Oxygen plasma modification of pitch-based isotropic carbon fibres

An isotropic carbon fibre was surface-treated by microwave oxygen plasma at different conditions and characterised by scanning electron microscopy (SEM), scanning tunneling microscopy (STM), N₂/CO₂ adsorption, Raman spectrometry, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is shown that the structure of the fibre suffers only limited alterations upon plasma treatment in such a way that the local disorder on its surface, which was already large in the fresh material, barely increases after the plasma exposure, as detected by Raman measurements. At the nanometre scale, STM images revealed a moderate increase in surface roughness. Evidence for chemical changes undergone by the fibre following the etching was provided by XPS and TPD, showing that stable oxygen functionalities were introduced by the plasma exposure, a result of practical importance for the application of this treatment not only to this type of carbon fibre, but to carbon materials in general. It was also observed that very gentle plasma exposures were generally sufficient to provide the fibre surface with a large amount of oxygen functional groups and that more intense treatments had a negative effect in this respect (i.e. they were not able to supply oxygen to the surface in larger amounts than the softer treatments did).     

Surface treatments of vapor-grown carbon fibers produced on a substrate

Vapor-grown carbon fibers (VGCF) were grown from a methane-hydrogen mixture on a reconstituted graphite support using different catalyst precursors, the [Fe₃(CO)₁₂] complex being found to be the most efficient for the production of VGCF. The fibers thus produced were characterized and submitted to different oxidative treatments, namely nitric acid, plasma, air and carbon dioxide. From analysis performed by scanning electron microscopy (SEM), nitrogen adsorption (BET) and X-ray photon spectroscopy (XPS) it appears that the air and carbon dioxide treatments do not lead to significant increase either of the surface area, or of the quantity of surface oxygen containing groups, despite the considerable weight loss attained (50%). This peculiar observation has been interpreted by considering the presence of traces of iron at the fibers surface, which can catalyze the gasification of carbon. The presence of iron on the VGCF has been evidenced for the first time by the time-of-flight secondary ion mass spectrometry technique. The cleansing of the fibers surface with concentrated hydrochloric acid results in the removal of the iron and leads, after CO₂ activation, to an improvement of the BET surface area. The use of nitric acid or plasma as oxidation agents does not affect significantly the surface morphology of the fibers, but greatly increases the number of surface oxygen functions.     

Evolution of the surface composition and topography of perfluorinated polymers following ammonia-plasma treatment

Treatment of fluorinated ethylene propylene (FEP) and polytetrafluoroethylene (PTFE) in ammonia plasmas produced surfaces with very high wettability by water, but on storage in air at ambient temperature, the air/water contact angles increased markedly. The evolution of the surface composition and topography was studied by angle-dependent X-ray photoelectron spectroscopy (XPS), derivatization of amine groups with fluorescein isothiocyanate, scanning tunelling microscopy (STM), and atomic force microscopy (AFM). XPS demonstrated a continuous increase in the oxygen content over periods of weeks; this was assigned to oxidation of trapped radicals and subsequent secondary reactions. In addition, the fluorine content also changed markedly on storage; the XPS fluorine signal suggested that there was a substantial amount of fluoride in the freshly treated surfaces, and this component disappeared rapidly on storage. STM and AFM showed no changes in topography with aging but suggested surface hardening on plasma treatment. The events following treatment of FEP and PTFE in ammonia plasmas are not adequately described by a model involving plasma-induced, instantaneous chemical modification followed by surface restructuring; the surface and sub-surface compositions evolve over a period of several weeks due to the occurrence of oxidative reactions, and these chemical changes interact with the physical process of surface restructuring.     

