Contributions of restructuring and oxidation to the aging of the surface of plasma polymers containing heteroatoms

The properties and composition of plasma polymer surfaces stored in air can change considerably over time, especially as a result of oxidative reactions. When plasma polymers contain an element other than O, it is possible to probe for mechanisms in addition to oxidation that contribute to the aging of the surface. Plasma polymers containing N were fabricated from either 1,3-diaminopropane (DAP),n-heptylamine (nHA), or allylamine (AA), and studied by X-ray photo-electron spectroscopy (XPS) and air/water contact angles (CA). For each of the plasma polymers, a multiexponential increase in the O/C ratio was observed over time using XPS. The N/C ratios remained constant (AA) or decreased somewhat (nHA and DAP). In contrast, the trends in CA values differed, declining for the nHA surfaces, rising for the AA, and changing little for the DAP. Surface roughness, assessed by scanning tunnelling or atomic force microscopy, did not change over time. The diverse adjustments in the polarity of each surface and the similar compositional changes between them are reconcilable if the aging of the plasma polymer surface is a manifestation of the superposition of concurrent oxidative reactions and partial surface reorientation; the former introduce polar groups and the latter transports then from the surface to deeper regions beyond the CA probe depth but within the XPS analysis depth. These processes vary between different plasma polymers. Data for the alkylamine plasma polymers is also compared with that for two plasma polymers fabricated from methanol. The change in composition, but not polarity, of the DAP surface after 4 days of storage demonstrates the importance of using multiple techniques to characterize the aging of plasma polymer surfaces.     

SURFACE REARRANGEMENTS OF OXYGEN PLASMA TREATED POLYSTYRENE： SURFACE DYNAMICS AND HUMIDITY EFFECT

The time evolution of oxygen plasma treated polystyrene(PS)surfaces was investigated upon storing them in theair under controlled humidity conditions.The methods of water contact angle,X-ray photoelectron spectroscopy(XPS),sumfrequency generation(SFG)vibrational spectroscopy,and atomic force microscopy(AFM)were used to infer the surfaceproperties and structure.Chemical groups containing oxygen were formed on the PS surface with the plasma treatment,demonstrated by water contact angle and XPS.The surface polarity decayed markedly on time,as assessed by steady increasein the water contact angle as a function of storage time,from zero to around 60°.The observed decay is interpreted as arisingfrom surface rearrangement processes to burying polar groups away from the uppermost layer of the surfaces,which is incontact with air.On the other hand,XPS results show that the chemical composition in the first 3 nm surface layer isunaffected by the surface aging,and the depth profile of oxygen is essentially the same with time.A possible change of PSsurface roughness was examined by AFM,and it showed that the increase of water contact angle during surface aging couldnot be attributed to surface roughness.Thus,it is concluded that surface aging is attributable to surface reorganization andthe motion of oxygen containing groups is confined within the XPS probing depth.SFG spectroscopy,which is intrinsicallyinterface-specific,was used to detect the chemical structure of PS surface at the molecular level after various aging times.The results are interpreted as follows.During the aging of the plasma treated PS surfaces,the oxygen containing groupsundergo reorientation processes toward the polymer bulk and/or parallel to the surface,while the CH_2 moiety stands up onthe PS surface.Our results indicate that the surface configuration changes do not require large length scale segmentalmotions or migration of macromolecules.Motions that are responsible for surface configuration changes could be relativelysmall rotational motions.The aging behaviors under different relative humidity conditions were shown to be similar from18% to 91%,whereas the kinetics of surface polarity decays were faster in higher relative humidity.Here,the surfacerearrangement of polystyrene films that were previously treated by oxygen plasma and aged,and was investigated in terms ofcontact angle after the water immersion.The contact angles of the water-immersed samples were found to change andapproach the initial values before the immersion asymptotically.     

