AO-resistant properties of polyimide fibers containing phosphorous groups in main chains

A series of polyimide fibers containing phosphorus element derived from (3-aminophenyl) methyl phosphine oxide (DAMPO) diamine was exposed to an artificial atomic oxygen environment which simulated the space environment in low earth orbit (LEO). The mass loss, surface morphology, chemical composition, and mechanical properties of the fibers before and after atomic oxygen (AO) exposure were compared in detail with a blank sample. Results showed that the phosphor-containing fibers demonstrated lower mass change and less tensile strength reduction. SEM results showed that the fibers with phosphorous element had relatively dense surface after AO exposure. Meanwhile, XPS results indicated that a passivated phosphate layer, which could protect the following under-layer from attacking by AO, was formed on the surface of the fibers. These results indicated that the incorporation of diamine (DAMPO) into the main chains could protect the fibers for avoiding further erosion from AO exposure. Hence, the phosphor-containing PI fibers exhibits potential application in space fields.     

The effect of low earth orbit atomic oxygen exposure on phenylphosphine oxide-containing poly(arylene ether)s

Unoriented thin films of phenylphosphine oxide-containing poly(arylene ether)s were exposed to low Earth orbit aboard the space shuttle Atlantis (STS-51) as part of a flight experiment designated Limited Duration Candidate Exposure (LDCE 4–5). The samples were exposed to primarily atomic oxygen (!10,\7×10¹⁹ atoms/cm²). Based on post-flight analyses using atomic force microscopy, X-ray photoelectron spectroscopy, gel permeation chromatogrpahy and weight loss data, it was proposed that atomic oxygen exposure of these materials produces a phosphate layer at the surface of the samples, apparently by the reaction of atomic oxygen with the phosphorus in the polymer backbone. Ground-based oxygen plasma exposure experiments have previously shown that this phosphate layer provides a barrier against further attack by atomic oxygen [1]. The results obtained from these analyses compare favorably with those obtained from samples exposed to an oxygen plasma in ground-based exposure experiments [1]. © 1998 John Wiley & Sons, Ltd.     

The antioxygen radiation properties of the nanocomposite film consisted of hydrogenated amorphous carbon (a-C:H) and MoS2

MoS₂/a-C:H multilayer film and MoS₂/a-C:H composite film exhibit excellent tribological properties in vacuum, which can be used as the potential space lubricant. The radiation-protective properties of these two films in atomic oxygen (AO) are evaluated. The influences of AO radiation on structure, morphology, and tribological properties of the films were investigated. The results show that AO radiation mainly causes oxidation and increases sp² C content in both of the films. Furthermore, the MoS₂ sublayer on the surface of the multilayer film is oxidized heavily, whereas both the MoS₂ and the a-C:H matrix on the surface were oxidized in the composite film. As a result of this, the multilayer film exhibits high friction coefficient and short sliding lifetime in vacuum after AO radiation. Compared with that, the composite film exhibits lower friction and longer sliding time more than 3600 seconds in vacuum, which illustrates it has a good AO radiation protection. This indicates that MoS₂/a-C:H composite film is more likely to be used as a potential space lubricant.     

Ultrathin Rh films on Ru(0001): oxidation in confinement

Ultrathin rhodium films with a thickness ranging from 1 to a few monolayers were deposited on a single-crystal Ru(0001) surface in order to investigate the oxidation behavior of ultrathin epitaxial films on a dissimilar substrate. It is found that rhodium grows on Ru(0001) initially layer by layer, adapting the in-plane lattice parameters of Ru(0001). When exposing Rh films to oxygen environment (approximately 4.8 x 10(6) L O2 exposure) at 660 K, 2-4 ML Rh films form a surface oxide composed of (9 x 9) O-Rh-O trilayers. Quite in contrast, oxidation of the 1 ML RhRu(0001) film leads to a poorly ordered oxide with a rutile structure reminiscent of RuO2(110) on Ru(0001). The oxidized 1 ML RhRu(0001) film contains much more oxygen than the oxidized thicker Rh films. Lower temperatures (535 K) and high doses of oxygen lead to a (1 x 1)-O overlayer on the 1 ML RhRu(0001) surface, whose atomic geometry resembles closely that of the (1 x 1)-O phase on clean Ru(0001).     

