Molecular orientation in electrodeposited polypyrrole films

The molecular arrangement in electrodeposited polypyrrole films was studied by means of linear dichroism in the near-edge X-ray absorption fine structure (NEXAFS) spectra measured at the K absorption edges of carbon and nitrogen. It has been found that the change of the exciting radiation incidence from normal to grazing leads to an increase in the intensity of π*-related resonances with simultaneous decrease in the intensity of σ*-related resonances in the spectra. Similar changes in the spectra measured for both absorption edges indicate a pronounced conjugation of π-bonds in the polypyrrole chains in the grown films. Preferential in-plane orientation of pyrrole rings relative to the substrate surface is observed for all the deposited films. The linear dichroism is more pronounced at the initial stages of deposition (2D growth) than at later stages characterized by “cauliflower”-like morphology of the grown film.     

Nucleobase orientation and ordering in films of single-stranded DNA on gold

We demonstrate how the orientation and ordering of DNA bases in ultrahigh vacuum (UHV) and ambient environments can be determined using complementary spectroscopic methods. Near-edge X-ray absorption fine structure (NEXAFS) with fluorescence detection, X-ray photoelectron (XPS), and Fourier transform infrared (FTIR) spectroscopies are used to quantify the coverage, chemical composition, orientation, and ordering of thymine bases in model self-assembled monolayers of thymine homo-oligonucleotides [oligo(dT)] on gold. We find that, in monolayers of thiol-modified oligo(dT), thymine bases tend to orient parallel to the Au substrate, and this preferential orientation is significantly more pronounced in monolayers of thiolated 5-mers compared to 25-mers. We interpret this preferential orientation as a signature of significant correlations (local ordering) between individual nuleobases, which offers a way to quantify and compare nucleobase interactions in films under both ambient and UHV conditions.     

Molecular and lamellar orientation in polyethylene thin films

Summary A combined wide (WA) and small angle X-ray diffraction (SAXS) study of melt compressed low density PE samples into the form of very thin films is reported. The WAXD patterns show an uniaxialb axis orientation normal to the film surface which can be interpreted in terms of a row structure in the plane of the film. The analysis of SAXS data indicates, in addition, a preferential orientation of bundles of stacked lamellae parallel to the film surface separated by longitudinal microvoids.With 3 figures     

Molecular ordering of ethanol at the calcite surface

To produce biominerals, such as shells, bones, and teeth, living beings create organic compounds that control the growth of the solid phase. Investigating the atomic scale behavior of individual functional groups at the mineral-fluid interface provides fundamental information that is useful for constructing accurate predictive models for natural systems. Previous investigations of the activity of coccolith-associated polysaccharides (CAP) on calcite, using atomic force microscopy (AFM) [Henriksen, K., Young, J. R., Bown, P. R., and Stipp, S. L. S. Palentology 2004, 43 (Part 3), 725-743] and molecular dynamics (MD) modeling [Yang, M., Stipp, S. L. S., and Harding, J. H. Cryst. Growth Des. 2008, 8 (11), 4066-4074], have suggested that OH functional groups control polysaccharide attachment. The purpose of this work was to characterize, using X-ray reflectivity (XR) combined with molecular dynamics (MD) simulations, the structuring on calcite of a layer of the simplest carbon chain molecule that contains an OH group, ethanol (CH(3)-CH(2)-OH). We found evidence that EtOH forms a highly ordered structure at the calcite surface, where the first layer molecules bond with calcite. The ethanol molecules stand up perpendicularly at the interface or nearly so. As a consequence, the fatty, CH(3) ends form a new surface, about 6 ? from the termination of the bulk calcite, and beyond that, there is a thin gap where ethanol density is low. Following is a more disordered layer that is two to three ethanol molecules thick, about 14 ?, where density more resembles that of bulk liquid ethanol. The good agreement between theory and experiment gives confidence that a theoretical approach can offer information about behavior in more complex systems.     

Determination of the molecular orientation of very thin films on solid substrates: surface liquid crystal layers and rubbed polyimide films

The molecular orientation of very thin films on solid substrates can be determined quantitatively by measuring the polarized infrared (IR) absorption spectra of samples as a function of angle of incidence. The quantitative molecular orientation is derived by fitting the incident angle dependence and the dichroic ratio with theoretical calculations. We applied this method to a technologically important system: liquid crystal (LC)/rubbed polyimide film. To understand the alignment mechanism of LC molecules in contact with rubbed polyimide films, we have quantitatively determined the molecular orientation of rubbed polyimide films and a surface LC layer in contact with a rubbed polyimide film. In this paper two relations are discussed: (1) correlation between the inclination angle of polyimide backbone structures in rubbed films and the pretilt angle of bulk LC in contact with them, and (2) relation among the molecular orientation of a rubbed polyimide film and those of surface and bulk LC layers in contact with it.     

