Лако-красочные материалы — производство

Several recent overviews of principles and applications of Raman, FTIR, and HREELS spectroscopies are available in the literature [35-37, 124]. The use of all major surface and interface vibrational spectroscopies in adhesion studies has recently been reviewed [38]. Infrared spectroscopy is undoubtedly the most widely applied spectroscopic technique of all methods described in this chapter because so many different forms of the technique have been developed, each with its own specific applicability. Common to all vibrational techniques is the capability to detect functional groups, in contrast to the techniques discussed in Sec. III. A, which detect primarily elements. The techniques discussed here all are based in principle on the same mechanism, namely, when infrared radiation (or low — energy electrons as in HREELS) interacts with a sample, groups of atoms, not single elements, absorb energy at characteristic vibrations (frequencies). These absorptions are mainly used for qualitative identification of functional groups in the sample, but quanti­tative determinations are possible in many cases.

(a) IR Techniques. Infrared spectroscopy in its original dispersive form, in which the absorption of infrared (IR) light (the intensity of transmitted energy) when passing through a sample is measured by scanning through the spectrum, has been in use for a long time. The introduction of the Fourier transform principle in IR spectroscopy has prompted an interest in the use of the technique for surface analysis. The Fourier transform (FTIR) instruments use mirrors instead of slits. Each scan thus gives informa­tion over the entire spectrum resulting in higher throughput, sensitivity, and signal-to — noise ratio. The many different (and sometimes confusing!) acronyms used in IR mainly refer to different forms of sampling techniques that have been developed in recent years and all use the FT (or interferometric) principle. The most common of these acronyms are listed in Table 1. This list is not complete because some researchers use their own acronyms.

Basically, there are two categories of FTIR spectroscopies: reflection and nonreflec­tion techniques [38]. The latter class comprises either acoustic detection or emission from the sample itself. The techniques recognized here are photoacoustic spectroscopy (PAS), emission spectroscopy (EMS), and photothermal beam deflection spectroscopy (PBDS). These techniques will not be considered further in this chapter. The reader is referred to the literature [39-42]. For adhesion studies the reflection techniques (SRIRS) are more important. The major classes of sampling techniques in SRIRS are:

Internal reflection IR (IRS); commonly known as MIR (multiple internal reflection) or ATR (attenuated total reflection).

External reflection IR (ERS); the techniques in this category can be a single reflection setup (reflection-absorption IR, RAIR, or grazing incidence reflection IR, GIR) or a multireflection setup (MRAIR). The single reflection technique is also frequently referred to as specular reflectance IR.

Diffuse reflectance IR (commonly called DRIFT).

The principles of these three major categories are shown in Fig. 12. ATR is useful primarily for identification of polymer films, liquids, or other materials that can be coated onto a crystal of high refractive index, such as Ge. The sample must be in very good contact with the crystal for good spectra to be obtained. IR light is shone into the crystal at an angle of incidence that is higher than the critical angle of reflection for the crystal, so internal reflection of the radiation occurs and its intensity is attenuated as a result of absorption by the sample. Mostly rectangular crystals are used, but circular crystals have recently been developed for the study of aqueous solutions, films, and fibers [43-46]. Although the requirement of intimate contact between crystal and sample is severely limiting, the tech­nique has the useful capability of providing a depth profile by varying the angle of incidence or by using crystals with different optical densities. As an example, the surface crystallinity or orientation of fibers with respect to their axis has been reported [46]. Further, the technique is uniquely suited for the study of solid/aqueous interfaces.

The technique of choice for studying thin films on metals (or certain other substrates) directly is single reflection RAIR [47-54]. The limitation here is that the substrate must be very smooth, but this can be easily achieved by polishing the metal before deposition of the film. Characterizations of thin organic layers on metal (oxide) surfaces, such as occur in lubricants, corrosion inhibitors, adhesives, polymers, paints, and so forth, are specific applications of this rather recent form of FTIR. It should be noted that the relative band positions and shapes may be different in this technique than in conventional trans­mission IR. The spectrum may also change with the thickness of the organic film, which implies that polymer/metal interactions are in principle observed [47,51]. The technique is so surface sensitive that oxidation of metals can be determined in situ [51] and the packing structure of monolayers of organic molecules or Langmuir-Blodgett films can be studied [52, 53]. In such studies the metallic substrate must have a high reflectivity. Ideal substrates are thus silver and copper. The technologically important substrates aluminum and steel have lower reflectivity or sensitivity.