热辅助存储磁盘硅掺杂非晶碳薄膜氧化的ReaxFF反应力场分子动力学模拟

Heat-assisted magnetic recording (HAMR) is one of the promising ways to extend the magnetic recording area density to 1 Tb·in^-2 in hard disk drives (HDDs).High temperature induced by laser heating can cause carbon overcoat (COC) oxidation.Reactive molecular dynamics (MD) simulations are performed to investigate the oxidation process of silicon-doped amorphous carbon (a-C:Si) films for HAMR application.The atomic details of the structure evolution and oxidation process are investigated, and, the oxidation mechanism of the a-C:Si film is clarified.The effect of the duration of laser irradiation on the oxidation of the a-C:Si film is investigated.The oxidation occurs during heating and the beginning of cooling process.Both volume expansion during heating process and cluster of carbon atoms during cooling process increase the rate of sp² carbon.Because of the decrease in the amount of unsaturated silicon atoms and low diffusion coefficient of atomic oxygen, the oxidation rate of the a-C:Si film decreases with laser irradiation cycles.The molecular oxygen is the oxidant due to surface defect of a-C:Si film.The atomic strains break the O-O bonds in Si-O-O-Si linkages and rearrange the surface oxide layers, and process the oxidation of the a-C:Si film.     

Response of Linear,Branched or Crosslinked Polyethylene Structures on the Attack of Oxygen Plasma

Linear, branched and crosslinked polyethylenes (PE) were exposed to the low-pressure oxygen plasma for 2–120 s. In the following the samples were washed with solvents to remove low-molecular weight oxidized material and to excavate the subjacent polymer structure for microscopic characterization. X-ray photoelectron spectroscopy (XPS) measurements provided information about changes in elemental composition and chemical structure of PE after plasma exposure and washing. The calculation of the concentration of tertiary C atoms using XPS data was a measure of branches and crosslinking in the polymer before and after exposure to oxygen plasma. Linear PE was most sensitive towards oxygen plasma and showed the highest concentration in tertiary C atoms after plasma exposure. On the other hand branched PE types, which possess originally more tertiary carbon atoms, have lost two-third of them after 2 s oxygen plasma exposure. Branched PE show also topological changes at their surface as detected by atomic force microscopy. Differential scanning calorimetry measurements confirmed strong changes in crystallinity and molecular orientation of linear PE already after 120 s exposure to the oxygen plasma interpreted as amorphization. These effects should be interpreted as result of crosslinking caused by the recombination of dangling bond sites.     

A study of a simulated low Earth environment on the degradation of FEP polymer

Spacecraft flying in a low Earth orbit environment require thermal blankets to provide protection from direct solar heat from the sun. Fluorinated ethylene propylene copolymer is one of the major components of these thermal blankets. In this study, the effect of a simulated low Earth orbit environment on FEP was investigated. UV and VUV degradation of fluorinated ethylene propylene (FEP) copolymer was studied using ESR and XPS. The ESR study revealed the formation of a terminal polymer chain radical. The stability of this radical has been investigated under different environments. An XPS study of FEP film exposed to VUV and atomic oxygen showed that oxidation takes place on the polymer surface. The study revealed also that the percentage of CF₂ in the polymer surface decreased with exposure time and the percentage of CF, CF₃ and carbon attached to oxygen increased. SEM micrographs of FEP film exposed to VUV and atomic oxygen produced a rough surface with regular undulations similar to sand dunes. © 1998 John Wiley & Sons, Ltd.     

