Thomas Debies

Surface functionalization of multiwalled carbon nanotubes with UV and vacuum UV photo-oxidation

Multiwalled carbon nanotubes (MWNTs) were surface photo-oxidized at room temperature with UV (253.7 and 184.9 nm) and vacuum UV (VUV) (106.7 and 104.8 nm) radiation. X-ray photoelectron spectroscopy (XPS) showed rapid UV photo-oxidation during the first hour of treatment and then an increase that was directly proportional to the time of treatment up to 4 h where the oxygen concentration was 7.5 at%. VUV photo-oxidation resulted in an oxygen concentration up to 9.5 at% with exposure time for the initial 2 h of treatment. Beyond 2 h, the oxygen concentration decreased with exposure to VUV photo-oxidation. Curve fitting of the XPS C1s spectra revealed mainly C–O–C, as ether or epoxy functional groups, with the presence of C=O, O–C=O and O=C–O–C=O or carbonate moieties. SEM micrographs showed no apparent effect on the structure or appearance of the MWNTs as expected from surface modification. Gas-phase UV and VUV photo-oxidation effectively functionalize MWNTs for potential adhesion improvement without resulting in liquid waste as from the traditional bulk processing method involving acidic oxidation.

Adhesion of copper to poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) surfaces modified by vacuum UV photo-oxidation downstream from Ar microwave plasma

Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) surfaces were exposed to vacuum UV (VUV) photo-oxidation downstream from Ar microwave plasma. The modified surfaces showed the following: (1) an improvement in wettability as observed by water contact angle measurements; (2) surface roughening; (3) defluorination of the surface; and (4) incorporation of oxygen as CF—O—CF2, CF2—O—CF2 and CF—O—CF3 moieties. With long treatment times, a cohesive failure of copper sputter-coated onto the modified surface occurred within the modified FEP and not at the Cu–FEP interface.

Surface characterization of polystyrene treated with ozone and grafted with poly(acrylic acid)

Polystyrene (PS) is one of the most widely used thermoplastic polymers and is often not recycled because of its light weight and low scrap value. The discarded PS in landfill sites has limited capacity for water absorption, and physical and chemical properties that make it relatively inert. To increase wettability and introduce oxygen-containing functional groups, PS was treated at room temperature with ozone in the absence of the UV radiation which was used to make the ozone from the photodissociation of oxygen. X-ray photoelectron spectroscopy detected the increase of oxygen content on the PS surface and the formation of functional groups with reaction time. Advancing contact angle measurements showed an increase of hydrophilicity with treatment time. The superabsorbent polymer poly(acrylic acid) was partially and/or thinly grafted onto the modified PS surface.

Surface modification of poly(biphenyl dianhydride-para-phenylene diamine) (BPDA-PDA) polyimide by UV photo-oxidation

Poly(biphenyl dianhydride-para-phenylene diamine) (BPDA-PDA, Upilex-S) polyimide (PI), was exposed to 185 and 254 nm UV radiation in the presence of oxygen at atmospheric pressure. SEM micrographs revealed only a small change in surface morphology following treatment. XPS showed an approximate doubling of the O/C ratio on the modified surfaces which appeared mostly as the carbonyl moiety. UV photo-oxidation degraded Upilex-S to produce a locus of failure below the interface (cohesive failure). XPS and TOF-SIMS results indicated that the Upilex-S thickness remaining on sputtered Cu after the tape test on the Cu-modified surface was ≤ 10 nm. Cohesive failure occurred at shorter treatment times when Cu was sputter coated on modified BPDA-PDA than on poly(pyromellitic dianhydide-oxydianiline) (PMDA-ODA) PI.

Surface Characterization of Poly(acrylic acid) Grafted to Photo-oxidized Perfluorosulfonic Acid Membrane Used in Fuel Cells

Perfluorosulfonic acid membrane (Nafion®-117) was first surface modified with atmospheric pressure UV photo-oxidation or low-pressure vacuum UV photo-oxidation downstream from an Ar microwave plasma, and then graft polymerized with acrylic acid. X-ray photoelectron spectroscopy (XPS) was used to analyze the modified Nafion surface and poly(acrylic acid) grafted to the modified surface.