Excitation Frequency Dependence of the Surface Properties and Composition of Plasma Polymers from Aldehyde Monomers

The effect was investigated of varying the plasma excitation frequency in the range 125 to 375 kHz upon the air/water contact angles of plasma polymers prepared from the monomers ethylbutyraldehyde (EBA), acetaldehyde (AA), capronaldehyde (CA), and nonyl aldehyde (NA). The surfaces of EBA, CA, and AA plasma films were more hydrophilic when deposited at lower excitation frequencies whereas little frequency dependence was observed in the case of NA plasma polymers. Lower contact angles correlated with increased amounts of C=O and O–C=O groups measured by FTIR and XPS analyses. The effects of storage (aging) in air at room temperature upon the properties of the plasma polymers were also studied. Contact angles of EBA plasma polymers decreased during aging. The contact angles of NA plasma polymers were stable over a 3 months storage period. In the case of CA plasma polymers, the contact angles decreased on aging for films deposited at higher frequencies, whereas CA films deposited at lower frequencies showed increasing contact angles on storage. This aging behavior is interpreted as a result of competition between post-deposition oxidative reactions with atmospheric oxygen and reorientation of polar polymer segments away from the air interface.     

Improvement of interfacial adhesion for plasma‐treated aramid fiber‐reinforced poly(phthalazinone ether sulfone ketone) composite and fiber surface aging effects

Interfacial adhesion between the fiber and the matrix in a composite is a primary factor for stress transfer from the matrix to the fiber. In this study, oxygen plasma treatment method was applied to modify the fiber surface for improving interfacial adhesion of aramid fiber‐reinforced poly(phthalazinone ether sulfone ketone) (PPESK) composite. Composite interfacial adhesion properties were determined by interlaminar shear strength (ILSS) using a short‐beam bending test. The composite interfacial adhesion mechanism was discussed by SEM. The changes of chemical composition and wettability for plasma‐treated fiber surfaces stored in air as long as 10 days were investigated by XPS and dynamic contact angle analysis (DCAA), respectively. Results indicated that oxygen plasma treatment was an effective method for improving interfacial adhesion; plasma‐treated fiber surface suffered aging effects during storage in air. Copyright © 2008 John Wiley & Sons, Ltd.     

A quantitative model for the surface restructuring of repeatedly plasma treated silicone rubber

Surface restructuring in ambient air of medical grade silicone rubber surfaces modified by repeated RF plasma treatments using various discharge gases including oxygen, argon, carbon dioxide and ammonia, was studied quantitatively. From advancing and receding water contact angle data, the fraction of the surface covered by mobile and immobile polar groups, and a characteristic time constant of the restructuring process were calculated. For argon plasma treated surfaces, the fraction of immobile polar groups increased with repeated plasma treatments, but remained relatively constant for samples repeatedly treated by an ammonia plasma. The use of an oxygen plasma only yielded incorporation of mobile polar groups but not of immobile polar groups. The increase in the restructuring time constants of argon and ammonia plasma treated silicone rubber with the number of plasma treatments suggested enhanced crosslinking of the silicone rubber by these plasmas. In contrast, when an oxygen plasma was repeatedly used, the restructuring time constant decreased suggesting chain cleavage by an oxygen plasma. Tentatively, the carbon dioxide plasma treatment of silicone rubber may initially (up to 3–4 repeated treatments) yield chain cleavage, while the occurrence of crosslinking is indicated after more repetitions.     

Aging of 1,3-diaminopropane plasma-deposited polymer films: Mechanisms and reaction pathways

In the course of plasma deposition of organic–polymeric thin films, radicals are incorporated into the growing film. These radicals initiate spontaneous oxidation reactions that continue over many weeks when the plasma polymers are stored in air. These reactions and their products have been previously studied in detail for spectroscopically simple, hydrocarbon-based plasma polymers. In this investigation, the aging of 1,3-diaminopropane (DAP) plasma polymer samples was monitored by XPS and FTIR in order to study how the oxidative reaction pathways might differ in a plasma-deposited material that is initially rich in amine groups. The freshly deposited DAP plasma polymer consisted of a random hydrocarbon network with a considerable amount of unsaturation and a high concentration of nitrogen-containing functional groups, mainly primary/secondary amines and imines. These groups strongly influenced the aging reactions: in contrast to hydrocarbon-based material where hydrogen abstraction and reaction of carbon-centered radicals with in-diffusing oxygen result in a wide range of oxidative products, both XPS and FTIR identified a rather narrow range of products (mainly amides and similar groups) in DAP plasma polymers even after extensive aging for more than 2 years. Reaction routes based on oxidation and/or hydrolysis of nitrogen functional groups, and involving primary as well as secondary reactions, are proposed to account for the spectroscopic data. The structure of the aged DAP plasma polymer appeared to be stable, and did not undergo more extensive oxidation, in contrast to hydrocarbon plasma polymers. In particular, carboxylic acid groups and carbamates were not detected. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2191–2206, 1999     