Multilayered ordering of the metal nanoparticles in polymer thin films under photoirradiation

Interference light-induced photogeneration of metal nanoparticle in polymer films was explored. The nanoparticle was obtained from metal complex homogeneously dispersed in the film. Standing waves resulting from light interference were generated by irradiating nearly monochromatic light to the sample placed on a reflective substrate. During irradiation metal nanoparticles were developed by photoreduction of the metal complexes forming layers rich with particles. These nanoparticle-enriched layers were found to align in parallel to the reflective substrate, and they were separated from each other by a constant spacing. This layer spacing was varied by changing the wavelength and/or the incident angle of the irradiating light. The observed results show that the spatial distribution of the nanoparticles is determined by the optical interference within the film. Surprisingly, regions exist between the nanoparticle-enriched layers where the metal species are not detected. Such regions extends for distances larger than tens of nanometers. This means that the metal complexes initially homogeneously dispersed within the polymer were transported away from certain regions upon photoirradiation. The metal precursors are preferentially photoreduced into the metal nanoparticles at the constructive interference regions. The spatially varying consumption rates of the precursors are considered to lead a concentration gradient, thereby causing a directional diffusion of the unreduced precursors toward the regions where constructive interference occurs.     

Molecular dynamics in thin (grafted) polymer layers

Broadband dielectric spectroscopy is employed to study the molecular dynamics in thin polymer layers of (spin-coated) cis-1, 4-polyisoprene (PI) and of (grafted and spin-coated) polydimethylsiloxane (PDMS). For the former, besides the segmental and the normal mode relaxation, a confinement-induced mode is observed, which is assigned to fluctuations of terminal subchains. It turns out that the uptake of water has a strong influence on the dynamics of this relaxation, which takes place in the immediate vicinity of the interface. Between grafted and spin-coated PDMS a pronounced difference in the dynamics is observed for layers that are comparable in thickness to the radius of gyration of the confined chain. This is attributed to the different conformation of grafted and spin-coated polymer layers.     

Polyelectrolyte multilayers and degradable polymer layers as multicompartment films

Polyelectrolyte multilayers are now a well established concept with numerous potential applications in particular as biomaterial coatings. To timely control the biological activity of cells in contact with a substrate, multicompartment films made of different polyelectrolyte multilayers deposited sequentially on the solid substrate constitute a promising new approach. In a first paper (Langmuir 2004, 20, 7298) we showed that such multicompartment films can be designed by alternating exponentially growing polyelectrolyte multilayers acting as reservoirs and linearly growing ones acting as barriers. In the present study, we first demonstrate however that these barriers composed of synthetic polyelectrolytes are not degraded despite the presence of phagocytic cells. We propose an alternative approach where exponentially growing poly(L-lysine)/hyaluronic acid (PLL/HA) multilayers, used as reservoirs, are alternated with biodegradable polymer layers consisting in poly(lactic-co-glycolic acid) (PLGA) and acting as barriers for PLL chains that diffuse within the PLL/HA reservoirs. We first show that these PLGA layers can be deposited alternatively with PLL/HA multilayers leading to polyelectrolyte multilayer/hydrolyzable polymeric layer films and acting as a reservoirs/barriers system. Bone marrow cells seeded on these films ending by a PLL/HA reservoir rapidly degrade it and internalize the PLL chains confined in this reservoir. Then the cells degraded locally the PLGA barrier and internalize the PLL localized in a lower (PLL/HA) compartment after 5 days of seeding. By changing the thickness of the PLGA layer, we hope to be able to tune the time delay of degradation. Such mixed architectures made of polyelectrolyte multilayers and hydrolyzable polymeric layers could act as coatings allowing us to induce a time scheduled cascade of biological activities. We are currently working on the use of comparable films with compartments filled by proteins or peptides and in which the degradation of the barriers results from a hydrolysis over tunable time scales.     

