External reflection absorption spectroscopy

This paper reports the study of the aging behavior of ultra-thin epoxy films on PVD layers of Al, Au, and Cu under varying environmental conditions by FTIR external reflection absorption spectroscopy (ERAS), to reveal influences of interphase interactions on crosslinking and on chemical aging processes at moderate temperature. 

Infrared spectroscopy experiments are performed at the University of the Saarland, Saarbriicken, using the external reflection absorption spectroscopy (ERAS) technique. Absorption peaks are attributed to molecular eigenvibrations which in some cases are localized at specific atom groups within the molecules. 

Eor IR-ERAS (external reflection absorption spectroscopy), steel substrates (1mm thick) are cut to 25x25 mm size. Spin-coating of EPl (directly after mixing, 10000 rpm, 10 s) produces thick ca. 5 pm layers on these substrates. Eor DSC, 7 mm steel disks (0.7 mm thick) are cut and spin-coating provides epoxy layers (ca. 10 pm) on the disks. All samples are first cured at RT for 48 h and subsequently post-cured at Tc=40°C for 24 h (PC40). 

In situ FTIR spectroscopy has proved to be very useful for the investigation of the reaction mechanisms of electrochemical reactions and structural changes of substances involved in these reactions. Two different methods are used external reflection absorption spectroscopy and internal reflection spectroscopy. The application of these methods to the study of the electrochemical doping process of polypyrrole, and the comparison of the results are described in this contribution. 

The setup for external reflection absorption spectroscopy is shown in Fig. 1. A PPy-covered platinum disc electrode is pressed against a ZnSe window, yielding a distance of some m between the electrode and the window. The electrochemical current of the oxidation process has to pass the thin electrolyte layer. Since only parallel-polarized light interacts with substances near a reflecting metal surface, the nt light was polarized in this way. The IR beam permeates the window, the electrolyte and the polymer and is reflected at the Pt surface. Only this part of the radiation (a in Fig. 1) contains information on the polymer absorption. Reflections at the window and the polymer surface b, c and d in Mg. 1), which also reach the detector, can lead to disturbing spectral features and have to be eliminated or corrected. ... 

Fig. 2. Spectral changes during the dectrochemical doping process of PPy using external reflection absorption spectroscopy. 

In addition to the polymer bands, spectral features at 2259, 1452 and 1376 cm " can be seen. Using external reflection absorption spectroscopy, absorption bands of the electrolyte cannot be compensated completely, leading to these disturbing effects. The bands at 1110 and 624 cm are bands of the C104 -ion. The electrochemical current of the oxidation process causes the migration of these ions from the surrounding bulk electrolyte into the thin electrolyte layer, resulting in an increase in the concentation and in IR absorption of this species. 

Both methods — external reflection absorption spectroscopy and internal reflection spectroscopy — can be used to obtain in situ spectral information on the electrochemical doping process of poly pyrrole. 

The main problem using external reflection absorption spectroscopy shows to be the thin electrolyte layer. The absorption bands of the electrolyte cannot be compensated completely, resulting in spectral disturbances. Besides difficulties with the ohmic resistance for the electrochemical current, concentration changes in the thin electrolyte layer occur during electrochemical reactions. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

Most of the solvents used in electrochemistry, and particularly water, present strong absorption in the mid-IR range. Therefore the use of external reflection IR spectroscopy for the in-situ observation of electrode processes requires a considerable reduction in the solution thickness in the path of the IR beam. Only a very thin layer of electrolyte between electrode and IR window is allowed in order to have enough energy reaching the electrode surface. Typically, the thickness of the solution layer produced by a well-positioned, flat-polished electrode is of the order of 1 - 5 pm. Within this cavity, which has been described by Yeager et al. as diffusionally decoupled, migration is the predominant form of mass transport [26]. 

Information on the conformational state of the hydrocarbon chains and their orientation has been obtained from external infrared reflection absorption spectroscopy (IRRAS). The first systematic IRRAS studies on phospholipid Langmuir monolayers were reported by Dluhy et al ) (see, for instance fig. 3.62). For DPPC monolayers in the LE phase the positions of the conformation-sensitive symmetric and anti-symmetric C-H stretching bands in the IRRAS spectra were found to be at the same positions as for bilayer systems of DPPC above the Kralft temperature. In the LC phase the frequencies of these bands indicate that the hydrocarbon chains of the lipid molecules are in the all-trans ) conformation (i.e. zig-zag) and analysis of polarized IRRAS spectra show that their average tilt is ca 35° relative to the monolayer normal. This is in reasonable agreement with the tilt angle of 30° obtciined from X-ray diffraction on DPPC monolayers (30°). 

The infrared reflection-absorption spectroscopy was performed on a Bruker IFS 66 spectrometer (Karlsruhe, Germany) equipped with a MCT detector and a modified external reflection attachment P/N 19650 of SPECAC (Orpington, UK). This included a miniaturized Langmuir-trough, permitting thermostatic measurements. An extensive description of the method can be found in Gericke et al. (1993). The IRRAS set-up as well as the experimental approach can be inferred from the schematic sketch shown in Fig. 2. 

Alpers W and Hiihnerfuss H (1989) The damping of ocean waves by surface films a new look at an old problem. J Geophys Res 94 6251-6265 Benvegnu DJ and McConnell HM (1992) Line tension between liquid domains in lipid monolayers. J Phys Chem 96 6820-6824 Gericke A, Michailov AV, and Hiihnerfuss H (1993) Polarized external infrared reflection-absorption spectroscopy at the air/water interface comparison of experimental and theoretical results for different angles of incidence. Vib Spectroscop 4 335-348... 

One of the most commonly applied IR techniques developed to overcome these problems is the external reflectance technique. In this method, the shong solvent absorption is minimized by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The sensitivity problem, that is, the enhancement of the signal/noise ratio in the case of external reflectance techniques is solved by various approaches. These are, for instance, electrochemically modulated infrared spectroscopy (EMIRS), in situ FTIR (which use potential modulation), and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS, FTIR) [86,117-123]. 

Figure 9 Optical setup for external reflection infrared measurements of Langmuir-Blodgett monolayers on liquid water surfaces. The arrow indicates the shuttling direction of the Langmuir trough in order to switch between the sample (monolayer on water) and the reference (pure water) position. (Reprinted with permission from Flach CR, Gericke A, and Mendelsohn R (1997) Quantitative determination of molecular chain tilt angles in mono-layer films at the air/water interface infrared reflection/absorption spectroscopy of behenic acid methyl ester. Journal of Physical Chemistry S 101 58-65 American Chemical Society.)...

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