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Paper des Monats
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A correlation of adhesion force and electrical conductivity is established for a vanadium oxide and highly oriented pyrolytic graphite (HOPG). Adhesion forces were determined by analysis of pull-off forces obtained from force–distance curves with atomic force microscopy in ultrahigh vacuum, on clean surfaces and with defined contact conditions. The investigated samples include (i) different stoichiometries of Magnéli-type vanadium oxides (V3O5, V4O7, V6O11, and V7O13), which undergo metal-insulator-transitions as a function of temperature, (ii) the (0001) basal plane and the (10–10) prism plane of highly oriented pyrolytic graphite (HOPG), which differ in their respective perpendicular-plane electrical conductivities by several orders of magnitude, and (iii) the (100) surfaces of pure metal single crystals, i.e., silver (Ag), copper (Cu), iron (Fe) and gold (Au). It is shown, that the vanadium oxides as well as the graphite exhibit significantly lower adhesion forces in their electrical conductive state than in their non-electrical conductive state. The values obtained for the electrical conductive states are quantitatively confirmed by the measurements on the single crystal metals.
Scanning force microscopy (SFM) has been used to assess the local mechanical properties of fibre-reinforced polymers. Using a sinusoidal displacement modulation (DM) and lock-in technique the method allows to characterize local viscoelastic properties with a high lateral resolution. The simultaneous measurement of the local electrical conductivity is proposed which facilitates the interpretation of the mechanical data. The investigation of cross-sections perpendicular to the axis of carbon fibres embedded in PPS delivers some information about the change in local stiffness within the interfacial region. As a first approach, assuming a single-exponential decrease in local stiffness along a radial line from fibre to polymer we find characteristic decay lengths which are distributed in a range between 20 and 80nm. Further, a modified DM-mode is proposed which is expected to provide a contrast enhancement of the signal which is related to local stiffness. This can be achieved by installing an additional feedback loop which keeps constant the amplitude of dynamic indentation (CDI-mode).
The physical properties as well as the thickness of the interphase between polymer and reinforcing phase are generally expected to show a strong influence on the macroscopic mechanical properties of composite materials. In this study the scanning force microscope (SFM) has been applied for detecting stiffness gradients near the interface between copper and epoxide. In displacement modulation mode the load exerted by the tip is modulated sinusoidally by vibrating the normal position of the cantilever or the sample. The corresponding amplitude of dynamic cantilever bending is related to local surface stiffness. Thus, using the SFM, the local mechanical properties of the surface under investigation can be probed. When scanning the tip across the reinforcement/polymer interface, the polymeric interphasial zone can be identified with the observed stiffness gradient. For a commercial epoxide based on diglycidyl ether of bisphenol A (DGEBA) cured with an amine-containing hardener, a stiffness gradient was detected that could be fitted by a Gaussian profile typical for diffusional processes. The width of the stiffness gradient was 280 nm. The observation is discussed in terms of interface-induced segregation processes between resin and hardener, as well as heat conduction mechanisms that can occur during the exothermic curing reaction. Both mechanisms are diffusional in nature and, via the local network structure, are expected to show some influence on the local stiffness of the cured epoxy. Copyright © 2000 John Wiley & Sons, Ltd.
We present dynamic plowing nanolithography on polymethylmethacrylate films, performed with a scan-linearized atomic force microscope able to scan up to 250 μm with high resolution. Modifications of the surface are obtained by plastically indenting the film surface with a vibrating tip. By changing the oscillation amplitude of the cantilever, i.e., the indentation depth, surfaces can be either imaged or modified. A program devoted to the control of the scanning process is also presented. The software basically converts the gray scale of pixel images into voltages used to control the dither piezo driving cantilever oscillations. The advantages of our experimental setup and the dependence of lithography efficiency on scanning parameters are discussed. Some insights into the process of surface modifications are presented.
Antimony clusters of different shapes and dimensions have been obtained by evaporating antimony on graphite. The dependence of the shape and dimensions of the particles on the evaporation parameters (effective layer thickness, temperature, pressure) is discussed. A characterisation of the different structures is presented. In particular, the decoration of graphite steps is discussed. Clusters have been dislocated by means of dynamic plowing nanolithography, both in vector and in image pattern mode. The dependence of the energy needed to dislocate a cluster on its dimensions and position is discussed.
Bulk samples and thin films of polyamides (PA6 and PA12) were exposed to fluorine (1 - 10 vol.-% F2 in N2) and analysed with photoelectron (XPS) and infrared spectroscopy. Fluorination affects both, the amide and the hydrocarbon parts of the polymers. However, only the carbon atom next to the carbonyl is readily fluorinated. Chemical modification of the amide group is apparent in a large binding energy shift (+5 eV) of the N1s level and the appearance of a CO band at 1734 cm-1. It is concluded that the amide C-N bond is cleaved in the fluorination process and that COOH and NF2 end groups are formed. This conclusion is corroborated by the appearance of ester oxygen in the XPS and by the 19F NMR spectra of the volatile products that show fluorine signals chemically shifted about 200 ppm towards lower field as compared with the CHF environment.