Molecular orientation and lattice ordering of C60 molecules on the polar FeO/Pt(111) surface

C(60) molecules assemble into close packing layer under the domination of the intermolecular interaction when deposited onto Pt(111)-supported FeO layer kept at 400 K. From corresponding high resolution scanning tunneling microscopy (STM) image, a kind of C(60) molecular orientational ordering stabilized by the intermolecular interaction is revealed as C(60)/FeO(111)-(√133 × √133) R17.5° structure and determined from the commensurability between the C(60) nearest-neighbor distance and the lattice of the underlying oxygen layer. Moreover, due to the inhomogeneously distributed work function of the underlying FeO layer, the C(60) molecular electronic state is periodically modulated resulting in a bright-dim STM contrast. In addition, one coincidence lattice ordering is determined as 8 × 8 superstructure with respect to the C(60) primitive cell, which overlays a 3 × 3 moiré cell of the underlying FeO layer.     

Thickness-dependent ordering of water layers at the NaCl(100) surface

We have determined the ordering properties of water adsorbed at room temperature on the rock salt (100) surface under four different conditions: ultrahigh vacuum, dry nitrogen atmosphere, 45% and 75% relative humidity. Details of the atomic structure are determined for both sides of the solid-liquid interface. The top most layer of NaCl shows a small relaxation that changes from an expansion to a contraction with increasing humidity. Under all measured conditions water monolayers with different ordering properties are present at the interface. Surprisingly, we find that the amount of ordering in the first layer is increasing with increasing thickness of the water film. At a solid-liquid interface, the ordering appears to be correlated with the solubility.     

Direct observation of smectic layers in side chain liquid crystal polymer films

By applying stringent conditions to the imaging process it has been possible to produce electron micrographs of the smectic planes in a highly beam sensitive polymethcrylate side chain liquid crystal polymer. These planes show a high degree of internal perfection, while, at the same time, showing clear evidence of curvature. The director orientation was determined over a fairly large region.     

Unconventional spinodal surface fluctuations on polymer films

We study the temporal growth pattern of surface fluctuations on a series of spinodally unstable polymer films where the instability can be adjusted with the film thickness, h0. For the most unstable film studied (whose /(h0 - h(sp))/h(sp)/ = 0.988; h(sp) is the thickness where the second derivative of the interfacial potential of the film equals zero), the growth rate function of the surface modes as a function of the wavevector fits well to the mean-field theory. When the film thickness is increased such that /(h0 - h(sp))/h(sp)/ < or = 0.977, the mean-field theory demonstrates marked disagreement with experiment, notwithstanding the provision of the known corrections from nonlinear effects and thermal noise. We show that the deviations arise from large-amplitude fluctuations induced by homogeneous nucleation, which are not considered in the conventional treatments.     

Hydrated-ion ordering in electrical double layers

In this work we revisit the surface forces measured between two atomically flat mica surfaces submerged in a reservoir of potassium nitrate (KNO(3)) solution. We consider a comprehensive range of concentrations from 0.08 mM to 2.6 M. The significantly improved resolution available from the extended surface force apparatus (eSFA) allows the distinction of hydration structures and hydrated-ion correlations. Above concentrations of 0.3 mM, hydrated-ion correlations give rise to multiple collective transitions (4 ± 1 ?) in the electrical double layers upon interpenetration. These features are interpreted as the result of hydrated-ion ordering (e.g. layering), in contrast to the traditional interpretation invoking water layering. The hydrated-ion layer adjacent to the surface (i.e. outer Helmholtz layer) is particularly well defined and plays a distinctive role. It can be either collectively expelled in a 5.8 ± 0.3 ? film-thickness transition or collectively forced to associate with the surface by external mechanical work. The latter is observed as a characteristic 2.9 ± 0.3 ? film-thickness transition along with an abrupt decrease of surface adhesion at concentrations above 1 mM. At concentrations as low as 20 mM, attractive surface forces are measured in deviation to the DLVO theory. The hydration number in the confined electrolyte seems to be significantly below that of the bulk. A 1-3 nm thick ionic layer solidifies at the surfaces at concentrations >100 mM, i.e. below bulk saturation.     