A study of the effect of oxygen plasma treatment on the interfacial properties of carbon fiber/epoxy composites

In this work, effects of the interface modification on the carbon fiber‐reinforced epoxy composites were studied. For this purpose, the surface of carbon fibers were modified by oxygen plasma treatment. The surface characteristics of carbon fibers were studied by X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), dynamic contact angle analysis (DCAA), and dynamic mechanical thermal analysis (DMTA), respectively. The interlaminar shear strength (ILSS) was also measured. XPS and AFM analyses indicated that the oxygen plasma treatment successfully increased some oxygen‐containing functional groups concentration on the carbon fiber surfaces, the surface roughness of carbon fibers was enhanced by plasma etching and oxidative reactions. DCAA and DMTA analyses show that the surface energy of carbon fibers increased 44.9% after plasma treatment for 3 min and the interfacial bonding intensities A and α also reached minimum and maximum value respectively. The composites exhibited the highest value of ILSS after oxgen plasma treated for 3 min. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of Atmospheric Plasma Treatment on Carbon Fiber/Epoxy Interfacial Adhesion

In this work the effect of atmospheric plasma treatment on carbon fiber has been studied. The carbon fibers were treated for 1, 3 and 5 min with a He/O₂ dielectric barrier discharge atmospheric pressure plasma. The fiber surface morphology, surface chemical composition and interfacial shear strength between the carbon fiber and epoxy resin were investigated using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and the single fiber composite fragmentation test. Compared to untreated carbon fibers, the plasma treated fiber surfaces exhibited surface morphological and surface composition changes. The fiber surfaces were found to be roughened, the oxygen content on the fiber surfaces increased, and the interfacial shear strength (IFSS) improved after the atmospheric pressure plasma treatment. The fiber strength showed no significant changes after the plasma treatment.     

Influence of oxygen plasma treatment parameters on the properties of carbon fiber

This study is focused on the impact of oxygen plasma treatment on properties of carbon fibers and interfacial adhesion behavior between the carbon fibers and epoxy resin. The influences of the main parameters of plasma treatment process, including duration, power, and flow rate of oxygen gas were studied in detail using interlaminar shear strength (ILSS) of carbon fiber composites. The ILSS of composites made of carbon fibers treated by oxygen plasma for 1 min, at power of 125 W, and oxygen flow rate of 100 sccm presented a maximum increase of 28% compared to composites made of untreated carbon fibers. Furthermore, carbon fibers were characterized by scanning electron microscopy (SEM), tensile strength test, attenuated total reflectance Fourier transform infrared (ATR-FTIR), and Raman spectroscopy analyses. It was found that the concentration of reactive functional groups on the fiber surface was increased after the plasma modification, as well the surface roughness, which finally improved the interfacial adhesion between carbon fibers and epoxy resin. However, high power and long exposure times could partly damage the surface of carbon fibers and decrease the tensile strength of filaments and ILSS of treated fiber composites.     

Structural defects created on natural graphite surface by slight treatment of oxygen plasma: – STM observations –

The creation of structural defects on natural graphite surfaces by slight treatment of oxygen plasma was studied by scanning tunneling microscopy (STM) at nanoscopic scale. Most of the defects were vacancies with the depth of one or two layers, while the mean number of defects per μm² of graphite surface and the mean area of defects increased with the increase in input power, time and temperature of the irradiation. After very slight treatment at room temperature, single carbon atom vacancies were obtained on graphite surfaces with a percentage more than 50%, though some large vacancies were formed. After slight irradiation at a high temperature (400°C), an agglomeration of defects occurred (the mean area increases whereas the defect density decreases), while the defects once formed were difficult to be annealed by heating at high temperatures up to 400°C in vacuum. The present work showed the possibility to control the modification of natural graphite surfaces for further applications by changing the condition of oxygen plasma treatment.     

Toughening vinyl ester networks with polypropylene meso-fibers: Interface modification and composite properties

Polymer-polymer composites comprised of vinyl ester matrices (VE) and polypropylene (PP) fiber meshes were fabricated and tested in this investigation. Results indicated that PP fibers greatly enhanced fracture toughness; however, strength of the VE was significantly reduced as voids were observed at the interface of the PP and VE. A two-step surface modification, oxygen plasma treatment followed by grafting vinyltrimethoxysilane (VTMS), was conducted on PP fibers in an effort to improve interfacial strength. Interfacial discontinuities of composites were improved after surface modification of PP. The oxygen plasma treatment added hydrophilic functional groups but caused surface roughness. Surface treatment of PP slightly increased fracture toughness of the PP-VE composite by enhancing energy absorption capacity at the interface. However, mechanical strength and modulus did not significantly increase for the composite using VTMS grafted PP fibers due to the weak fiber material. Small PP fibers with higher strength may attain the expected improvement in mechanical properties after surface treatment.     

