on water studied by Interfacial Shear Rheology

We report the details of the construction and calibration of a sensitive surface rheometer, inspired by an instrument described in the literature, adapted to the study of photosensitive polymeric materials. By high resolution video tracking of the motion of a floating magnetized needle we are able to measure the viscoelastic complex shear modulus G of a Langmuir monolayer with an accuracy of 5*10 − 6 N/m. This instrument is then employed for the rheological characterization of a Langmuir monolayer of a photosensitive azobenzene polymer, which can be brought out of equilibrium by a suitable photoperturbation. The shear modulus is measured as a function of temperature, illumination power and wavelength. The reversible rheological changes induced in the film by photo-perturbation are monitored during time, observing a transition from a predominantly elastic (G mainly real) to a viscoelastic regime (real and imaginary parts of G comparable). These results are confirmed by a comparison with independent measurements performed using other rheological techniques. Finally a discussion is made, taking into account the results of a recent X-ray photon correlation spectroscopy (XPCS) experiment on the same polymer in equilibrium and out of equilibrium.

Langmuir and Langmuir−Blodgett Films of a Maleic Anhydride Derivative: Effect of Subphase Divalent Cations

We report the study of the equilibrium and dynamic properties of Langmuir monolayers of poly(styrene-co-maleic anhydride) partial 2-buthoxyethyl ester cumene terminated polymer and the effect of the Mg(NO(3))(2) addition in the water subphase on the film properties. Results show that the polymer monolayer becomes more expanded when the electrolyte concentration in the subphase increases. Dense polymer films aggregate at the interface. The aggregates are transferred onto silicon wafers using the Langmuir-Blodgett methodology and the morphology is observed by AFM. The structure of aggregates depends on the subphase composition of the Langmuir film transferred onto the silicon wafer.

Calcium Phosphate Mineralization beneath a Polycationic Monolayer at the Air–Water Interface

The self-assembly of the amphiphilic block copolymer poly(n-butyl methacrylate)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface has been investigated at different pH values. Similar to Rehfeldt et al. (J. Phys. Chem. B 2006, 110, 9171), the subphase pH strongly affects the monolayer properties. The formation of calcium phosphate beneath the monolayer can be tuned by the subphase pH and hence the monolayer charge. After 12 h of mineralization at pH 5, the polymer monolayers are still transparent, but transmission electron microscopy (TEM) shows that very thin calcium phosphate fibers form, which aggregate into cotton ball-like features with diameters of 20 to 50 nm. In contrast, after 12 h of mineralization at pH 8, the polymer film is very slightly turbid and TEM shows dense aggregates with sizes between 200 and 700 nm. The formation of calcium phosphate is further confirmed by Raman and energy dispersive X-ray spectroscopy. The calcium phosphate architectures can be assigned to the monolayer charge, which is high at low pH and low at high pH. The study demonstrates that the effects of polycations should not be ignored if attempting to understand the colloid chemistry of biomimetic mineralization. It also shows that basic block copolymers are useful complementary systems to the much more commonly studied acidic block copolymer templates.

Communication: Evidence of dynamic heterogeneity in glassy polymer monolayers from interface microrheology measurements

We have developed a novel nanoparticle tracking based interface microrheology technique to perform in situ studies on confined complex fluids. To demonstrate the power of this technique, we show, for the first time, how in situ glass formation in polymers confined at air-water interface can be directly probed by monitoring variation of the mean square displacement of embedded nanoparticles as a function of surface density. We have further quantified the appearance of dynamic heterogeneity and hence vitrification in polymethyl methacrylate monolayers above a certain surface density, through the variation of non-Gaussian parameter of the probes.

Polymer Behavior at the Air-Water Interface

AbstractThis article reviews polymer behavior at the air-water interface with a focus on the Langmuir technique and spreading processes to gain insight into this interface. The influence of the solvent and the water subphase on some polymer monolayers is discussed. The surface pressure–area isotherms (π–A) are described in terms of the chemical structure and the hydrophilic-hydrophobic balance of selected polymeric systems, with an emphasis on the behavior of the specific diblock copolymer films at the air-water interface. In some cases, molecular dynamic simulation studies can be used to visualize the organization and orientation of polymers at the air-water interface and to identify the main molecular interactions involved in such processes.

