Reaktionskinetische Vorgänge an der Grenzfläche zwischen nichtisothermem Plasma und Feststoff

On the boundary between nonthermal reactive plasma of molecular gases and solids gasspecific functional groups are formed by addition of plasmaspecies as pre-stages of destructive processes (chemical reactions and sputtering) or of formation of layers (polymeric). At various polymers it was observed that the vacuum ultra violet radiation occurs progressive changes in structure of layers near the surface (up to 100 μ). The formation of functional groups on the surface of solids is a quick process. It follows a period of slow penetration of plasma species in the first layers of the solid (some nm). The destruction of solids is not linear with time if the structure of solid is build up from some layers.     

Polymer-like Behavior of Inorganic Nanoparticle Chain Aggregates

Studies of the behavior of nanoparticle chain aggregates (NCA) have shown properties similar to those of molecular polymers. Like polymer chains, NCA tend to gather up and become more compact when heated. Under tensile stress, folded chain segments pull out and the NCA elongates. When the tension is relaxed, the chains contract. The stretching of NCA may contribute to the ductility of compacts made from nanoparticles, a subject of current research interest. In a well established technological application, carbon black and pyrogenic silica NCA produce remarkable increases in elastic modulus and tensile strength when added to commercial rubber. This may be due to the mechanical interaction between the polymer chains and NCA. However, basic mechanisms of NCA elasticity differ from those of molecular polymers. The alignment of chain segments when the NCA are subjected to tension probably results from rotation and translation at grain boundaries between neighboring nanocrystals. The elastic properties depend on the van der Waals forces between segments of the chain that fold to minimize surface free energy. Under tension, these segments pull out, but tend to reform when the tension is relaxed. The processes that lead to NCA formation and control the strength of interparticle bonds are briefly reviewed.     

Phase behavior and charge regulation of weak polyelectrolyte grafted layers

The stability of weak polyelectrolytes end grafted to a planar surface has been studied with a molecular theory. The effective quality of the solvent is found to depend on the interplay between polymer grafting density, acid-base equilibrium, and salt concentration. Our results reveal that increasing salt concentration results in a thermodynamically more stable layer. This reverse salt effect is due to the competition between the solvent quality and the dual role of the ionic strength in screening the electrostatic interactions (reducing stability with increasing salt concentration), and regulating the charge on the polymer (increasing charge with increasing salt concentration). Grafted weak polyelectrolyte layers are found to be thermodynamically unstable at intermediate surface coverages. Additionally, it is established that the increased solubility of the layer at low surface coverage is due to the relatively large charge of the grafted polymers. The range of stability of the film with regard to polymer surface coverage, temperature, bulk pH and salt concentration is demonstrated.     

Mean chain length of adsorbed supramolecular polymers

We present a theoretical study of reversible supramolecular polymers near an adsorbing surface. Mean chain lengths for free and adsorbed supramolecular polymers were calculated for a broad range of concentrations. As far as we know, this is the first report that describes a regime where the mean chain length decreases with increasing monomer concentration. It is shown that this anomalous behavior is caused by a change of the structure of the adsorbed layer.     

Hybrid complex polymers of boron and imidazole hydroxide

The possibility of forming supramolecular structures of a polymer complex consisting of products of polycondensation of boric acid and imidazole from aqueous solutions and melts of the components is shown. Depending on the ratio of the components, the heat resistance of the polymer complex lies within 60 and 480°C. The effect of sharp decrease in the heat resistance of the complex and the degree of polycondensation of boric acid upon a slight increase in the imidazole concentration above the stoichiometric ratio is revealed, which was explained by the “blocking” of hydroxyl groups by imidazole molecules and, correspondingly, by a significant decrease in their chemical activity. The energies of the intermolecular bonds in the complexes are calculated.     

Low-dimensional physics of ultracold gases with bound states and the sine-Gordon model

We present 2D steady concentration profiles of confined layers of off-critical polymer blends. The layer rests on a solid substrate and has a flat free surface due to very high surface tension. The profiles correspond to non-linear steady solutions of the Cahn-Hilliard equation in a rectangular domain. The free polymer-gas interface is considered to be sharp, while the internal interfaces are diffuse. We explore the rich solution structure (including laterally structured layers, stratified layers, checkerboard structures, oblique states and droplets) as a function of mean concentration.     