Characterization of oxygen and argon ion flux interaction with PET surfaces by in-situ XPS and ex-situ FTIR

The effects of low (2.5, 0.2 keV) energy reactive oxygen ion bombardment and argon ion bombardment on poly(ethylene terephthalate) thin film (PET) surface chemical composition were studied. PET films have a high potential as a material for biomedical and electrical industries. The source of ions was an ECR Ion Gun with settable acceleration voltages. PET films were sputtered by ion bombardment for variable process time and the modified films were investigated by in-situ X-ray Photoelectron Spectroscopy (XPS) and ex-situ Fourier transform infrared spectroscopy (FTIR). The significant changes in the chemical composition of surface layers were quantitatively studied by XPS. The ion bombardment scissions the chains in PET film surface layers. Selective sputtering of oxygen atoms from PET surface was observed when argon ion flux used. The 0.2 keV and 2.5 keV argon ion decreased O/C ratio from 0.37 to 0.25, 0.04 respectively. This phenomenon is responsible for the creation of carbon-rich up 96 at.% surface layer and the oxygen in ester bonds is detached first. The oxygen 2.5 keV ion bombardment had similar effect as argon ion bombardment; the ratio O/C was decreased. The ester bond was broken first. But oxygen 0.2 keV ion flux irradiation created an oxygen rich surface; the O/C ratio was in increased from 0.37 to 0.46. The changes in surface conductivity were investigated by shifts in C1s binding energy. Good agreement with atomic concentration of carbon in C-C bonds on the films surface was found. The FTIR analyses identified changes in chemical composition but with no obvious correlation to surface changes. Photons from the ion source irradiating the PET film during ion bombardment probably caused the observed changes in FTIR spectra.     

Aggregation of photolytic gold nanoparticles at the surface of chitosan films

Formation and aggregation of photolytic gold nanoparticles at the surface of chitosan (CTO) films have been investigated. When thin films of chloroauric acid salt of CTO were irradiated with UV light in wet air at room temperature for 10 min, gold nanoparticles of approximately 10 nm size are formed at the film surface. Detailed X-ray photoelectron spectroscopy (XPS) study and field emission type scanning electron microscopy (FE-SEM) observation have been carried out to characterize gold nanoparticles at the film surface. The shift of Au(4f) peak to the higher energy side and broadening of full width at half-maximum in the XPS spectrum are the direct evidence of the existence of gold atoms and small clusters in the early stage of photolysis. According to FE-SEM observation, growth in the particle diameter and aggregation of nanoparticles were observed after prolonged irradiation, and, finally, the film surface was densely covered with gold particles of 20-100-nm size. Gold atoms and clusters could move in the film and precipitate to the irradiated surface. Chemical composition analysis further suggests that gold particles at the surface are covered with an ultrathin CTO layer, which is partly oxidized by oxygen and chlorinated by chlorine during photochemical reactions.     

Kinetics of CO oxidation on high-concentration phases of atomic oxygen on Pt(111)

