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Electroluminescence and photoluminescence of conjugated polymer films prepared by plasma enhanced chemical vapor deposition of naphthalene  [PDF]
Mojtaaba Rajabi,Amir Reza Ghassami,Marzieh Abbasi Firouzjah,Seyed Iman Hosseini,Babak Shokri
Physics , 2012,
Abstract: Polymer light-emitting devices were fabricated utilizing plasma polymerized thin films as emissive layers. These conjugated polymer films were prepared by RF Plasma Enhanced Chemical Vapor Deposition (PECVD) using naphthalene as monomer. The effect of different applied powers on the chemical structure and optical properties of the conjugated polymers was investigated. The fabricated devices with structure of ITO/PEDOT:PSS/ plasma polymerized Naphthalene/Alq3/Al showed broadband Electroluminescence (EL) emission peaks with center at 535-550 nm. Using different structural and optical tests, connection between polymers chemical structure and optical properties under different plasma powers has been studied. Fourier transform infrared (FTIR) and Raman spectroscopies confirmed that a conjugated polymer film with a 3-D cross-linked network was developed. By increasing the power, products tended to form as highly cross-linked polymer films. Photoluminescence (PL) spectra of plasma polymers showed different excimerc emissions, resulted from crosslinked architecture. Further analysis showed an interesting change in dominance of excimeric emissions by increasing the power. In fact, as the plasma power increased, the optical properties showed two different domains; up to 200 w, EL, PL and UV-Vis spectra red-shifted and broadened significantly. At higher powers, a reverse behavior was observed. Also, the relation between the film structure and plasma species was investigated using Optical Emission Spectroscopy (OES).
PEO/CHCl3: Crystallinity of the polymer and vapor pressure of the solvent - Equilibrium and non-equilibrium phenomena -  [PDF]
A. KHassanova,B. A. Wolf
Physics , 2003,
Abstract: Vapor pressures were measured for the system chloroform/polyethylene oxide (peo, weight average molar mass = 1000 kg/mol) at 25 degrees centigrade as a function of the weight fraction w of the polymer by means of a combination of head space sampling and gas chromatography. The establishment of thermodynamic equilibria was assisted by employing thin polymer films. The degrees of crystallinity alpha of the pure peo and of the solid polymer contained in the mixtures were determined via dsc. An analogous degree of polymer insolubility, beta, was calculated from the vapor pressures measured in this composition range. The experiments demonstrate that both quantities and their concentration dependence are markedly affected by the particular mode of film preparation. These non-equilibrium phenomena are discussed in terms of frozen local and temporal equilibria, where differences between alpha and beta are attributed to the occlusion of amorphous material within crystalline domains. Equilibrium information was obtained from two sources, namely from the vapor pressures in the absence of crystalline material (gas/liquid) and from the saturation concentration of peo (liquid/solid). The thermodynamic consistency of these data is demonstrated using a new approach that enables the modeling of composition dependent interaction parameters by means of two adjustable parameters only.
SAXS Studies of TiO2 Nanoparticles in Polymer Electrolytes and in Nanostructured Films  [PDF]
Aleksandra Turkovi?,Pavo Dub?ek,Krunoslav Jurai?,Antun Dra?ner,Sigrid Bernstorff
Materials , 2010, DOI: 10.3390/ma3114979
Abstract: Polymer electrolytes as nanostructured materials are very attractive components for batteries and opto-electronic devices. (PEO) 8ZnCl 2 polymer electrolytes were prepared from PEO and ZnCl 2. The nanocomposites (PEO) 8ZnCl 2/TiO 2 themselves contained TiO 2 nanograins. In this work, the influence of the TiO 2 nanograins on the morphology and ionic conductivity of the nanocomposite was systematically studied by transmission small-angle X-ray scattering (SAXS) simultaneously recorded with wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) at the synchrotron ELETTRA. Films containing nanosized grains of titanium dioxide (TiO 2) are widely used in the research of optical and photovoltaic devices. The TiO 2 films, prepared by chemical vapor deposition and e-beam epitaxy, were annealed in hydrogen atmospheres in the temperature range between 20 °C and 900 °C in order to study anatase-rutile phase transition at 740 °C. Also, grazing-incidence small angle X-ray scattering (GISAXS) spectra for each TiO 2 film were measured in reflection geometry at different grazing incident angles. Environmentally friendly galvanic cells, as well as solar cells of the second generation, are to be constructed with TiO 2 film as working electrode, and nanocomposite polymer as electrolyte.