Thin polyamide-6 films were exposed to fluorine gas and analysed with photoelectron and infrared spectroscopies. Fluorine cleaves the amide CN bond resulting in the formation of the COOH and NF2 terminal groups. This is evident from large shifts in the N1s binding energy (+5 eV) and C=O stretching frequency (~80 cm-1), appearance of ester oxygen in the XPS spectra, as well as by the 19F nmr spectra of volatile products consistent with a terminal NF2 group.
Chemical, morphological and accumulation phenomena in ultrashort-pulse laser ablation of TiN in air
(2000)
Ultrashort-pulse laser ablation (F=130 fs, 5=800 nm, repetition rate 2-20 Hz) of titanium nitride was investigated for laser fluences between 0.3 and 4.5 J/cm2 using the direct focusing technique in air. The influence of the laser pulse number and the peak fluence was investigated by means of several surface analytical techniques (optical microscopy, dynamic friction atomic force microscopy, scanning Auger electron microscopy and small-spot electron spectroscopy for chemical analysis). The correlation of the results about optical, physical and chemical properties of the irradiated areas allows us to propose a simple oxidation model, which explains different observed phenomena associated with surface damage such as mound formation and crater widening and clarifies the incubation behavior reported earlier for this material.
Two different methods of nanolithography, namely dynamic plowing lithography (DPL) and indentation by means of force-displacement curves (FDI), have been compared by performing them on two different polymers, poly (methylmethacrylate) and polystyrene. No fundamental differences can be found out in the efficiency of the two methods, i.e., in the depth of the lithographed structure as a function of the scanning parameters. The main drawback of FDI is that it is much more time consuming than DPL. On the other hand, when the sample is lithographed with DPL, the border walls that surround the lithographed structure are very much bigger than the border walls created through FDI. The physicochemical properties of the border walls created through DPL have been investigated. Several experimental data reveal that such border walls are very much softer and looser than unmodified polymers and suggest that during DPL the fast oscillating tip is able to break polymer chains. This does not happen with FDI, where border walls are simply made up of the material carved out by the tip. A method to eliminate these undesirable border walls is suggested.
Ti:sapphire laser pulses of 130 fs and 800 nm were focused on 3.2-7m-thick TiN films by a 60-mm focal length lens in air. The morphology of the ablated areas generated by laser pulses at a fluence slightly above the ablation threshold was characterized in dependence on the pulse number by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The cavity profiles, depths, diameters, and volumes were quantitatively evaluated by AFM. The polarization state of the laser light is discussed as a further parameter, in addition to fluence and pulse number, that influences and controls the ablation precision of these materials. It was observed that circularly polarized radiation enhances the average ablation rates and reduces the roughness in the cavities by a factor of 2-3 as compared to linearly polarized radiation of the same incident laser fluence. Special attention was paid to the interfacial region between the coating and substrate. Ultrashort-pulse laser drilling into the Si substrate revealed the generation of columnar features which even may surmount the original coating under laser conditions.
A broadband dielectric study of the dispersions of silica spheres (Aerosil 380) in octylcyanobiphenyl (8CB) with densities between 0.083 and 10 g of silica per 1 cm3 of liquid crystal (LC) is reported. High values of silica densities were achieved for the first time to observe the behaviour of the LC monolayer on the surface of the silica particles. The relaxation characteristic of the bulk LC is noticed especially for the samples with low silica densities. Additionally, a slow relaxation was detected, even at temperatures for which the bulk LC is in the crystalline state. It was assigned to the surface layer of LC molecules. Remarkably, the temperature dependence of the relaxation rates for this slow process shows a behaviour typical for glass-forming liquids.
The adhesion of poly(methyl methacrylate) (PMMA) and polystyrene (PS) films, whose surface has been previously structured by dynamic plowing lithography (DPL), has been measured by means of forcedisplacement curves. The different adhesion of modified and unmodified PS leads to the assumption that polymer chains are broken during DPL. After measuring the energy dissipated by the tip during DPL, in order to check that the transferred energy is sufficient to break covalent bonds, the polymer chain scission caused by the lithographic process has been definitely confirmed by size exclusion chromatography measurements of the lithographed films.
Poly(methyl methacrylate) (PMMA) surfaces have been modified by dynamic plowing lithography. The resulting modified structures, presenting regions of different densities and stiffnesses, have been studied by forcedisplacement curves. The elasto-plastic response, the stiffness, and the adhesion of the modified samples have been characterized. Forcedisplacement curves are able to distinguish between unmodified PMMA and two different structures resulting from the lithography, i.e. compressed PMMA and low density PMMA. In particular, the low density regions are made up of cluster-like globular particles, whose elasto-plastic response and adhesion are measured and whose contribution to the total elasto-plastic response and to the adhesion has been studied in detail.
Based on an analysis of the diffusive heat flow equation, we determine limits on the localization of heating of soft materials and biological tissues by electromagnetically excited nanoparticles. For heating by rf magnetic fields or heating by typical continuous wave lasers, the local temperature rise adjacent to magnetic or metallic nanoparticles is negligible. However, heat dissipation for a large number of nanoparticles dispersed in a macroscopic region of a material or tissue produces a global temperature rise that is orders of magnitude larger than the temperature rise adjacent to a single nanoparticle. One approach for producing a significant local temperature rise on nanometer length scales is heating by high-power pulsed or modulated lasers with low duty cycle.