Stability of polycarbonate and polystyrene surfaces after hydrophilization with high intensity oxygen RF plasma

A general drawback observed with plasma treatment is the limited stability of the hydrophilic-treated surfaces toward washing, storage, or heating. It has recently been found that oxygen, air, or argon radiofrequency plasmas with higher intensities than normally used can give hydrophilic surfaces having good wash stability. High intensity oxygen plasma treatment of polystyrene and polycarbonate surfaces was therefore carried out using two different capacitively coupled RF reactors with internal shelf electrodes. The obtained surface characteristics and stability were evaluated using contact angle measurements, XPS, AFM, and nanoindentation. For both materials, low water contact angles were found to correlate with high surface oxygen content. Only the surfaces exposed to relatively intense treatments, with self-bias voltages above 140 V (polystyrene) or 240 V (polycarbonate), could withstand washing in ethanol and remain highly hydrophilic. Substantial amounts of nonsoluble material were observed on the plastic substrates after treatment. Furthermore, for polycarbonate Young's modulus of the surface was found to increase with increasing intensity of the plasma. These observations were taken as an indication that extensive cross-linking of the surface layer took place. After more than 6 months of storage, the samples treated with the most intense plasmas (self-bias voltages in the range of 480-600 V) still had water contact angles around 20 degrees .     

Surface modification of PET films by atmospheric pressure plasma exposure with three reactive gas sources

The surface modification of poly (ethylene terephthalate) (PET) film was carried out using an atmospheric pressure plasma (APP) jet device with three reactive gases: air, N₂, and Ar. The water contact angles on the PET film were found to decrease considerably after the APP exposure. The changes in the advancing and receding contact angles of water on the APP-exposed PET film with aging time were examined by the wetting force measurements employing the Wilhelmy method. The hydrophobic recovery due to the rinsing with water as well as the aging in air was observed only for the advancing angle, which was probably caused by the dissolution of low molecular weight oxidized materials into water, the loss of volatile oxidized species to the atmosphere and the reorientation and the migration of polymer chains. The wettability and the surface free energy of the APP-exposed PET film after diminishing hydrophobic recovery was sufficiently large compared with the untreated film. X-ray photoelectron spectroscopy confirmed that the PET film surface was oxidized due to the APP exposure. When N₂ gas was used for the APP exposure, the surface nitrogen concentration was found to increase with decreasing D. The surface oxygen concentration on the APP-exposed PET film was reduced by rinsing with water, in accordance with the hydrophobic recovery behavior. From atomic force microscopy, surface topographical change due to the APP exposure was observed. The changes in the PET surface properties due to the APP exposure as mentioned above were remarkable for using N₂ gas.     

In-situ investigation of surface processes on AlGaAs/GaAs cleavage edges as studied by atomic force microscopy

In-situ observations of surface processes on a freshly cleaved multiquantum well (MQW) cleavage edge allow to obtain chemical information in addition to the surface topography primarily seen in AFM images. Under air the cleavage surface shows a corrugation of about 0.5 nm due to a varying degree of oxidation on the different layers. This oxidation process could be avoided by preparing and imaging the cleavage surface under inert toluene without any contact to ambient atmosphere. After removing the toluene and purging the cell with air, oxidation products developed along the expected AlGaAs layers. A treatment of the oxidized surface with 1 and 10 mmol/L HCl has led to crater formation, which was more pronounced in areas of chemical inhomogeneities and crystallographic defects. 0.1 mol/L HCl has led to an inversion of the original contrast over the whole investigated area, which could be monitored directly in the AFM liquid cell.     