Molecular orientation and composition at the surface of spin‐coated polyfluorene:Fullerene blend films

The surface composition in spin‐coated films of polyfluorene:fullerene blends was determined quantitatively by near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy. By comparing partial and total electron yield spectra, we found vertical compositional differences in the surface region. Furthermore, the orientation of the polymer chains was investigated by variable‐angle NEXAFS. Blend films of poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole] with [6,6]‐phenyl‐C61‐butyric acid methyl ester in two different blend ratios were studied. Results showed polymer enrichment of the surfaces for films with a polymer:fullerene weight ratio of 20:80 and of 50:50, spin‐coated from both chlorobenzene and chloroform solutions. The angular dependence of the NEXAFS spectra of the pure polymer films showed a preferential plane‐on orientation, which was slightly stronger in the subsurface region than at the surface. In blend films, this orientational preference was less pronounced and the difference between surface and subsurface vanished. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Photo-fabrication of surface relief gratings on polymer films

Surface relief gratings were optically produced on a number of azobenzene-based polymer films. The surface grating formation was investigated by monitoring the diffraction efficiency and using atomic force microscopy. The effect of the structure of the chromophores on surface modulation was investigated. The surface deformation process depended on the polarization state of the writing beams. The localized variations of the light intensity and alteration of the resulting electric field polarization were essential writing conditions to the formation of the surface relief gratings. The surface pattern from straight edge diffraction established that the surface profile of the recorded gratings is proportional to negative gradient of the intensity pattern incident on the polymer film.     

NEXAFS measurements of the molecular ordering in the boundary layers of liquid crystalline free standing films

By near edge X-ray absorption fine-structure (NEXAFS) spectroscopy a finite molecular tilt angle in the surface layer of a free standing film in the liquid crystalline smectic A phase of C7 was directly detected. Analysis of the angular dependent intensities of the oxygen K edge NEXAFS spectra leads to an average tilt angle of the molecules in the surface layer of about 34°, which is characteristic for the bulk smectic C* phase of C7.     

Liquid-crystalline ordering in polymer brushes

The mean-field theory of liquid crystalline (LC) ordering is developed for a polymer brush immersed in a solvent. Additional attraction between neighbouring parallel mesogenic segments of the grafted chains is taken into account. It is shown that LC ordering in this brush is connected with the loss of solubility and occurs as a discrete first-order phase transition which is similar or even identical to the transition in polymer solution.     

All-optical orientation of photoisomerizable octupolar zinc(II) complexes in polymer films

A new type of 4,4'-bis(styryl)-2,2'-bipyridine functionalized by a dialkylamino-azobenzene group has been prepared. This ligand has allowed the preparation of photoisomerizable octupolar tris(bipyridyl)zinc(II) complexes and the corresponding star-shaped polymer by atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). The photoisomerization properties of such new metallo-chromophores have been studied. The macroscopic molecular orientation of the corresponding doped and grafted NLO-polymer films is reported for the first time.     

Nano-organized collagen layers obtained by adsorption on phase-separated polymer thin films

The organization of adsorbed type I collagen layers was examined on a series of polystyrene (PS)/poly(methyl methacrylate) (PMMA) heterogeneous surfaces obtained by phase separation in thin films. These thin films were prepared by spin coating from solutions in either dioxane or toluene of PS and PMMA in different proportions. Their morphology was unraveled combining the information coming from X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and water contact angle measurements. Substrates with PMMA inclusions in a PS matrix and, conversely, substrates with PS inclusions in a PMMA matrix were prepared, the inclusions being either under the form of pits or islands, with diameters in the submicrometer range. The organization of collagen layers obtained by adsorption on these surfaces was then investigated. On pure PMMA, the layer was quite smooth with assemblies of a few collagen molecules, while bigger assemblies were found on pure PS. On the heterogeneous surfaces, it appeared clearly that the diameter and length of collagen assemblies was modulated by the size and surface coverage of the PS domains. If the PS domains, either surrounding or surrounded by the PMMA phase, were above 600 nm wide, a heterogeneous distribution of collagen was found, in agreement with observations made on pure polymers. Otherwise, fibrils could be formed, that were longer compared to those observed on pure polymers. Additionally, the surface nitrogen content determined by XPS, which is linked to the protein adsorbed amount, increased roughly linearly with the PS surface fraction, whatever the size of PS domains, suggesting that adsorbed collagen amount on heterogeneous PS/PMMA surfaces is a combination of that observed on the pure polymers. This work thus shows that PS/PMMA surface heterogeneities can govern collagen organization. This opens the way to a better control of collagen supramolecular organization at interfaces, which could in turn allow cell-material interactions to be tailored.