Force-displacement curves have been obtained with a commercial atomic force microscope (AFM) at different temperatures and probe rates on a thick film of poly(n-butyl methacrylate) (PnBMA). The analysis of the force-displacement curves has been focused on the contact portion of the curves, giving information about the stiffness of the sample and its Young's modulus. A novel model of sample deformations that extends the basic equations of the elastic continuum contact theories to the plastic deformations is presented. This model gives several insights into the processes of deformation of soft samples and permits to calculate not only the parameters of the Williams-Landel-Ferry equation but also the Young's modulus and the yielding force of the polymer as a function of temperature and/or probe rate. These quantities have been measured in a wide range of temperatures (70 K) and probe rates (6 decades) for the first time with the AFM, and the results are in very good agreement with measurements performed with customary techniques, such as broadband spectroscopy and dynamic mechanical analysis.
Structure formation on the surface of indium phosphide irradiated by femtosecond laser pulses
(2005)
Laser-induced periodic surface structures (LIPSS; ripples) with different spatial characteristics have been observed after irradiation of single-crystalline indium phosphide (c-InP) with multiple linearly polarized femtosecond pulses (130 fs, 800 nm) in air. With an increasing number of pulses per spot, N, up to 100, a characteristic evolution of two different types of ripples has been observed, i.e., (i) the growth of a grating perpendicular to the polarization vector consisting of nearly wavelength-sized periodic lines and (ii), in a specific pulse number regime (N = 530), the additional formation of equally oriented ripples with a spatial period close to half of the laser wavelength. For pulse numbers higher than 50, the formation of micrometer-spaced grooves has been found, which are oriented perpendicular to the ripples. These topographical surface alterations are discussed in the frame of existing LIPSS theories.
Observation of nanoscale hot-spot generation on a 2, 4, 6-trinitrophenol (TNP) single crystal
(2011)
An energetic material (EM) creates a local thermal point when it accepts energy and the EM will explode due to thermal decomposition when the hot-spot temperature reaches the explosive temperature. The initial stage of the hot-spot formation on 2, 4, 6-trinitrophenol (TNP) single crystal has been observed at the nanometer scale using atomic force microscopy (AFM). The hot-spot generation has been directly initiated by means of an AFM tip. The phenomenon is accompanied by producing agglomerated spherical-like nanoparticles due to a low-temperature decomposition of TNP. It has been observed that the agglomerated particles are produced outside of the stimulated region. The observed decomposition process at the nanoscale doesn't lead to a self-sustaining shock as detonation or explosion. The results are of interest in relation to explore the origin of deformation-induced chemical decomposition or detonation of EM.
Layered silicate epoxy nanocomposites: formation of the inorganic-carbonaceous fire protection layer
(2011)
The layered silicate (LS) modification and processing parameters applied control the morphology of the LS/polymer composites. Here, increasing the surface area of the LS particles by using alternative drying processes increases dispersion towards a more typical nanocomposite morphology, which is a basic requirement for promising flame retardancy. Nevertheless, the morphology at room temperature does not act itself with respect to flame retardancy, but serves as a prerequisite for the formation of an efficient surface protection layer during pyrolysis. The formation of this residue layer was addressed experimentally for the actual pyrolysis region of a burning nanocomposite and thus our results are valid without any assumptions or compromises on the time period, dimension, surrounding atmosphere or temperature. The formation of the inorganic-carbonaceous residue is influenced by bubbling, migration, reorientation, agglomeration, ablation, and perhaps also delamination induced thermally and by decomposition, whereas true sintering of the inorganic particles was ruled out as an important mechanism. Multiple, quite different mechanisms are relevant during the formation of the residue, and the importance of each mechanism probably differs from one nanocomposite system to another. The main fire protection effect of the surface layer in polymer nanocomposites based on non-charring or nearly non-charring polymers is the increase in surface temperature, resulting in a substantial increase in reradiated heat flux (heat shielding).
Analytical and numerical analysis of imaging mechanism of dynamic scanning electron microscopy
(2012)
The direct observation of small oscillating structures with the help of a scanning electron beam is a new approach to study the vibrational dynamics of cantilevers and microelectromechanical systems. In the scanning electron microscope, the conventional signal of secondary electrons (SE, dc part) is separated from the signal response of the SE detector, which is correlated to the respective excitation frequency for vibration by means of a lock-in amplifier. The dynamic response is separated either into images of amplitude and phase shift or into real and imaginary parts. Spatial resolution is limited to the diameter of the electron beam. The sensitivity limit to vibrational motion is estimated to be sub-nanometer for high integration times. Due to complex imaging mechanisms, a theoretical model was developed for the interpretation of the obtained measurements, relating cantilever shapes to interaction processes consisting of incident electron beam, electron–lever interaction, emitted electrons and detector response. Conclusions drawn from this new model are compared with numerical results based on the Euler–Bernoulli equation.