Surface restructuring of polymers

Time-dependent contact angle measurements are employed to follow the dynamics of surface modifications of various polymeric surfaces of different hydrophilicities. The equilibration of a hydrophilic polymer in a strong polar environment (such as water) induces an increase in the polarity of the surface; the subsequent exposure of the restructured solid to a nonpolar environment decreases the polarity of the surface. The dynamics of these processes depends on the history of the specimen. Various phenomena, such as surface restructuring by the reorientation of the buried polar or nonpolar moieties, water penetration into the polymer, and the reorganization of water in the neighborhood of the surface, are suggested to be responsible for the time evolution of the dynamic contact angles.     

Plasma surface modification and characterization of POSS-based nanocomposite polymeric thin films

The effect of a remote oxygen plasma on nanocomposite hybrid polymer thin films of poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsequioxane)-co-(methylmethacrylate)] (POSS-MA) has been examined by advancing contact angle, X-ray photoelectron spectroscopy (XPS), and variable-angle spectroscopic ellipsometry (VASE). Exposure to a 25 W remote oxygen-containing plasma was found to convert the surface of POSS-MA films from hydrophobic to hydrophilic within 20 s. The exposure time needed for this conversion to occur decreased as the O2/N2 ratio in the plasma environment increased, indicating a positive correlation between the hydrophilicity and the presence of oxygen in the plasma. Local bonding information inferred from high-resolution XPS data showed that the isobutyl bonding to the POSS moiety is replaced with oxygen as a result of plasma exposure. Finally, VASE data demonstrates that increasing the weight percent of POSS in the copolymer significantly impedes the oxygen plasma degradation of POSS-MA films. On the basis of these results, a model is presented in which the oxygen plasma removes isobutyl groups from the POSS cages and leaves a SiO2-like surface that is correspondingly more hydrophilic than the surface of the untreated samples and is more resistant to oxidation by the plasma. The ability to modify surfaces in this manner may impact the utility of this material for biomedical applications such as microfluidic devices in which the ability to control surface chemistry is critical.     

Introduction of Primary Amino Groups on Poly(ethylene terephthalate) Surfaces by Ammonia and a Mix of Nitrogen and Hydrogen Plasma

With the aim of introducing primary amino groups on the surface of poly(ethylene terephthalate) (PET), two methods were compared—the use of ammonia or a combination of nitrogen and hydrogen low-pressure microwave plasma. Several plasma parameters were optimized on the reactor to increase the –NH₂ surface density, which was estimated by colorimetric titration and X-ray photoelectron spectroscopy (XPS). These techniques show that whatever the plasma treatment, almost 2 –NH₂/nm2 are incorporated on PET films. Emission spectroscopy highlighted a correlation between the density of primary amino groups and the ratio between an NH peak intensity and an Ar peak intensity (I_NH/I_Ar). Variation in surface hydrophilicity with aging in air after plasma treatment was monitored with contact angle measurements and showed a hydrophobic recovery. This was confirmed by XPS, which suggests also that surfaces treated by NH₃ plasma are more stable than surfaces treated by N₂/H₂.     

Surface modification of EPDM rubber by plasma treatment

The effect of argon, oxygen, and nitrogen plasma treatment of solvent cast EPDM rubber films has been investigated by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and surface energy measurements. Plasma treatment leads to changes in the surface energy from 25 to 70 mN/m. Treatment conditions influenced both the changes in surface energy and the stability, and it became more difficult to obtain good contact angle measurements after longer (> ca. 4 min) treatment times, probably because of an increasingly uneven surface structure. XPS analyses revealed that up to 20 at. % oxygen can be easily incorporated and that variations of approximately 5% can be controlled by the plasma conditions. Oxygen was mainly found in hydroxyl groups, but also as carbonyl and carboxyl. XPS analyses showed more stable surfaces than expected from contact angles, probably because XPS analysis is less surface sensitive than contact angle measurements. AFM measurements revealed different surface structures with the three gases. The surface roughness increased generally with treatment time, and dramatic changes could be observed at longer times. At short times, surface energy changes were much faster than the changes in surface structure, showing that plasma treatment conditions can be utilized to tailor both surface energies and surface structure of EPDM rubber.     

