G. A. Takacs

Atmospheric photochemistry of alkyl nitrates

Abstract Photoabsorption spectra of gaseous ethyl nitrate, n -propyl nitrate, 2-propyl nitrate, n -butyl nitrate and 3-methyl-1-butyl nitrate were quantitatively measured in the laboratory at room temperature for λ ⩾ 185 nm. The spectra are continuous and exhibit low- and high-intensity absorption bands. The maximum for the high-intensity band occurs at 188 ± 2 nm. Atmospheric photodissociation rate coefficients for alkyl nitrates were estimated in the range 0 – 50 km and were found to have a strong dependence on altitude. Photodissociation lifetimes were on the order of days and hours for the proposphere and stratosphere respectively. Compared with reaction with OH, photodissociation is the dominant removal process for alkyl nitrates throughout the atmosphere.

Surface composition and distribution of fluorine in plasma‐fluorinated polyimide

Surface composition, fluorine distribution, and morphology were determined for polyimide films modified downstream from microwave plasmas containing CF4/O2. Complementary analytical techniques including x‐ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, and scanning electron microscopy yielded a more complete understanding of polyimide fluorination and subsequent etching of the modified film. Depth of fluorination increased nonlinearly with treatment time for films exposed downstream from a CF4‐rich plasma. Exposure downstream from an O2‐rich plasma resulted in a reduction of thickness in both the fluorinated layer and the unmodified polyimide during etching. Finally, a model for fluorination of polyimide and subsequent removal is proposed.

Photoetching and modification of poly(tetrafluoroethylene-co-hexafluoropropylene) polymer surfaces with vacuum UV radiation

G.A.T. is grateful for an RIT Faculty Development Grant that allowed presentation of this paper at the XIXth IUPAC Symposium on Photochemistry, Budapest, Hungary, 14–19 July 2002.

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.

Photoetching and modification of organic polymer surfaces with vacuum UV radiation

Abstract Commercially available organic polymer films were treated with radiation from 6·7 × 10 4 Pa He and Ar plasmas that were made to rotate inside a graphite tube by the application of an auxiliary magnetic field. To investigate modification with vacuum ultraviolet (VUV) photons, the films were covered with LiF, CaF 2 , fused silica, KBr and crown glass optical filters having different cut-off wavelengths. The weight loss for polytetrafluoroethylene (PTFE) was found to be linear with exposure time and dependent upon cut-off wavelength, inert gas, exposure temperature and arc current. Photoetch rates up to 105 nm min −1 were observed for PTFE using unfiltered radiation from a He arc with a current of 7 A and temperatures up to 240°C. Kapton-H ® showed nearly linear weight loss, although there was some curvature at longer exposure times. Upilex-S ® and -R ® showed rapid initial weight loss that increased with decreasing wavelength and then negligible loss with increasing time of exposure. XPS analysis of treated samples revealed defluorination for PTFE and formation of SiO 2 , probably resulting from photo-oxidation of a Si, O, C-containing additive, on Upilex-S which prevented further photoetching. Receding contact angles of the polymers decreased rapidly with exposure time to minimum saturation values. In agreement with known photoabsorption spectra, modification for PTFE was induced with wavelengths between 160 and 200 nm while λ > 160 nm affected Kapton and Upilex surfaces. Surface roughening was observed with exposure to VUV radiation.

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.

Adhesion of copper to poly(tetrafluoroethylene) surfaces modified with vacuum UV radiation from helium arc plasma

Poly(tetrafluoroethylene) (PTFE) film surfaces were exposed to vacuum UV (VUV) radiation from He dc arc plasmas that were made to rotate inside a graphite tube by the application of an auxiliary magnetic field. The films were covered with optical filters having different cutoff wavelengths to vary the VUV radiation that modified the fluoropolymer surface. Photo-etching was detected, as well as surface modification that showed the following: (1) water contact angles decreasing with wavelengths of 173 nm or shorter; (2) surface roughening; (3) defluorination of the surface and formation of cross-linking bonds in the top 10 nm of the surface as detected by XPS analysis; and (4) incorporation of oxygen upon exposure to air. An improvement in the adhesion of copper to these modified surfaces was observed.

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.