In this work, Pb(Zr,Ti)O3 thin films were used to fabricate well-ordered nanodot arrays by means of nanosphere lithography. This technique is based on a two-step etch process that enables excellent control of the fabrication of ordered nanodisc arrays of defined height, diameter, and pitch. Piezoresponse force microscopy was used to investigate both non-patterned and patterned films. The topography and both the out-of-plane and the in-plane polarization were deduced in this mode. Grains of nanodots with a low aspect ratio form domain structures comparable to domains in non-patterned two-dimensional films. In contrast, nanodots with a higher aspect ratio form particular structures like bi-sectioned domain assemblies, c-shaped domains or multi-domains surrounding a center domain. The patterning of the ferroelectric material was shown to affect the formation of ferroelectric domains. The initial polycrystalline films with random polarization orientation re-orient upon patterning and then show domain structures dependent on the nanodisc diameter and aspect ratio.
Stiffness variation in the interphase of amine-cured epoxy adjacent to copper microstructures
(2007)
In an effort to expand the understanding of the mechanical properties of the polymeric interphase on a metal surface, a composite consisting of epoxy and copper was prepared and analyzed. Scanning force microscopy-based force modulation microscopy (SFM-FMM) was employed along with dynamic mechanical analysis (DMA) and energy dispersive X-ray analysis (EDX). Diglycidyl ether of bisphenol A (DGEBA)-based epoxy resins were applied with amine curing agents. The samples were made taking advantage of electron beam lithography (EBL) in order to produce sharp edges of copper structures and a flat surface suitable for the SFM-FMM analysis, which was able to depict the stiffness within the interphase. It is considered significant information because the mechanical characteristic within the narrow interphase was revealed. Comparing with DMA and EDX, the stiffness information of SFM-FMM demonstrated a matching correlation and agreement in terms of preferential adsorption of the curing agent in the vicinity of the interface. The stiffness profiles of the two epoxy systems turned out to be different, and it shows the material dependence of the interphase characteristics.
Low-energy secondary electrons are the most abundant radiolysis species which are thought to be able to attach to and damage DNA via formation and decay of localized molecular resonances involving DNA components. In this study, we analyze the consequences of low-energy electron impact on the ability of DNA to hybridize (i.e., to form the duplex). Specifically, single-stranded thymine DNA oligomers tethered to a gold surface are irradiated with very low-energy electrons (E = 3 eV, which is below the 7.5 eV ionization threshold of DNA) and subsequently exposed to a dye-marked complementary strand to quantify by a fluorescence method the electron induced damage. The damage to (dT)25 oligomers is detected at quite low electron doses with only about 300 electrons per oligomer being sufficient to completely preclude its hybridization. In the microarray format, the method can be used for a rapid screening of the sequence dependence of the DNA-electron interaction. We also show for the first time that the DNA reactions at surfaces can be imaged by secondary electron (SE) emission with both high analytical and spatial sensitivity. The SE micrographs indicate that strand breaks induced by the electrons play a significant role in the reaction mechanism.
Moderne Leichtbaustrukturen (Luftfahrt, Eisenbahn, Automotive) aus Faserverbundwerkstoffen (FVW) sind hohen mechanischen und klimatischen Beanspruchungen ausgesetzt. Aussagen über die Lebensdauer und Funktionssicherheit der FVW-Bauteile können eher aus Untersuchungen der mikromechanischen Eigenschaften gewonnen werden als aus der globalen Festigkeit. Die Röntgenrefraktionstopografie ist ein Verfahren zur zerstörungsfreien Bestimmung von inneren Oberflächen in Werkstoffen. Angewendet auf FVW lassen sich Aussagen über die Faserorientierungen, die Fertigungsqualität sowie über Faser-Matrix-Ablösungen und Matrixrisse infolge mechanischer Beanspruchung gewinnen. Nachfolgend wird ein neues Verfahren zur Bestimmung des Zwischenfaserbruchversagens am komplexen Laminat vorgestellt. Hierbei wird das Röntgenrefraktionsverfahren "online bei Zug-Belastung der Probe angewendet.
Laser ablation of single-crystalline indium phosphide (InP) was performed in air by means of linearly polarized Ti : sapphire femtosecond pulses (800 nm, 130 fs, 10 Hz). As a result of the irradiation with a variable number of laser pulses per spot (N /spl les/ 5), several morphological changes (crater formation, rim formation, ripple structures, and cones) were observed. These effects were explored using force modulation microscopy (FMM), a technique based on scanning force microscopy, allowing the simultaneous imaging of both topography and local stiffness at a high lateral resolution. The first laser pulse induces the formation of a protruding rim (height < 20 nm, width /spl ap/ 300 nm) bordering the ablated crater. A Fourier analysis of the multipulse generated topographies reveals the formation of wavelength-sized periodic ripples (modulation depth < 100 nm) with an orientation perpendicular to that of the electric field vector of the laser radiation. Besides these morphological alterations, material modifications were also observed in the irradiated regions by means of the FMM technique. Within the ablated craters, local stiffness variations were found revealing an inhomogeneous material composition/structure as a consequence of the femtosecond pulse laser treatment.