Aging behavior of PBO fibers and PBO‐fiber‐reinforced PPESK composite after oxygen plasma treatment

Aging behavior of poly(p‐phenylene benzobisoxazole) (PBO) fibers and PBO‐fiber‐reinforced poly(phthalazinone ether sulfone ketone) (PPESK) composites after oxygen plasma treatment was investigated. Surface chemical composition, surface roughness and surface morphologies of oxygen‐plasma‐treated PBO fibers before and after aging in air for 1, 3, 5 and 10 days were analyzed by XPS and atomic force microscopy (AFM). The effects of aging on the material were examined by interlaminar shear strength (ILSS) and water absorption measurements. The results indicate that the major aging behavior of the fibers and the composite appeared in the first few days after oxygen plasma treatment, whereas minor aging effects were observed with prolonged aging. Copyright © 2008 John Wiley & Sons, Ltd.     

Wettability interpretation of oxygen plasma modified poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) has been modified via a dc pulsed oxygen plasma for different treatment times. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), optical profilometer, zeta potential, and advancing contact angle measurements. The measured advancing contact angles of water decreased considerably as a function of discharge. Several oxygen-based functionalities (carbonyl, carboxyl, carbonate, etc.) were detected by XPS, while zeta potential measurements confirmed an increase in negative charge for the treated PMMA surface. Evaluating the correlation between the concentration of polar chemical species and zeta potential, we found that increase in surface hydrophilicity results from the coeffect due to incorporation of oxygen functional groups and creation of charge states. The electrical double layer (EDL) effect was also considered in contact angle interpretation by introducing an additional surface tension term into Young's equation. We also found that EDL contribution to the solid-liquid interfacial tension is negligible and can be safely ignored for the systems considered here.     

A chemical, morphological, and electrochemical (XPS, SEM/EDX, CV, and EIS) analysis of electrochemically modified electrode surfaces of natural chalcopyrite (CuFeS2) and pyrite (FeS2) in alkaline solutions

Electrodic surfaces of natural chalcopyrite and natural pyrite minerals (El Teniente mine, Chile) have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy including microanalysis (SEM/EDX). For comparison, fractured and polished mineral surfaces were also studied by XPS. In both electrodes, the formation of Fe(III) species containing oxygen were detected and Cu(II) species containing oxygen were additionally detected for chalcopyrite at advanced oxidation states. The presence of Cu(II) species containing oxygen was not detected by XPS for the initial oxidation states of the chalcopyrite. For pyrite, the present results do not allow confirmation of the presence of polysulfurs such as have been previously proposed. In both minerals, the measurements of SEM and EDX show relevant alterations in the respective surfaces when different potential values were applied. The chalcopyrite surface shows the formation of protrusions with a high concentration of oxygen. The pyrite surface shows a layer of modified material with high oxygen content. The modifications detected by XPS, SEM, and EDX allowed the explanation of the complexity of the equivalent circuit used to simulate the experimental EIS data. At high oxidation states, both minerals showed a pseudoinductive loop in the equivalent circuit, which was due to the active electrodissolution of the minerals which takes place through a surface film previously formed.     

Surface modification and characterization of indium-tin oxide for organic light-emitting devices

In this work, we used different treatment methods (ultrasonic degreasing, hydrochloric acid treatment, and oxygen plasma) to modify the surfaces of indium-tin oxide (ITO) substrates for organic light-emitting devices. The surface properties of treated ITO substrates were studied by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), sheet resistance, contact angle, and surface energy measurements. Experimental results show that the ITO surface properties are closely related to the treatment methods, and the oxygen plasma is more efficient than the other treatments since it brings about smoother surfaces, lower sheet resistance, higher work function, and higher surface energy and polarity of the ITO substrate. Moreover, polymer light-emitting electrochemical cells (PLECs) with differently treated ITO substrates as device electrodes were fabricated and characterized. It is found that surface treatments of ITO substrates have a certain degree of influence upon the injection current, brightness, and efficiency, but hardly upon the turn-on voltages of current injection and light emission, which are in agreement with the measured optical energy gap of the electroluminescent polymer. The oxygen plasma treatment on the ITO substrate yields the best performance of PLECs, due to the improvement of interface formation and electrical contact of the ITO substrate with the polymer blend in the PLECs.     