Comprehensive study of polymer fiber surface modifications Part 2: low‐temperature oxygen‐plasma treatment

Polyamide fibers were treated with a low‐temperature oxygen plasma and the effects on the morphology, chemistry and crystallinity of the material were studied. Topographical results illustrate that changes in the surface morphology of the oxygen‐plasma‐treated polyamide correlate well with the discharge power and treatment time. The effects can be categorized into three groups: surface cleaning resulting in a smoother surface, surface etching with formation of ‘ripple‐like’ structures of sub‐micrometer size, and surface melting with down grading of the material. Chemical studies show that the surface oxygen content of the polyamide increases after oxygen‐plasma treatment. The latter induces the formation of many hydroxyl and carboxylic acid functional groups. These groups mainly replace the hydrocarbon or carbonyl groups in the polyamide. Differential scanning calorimetry (DSC) results indicate that a short treatment time does not affect the degree of crystallinity of the polyamide material, while a long plasma‐treatment time slightly increases the crystallized fraction. Copyright © 2004 Society of Chemical Industry     

New atomic-scale features in graphite surfaces treated in a dielectric barrier discharge plasma

The effects of an air plasma generated by a dielectric barrier discharge (DBD) on highly oriented pyrolytic graphite (HOPG) have been investigated through scanning tunneling microscopy (STM), Raman and XPS spectroscopies. Three main types of nanometer/atomic scale features were identified by STM: (i) small bumps, attributed to surface atomic vacancies; (ii) smooth bumps, attributed to interstitial Ar or C atoms; and (iii) depressions not previously reported for graphite, and tentatively attributed to interstitial oxygen species. The evolution of the STM features suggests that the surface modification of HOPG by an air DBD plasma is dominated by physical processes (ion bombardment).     

Effects of Sputtering and Plasma Etching on the Surface Reactivity of Graphite

The surfaces of highly oriented pyrolytic graphite (HOPG) samples were treated using two different methods, exposure to an energetic oxygen ion beam and immersion in an oxygen ion plasma, and the reactions which occur during treatment were characterized using high-resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and scanning tunneling microscopy (STM). Both surface treatments result in similar oxidation species. The results of this investigation provide spectroscopic evidence for the presence of semiquinone functionalities on sputtered and oxidized HOPG. STM images are presented to quantify the increase in defect sites after oxygen ion sputtering and to correlate defect site density with reactivity.     

Surface composition of carbon fibers subjected to oxidation in nitric acid followed by oxygen plasma

Type II, PAN-based carbon fibers (unsized and commercially treated) have been exposed to nitric acid and oxygen plasma individually and also to combined nitric acid/oxygen plasma treatments and the surface compositions have been determined using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and ion scattering spectroscopy (ISS). Most of the oxygen on the as-received carbon fibers resides within the outermost 10-15 Å of the surface. Fiber exposure to nitric acid at 115°C for 20-90 min enhances the oxygen surface concentration to a point of saturation and the oxygen depth distribution is increased and becomes more uniform within the maximum XPS sampling depth (~60-100 Å). In addition, the fiber surface area is believed to be increased. After treating fibers to various degrees in nitric acid, subsequent exposure to oxygen plasma yields an additional increase in the surface oxygen content, particularly in the outermost fiber layers (10-15 Å). Under the conditions of the investigation, the maximum amount of surface oxidation occurs after sequential fiber exposure to nitric acid at 25°C for 30 s and oxygen plasma. As the extent of initial nitric acid treatment is increased, the synergism with subsequent plasma oxidation decreases, and the oxygen concentration becomes more uniform within the outer layers of the oxidized fibers. Overall, the data are consistent with a proposed oxidation mechanism in which oxygen plasma acts to enhance the surface density of oxygen on roughened and pitted nitric acid-oxidized fiber surfaces. As the duration of nitric acid exposure is increased, it is hypothesized that subsequent exposure to oxygen plasma smoothes the fiber surfaces but the surface density of oxygen remains essentially constant.     