Gas-phase surface functionalization of multi-walled carbon nanotubes with vacuum UV photo-oxidation

For bulk processing of carbon nanotubes, an important first step in adhesion to the nanotubes is often liquid-phase functionalization through chemical oxidation with acids (e.g., nitric and sulfuric), peroxides and/or potassium permanganate. In comparison, gas-phase photo-oxidation can be an alternative to introduce oxygenated functional groups on the surfaces of carbon nanotubes without the generation of liquid waste. In the present study, vacuum UV photo-oxidation of multi-walled carbon nanotube (MWNT) paper was investigated downstream from an Ar microwave plasma. X-ray Photoelectron Spectroscopy (XPS) was used to detect the carbon- and oxygen-containing functional groups in the top 2–5 nm of the samples surface. The current results are compared to previous investigations using MWNT powder and single-walled carbon nanotube (SWNT) paper showing decreased levels of oxidation in MWNT samples.

XPS studies of poly(acrylic acid) grafted onto UV photo-oxidized polystyrene surfaces

UV photo-oxidation of polystyrene was achieved with 253.7 and 253.7/184.9 nm radiation in the presence of an atmospheric pressure of oxygen and analyzed by X-ray photoelectron spectroscopy as a function of treatment time. A methodical increase in the atomic percentage (at.%) of oxygen occurred during 253.7 nm activation up to 2 h of treatment time. Photo-oxidation with the 253.7/184.9 nm lamps resulted in a saturation level of ca. 35 at.% O. Initially, C–O and C=O groups were formed and then O–C=O type moieties. Poly(acrylic acid) was partially and/or thinly grafted onto the oxidized polystyrene surfaces.

Vacuum UV photo-oxidation of polystyrene

Abstract Polystyrene (PS) was treated with vacuum UV (VUV) (λ = 104.8 and 106.7 nm) photo-oxidation and X-ray photoelectron spectroscopy detected a controlled increase in the atomic percentage of oxygen up to a saturation level of ca. 20 at% O. Initially, C–O and carbonyl groups are observed due to the formation of alcohols, ethers, esters, and ketones. Water contact angle measurements showed ca. 25% increase in hydrophilicity of the surface with oxidation. Atomic Force Microscopy observed little changes in surface roughness with treatment time. The super water absorbent polymer poly(acrylic acid) was thinly grafted to the modified PS surface.

Surface Modification of Poly(biphenyl dianhydride-para-phenylene diamine) (BPDA-PDA) and Poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA) Polyimides with UV Photo-Oxidation

Poly(biphenyl dianhydride-para-phenylene diamine) (BPDA-PDA), Upilex-S, and poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA), Kapton HN, polyimides (PIs), were exposed to 253.7 nm and broad band radiation centered at 300 nm UV radiation which photo-activated the polymer surfaces in the presence of oxygen at atmospheric pressure. XPS analysis showed an increase in the O/C ratio on the modified surfaces which appeared mostly as the carbonyl moiety, although, treated Upilex-S samples also displayed an increase in C–O–C groups. The O/C ratio saturated more quickly for Upilex-S than Kapton. Good practical adhesion of sputtered Cu to the treated surfaces occurred at short treatment times while extensive exposure resulted in cohesive failure within the substrates at shorter treatment times for Upilex-S than Kapton.

Surface modification of poly(tetrafluoroethylene-co-hexafluoropropylene) with vacuum UV radiation from rotating helium dc arc plasmas

Treatment of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) with vacuum ultraviolet (VUV) radiation from high pressures of helium in rotating dc arc plasmas was investigated. X-ray photoelectron spectroscopy (XPS) detected defluorination and appearance of the functional groups (C=O, O–C=O and C–O) on the surface. Fourier transform infrared (FT-IR) spectroscopy showed the appearance of a band at 1884 cm−1 indicating the formation of the carboxylic acid fluoride moiety, –(C=O)–F. Improvement in film wettability was observed by contact angle measurements while SEM micrographs showed an increased amount of cracking on the surface with VUV exposure. Adhesion measurements of Cu sputter-coated onto the photo-modified surfaces resulted in failure within the fluoropolymer (cohesive failure) and not at the Cu-fluoropolymer interface. Cohesive failure occurred with shorter treatment times than for VUV exposure downstream from low pressure Ar microwave plasmas.