Temperature-programmed reaction spectroscopy (TPRS) and direct, isothermal reaction-rate measurements were employed to investigate the oxidation of CO on Pt(111) covered with high concentrations of atomic oxygen. The TPRS results show that oxygen atoms chemisorbed on Pt(111) at coverages just above 0.25 ML (monolayers) are reactive toward coadsorbed CO, producing CO(2) at about 295 K. The uptake of CO on Pt(111) is found to decrease with increasing oxygen coverage beyond 0.25 ML and becomes immeasurable at a surface temperature of 100 K when Pt(111) is partially covered with Pt oxide domains at oxygen coverages above 1.5 ML. The rate of CO oxidation measured as a function of CO beam exposure to the surface exhibits a nearly linear increase toward a maximum for initial oxygen coverages between 0.25 and 0.50 ML and constant surface temperatures between 300 and 500 K. At a fixed CO incident flux, the time required to reach the maximum reaction rate increases as the initial oxygen coverage is increased to 0.50 ML. A time lag prior to the reaction-rate maximum is also observed when Pt oxide domains are present on the surface, but the reaction rate increases more slowly with CO exposure and much longer time lags are observed, indicating that the oxide phase is less reactive toward CO than are chemisorbed oxygen atoms on Pt(111). On the partially oxidized surface, the CO exposure needed to reach the rate maximum increases significantly with increases in both the initial oxygen coverage and the surface temperature. A kinetic model is developed that reproduces the qualitative dependence of the CO oxidation rate on the atomic oxygen coverage and the surface temperature. The model assumes that CO chemisorption and reaction occur only on regions of the surface covered by chemisorbed oxygen atoms and describes the CO chemisorption probability as a decreasing function of the atomic oxygen coverage in the chemisorbed phase. The model also takes into account the migration of oxygen atoms from oxide domains to domains with chemisorbed oxygen atoms. According to the model, the reaction rate initially increases with the CO exposure because the rate of CO chemisorption is enhanced as the coverage of chemisorbed oxygen atoms decreases during reaction. Longer rate delays are predicted for the partially oxidized surface because oxygen migration from the oxide phase maintains high oxygen coverages in the coexisting chemisorbed oxygen phase that hinder CO chemisorption. It is shown that the time evolution of the CO oxidation rate is determined by the relative rates of CO chemisorption and oxygen migration, R(ad) and R(m), respectively, with an increase in the relative rate of oxygen migration acting to inhibit the reaction. We find that the time lag in the reaction rate increases nearly exponentially with the initial oxygen coverage [O](i) (tot) when [O](i) (tot) exceeds a critical value, which is defined as the coverage above which R(ad)R(m) is less than unity at fixed CO incident flux and surface temperature. These results demonstrate that the kinetics for CO oxidation on oxidized Pt(111) is governed by the sensitivity of CO binding and chemisorption on the atomic oxygen coverage and the distribution of surface oxygen phases.     

Surface modification of plasma‐pretreated expanded poly (tetrafluroethylene) films by graft copolymerization

A two‐step process based on a low‐pressure helium plasma treatment followed by acrylic acid‐grafting copolymerization was used for the surface modification of expanded polytetrafluoroethylene (ePTFE) films. The effects of plasma treatment power and treatment time on the hydrophilicity of the film surface were investigated. The wettability of the ePTFE film surface was characterized by water contact angle, and the surface compositions of the untreated and treated ePTFE samples were evaluated by atomic force microscopy and XPS. Contact angle measurements revealed that the hydrophilicity of the ePTFE film surface was greatly enhanced by the combined actions of the plasma treatment and acrylic acid grafting, and the contact angle decreased from 145° to 66°. Atomic force microscopy analyses showed that the surface roughness increased after the plasma treatment. XPS analyses showed substantial increase in the concentration of carbon and oxygen atoms and a decrease in the concentration of fluorine atoms at the film surface. T‐peel strength showed an improved bonding strength between the film and an adhesive tape after the treatment. Copyright © 2012 John Wiley & Sons, Ltd.     

The effect of glow discharge sputtering on the analysis of metal oxide films

The potential of radiofrequency glow discharge optical emission spectrometry (rf-GD-OES) for the quantification and the solid-state speciation of metal oxide films has been investigated in this work. Two types of oxide coatings, an iron oxide film deposited on silicon and a chromate conversion coating (CCC), were studied at 700 Pa of pressure and 30 W of forward power. The metal to oxygen ratios in the quantitative depth profiles (Fe/O and Cr/O, respectively) were used to evaluate the oxidation states of iron and chromium in the oxide films, demonstrating the capability of GD-OES technique for depth-resolved solid-state speciation. Furthermore, the effect of glow discharge sputtering on the samples surface in terms of modifications in the surface morphology and species transformations, were investigated by using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The iron and chromium oxidation states were carefully studied by XPS at the original samples surface and at the bottom of GD craters, and a systematic reduction of metal elements was observed after rf-GD-OES analysis. In the case of thin oxide films, preferential sputtering can be considered as a critical factor since oxygen atoms can be preferentially sputtered, leaving a metal-enriched surface and, therefore, promoting the reduction of metal elements. In the present study preferential sputtering was found to be sample dependent, changing the proportion of the metal reduction in the oxide film with its composition. Additionally, alternative sputter-depth-profiling techniques such as secondary ion mass spectrometry (SIMS), femtosecond laser ablation (fs-LA), and XPS ion gun were used for the analysis of the CCC in order to evaluate the reduction of Cr⁶⁺ to Cr³⁺ depending on the sputtering mechanism.     