A stoichiometric amineepoxy formulation was cured in the presence of a thermoplastic, namely poly(vinylpyrrolidone) (PVP). The epoxy system consisted of the resin diglycidyl ether of bisphenol A (DGEBA) and the aromatic curing agent 4,4'-diaminodiphenylsulfone (DDS). As shown for this system in a former study by Oyama et al. [Oyama HT, Lesko JJ, Wightman JP. J Polym Sci B 1997;35:33146. [36]], preferential absorption of amine molecules by PVP can occur. In the present study, the focus is on the variations of local elastic properties within the epoxy interphase adjacent to the PVP layer. The curing was performed close to the glass transition temperature, Tα, of the PVP film, namely at 170 °C. Variations of the local amine concentration were tracked using energy-dispersive analysis of X-rays (EDX), by taking benefit of the sulfur contained in DDS. Using temperature-dependent dynamic mechanical analysis (DMA), a series of epoxy reference samples of different amineepoxy concentration ratios, r, was investigated in order to work out the relationship between r and the epoxy storage modulus at room temperature. In the excess-epoxy regime, r<1, the modulus is observed to increase with departure from the stoichiometric ratio, r=1. Considering the respective suppression of the ß-transition, the observed characteristic can be explained by an antiplasticisation effect. Depth-sensing indentation (DSI) experiments across the epoxy/PVP interphase provided evidence for strong modulus variations. In consistency with the EDX and the DMA data, in the vicinity of the PVP layer the local epoxy modulus is increased. The total change of the epoxy Young's modulus is ~1.1 GPa. However, the total width of the modulus decay of ~175 µm is ~2.5 times larger than the one of the DDS concentration gradient. This finding is discussed in terms of additional spatial variations of the DGEBA concentration as well as long-range diffusion currents of DDS induced by the interdiffusion processes and their effect on the final network of crosslinks.
Femtosecond time-resolved microscopy has been used to analyze the structural transformation dynamics (melting, ablation, and solidification phenomena) induced by single intense 130 fs laser pulses in single-crystalline (100)-indium phosphide wafers in air on a time scale from ~100 fs up to 8 ns. In the ablative regime close to the ablation threshold, transient surface reflectivity patterns are observed by fs microscopy on a ps to ns time scale as a consequence of the complex spatial density structure of the ablating material (dynamic Newton fringes). At higher fluences, exceeding six times the ablation threshold, optical breakdown causes another, more violent ablation regime, which reduces the energy deposition depth along with the time of significant material removal. As a consequence, ablation lasts longer in a ring-shaped region around the region of optical breakdown. This leads to the formation of a crater profile with a central protrusion. In the melting regime below the ablation threshold, the melting dynamics of indium phosphide has been quantified and subsequent superficial amorphization has been observed upon solidification on the ns time scale leading to amorphous layer thicknesses of the order of a few tens of nanometers.
The pyrolysis and fire behavior of glass-fiber reinforced poly(butylene terephthalate) (PBT/GF) with two different metal phosphinates as flame retardants in combination with and without melamine cyanurate (MC) were analyzed by means of thermogravimetry, thermogravimetry coupled with infrared spectroscopy, flammability, and cone calorimeter tests as well as scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray fluorescence spectroscopy. In PBT/GF, dosages of 13-20% of the halogen-free flame retardant aluminum phosphinate or aluminum phosphinate in combination with MC fulfill the requirements for electrical engineering and electronics applications (UL 94 = V-0; LOI > 42%), whereas the use of the same amount of zinc phosphinate or zinc phosphinate in combination with MC does not improve the fire behavior satisfactorily (UL 94 = HB; LOI = 27-28%). The performance under forced flaming conditions (cone calorimeter) is quite similar for both of the metal phosphinates. The use of aluminum and zinc salts results in similar flame inhibition predominantly due to the release of the phosphinate compounds in the gas phase. Both metal phosphinates and MC interact with the polymer changing the decomposition characteristics. However, part of the zinc phosphinate vaporizes as a complete molecule. Because of the different decomposition behavior of the metal salts, only the aluminum phosphinate results in a small amount of thermally stable carbonaceous char. In particular, the aluminum phosphinate-terephthalate formed is more stable than the zinc phosphinate-terephthalate. The small amount of char has a crucial effect on the thermal properties and mechanical stability of the residue and thus the flammability.