Dewetting dynamics of stressed viscoelastic thin polymer films  [PDF]
Falko Ziebert,Elie Raphael
Physics , 2008, DOI: 10.1103/PhysRevE.79.031605
Abstract: Ultrathin polymer films that are produced e.g. by spin-coating are believed to be stressed since polymers are 'frozen in' into out-of-equilibrium configurations during this process. In the framework of a viscoelastic thin film model, we study the effects of lateral residual stresses on the dewetting dynamics of the film. The temporal evolution of the height profiles and the velocity profiles inside the film as well as the dissipation mechanisms are investigated in detail. Both the shape of the profiles and the importance of frictional dissipation vs. viscous dissipation inside the film are found to change in the course of dewetting. The interplay of the non-stationary profiles, the relaxing initial stress and changes in the dominance of the two dissipation mechanisms caused by nonlinear friction with the substrate is responsible for the rich behavior of the system. In particular, our analysis sheds new light on the occurrence of the unexpected maximum in the rim width obtained recently in experiments on PS-PDMS systems.
Dewetting of thin polymer films: Influence of interface evolution  [PDF]
Falko Ziebert,Elie Raphael
Physics , 2009, DOI: 10.1209/0295-5075/86/46001
Abstract: The dewetting dynamics of ultrathin polymer films, e.g. in the model system of polystyrene on a polydimethylsiloxane-covered substrate, exhibits interesting behavior like a fast decay of the dewetting velocity and a maximum in the width of the built-up rim in the course of time. These features have been recently ascribed to the relaxation of residual stresses in the film that stem from the nonequilibrium preparation of the samples. Recent experiments by Coppee et al. on PS with low molecular weight, where such stresses could not be evidenced, showed however similar behavior. By scaling arguments and numerical solution of a thin film viscoelastic model we show that the maximum in the width of the rim can be caused by a temporal evolution of the friction coefficient (or equivalently of the slip length), for which we discuss two possible mechanisms. In addition, the maximum in the width is affected by the sample age. As a consequence, knowing the temporal behavior of friction (or slip length) in principle allows to measure the aging dynamics of a polymer-polymer interface by simple dewetting experiments.
Interplay of internal stresses, electric stresses and surface diffusion in polymer films  [PDF]
Fabien Closa,Falko Ziebert,Elie Raphael
Physics , 2010, DOI: 10.1103/PhysRevE.83.051603
Abstract: We investigate two destabilization mechanisms for elastic polymer films and put them into a general framework: first, instabilities due to in-plane stress and second due to an externally applied electric field normal to the film's free surface. As shown recently, polymer films are often stressed due to out-of-equilibrium fabrication processes as e.g. spin coating. Via an Asaro-Tiller-Grinfeld mechanism as known from solids, the system can decrease its energy by undulating its surface by surface diffusion of polymers and thereby relaxing stresses. On the other hand, application of an electric field is widely used experimentally to structure thin films: when the electric Maxwell surface stress overcomes surface tension and elastic restoring forces, the system undulates with a wavelength determined by the film thickness. We develop a theory taking into account both mechanisms simultaneously and discuss their interplay and the effects of the boundary conditions both at the substrate and the free surface.
Deformation of polymer films by bending forces  [PDF]
Gerald Paetzold,Thorsten Hapke,Andreas Linke,Dieter W. Heermann
Physics , 1996, DOI: 10.1016/S0040-6090(96)09308-X
Abstract: We study the deformation of nano--scale polymer films which are subject to external bending forces by means of computer simulation. The polymer is represented by a generalized bead--spring--model, intended to reproduce characteristic features of n--alkanes. The film is loaded by the action of a prismatic blade which is pressed into the polymer bulk from above and a pair of columns which support the film from below. The interaction between blade and support columns and the polymer is modelled by the repulsive part of a Lennard-Jones potential. For different system sizes as well as for different chainlengths, this nano--scale experiment is simulated by molecular dynamics methods. Our results allow us to give a first characterization of deformed states for such films. We resolve the kinetic and the dynamic stage of the deformation process in time and access the length scale between discrete particle and continuum mechanics behaviour. For the chainlengths considered here, we find that the deformation process is dominated by shear. We observe strangling effects for the film and deformation fluctuations in the steady state.