Surface modification of armos fibers with oxygen plasma treatment for improving interfacial adhesion with poly(phthalazinone ether sulfone ketone) resin

We introduce in this article oxygen plasma treatment as a convenient and effective method for the surface modification of Armos fibers. The effects of oxygen‐plasma‐treatment power on both the Armos fiber surface properties and Armos‐fiber‐reinforced poly(phthalazinone ether sulfone ketone) composite interfacial adhesion were investigated. The Armos fiber surface chemical composition, surface morphology and roughness, and surface wettability as a function of oxygen‐plasma‐treatment power were measured by X‐ray photoelectron spectroscopy, scanning electronic microscopy, atomic force microscopy, and dynamic contact angle analysis. The results show that oxygen plasma treatment introduced a lot of reactive functional groups onto the fiber surface, changed the surface morphology, increased the surface roughness, and enhanced the surface wettability. Additionally, the effect of the oxygen‐plasma‐treatment power on the composite interfacial adhesion was measured by interlaminar shear strength with a short‐beam bending test. Oxygen plasma treatment was an effective method for improving the composite interfacial properties by both chemical bonding and physical effects. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of plasma modification on the mechanical properties of carbon fiber/phenolphthalein polyaryletherketone composites

A study was carried out to investigate the effect of plasma modification on the mechanical properties of carbon fiber/phenolphthalein polyaryletherketone composites. The influence of oxygen plasma treatment on the surface properties of carbon fibers was investigated by X‐ray photoelectron spectroscopy and atomic force microscopy. The results indicated that oxygen plasma treatment was capable of increasing the concentrations of the oxygen‐containing groups of the carbon fiber surface as well as enhancing surface roughness. Both the chemical bonding and mechanical interlocking gave rise to an increase of the interlaminar shear strength of composite. Scanning electron microscope photographs showed that the destruction mode of composites was changed after the carbon fibers were treated by oxygen plasma. The results also indicated that the flexural properties of plasma‐treated carbon fiber composites were improved. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Influence of oxygen plasma treatment on interfacial properties of poly(p‐phenylene benzobisoxazole) fiber‐reinforced poly(phthalazinone ether sulfone ketone) composite

The influence of oxygen plasma treatment on both surface properties of poly(p‐phenylene benzobisoxazole) (PBO) fibers and interfacial properties of PBO fiber reinforced poly(phthalazinone ether sulfone ketone) (PPESK) composite were investigated. Surface chemical composition, surface roughness, and surface morphologies of PBO fibers were analyzed by X‐ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM), and scanning electron microscopy (SEM), respectively. Surface free energy of the fibers was characterized by dynamic contact angle analysis (DCAA). The interlaminar shear strength (ILSS) and water absorption of PBO fiber‐reinforced PPESK composite were measured. Fracture mechanisms of the composite were examined by SEM. The results indicated that oxygen plasma treatment significantly improved the interfacial adhesion of PBO fiber‐reinforced PPESK composite by introducing some polar or oxygen‐containing groups to PBO fiber surfaces and by fiber surface roughening. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

常压等离子体改善高性能纤维粘结性的研究

以氦气为载气,氧气为反应气体,对高强度聚乙烯和Twaron 1000芳纶两种高性能纤维进行常压等离子体处理,来改善纤维的粘结性能;采用单纤维抽拔实验测定等离子体处理前后纤维与环氧树脂之间的界面剪切力;利用原子力显微镜和X射线光电子能谱仪分析等离子体处理前后纤维表面形态和化学成分的变化。结果表明:高强度聚乙烯纤维和芳纶经常压等离子体处理后,纤维表面粗糙度增加,纤维表面碳元素含量下降,羟基、羧基等含氧或氮的极性基团增加,纤维粘结性能得到提高,但其强度无明显变化。     