Thermal, thermo-oxidative and fire residues of high impact polystyrene/magnesium hydroxide/red phosphorus (HIPS/Mg(OH)2/Pr) are investigated by solid-state NMR and compared with the results for the binary subsystem Mg(OH)2/Pr. The influences of oxygen, nitrogen and temperature are discussed. For a thermal decomposition and pyrolysis during combustion, the main pyrolysis of HIPS takes place while the remaining residue is a rather intact polymer, with a major share of the embedded Pr still present. Subsequently, mainly amorphous phosphates and a slight amount of crystalline Mg3(PO4)2 and Mg2P2O7 are formed at the highest temperatures. Only with increasing mass loss does the remaining polystyrene structure decompose and graphitic structures occur. The influence of oxygen on the decomposition mechanism is most obvious for the binary system Mg(OH)2/Pr. Pr vanishes more rapidly and crystalline, oxygen-rich magnesium phosphates are formed. In HIPS/Mg(OH)2/Pr systems the polymer acts as a barrier to reaction by the embedded particles, so that major characteristics of an anaerobic decomposition are found. Significant amounts of phosphorus are retained in the condensed phase through a reaction of Pr with Mg(OH)2 to mostly amorphous phosphates. This formation of amorphous inorganic magnesium phosphates can act as an additional physical barrier. This study outlines some advanced approaches for controlling the condensed-phase mechanisms of phosphorus and underlines that solid-state NMR is a most powerful tool for investigating the organic and inorganic residues.
Applying a high-frequency lateral vibration between the sample surface and the tip of a scanning force microscope (SFM), a harmonically modulated lateral (friction) force image can be obtained using lock-in techniques. The principles of operation are explained, in particular the dramatic decrease of image artefacts generally caused by topography cross-talk and laser beam interference. Flat interfaces between the two immiscible polymers, poly(methyl methacrylate) (PMMA) and polystyrene (PS), are prepared on a sodium chloride single crystal from the melt. These samples are used to evaluate the appropriate model for the tip-sample interaction geometry. The relationship between frictional and normal force does not follow Amonton's law. This shows that a single-asperity interaction between the tip and sample surface can be considered. Using the new technique, local measurements of shear strength and Young's modulus can be performed.
Applying a high-frequency lateral vibration between tip and sample in a scanning force microscope (SFM), a harmonically modulated lateral (friction) force image can be obtained using lock-in techniques. Harmonically modulated lateral force microscopy (HM-LFM) offers several advantages compared with standard lateral force microscopy (LFM). After a brief investigation of the scan velocity dependence of LFM and HM-LFM, two samples were investigated. First, the surface of a poly(acrylonitrile-co-styrene)/polybutadiene blend (ABS) was used to demonstrate the ability of the new technique to decrease the stick effects of the SFM tip. Second, an interface between two chemically very similar polymers was prepared by melting polypropylene (PP) and poly(propene-block-ethene) (PP-block-PE) films on each other. After cutting, the surface roughness of this sample was very high. It is shown that only HM-LFM clearly resolves the local micromechanical properties without artefacts.
The use of a fibre-based light sensor for the calibration of Scanning Probe Microscopy piezos
(1999)
After a delamination process in a C-fiber reinforced polymer composite, the identification of the polymer residue of poly-(phenylene sulphide) on carbon fiber surfaces gives important information on the composite failure mechanism. Using scanning force microscopy (SFM) in its lateral force imaging mode or with the stiffness imaging mode, it is almost impossible to distinguish the two composite components owing to a very low material-based contrast of the local friction and compliance. The new technique using contrast based on a local AC current measurement allows a clear identification of the conducting carbon fiber and the non-conducting polymer coverage.
We report surface plasmon resonance (spr) and confocal fluorescence results concerning the interaction of a gene-5-protein (g5p) with single-stranded DNA oligonucleotides (dT25) tethered to a gold surface. The spr data show that a highly stable g5pssDNA complex is readily formed on a gold surface with koff = 1.7 × 10-3 s-1. The extent of the complexion indicated involvement of the cooperative proteinprotein interactions within the binding to DNA. In the experiments where dT25 coexist with g5p in the solution, the fluorescence data show that g5p also mediates the binding between the non-complementary oligonucleotides in the solution and those immobilised on the surface.
For the first time, we report the direct observation of the nanoscale adsorption and desorption of 2,4,6-trinitrophenol (TNP) molecules on the crystal surface during atomic force microscopy scanning. Our results reveal the position instability of TNP molecules at the border of molecular layers under normal ambient conditions. The observed phenomena are of great interest for exploring the origin of hot-spot generation on the surface of energetic materials. On the basis of the explored properties of TNP crystal, a plausible mechanism of hot-spot formation upon weak initiation at the nanoscale is proposed.
Heterogeneous, gradual or structured morphology of fire residues plays an important role in fire retardancy of polymers. A scanning electron microscope with an attached energy dispersive X-ray spectrometer (SEM/EDX) is highlighted as a powerful tool for the advanced characterization of such complex fire residues, since it offers high resolution in combination with both good depth of field and analysis of chemical composition. Two examples are presented: First, comprehensive SEM/EDX investigation on a complex structured fire residue of glass fibre reinforced polyamide 6,6 (PA 66-GF) flame retarded by diethylaluminium phosphinate, melamine polyphosphate and some zinc borate. A multilayered surface crust (thickness ~ 24 µm) covers a rather hollow area stabilized by GF glued together. The resulting efficient thermal insulation results in self-extinguishing before pyrolysis is completed, even under forced-flaming combustion. Second, sophisticated, quasi online SEM/EDX imaging of the formation of residual protection layer in layered silicate epoxy resin nanocomposites (LSEC). Burning specimens were quenched in liquid nitrogen for subsequent analyses. Different zones were distinguished in the condensed phase characterized by distinct processes such as melting and ablation of organic material, as well as agglomeration, depletion, exfoliation and reorientation of the LS.