SIMS/XPS characterization of surface layers formed in 3 mass% Si‐steel by annealing in oxygen at low partial pressure

Selective oxidation in silicon steel shows several interesting phenomena, such as the formation of an internal oxidation zone that depends on the oxidation conditions and the steel composition. In this work, SIMS and XPS were used for characterizing the formation processes of surface layers formed during selective oxidation of a typical silicon steel. The starting material is a secondary‐recrystallized 3 mass% Si‐steel sheet with a surface orientation of (011). Sample sheets were annealed at a temperature of 948–1023 K under an atmosphere with a low partial pressure of oxygen. The SIMS depth profiles show that the internal oxidation zone thickens and an iron‐rich layer that formed on the internal oxidation zone expands as the annealing temperature increases. Manganese and chromium levels increase outside the internal oxidation zone, whereas tin exists in the internal oxidation zone. The XPS results of the sample surface show that silicon and manganese levels increase on the sample surface to form oxides, and the chemical composition and state of these elements depend on the annealing temperature. In addition, tin increases on the surface of a relatively thick iron‐rich layer that formed on the internal oxidation layer. These experimental results are discussed on the basis of the thermodynamic characteristics of the elements. Copyright © 2003 John Wiley & Sons, Ltd.     

Surface characterization and barrier properties of plasma‐modified polyethersulfone/layered silicate nanocomposites

This study describes the preparation of polyethersulfone (PES)/layered silicate nanocomposites (PLSNs) by mixing PES polymer chain into organically‐modified layered silicate in 1‐methyl‐2‐pyrrolidinone (NMP) solution. Both X‐ray diffraction data and transmission electron microscopy images of PLSNs indicate that the silicate layers were almost exfoliated and randomly distributed into the PES matrix. The mechanical and barrier properties of PLSNs show remarkable enhancement in the storage modulus and water/oxygen permeability when compared with that of neat PES matrix. Surfaces modification of PES and PLSN films with various treated times, system pressures, and radio frequency (RF) powers were performed using a mixture of oxygen (O₂) and nitrogen (N₂) plasmas. The topographical and physical properties of plasma‐modified PES and PLSN surfaces were investigated using scanning probe microscopy (SPM), contact‐angle measurements, and X‐ray photoelectron spectroscopy (XPS). These results indicate that the surface roughness of PLSNs with the same condition of plasma modification is lower than that of neat PES matrix and is probably due to the increase of stiffness with the presence of inorganic layered silicates in PES matrix. The surface properties of the PES and PLSNs are also changed from hydrophobic to hydrophilic. The XPS spectra suggest that the exposure of the PES and PLSNs to the plasmas led to the combination of etching reactions of polymer surface initiated by plasma and the following addition reactions of new oxygen‐ and nitrogen‐containing functional groups onto polymer surfaces to change their surface properties. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3185–3194, 2006     

Surface properties of polyethylene after low-temperature plasma treatment

The effect of oxygen and ammonia plasma treatments on changes of the surface properties of linear high-density polyethylene (HDPE) was studied. Surface energies of the polymer substrates were evaluated by contact angle measurements using Lifshitz-van der Waals acid-base approach. The surface energy of untreated HDPE is mainly contributed by Lifshitz-van der Waals interactions. After 5 min of plasma treatment, hydrogen bonds are formed on the surface, which is reflected in predominant acid-base interactions. The SEM results obtained demonstrate considerable changes of the surface roughness due to different types of the plasma gas used. Evolution of oxygen- or amino-containing moieties was detected by XPS and ATR FT IR. The prepared polyethylene surfaces were used as a basic support for further fabrication of novel hybrid biocomposite sandwich structures.