Effect of different solid matrixes on surface free energy of EGDMA and TRIM polymers

Advancing and receding contact angles of water, formamide, glycerol and diiodometane were measured on the two polymers; EGDMA (dimethacrylate of ethylene glycol) and TRIM (trimethacrylate-1,1,1-trihydroksymethylopropane) which were polymerized next to glass, silanized glass, stainless steel, mica and silicon surfaces as the matrices. Then from the contact angle hystereses (CAH) and van Oss, Good, Chaudhury (LWAB) approaches the apparent surface free energies were evaluated. The measured contact angles not only depend solely on the polymer chemical structure but also, to some extent, on the solid matrix next to whose surface the sample has polymerized. Surface free energy of the polymer samples calculated from the LWAB approach shows that they interact mainly by dispersive forces. The apparent surface free energy of the polymers calculated from the diiodomethane contact angles hysteresis is practically the same irrespective of the kind of the matrix used. Therefore it can be concluded that the observed weak polar interactions in the surface free energy of the samples depend on the polymer surface preparation. The AFM images show that the obtained surfaces are of different roughness. The RMS values of roughness range between 3.7-90.2 nm for EDGMA, and 5.3-124.5 nm for TRIM. However, as reported in literature, rather protrusions bigger than 1 μm may significantly affect the contact angles, especially the receding ones.     

Auger electron spectroscopy - a local probe for solid surfaces

The Auger electron transition in solids is discussed under the aspect of a local excitation due to the strongly localized primary hole in an inner atomic core level. In first approximation the solid is represented by a cluster model, consisting of the excited atom and its neighbors. Using this simple model it is possible to describe the Auger electron energies, intensities and line shapes of transitions in solids in a satisfactory way. Only for the angular dependent Auger emission, characteristic long-range crystalline order has to be taken into account. It is the aim of this introductory review to point out that Auger spectra bear more information about the solid surface and particularly on its chemical bonds as has yet been exploited by surface spectroscopists.     

On one supramolecular mechanism of the nonlinear viscoelasticity of oriented polymers

A phenomenological model of the viscoelasticity of highly oriented polymer systems is developed based on the results of studying the relaxation of such systems (mainly, fibers of polyethylene terephthalate, polyamide-6, polyvinyl alcohol, and other polymers) in the loaded state. The effect of an applied load on their relaxation spectra agrees qualitatively with the deformation behavior of crystal-like bandles that are present in amorphous intercrystalline layers of the fibrillar supramolecular structure.     

Microscopic theory of desorption of neutral atoms due to high excitonic density at the surfaces of III–V compounds

A microscopic theory of desorption of neutral atoms from the surface of crystalline GaP is presented. Derived results suggest that bonds are broken at the surface due to high excitonic density, so that a pair of excited holes can get localised on the same bond because such an excited state has much lower energy than that of a free exciton state. Any bond with a pair of holes, instead of covalent electrons, will be broken. Strong exciton-lattice interaction is assumed. It is argued that the mechanism of atomic desorption from surfaces is analogous to that of polymer ablation; and the desorption of neutral atoms increases linearly to super linearly with the increase in laser fluence. This agrees well with experiments.     

Hydrophilic surface formation on polymers and its applications

A new surface modification technique, so-called ion assisted reaction (IAR) has been developed at the Korea Institute of Science and Technology (KIST) Ion Beam Laboratory while modifying the surface of polymer results in many of industrial applications. The IAR, in which a keV ion beam is irradiated on the surface of polymer in reactive gases environment, has been developed for improving wettability of polymer surface and enhancing adhesion of other materials. The contact angles of water drops with modified polymers were significantly reduced by Ar⁺ ion irradiation with flowing oxygen gas environment than without flowing oxygen gas. Change of contact angles for the modified polymers was explained by a two-step chemical reaction among polymer matrix, energetic ions and oxygen gas. X-ray photoelectron analysis showed that hydrophilic groups were formed on the surface of polymers by chemical reaction between the unstable chains induced by ion irradiation and the oxygen gas, and the hydrophilic groups were identified as –(C–O)–, –(CO)– and –(CO)–O– bonds. The enhanced adhesion between metal and modified polymers was explained by the formation of charge transfer complex in polymer and electron donors in metal. Possible industrial applications of the IAR are to be discussed.     