A specially designed scanning force microscope (SFM, atomic force microscope – AFM) was incorporated into the chamber of a commercial scanning electron microscope (SEM) to investigate vibrating SFM cantilevers at their resonance. Inherently, the spatial resolution of electron microscopy is higher than that of optical methods. In this paper we present vibration modes (eigenmodes) of two different SFM cantilevers. Their nonlinear behavior is also explored in order to depict their 2nd harmonics (twice the fundamental frequency). Imaging of the local vibration is performed by measuring the frequency- and phase-selective responses of the SE signal at different X and Y positions of the scanned electron beam.
We report a new sequence selective terpolymerisation in which three monomers (butylene oxide (BO) A, PhNCS B and phtalic thioanhydride (PTA) C) are selectively enchained into an (ABA′C)n sequence. PTA/PhNCS/BO ring-opening terpolymerisation ROTERP can be coupled with CS2 ROTERP to generate tetrapolymers and with εDL ROP in switchable catalysis for blockpolymer synthesis.
Filigree structures can be manufactured via two-photon-polymerization (2PP) operating in the regime of non-linear light absorption. For the first time it is possible to apply this technique to the powder processing of ceramic structures with a feature size in the range of the critical defect size responsible for brittle fracture and, thus, affecting fracture toughness of high-performance ceramics. In this way, tailoring of advanced properties can be achieved already in the shaping process. Traditionally, 2PP relies on transparent polymerizable resins, which is diametrically opposed to the usually completely opaque ceramic resins and slurries. Here we present a transparent and photocurable suspension of nanoparticles (resin) with very high mass fractions of yttria-stabilized zirconia particles (YSZ). Due to the extremely well dispersed nanoparticles, scattering of light can be effectively suppressed at the process-relevant wavelength of 800 nm. Sintered ceramic structures with a resolution of down to 500 nm were obtained. Even at reduced densities of 1 to 4 g/cm³, the resulting compressive strength with 4,5 GPa is equivalent or even exceeding bulk monolithic yttria stabilized zirconia. A ceramic metamaterial is born, where the mechanical properties of yttria stabilized zirconia are altered by changing geometrical parameters and gives access to a new class of ceramic materials.
Mussel-inspired polydopamine (PDA) initiated a multifunctional modification route that leads to the generation of novel advanced materials and their applications. However, existing PDA deposition techniques still exhibit poor spatial control, have a very limited capability of micropatterning and do not allow to locally tune PDA topography. Herein, we demonstrate PDA deposition based on Multiphoton Lithography (MPL) that enables full spatial and temporal control with nearly total freedom of patterning design. Using MPL, we achieve 2D microstructures of complex design with pattern precision of 0.8 μm without the need of a photomask or stamp. Moreover, this approach permits adjusting the morphology and thickness of the fabricated microstructure within one deposition step, resulting in a unique tunability of materials properties. The chemical composition of PDA is confirmed and its ability for protein enzyme immobilization is demonstrated. This work presents a new methodology for high precision and complete control of PDA deposition, enabling PDA incorporation in applications where fine and precise local surface functionalization is required. Possible applications include multicomponent functional elements and devices in microfluidics or lab-on-a-chip systems.
Though the reinforcing properties of inorganic particles in thermosetting nanocomposites, has been exploited, the integration of nanoparticles continues to be challenging in terms of their homogeneous distribution and their manipulation which can contribute to occupational hazards. Due to a second encapsulations of nanoparticles, electrospun nanocomposite fibers containing nanoparticles might be an alternative for overcoming these issues, as the fiber nonwovens contains the nanoparticles allowing for safer manipulation. Here, the morphology, and the thermal properties of electrospun polycarbonate fibers containing taurine modified boehmite nanoparticles (BNP) are investigated by means of small and wide-angle X-ray scattering as well as fast scanning and temperature modulated fast scanning calorimetry for the first time. The latter techniques allow the investigation of the thermal properties of single fibers at heating rates up to 10^4 K s^-1 keeping its structure intact. A quantitative analysis of the scattering data reveals a porous structure of the fibers. The porous structure is quantified regarding the pore volume and the pore size. A constant amount of aggregation is found even for the highly BNP loaded fibers. Thermal analysis on the fibers reveals a rigid amorphous fraction (RAF) where it is known that RAF determinates the properties of a nanocomposite to a large extent. For the fibers RAF amounts up to 40 wt%, which is essential higher compared to equally formulated PC/BNP composite cast films. The RAF in the case of the fibers, is not only due to the presence of particles in the polymer but also due to orientation effects induced by the electrospinning process.
