John J. Chera

Simulation of Titan haze formation using a photochemical flow reactor The optical constants of the polymer

Solar UV is the principal energy source impinging the atmosphere of Titan while the energy from the electrons in Saturn’s magnetosphere is less than 0.5% of the UV light. Titan haze analogs were prepared by the photolysis of a mixture of gases that simulate the composition of its atmosphere (nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene). The real ( n) and imaginary (k) parts of the complex refractive index of haze analogs formed from four different gas mixtures were calculated from the spectral properties of the solid polymer in UV-visible, near infrared and infrared wavelength spectral regions. The value of n was constant at 1.6 ± 0.1 throughout the 0.2–2.5 µm region. The variation of k with wavelength for the values derived for Titan has a lower error than the absolute values of k so the more significant comparisons are with the slopes of the k(λ) plots in the UV-VIS region. Three of the photochemical Titan haze analogs had slopes comparable to those derived for Titan from the Voyager data (Rages and Pollack, 1980, Icarus 41, 119–130; McKay and Toon, 1992, in: Proceedings of the Symposium on Titan, in: ESA SP, Vol. 338, pp. 185–190). The slopes of the k(λ) plots for haze analogs prepared by spark discharge (Khare et al., 1984, Icarus 60, 127–137) and plasma discharge (Ramirez et al., 2002, Icarus 156, 515–529) were also comparable to Titan’s. These finding show that the k(λ) plots do not differentiate between different laboratory simulations of atmospheric chemistry on Titan in the UV-VIS near IR region (0.2–2.5 microns). There is a large difference between the k(λ) in the infrared between the haze analogs prepared photochemically and analogs prepared using a plasma discharges (Khare et al., 1984, Icarus 60, 127–137; Coll et al., 1999, Planet. Space Sci. 47, 1331–1340; Khare et al., 2002, Icarus 160, 172–182). The C/N ratio in the haze analog prepared by discharges is in the 2–11 range while that of the photochemical analogs is in the 18–24 range. The use of discharges and UV light for initiating the chemistry in Titan’s atmosphere is discussed.

The photochemical formation of a titan haze analog. Structural analysis by x-ray photoelectron and infrared spectroscopy

The photochemical flow reactor (D.W. Clarke et al., 2000, Icarus 147, 282–291) has been modified to minimize the incorporation of oxygen and other impurities in the photoproducts. A mixture of gases that approximate their mixing ratios on Titan (N2, CH4, H2, C2H2, C2H4, and HC3N) (0.98, 0.018, 0.002, 3.5 × 10−4, 3 × 10−4, 1.7 × 10−5, respectively) was irradiated in the flow photochemical reactor using a 185-nm source to give a Titan haze analog as a solid product. X-ray photoelectron spectroscopy (XPS) gave a composition of 93.3% C, 5.3% N, and 1.4% O. Of the 93.3% carbon, high-resolution XPS revealed that 81.2% was present as CH, CC, and CC groups, 12.1% may be CO, CN, CN, CN, and/or CN groups, 5.3% as a CN group. The peak for N was symmetrical and was assigned to the CN while that for oxygen was assigned to the CO and/or the CO group. Some of these assignments were confirmed by FTIR spectroscopy. The polymeric product had a C:N ratio of 17.6, which is significantly greater than that for Titan haze analogs prepared in discharge reactions. When the polymer was exposed to air for seven days the oxygen content increased by 6% along with an increase in the infrared absorption at 1710 cm−1 assigned to the CO group of a ketone. The oxidation is attributed to the reaction of oxygen with free radicals trapped in the polymer matrix. It is proposed that the photochemical initiation of Titan haze formation from compounds formed from starting materials formed high in Titan’s atmosphere is a more plausible model than haze formed in reactions initiated by solely by discharges. These data will be helpful in the interpretation of the data returned from the Huygens probe of the Cassini mission.

Interfacial reactions at copper surfaces coated with polyimide films prepared from poly(amide–acid) precursors

Thin films of polyimide were prepared by spin coating the poly(amide–acid) precursor onto copper and aluminum substrates, followed by the usual heat treatment to promote imidization (curing) of the film. Films prepared on aluminum substrates were completely cured during the heat treatment, as shown by x‐ray photoelectron spectroscopy and infrared measurements. On copper substrates, the thinnest films (2000 A or less) showed considerable intermixing of copper ions in the polymer layer. This prevented the films from curing completely during heat treatment. The formation of a copper carboxylate at the acid site in the polyimide precursor is postulated.

Interfacial reactions at copper surfaces coated with polymer films

Polymeric coatings on copper surfaces are known to degrade at a faster rate than identical materials on other metals such as aluminum, particularly during thermal aging. We have studied the interfacial reactions occurring at copper surfaces coated with poly(esterimide) and polyimide wire enamels. Thin coatings (100 A–1 μm) were heat treated at temperatures from 200 to 240 °C. Interfacial reactions were studied by x‐ray photoelectron spectroscopy (XPS), Auger profiling, and reflectance infrared spectroscopy. In addition to copper oxide growth at the interface, thermal oxidative degradation of the polymer leads to thinning of the coating. This reaction is catalyzed by the copper (oxide) surface and material loss occurs primarily at the polymer/copper (oxide) interface. Migration of mobile copper species into the bulk of the coating is observed by its appearance at the surface and by depth profiling. For polyimide films prepared from poly(amide‐acid) precursors, an interfacial reaction occurs during initial ...