Atomic-force microscopy of submicron films of electroactive polymer

Atomic-force microscopy is used to study the supramolecular structure of submicron films of electroactive thermally stable polymer (polydiphenylenephthalide (PDP)). It has been demonstrated that PDP films produced using centrifuging are solid homogeneous films with thicknesses down to several nanometers, which correspond to two or three monomolecular layers. The film volume is structurized at thicknesses greater than 100 nm. The study of the rheological properties of solutions used for film production yields a crossover point that separates the domains of strongly diluted and semidiluted solutions. A transition from the globular structure to the associate structure is observed in films that are produced using solutions with a boundary concentration. A model of the formation of polymer film that involves the presence of associates in the original solution is discussed.     

Influence of complexation and solid environment on the vibrational coherence of DCl

The vibrational dynamics of DCl molecules embedded in different cryogenic matrices is studied by time resolved infrared Degenerate Four Wave Mixing experiments using the infrared free electron laser CLIO. The coherence behaviour of the D-Cl stretch in pure argon as well as in mixed argon/nitrogen van der Waals solids is investigated. Different interactions between the excited molecules and their environment are probed: the van der Waals interaction between the trapped molecules and the solid lattice and the interaction binding complexes trapped in the matrix (either the vdW interaction between DCl and nitrogen or weak H bonds in (DCl)_n clusters). The dependence of the dephasing time on the solid and on specific interactions is clearly observed.     

Controlling Surface Interactions with Grafted Polymers

The interactions between surfaces modified with grafted polymers is studied theoretically. The aim of this work is to find polymer surface modifications that will result in localized attractive interactions between the surfaces. The practical motivation of the work is to find means to control the distance between bilayers and solid supports in supported membranes. Two theoretical approaches are used, the analytical treatment of Alexander and a molecular theory. It is found that grafting each end of the polymer to each surface results in an interaction with a well defined minimum. The location of the minima is found to be very close to the thickness of the polymer layer when the chains are grafted to only one of the surfaces. The predictions of the analytical theory are in excellent agreement with the molecular approach in this case. It is found that increasing the surface coverage increases the strength of the interaction. However, increasing the polymer chain length at fixed surface coverage results in a decrease of the free energy cost associated with separating the surfaces from their optimal distance. For the cases in which grafting to both surfaces is not possible, the molecular theory is used to study the effect of functionalizing segments of the chain to achieve an attractive well. It is found that by functionalizing the free end-groups of the polymers with segments attracted to the membrane, the range of the attractive interaction is significantly larger than the thickness of the unperturbed layer. Functionlizing the middle segments of the chains results in a shorter range attraction but of the same strength as in the end-functionalized layers. The optimal polymer modification is found to be such that the functionlized groups are attracted to the bare surface but are not attracted to the grafting surface. The relevance of the results to the design of experimental surface modifiers is discussed.     

Local entropic effects of polymers grafted to soft interfaces

In this paper, we study the equilibrium properties of polymer chains end-tethered to a fluid membrane. The loss of conformational entropy of the polymer results in an inhomogeneous pressure field that we calculate for Gaussian chains. We estimate the effects of excluded volume through a relation between pressure and concentration. Under the polymer pressure, a soft surface will deform. We calculate the deformation profile for a fluid membrane and show that close to the grafting point, this profile assumes a cone-like shape, independently of the boundary conditions. Interactions between different polymers are also mediated by the membrane deformation. This pair-additive potential is attractive for chains grafted on the same side of the membrane and repulsive otherwise. Received 20 April 2000     

Microscopic degradation mechanism of polyimide film caused by surface discharge under bipolar continuous square impulse voltage