Ultrathin Carbon Film Electrodes from Vacuum‐Carbonised Cellulose Nanofibril Composite

A novel way to produce ultrathin transparent carbon layers on tin‐doped indium oxide (ITO) substrates is developed. The ITO surface is coated with cellulose nanofibrils (from sisal) via layer‐by‐layer electrostatic binding with poly(diallyldimethylammonium chloride) or PDDAC acting as the binder. The cellulose nanofibril‐PDDAC composite film is then vacuum‐carbonised at 500 °C. The resulting carbon films are characterised by atomic force microscopy (AFM), small angle X‐ray scattering (SAXS), wide‐angle X‐ray scattering (WAXS), and Raman methods. Smooth carbon films with good adhesion to the ITO substrate are formed. The electrochemical characterisation of the carbon films is based on the oxidation of hydroquinone and the reduction of benzoquinone in aqueous phosphate buffer media. A modest effect of the cellulose nanofibril‐PDDAC film on the rate of electron transfer is observed. The effect of the film on the rate of electron transfer after carbonisation is more dramatic. For a 40‐layer cellulose nanofibril‐PDDAC film after carbonisation a two‐order of magnitude change in the rate of electron transfer occurs presumably due to a better interaction of the hydroquinone/benzoquinone system with the electrode surface.     

Surface modification and characterization of SEBS films obtained by in situ and ex situ oxidization with potassium permanganate

The surface morphologies and compositions of the asymmetric films of polystyrene‐b‐poly(ethylene‐co‐butylene)‐b‐polystyrene (SEBS) prepared by in situ and ex situ oxidization with the KMnO₄ aqueous solution and KMnO₄/H₂SO₄ mixed solution were investigated by using scanning electron microscope, atomic force microscope (AFM), X‐ray photoelectron spectroscopy (XPS) and attenuated total reflectance infrared spectroscopy (ATR‐FTIR). The effect of the oxidization reagents on morphological changes and the influence of in situ and ex situ preparation methods on surface compositions were discussed. Different from the in situ oxidation by degrading the copolymers to form a gradient film, the ex situ oxidation preferentially degraded the uppermost layer of the film. Although both the KMnO₄ oxidation and the KMnO₄/H₂SO₄ oxidation gave hierarchical structures, distinctive differences were found that large ridges and smaller granules were fabricated in the former film and the latter produced large and deep ravines and fine sponge‐like morphologies. Additionally, the oxygen concentration and the oxo‐species implanted by these oxidation treatments were characterized and evaluated to provide a quantitative comparison. Oxygen, as well as manganese was found to be implanted in the surface layer of the oxidized film, forming predominantly O? C and O? C?O groups, as well as a small fraction of O? H and Mn? O compounds. Changes in contact angle of water on these films and total surface oxygen content are related but not directly. The hystereses of water contact angle at a value of 119 ± 3° due primarily to surface roughness and at a value of 63 ± 3° due primarily to chemical heterogeneity are led by different oxidation degrees and oxidation methods. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Mechanical properties degradation of polyimide films irradiated by atomic oxygen