Spinodal Surface Fluctuations on Polymer Films  [PDF]
Y. J. Wang,Ophelia K. C. Tsui
Physics , 2005,
Abstract: We study the temporal growth pattern of surface fluctuations on a series of spinodally unstable polymer films where the degree of instability is controlled by the film thickness. For films in the deep spinodal region, the growth rate function of the surface modes as a function of the wavevector,, fits well to the mean-field theory. As the film thickness is increased and the film instability decreases, the mean-field theory demonstrates marked disagreement with experiment, notwithstanding provision of the known corrections for random thermal noise. We show that the deviations arise from large-amplitude fluctuations induced by homogeneous nucleation, which has not been considered in conventional treatment of thermal noise.
Adsorption mechanisms in layer-by-layer films
Raposo, Maria;Oliveira Jr, Osvaldo N.;
Brazilian Journal of Physics , 1998, DOI: 10.1590/S0103-97331998000400014
Abstract: adsorption processes in layer-by-layer films are discussed using poly(o-methoxyaniline) (poma) as a case study and also comparing with results for other polymers from the literature. this follows a brief overview of the materials and characterization techniques employed for self-assembled films, including their possible applications. the original paradigm of the self-assembly method is associated with spontaneous adsorption of oppositely charged polymer layers. while this rationale has been successful in explaining adsorption mechanisms for some polyelectrolytes, for polyanilines other interactions must be included. for poma, in particular, at least three types of interactions are identified, namely van der waals forces, ionic interactions and h-bonding. furthermore, h-bonding is responsible for a number of effects even for charged poma where electrostatic attraction was expected to predominate. such effects include poma dedoping upon contact with a glass substrate at early stages of adsorption, and the non-linear increase in the adsorbed amount with the number of poma/poly(ethenesulfonic acid) (pvs) bilayers deposited in a multilayer structure. adsorption of a poma layer on a glass substrate or on an already formed poma/pvs film occurs in two steps: a fast, first-order kinetics process with a characteristic time of a few seconds and a slower process represented by a johnson-mehl-avrami function with a characteristic time of hundreds of seconds. these correspond basically to nucleation and growth mechanisms which is corroborated by atomic force microscopy measurements. the amount of material adsorbed in any given layer depends on experimental parameters, especially polymer concentration and ph, owing to the different extents of h-bonding that may allow poma to adsorb on itself or on pvs molecules.
Adsorption mechanisms in layer-by-layer films  [cached]
Raposo Maria,Oliveira Jr Osvaldo N.
Brazilian Journal of Physics , 1998,
Abstract: Adsorption processes in layer-by-layer films are discussed using poly(o-methoxyaniline) (POMA) as a case study and also comparing with results for other polymers from the literature. This follows a brief overview of the materials and characterization techniques employed for self-assembled films, including their possible applications. The original paradigm of the self-assembly method is associated with spontaneous adsorption of oppositely charged polymer layers. While this rationale has been successful in explaining adsorption mechanisms for some polyelectrolytes, for polyanilines other interactions must be included. For POMA, in particular, at least three types of interactions are identified, namely van der Waals forces, ionic interactions and H-bonding. Furthermore, H-bonding is responsible for a number of effects even for charged POMA where electrostatic attraction was expected to predominate. Such effects include POMA dedoping upon contact with a glass substrate at early stages of adsorption, and the non-linear increase in the adsorbed amount with the number of POMA/poly(ethenesulfonic acid) (PVS) bilayers deposited in a multilayer structure. Adsorption of a POMA layer on a glass substrate or on an already formed POMA/PVS film occurs in two steps: a fast, first-order kinetics process with a characteristic time of a few seconds and a slower process represented by a Johnson-Mehl-Avrami function with a characteristic time of hundreds of seconds. These correspond basically to nucleation and growth mechanisms which is corroborated by atomic force microscopy measurements. The amount of material adsorbed in any given layer depends on experimental parameters, especially polymer concentration and pH, owing to the different extents of H-bonding that may allow POMA to adsorb on itself or on PVS molecules.
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