Polyimide （PI） film is an important type of insulating material used in inverter-fed motors. Partial discharge （PD） under a sequence of high-frequency square impulses is one of the key factors that lead to premature failures in insulation systems of inverter-fed motors. In order to explore the damage mechanism of PI film caused by discharge, an aging system of surface discharge under bipolar continuous square impulse voltage （BCSIV） is designed based on the ASTM 2275 01 standard and the electrical aging tests of PI film samples are performed above the partial discharge inception voltage （PDIV）. The chemical bonds of PI polymer chains are analyzed through Fourier transform infrared spectroscopy （FTIR） and the dielectric properties of unaged and aged PI samples are investigated by LCR testers HIOKI 3532-50. Finally, the micro-morphology and micro-structure changes of PI film samples are observed through scanning electron microscopy （SEM）. The results show that the physical and chemical effects of discharge cut off the chemical bonds of PI polymer chains. The fractures of ether bond （C-O-C） and imide ring （C-N-C） on the backbone of a PI polymer chain leads to the decrease of molecular weight, which results in the degradation of PI polymers and the generation of new chemical groups and materials, like carboxylic acid, ketone, aldehydes, etc. The variation of microscopic structure of PI polymers can change the orientation ability of polarizable units when the samples are under an AC electric field, which would cause the dielectric constant e to increase and dielectric loss tan ~ to decrease. The SEM images show that the degradation path of PI film is initiated from the surface and then gradually extends to the interior with continuous aging. The injection charge could result in the PI macromolecular chain degradation and increase the trap density in the PI oolvmer bulk.     

Effect of strain on geometric and electronic structures of graphene on Ru(0001) surface

The atomic and electronic structures of a graphene monolayer on a Ru(0001) surface under compressive strain are investigated by using first-principles calculations. Three models of graphene monolayers with different carbon periodicities due to the lattice mismatch are proposed in the presence and the absence of the Ru(0001) substrate separately. Considering the strain induced by the lattice mismatch, we optimize the atomic structures and investigate the electronic properties of the graphene. Our calculation results show that the graphene layers turn into periodic corrugations and there exist strong chemical bonds in the interface between the graphene N×N superlattice and the substrate. The strain does not induce significant changes in electronic structure. Furthermore, the results calculated in the local density approximation (LDA) are compared with those obtained in the generalized gradient approximation (GGA), showing that the LDA results are more reasonable than the GGA results when only two substrate layers are used in calculation.     

Evolution of multilevel order in supramolecular assemblies

The process of self-assembly at multiple length scales of bis-urea substituted toluene on a Au(111) surface was studied by low temperature scanning tunneling microscopy. Pattern formation is controlled by specific hydrogen bonds between these molecules but also by significantly weaker lateral coupling between the resulting supramolecular polymers and a quasiepitaxial interlocking with the substrate. The ordered assemblies exhibit a tunnel transparency. Our experiments indicate the necessity of multiple interactions of different strengths for obtaining ordered structures with hierarchical levels of organization.     

Fluorescence Enhancement of a Polymer Planar Waveguide Doped with Rhodamine B and Ag Nanoparticles

Planar polymer multi-model waveguides doped with Ag nano-particles and rhodamine B are fabricated and investigated by the spectroscopy analysis method as well as the M-line method. Experimental results shown that fluorescence enhancement occurs when excited by a wide band wavelength with Ag nano-particle concentration at a certain level. The maximum enhancement factor in our experiment is obtained to be about 3.8 when excited by 350 nm. Our study may have potential applications in polymers optical elements, such as polymer waveguide lasers and amplifiers.     

Study of carboxylic functionalization of polypropylene surface using the underwater plasma technique

Non-equilibrium solution plasma treatment of polymer surfaces in water offers the possibility of more dense and selective polymer surface functionalization in comparison to the well-known and frequently used low-pressure oxygen plasma. Functional groups are introduced when the polymer surface contacts the plasma moderated solution especially water solutions. The emission of ions, electrons, energy-rich neutrals and complexes, produced by the ion avalanche are limited by quenching, with the aid of the ambient water phase. The UV-radiation produced in plasma formation also helps to moderate the reaction solution further by producing additional excited, ionized/dissociated molecules. Thus, monotype functional groups equipped polymer surfaces, preferably OH groups, originating from the dissociated water molecules, could be produced more selectively. An interesting feature of the technique is its flexibility to use a wide variety of additives in the water phase. Another way to modify polymer surfaces is the deposition of plasma polymers carrying functional groups as carboxylic groups used in this work. Acetic acid, acrylic acid, maleic and itaconic acid were used as additive monomers. Acetic acid is not a chemically polymerizing monomer but it could polymerize by monomer/molecular fragmentation and recombination to a cross linked layer. The other monomers form preferably water-soluble polymers on a chemical way. Only the fragmented fraction of these monomers could form an insoluble coating by cross linking to substrate. The XPS analysis was used to track the alterations in –O-CO- bond percentage on the PP surface. To identify the -COOH groups on substrate surface unambiguously, which have survived the plasma polymerization process, the derivatization with trifluoroethanol was performed.