Molecular Weight Dependence of Surface Dilatational Moduli of Poly(n-hexyl isocyanate) Films Spread at the Air−Water Interface

The surface dilatational modulus of the monolayer of a well-known semiflexible polymer, poly(n-hexyl isocyanate) (PHIC), at the air-water interface was measured as a function of the molecular weight of PHIC. In the dilute regime, the surface dilatational modulus of PHIC showed a linear response over a measured strain range of 5%-20%. The modulus could be well-separated into the elastic component E' and the viscous component E'', irrespective of molecular weight of PHIC. The E' and E'' components, respectively, are mainly attributed to static compression modulus caused by a change in the area covered by the PHIC monolayer and the semiflexibility of the PHIC chain. In the semidilute regime, the surface dilatational modulus of PHIC showed a nonlinear response to even 1% of strain due to the semiflexibility of the PHIC chain. Moreover, the surface dilatational modulus of PHIC depended on its molecular weight: at the smaller strain, the surface dilatational modulus of the high molecular weight PHIC was larger than that of the low molecular weight PHIC. The surface dilatational modulus of the high molecular weight PHIC increased with the surface concentration, whereas that of the low molecular weight PHIC remained constant. The molecular weight dependence in the semidilute regime is caused by the difference in chain entanglement between the high and low molecular weight PHICs at the air-water interface.

Anthraquinone Sulfonate Modified, Layered Double Hydroxide Nanosheets for Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) have been extensively investigated for solar energy conversion by using various combinations of inorganic semiconductors and organic sensitizers because of their low cost, easy production, and high efficiency. For efficient visible-light absorption, various organic dyes have been intensively exploited because of their advantages, such as their high molar extinction coefficients and tunable optical bands, in which they are anchored on mesoscopic TiO2 semiconductors. Recently, inorganic semiconducting materials, such as quantum dots (CdSe, CdS,) and organometal perovskites, have been proposed as inorganic sensitizers in photovoltaic cells to overcome some drawbacks of the organic dyes such as their relatively low heat stability and narrow absorption bands. One possible approach to improving the inherent light-harvesting ability of the organic dyes is to hybridize them with nanosized multifunctional inorganic materials such as layered double hydroxides (LDHs), which can provide a stable chemical environment, higher heat or photostability, and are environmentally friendly. The LDHs, also known as anionic or hydrotalcite-like clays, are useful in new multifunctional systems such as biological carriers, catalysts, and hybrid optical layers. Recently, Duan et al. reported an ultrathin hybrid film consisting of LDH nanosheets and luminescent polyanions, in which the LDH nanosheets induced a welldefined photoluminescence of polymer monolayers that are individually separated by the exfoliated LDH nanosheets. Moreover, the LDHs provide a stable chemical environment to increase the photochemical function and thermal stability of the intercalated organic photochromic dyes. In this study, LDH nanosheets are suggested as the inorganic matrix in an attempt to induce an intense photochromic function of the organic photochromic dyes, anthraquinone sulfonate anion (AQS), that are chemically immobilized on the surface of the LDH nanosheets. Herein, we report a new hybrid light sensitizer for DSCs, in which the AQS anion is selected as the organic sensitizer and the LDH nanosheets as the inorganic host. This is believed to be the first example of hybrid LDH/organic nanosheets used as a light sensitizer in photovoltaic devices. The chemical structure and photochromic behavior of the LDH AQS nanosheets in formamide are shown in Figure 1a. The powder X-ray diffraction (XRD) pattern of LDH AQS microcrystals indicated a well-crystallized rhombohedral hydrotalcite-like, 3R1 phase with lattice parameters of a=3.05 and c=60.0 . An antiparallel arrangement of the AQS would be the best model by assuming that the length of the AQS anion was 12.9 . Notably, a transparent solution was obtained by ultrasound treatment for 10 min, indicating the successful exfoliation of the platelike LDH AQS microcrystals. Typical Tyndall light scattering of the resulting solution demonstrated the presence of exfoliated LDH nanosheets as shown in Figure 1a. Interestingly, the suspension showed a strong photoinduced coloration that was not seen in the AQS–formamide solution. In (4) of Figure 1b, the UV/Vis absorption spectra for the irradiated suspension of the LDH AQS nanosheets show extremely enhanced absorption bands in the range of 400–600 nm, where the absorption bands at 435 and 525 nm are characteristic signals for the reduction state of anthraquinone sulfonate (AQS ) that have a long-term stability in a high pH condition, whereas formamide (the solvent) might be oxidized during the photoreaction. Under continuous irradiation of the exfoliated LDH AQS solution shown in (2) of Figure 1b, the band intensities gradually increased as a function of the irradiation time and then decreased after the light was cut off. The rate constants for photocoloration and decay were 0.072 and 0.059 min , respectively. This may be [a] Dr. J. H. Lee, J. Chang, J.-H. Cha, Prof. D.-Y. Jung, S. S. Kim, Prof. J. M. Kim Department of Chemistry-BK21 and Sungkyunkwan Advanced Institute of Nanotechnology Institute of Basic Sciences, Sungkyunkwan University Suwon, 440-746 (Korea) Fax: (+82)31-290-7075 E-mail : dyjung@skku.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000703.