Mechanical properties of polyimide films are degraded by exposure to a low earth orbit environment. The main environmental factor for that degradation is atomic oxygen (AO). Using tensile tests, AO-irradiated surface topography observations, and fracture surface analyses, this study investigated the degradation behavior of polyimide films’ mechanical properties by increased AO fluence and its accompanying degradation mechanisms. Tensile strength and elongation of polyimide films were reduced concomitantly with increased AO fluence. Furthermore, AO-irradiated polyimide films fractured from the AO-irradiated surfaces, of which roughness became marked as AO fluence increased. These results reflect that reduction of mechanical properties is attributable to the roughness increase in AO-irradiated surfaces. Polyimide films coated with indium tin oxide (ITO) were also evaluated to confirm the degradation behavior of AO protective films. Surfaces of ITO-coated polyimide films remained smooth even after AO irradiation. However, undercut cavities were formed at ITO coating defect sites. Rupture of ITO-coated polyimide films initiates from the undercut cavities, engendering large reduction of tensile strength and elongation. The degradation of the mechanical properties of ITO-coated polyimide films increased substantially until the undercut cavities fully penetrated the film.     

Resistance of Nanoclay Reinforced Epoxy Composites to Hyperthermal Atomic Oxygen Attack

Due to outstanding mechanical properties, heat resistance, and relatively facile production, nanoclay reinforced epoxy composites (NCRE composites) have been suggested as candidate materials for use on external surfaces of spacecraft residing in the low Earth orbit (LEO) environment. The resistance of the NCRE composites to bombardment by atomic oxygen (AO), a dominant component of the LEO environment, has been investigated. Four types of samples were used in this study. They were pure epoxy (0 wt% nanoclay content), and NCRE composites with different loadings of nanoclay—1 wt%, 2 wt%, and 4 wt%. Etch depths decreased with increasing nanoclay content, and for the 4 wt% samples it ranged from 28% to 37% compared to that of pure epoxy. X-ray photoelectron spectroscopy (XPS) indicates that after AO bombardment, relative area of C-C/C-H peak decreased, while the area of the C-O, ketones peaks increased, and the oxidation degree of surfaces increased. New carbon-related component carbonates were detected on nanoclay containing composite surfaces. Scanning electron microscopy indicates that aggregates formed on nanoclay-containing surfaces after AO bombardment. The sizes and densities of aggregates increased with nanoclay content. The combined erosion depths, XPS and SEM results indicate that although all the studied surfaces got eroded and oxidized after AO bombardment, the nanoclay containing composites showed better AO resistance compared to pure epoxy, because the produced aggregates on surface potentially act as a physical “shield”, e ectively retarding parts of the surface from further AO etching.     

Nanoscale degradation of polypyrrole films under oxidative stress: An atomic force microscopy study and review

Atomic force microscopy (AFM) is employed to monitor the surface morphology of polypyrrole (PPy) films grown on vitreous carbon substrates during the catalytic reduction of Cr(VI) to Cr(III). The morphology of freshly-prepared films depends on substrate characteristics. Upon reaction, uniform nodules of aggregated PPy clusters appear. No significant differences in surface morphology are found between its oxidized and reduced forms. Loss of catalytic activity after 8-9 oxidation/reduction cycles of exposure to the chromate solution (oxidation) and electrochemical recharging of the film at negative potentials (reduction) correlates well with the observed polymer film dissolution/detachment from the carbon substrate. Formation of well-defined circular features (PPy rings) at different stages leads to a model for the film degradation process that includes formation of Cl₂ gas inside the polymer matrix. In the final stages, the bulk of the film typically fractures and detaches from the electrode. A catalytically inactive, ultrathin PPy layer remains on the substrate even after prolonged exposure to the target solution. A review of techniques for the study of PPy aging/degradation is given.     