Charge correction of the binding energy scale in XPS analysis of polymers using surface deposition of PDMS

A method is described for correcting the binding energy scale for specimen charging that occurs during XPS analysis of insulating samples. A small quantity of polymeric poly (dimethyl silicone) (PDMS) is deposited from solution onto the surface of a series of polymers. After XPS analysis, the binding energy scale is then adjusted to align the Si 2p signal of the adsorbed PDMS to the value observed on conducting samples. A model is proposed that shows that the binding energies of insulating specimens are measured with respect to the sample Fermi level. Using this method, the C 1s binding energy for aliphatic hydrocarbon (CH2)x in polyethylene is measured at 284.97 eV, in excellent agreement with previously reported values based on other correction schemes. Measured energies for 12 other materials are also presented. Copyright

Study of the enhanced oxidative degradation of polymer films at polymer/copper (oxide) interfaces using depth profile and inert marker techniques

The degradation of poly(ester imide) films on metal substrates was monitored by determining the changes in polymer film thickness using x‐ray photoelectron spectroscopy combined with ion sputter depth profiling. Thin films (1000–7000 A) of poly(ester imide) wire enamel were spin coated onto metal substrates and aged in air at 200 °C for several hours. For films on copper substrates, thermal oxidative degradation of the polymer led to thinning of the film (2.5–3 A/min), whereas films on aluminum substrates did not exhibit a decrease in thickness. The catalytic effect of copper upon the degradation of the polymer occurs primarily at the copper (oxide)/polymer interface, as revealed by the location of an inert gold marker layer within the polymer film before and after aging.

Studies on metal/benzocyclobutene (BCB) interface and adhesion

Interfacial characteristics such as chemical reaction, metal diffusion, and morphology were investigated for Cu/BCB, Cr/BCB and Ti/BCB structures. Using Auger and XPS depth profiling, the formation of titanium carbide and chromium oxide was confirmed at the metal/BCB interface. Annealing at 250°C for extended periods resulted in the diffusion of Cu, Cr and Ti into the BCB and subsequent formation of Cu-Si, CrSi2 and Ti-Si compound precipitates. The reaction is a thermal diffusion controlled process which is dependent on time and temperature. Ar sputtering treatment of BCB film before metallization was found to roughen the surface, resulting in metal spikes which penetrate into the roughened BCB film. However, the peel strength of metals on BCB was only about 177 g cm_1presumably due to the brittleness of the BCB film. The etch rates of the BCB film in a reactive ion etcher (RIE) and a plasma etcher were measured using Ar, O2, O2 + CF4, and O2 + SF6 gas mixtures. Faster etch rates were obtained when CF4 an...

Copper deposition onto polyetherimide: Interface composition and adhesion

The chemical composition and adhesion of the interface between copper (applied via an electroless bath or by vacuum evaporation) and polyetherimide substrates are investigated. Electroless copper deposition requires a chemical pretreatment to ensure final adhesion values of 180–220 g/mm, whereas comparable adhesion is obtained for evaporated copper without pretreatment. Both metallization methods require a postdeposition heat treatment to maximize the copper/polyetherimide adhesion. The interface side of the metal deposits are exposed by a substrate dissolution technique and analyzed using x‐ray photoelectron spectroscopy. A thin (∼30 A) insoluble polymer layer remains which is believed to chemically interact with the metal. In addition, interfacial oxidation of the metal deposit increases with the application of the heat treatment. The role of this interfacial chemistry on copper/polyetherimide adhesion is discussed.

Surface studies of polyether–polyester copolymers and blends

The surface composition and morphology of a series of thermoplastic elastomers based on polyester (PBT)–polyether (PPG) copolymers have been studied using x‐ray photospectroscopy (XPS) and static secondary‐ion‐mass spectroscopy (SIMS). In all cases, an enrichment of the polyether soft block at the surface is observed. The degree of preferential surface adsorption is greater for molded specimens compared ot the solvent cast films. The enrichment factor (surface content/bulk content) is greater for speciment of highest PBT content. Angle‐dependent XPS measurements are fit with an enriched overlayer model and a continuous profile model. Static SIMS measurements are consistent with the XPS results for molded versus solvent cast films, but indicate that some PBT is present at the outer surface of all specimens analyzed.

Polycarbonate Spin Cast Films by XPS

A spin cast film of poly(bisphenol-A carbonate) was analyzed by XPS using monochromatic Al Kα excitation. The measured atomic composition closely matched the expected stoichiometry of the polymer. Peak positions of the individual chemically shifted C 1s and O 1s components are reported. The results are in general agreement with other reported spectra of polycarbonate polymers: experimental, C 83.4% and O 16.6%; theoretical, C 84.2% and O 15.8%.