Atomic force microscopy on polymers and polymer related compounds

Summary Results of Atomic Force Microscopy (AFM) on carbon fibers from polyacrylonitrile and pitch are presented in comparison with Scanning Electron Microscopy (SEM) and Scanning Tunneling Microscopy (STM) images. Single fiber surfaces and their crosssections have been imaged on scales from microns to nanometers. Morphological details beyond the resolution of SEM were revealed by AFM and STM. Grain-type structure was verified on surface of numerous nanofibrils orlented along the main fiber direction. Grains are bigger on pitch-based fibers generally, and on fibers of both types after treatment at higher temperatures. In the atomic scale AFM images traces of graphitic structure were recorded. AFM artefacts on rough surfaces are demonstrated. ac19920414     

Surface modification of ultra high modulus polyethylene fibers by an atmospheric pressure plasma jet

To improve their adhesion properties, ultra high modulus polyethylene (UHMPE) fibers were treated by an atmospheric pressure helium plasma jet (APPJ), which was operated at radio frequency (13.56 MHz). The surface properties of the fibers were investigated by X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and contact angle measurement. The surface dyeability improvement after plasma treatments was investigated using laser scanning confocal microscopy (LSCM). The adhesion strengths of the fibers with epoxy were evaluated by microbond tests. In addition, the influence of operational parameters of the plasma treatment including power input and treatment temperature was studied. XPS analysis showed a significant increase in the surface oxygen content. LSCM results showed that the plasma treatments greatly increased fluorescence dye concentrations on the surface and higher diffusion rate to the fiber center. The tensile strength of UHMPE fiber either remained unchanged or decreased by 10–13.6% after plasma treatment. The contact angle exhibited a characteristic increase in wettability, due to the polar groups introduced by plasma treatment. The microbond test showed that the interfacial shear strengths (IFSS) increase significantly (57–139%) after plasma treatment for all groups and the optimum activation is obtained at 100°C and 5 W power input. SEM analysis showed roughened surfaces after the plasma treatments. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Application of the microbond technique. IV. Improved fiber–matrix adhesion by RF plasma treatment of organic fibers

The microbond technique is a modification of the single-fiber pullout test for measuring interfacial shear strength. Briefly, a cured microdroplet of material is debonded in shear from a single fiber. Ultra-high modulus polyethylene (Spectra) fibers and aramid fibers (Kevlar) were treated using a radio frequency plasma in order to increase the interfacial bond between the fibers and an epoxy resin. The treated fiber surface was subsequently analyzed by X-ray photoelectron spectroscopy (XPS). Plasma treatment resulted in an increased concentration of oxygen containing functionalities on the fiber surface. The interfacial shear strength as determined by the microbond test increased by 118% for the Spectra fibers and by 45% for the Kevlar fibers with the same epoxy resin. Scanning electron microscopy indicated little change of the surface topography of either fiber following plasma treatment. Effects of friction and surface composition of the plasma-treated fibers is discussed. © 1993 John Wiley & Sons, Inc.     

Effect of fiber surface morphology on the hydrophilicity modification of cold plasma‐treated polypropylene nonwoven fabrics

Cold oxygen plasma was employed to give hydrophilicity modification to polypropylene (PP) nonwoven fabric (NWF). It was found that, after plasma treatment, PP NWF made from fibers with smooth surfaces can only keep its hydrophilicity for a short time and then shows a quick hydrophobic recovery at room temperature. However, this hydrophilic property can last for a long time in the case of the PP NWF made from fibers with rough surfaces. To prove the contribution of the rough surface to the long‐term hydrophilicity, this PP NWF was treated in an organic solvent to smooth the fiber surface. The hydrophilic feature of this PP NWF no longer lasts for a long time after the same plasma treatment. This observation strongly supports our opinion that the fiber surface morphology of PP NWF is a critical factor for long‐term hydrophilicity improvement after plasma treatment, which gives a positive solution to overcoming the aging effect of hydrophilicity modification often found in this technique. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010