Deep understanding of the effects associated with fabrication parameters and their influence on the resulting structures shape is essential for the further development of direct laser writing (DLW). In particular, it is critical for development of reference materials, where structure parameters are precisely fabricated and should be reproduced with use of DLW technology. In this study we investigated the effect of various fabrication and preparation parameters on the structural precision of interest for reference materials. A well-studied photo-curable system, SZ2080 negative photo-resist with 1 wt.% Michler's ketone (Bis) photo-initiator, was investigated in this work. The correlation between applied laser power, laser velocity, fabrication direction on the deviations in the structure shape were observed by means of white light interferometry microscopy. Moreover, influence of slicing and hatching distances as well as prebake time were studied as function of sample shape. Deviations in the structure form between the theoretically expected and the one detected after DLW fabrication were observed in the range up to 15%. The observed shape discrepancies show the essential importance of fine-tuning the fabrication parameter for reference structure production.
The reinforcing effect of boehmite nanoparticles (BNP) in epoxy resins for fiber composite lightweight construction is related to the formation of a soft but bound interphase between filler and polymer. The interphase is able to dissipate crack propagation energy and consequently increases the fracture toughness of the epoxy resin. Usually, the nanoparticles are dispersed in the resin and then mixed with the hardener to form an applicable mixture to impregnate the fibers. If one wishes to locally increase the fracture toughness at particularly stressed positions of the fiber-reinforced polymer composites (FRPC), this could be done by spraying nanoparticles from a suspension. However, this would entail high costs for removing the nanoparticles from the ambient air. We propose that a fiber fleece containing bound nanoparticles be inserted at exposed locations. For the present proof-of-concept study, an electrospun polycarbonate nonwoven and taurine modified BNP are proposed. After fabrication of suitable PC/EP/BNP composites, the thermomechanical properties were tested by dynamic mechanical analysis (DMA). Comparatively, the local nanomechanical properties such as stiffness and elastic modulus were determined by atomic force microscopy (AFM). An additional investigation of the distribution of the nanoparticles in the epoxy matrix, which is a prerequisite for an effective nanocomposite, is carried out by scanning electron microscopy in transmission mode (TSEM). From the results it can be concluded that the concept of carrier fibers for nanoparticles is viable.
The paper presents the first complex study of Gamma radiation effects on a low-loss perfluorinated polymer optical fiber (PF-POF) based on Cytop® polymer. Influence of gamma radiation on fiber’s optical, mechanical and climatic performance is investigated. The radiation-induced attenuation (RIA) in the visible and near-infrared region (0.4 μm-1.7 mm) is measured and its origins are discussed. Besides attenuation increase, radiation is also shown to decrease the thermal degradation stability of the fiber and to increase its susceptibility to water. With regard to complex fiber transmission performance upon irradiation, the optimal operation wavelength region of PF-POF-based systems intended for use in Radiation environments is determined to be around 1.1 μm. On the other hand, the investigated fiber holds potential for low-cost RIA-based optical fiber dosimetry applications with sensitivity as high as 260 dBm⁻¹/kGy in the visible region.
Central venous port devices made of two different polymeric materials, thermoplastic polyurethane (TPU)and silicone rubber (SiR), were compared due their material properties. Both naïve catheters as well as catheters after removal from patients were investigated. In lab experiments the influence of various chemo-therapeutic solutions on material properties was investigated, where as the samples after removal were compared according to the implanted time inpatient. The macroscopic,mechanical performance was assessed with dynamic, specially adapted tests for elasticity. The degradation status of the materials was determined with common tools of polymer characterisation, such as infrared spectroscopy, molecular weight measurements and various methods of thermal analysis. The surface morphology was an alysed using scanning electron microscopy. A correlation between material properties and clinical performance was proposed. The surface morphology and chemical composition of the polyurethane catheter materials can potentially result in increased susceptibility of the catheter to bloodstream infections and thrombotic complications. The higher mechanic failure,especially with increasing implantation time of the silicone catheters is related to the lower mechanical performance compared to the polyurethane material as well as loss of barium sulphate filler particles near the surface of the catheter. This results in preformed microscopic notches, which act as predetermined sites of fracture.
Waste disposal of carbon nanotube (CNT) containing products is expected to be the most important pathway for release of CNTs into the environment. In the present work, the use of radiolabelled CNTs (14C-CNT) for polycarbonate polymer nanocomposites with 1 wt% 14C-CNT content allowed for the first time to quantify and differentiate the CNT release according to the type of impact along the materials' ageing history. After an initial exposure of the nanocomposite by solar-like irradiation, further environmental impacts were applied to composite material. They aimed at mimicking disposal site conditions that may induce further ageing effects and CNT release. This study included shaking in water, rapid temperature changes, soaking in humic acid solution as well as waste water effluent, and, finally, gentle mechanical abrasion. All ageing impacts were applied sequentially, both on pristine (control) and on solar-irradiated nanocomposites. All experiments were accompanied by absolute quantification of radioactive release as well as chemical and morphological analyses of the nanocomposite surfaces using infra-red (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The morphological analysis showed that spectral irradiation can uncover CNT networks on the outer nanocomposite surface layers by polymer degradation. After having subjected the solar-irradiated nanocomposite to all studied disposal site effect, the total radioactive release was quantified to amount to 64 mg CNT/m2, whereas only 0.8 mg CNT/m2 were found for the un-irradiated control sample. Solar degradation of polymers was thus found to significantly increase the propensity of the studied polymer nanocomposites to release CNTs during ageing effects at the product's end-of-life typical for disposal sites.