Surface molecular degradation of 3D glass polymer composite under low earth orbit simulated space environment

Epoxy resin reinforced with 3D parabeam glass fibre was subjected to low earth orbit (LEO) simulation conditions comprising ultra high vacuum, temperature cycling (TC), and ultraviolet (UV) radiation and atomic oxygen (AO) bombardment. Inspection of the same composite using only a selection of these hazardous conditions provided comparison measures to identify the effect of each condition on the surface degradation of the resin composite. Each of the individually selected conditions showed a different degradation mechanism that is accelerated by the presence of other conditions. X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy (SEM) were used to provide surface information. The resin composite samples suffered surface oxidation that increased the oxygen content to 17.24% in comparison with the untreated sample (only 14.2%). The samples that were treated with AO showed higher C-O and CO functional groups on the surface in comparison with the rest of the samples (as indicated by XPS). Molecular information (from ToF-SIMS) showed that surface oxidation differs with different conditions and in comparison with the use of all conditions. All treated samples were shown to suffer significant chain scission and loss of volatiles as a result of the LEO conditions. The extent of the chain scission reaction for each condition can be indicated by the extent of the reduction of the relative concentration of the aliphatic hydrocarbon ions. The relative intensity of the C₄H₁₁N₄O₂⁺ ion showed that AO bombardment accelerated the oxidation of the surface. The AO effect is doubled when UV and TC are also present. SEM results indicated that sample surfaces were eroded and roughened upon exposure to LEO conditions. Presence of AO and UV in the LEO conditions introduced white deposits onto the surface, believed to be crosslinked formations.     

Competitive reaction pathways for functionalization and volatilization in the heterogeneous oxidation of coronene thin films by hydroxyl radicals and ozone

X-ray photoelectron spectroscopy (XPS) is used to monitor the heterogeneous reaction of hydroxyl radicals (OH) and ozone with thin films (～5 ?) of coronene. Detailed elemental and functional group analysis of the XPS spectra reveals that there is a competition between the addition of oxygenated functional groups (functionalization) and the loss of material (volatilization) to the gas phase. Measurements of the film thickness and elemental composition indicate that carbon loss is as important as the formation of new oxygenated functional groups in controlling how the oxygen-to-carbon ratio (O/C) of the coronene film evolves during the surface reaction. When the O/C ratio of the film is small (～0.1) the addition of functional groups dominates changes in film thickness, while for more oxygenated films (O/C > 0.3) carbon loss is an increasingly important reaction pathway. Decomposition of the film occurs via the loss of both carbon and oxygen atoms when the O/C ratio of the film exceeds 0.5. These results imply that chemically reduced hydrocarbons, such as primary organic aerosol, age in the atmosphere by forming new oxygenated functional groups, in contrast to oxygenated secondary organic aerosol, which decompose by a heterogeneous loss of carbon and/or oxygen.     

Nanoscale organization of adsorbed collagen: influence of substrate hydrophobicity and adsorption time

The adsorption of collagen on polystyrene (PS) and polystyrene oxidized by oxygen plasma discharge (PSox) was studied as a function of time using radiolabeling, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Radiolabeling and XPS indicated that the initial step of adsorption was faster on PS than on PSox. AFM imaging under water revealed very different supramolecular organization of the adsorbed films depending on time and on the nature of the substrate: PS showed patterns of collagen aggregates at all adsorption times (from 1 min to 24 h); PSox was covered with a smooth layer except at long adsorption times (24 h), for which a mesh of collagen structures was observed. After fast drying, the collagen layer remained continuous and showed a morphology which recalled that observed under water. The mechanical stability of the adsorbed films was assessed under water by scraping with the AFM probe at different loading forces: no perturbations were created on PSox; in contrast, the layer adsorbed on PS was sensitive to scraping, the minimum force required to alter the collagen layer morphology increasing with time. These differences in the film properties were correlated with force measurements upon retraction: multiple adhesion forces were observed with collagen adsorbed on PS samples, whereas such an effect was never observed on PSox. The results show that the amount adsorbed and the organization of the adsorbed film respond differently to the adsorption time and that this is influenced by surface hydrophobicity. The quick initial adsorption on PS, compared to PSox, is thought to leave dangling collagen segments that are responsible for the observed morphology, for adhesion forces, and for lower mechanical resistance of the adsorbed layer.