In diffuse reflectance IR spectroscopy (DRIFT) light impinges on a solid powdered sample and is scattered in all directions. This light is collected and redirected to a detector. The powder must be very fine and is mixed with or dispersed in a suitable matrix, such as KCl or KBr with a particle size of less than 10 mm. It is not suitable for large powders or lumps. By using special sample preparation techniques, e. g., by placing the KBr powder over the sample, monolayers of adsorbed silanes have been studied [55], or water adsorbed on polymer surfaces has been detected [56].

In summary of this section, it can be stated that there are now numerous FTIR spectroscopy techniques, which as a result of their enhanced sensitivity and signal-to-noise ratio have contributed immensely in recent years to the understanding of molecular phe­nomena at surfaces and interfaces as they are related to adhesion. The development of new sampling techniques still continues, and much activity can be expected to occur in the near future in the FTIR arena, along with another promising vibrational spectroscopy tool, Raman spectroscopy, to be discussed below.

(b) Raman Techniques. This vibrational spectroscopy is related and complementary to FTIR. Although its usage is currently not so widespread as that of the various IR tech­niques, primarily because of its much lower sensitivity, this may well change in the future as some exciting new developments have recently been published.

Raman spectroscopy is a long-established technique for the study of bulk materials. In principle, the technique is straightforward [57-63]. A small region of a transparent sample is illuminated by a monochromatic laser beam, and light that is scattered at a 90° angle with the incident beam is collected and directed into a spectrometer. Most of the scattered light is elastically scattered and has the same frequency vo as the incident light. This is known as Rayleigh scattering. A small fraction of the scattered light, however, is inelastically scattered and thus contains new frequencies vo ± vk. These frequency differ­ences are associated with the transitions between the various vibrational levels in the sample molecules; hence the frequencies observed are in many cases similar to the wave numbers in FTIR. The lines with the lower frequencies are the Stokes lines; those with the higher frequencies are referred to as the anti-Stokes lines. The former series is usually measured in the Raman experiment. The intensities, frequencies, and polarization char­acter of these lines can be determined. The lines are observed in a direction perpendicular to the incident beam, which is plane-polarized.

Raman spectroscopy is useful for studying aqueous solutions, e. g., of polymers, because the spectra are hardly affected by the presence of water. The two major problems of the technique are: (1) the low intensities of the Stokes lines (hence data acquisition is slow), and (2) laser-induced fluorescence effects, which may be so intense that they can completely wipe out all Raman scattering of interest.

The recent developments in the Raman technique, which are very promising and which may lead to its emergence as one of the most surface-sensitive techniques available in the near future, are the following.

Development of Fourier transform [64-66] and Hadamer transform [67] spectro­meters as detectors in Raman spectroscopy, similar to FTIR; it has recently been shown that with the proper masking of the Rayleigh scattered radiation and special optical filters, fluorescence-free Raman spectra can be obtained [64].

Resonance Raman spectroscopy (RRS); if the wavelength of the incident radiation is chosen so that it coincides with an absorption band of the scattering molecules, the resonant Raman scattering cross-sections may be up to 106 times the cross­sections for normal Raman scattering. In such cases it is possible to detect monolayers (e. g., of dye molecules) at surfaces. This has indeed been demon­strated [68,69]. Recently RRS has found many new applications, mainly in biological studies.

Surface-enhanced Raman spectroscopy (SERS); in this resonance scattering the enhancement is caused by the substrate rather than by the adsorbed molecules.

SERS is thus a surface technique that has so far been restricted to only a few substrate metals, namely copper, silver, and gold [70-75], but it is applicable to almost any adsorbate. It is also noteworthy that the two mechanisms in RRS and SERS are additive, so that certain dyes on SERS substrates can be detected at extremely low coverage [76]. It is generally accepted now that two mechan­isms contribute to the strong enhancement (up to a factor of 106) of Raman scattering by molecules adsorbed at the roughened surface of Cu, Ag, or Au. One factor is the generation of large oscillating dipoles and therefore oscillating electrical fields at the metal surface. The other factor is the enhancement of the polarizability of the adsorbed molecules at optical wavelengths by the substrate, i. e., a charge-transfer mechanism [35]. In addition to the substrates already mentioned, enhancement has been observed for the alkali metals, aluminum, indium, palladium, platinum, and even some oxides (NiO and TiO2), though with lower intensities than for the metals Ag, Cu, or Au.

The range at which molecules contribute significantly to the SERS signal varies between 5 and 50 A, i. e., comparable to the sampling depth in XPS. The SERS signal does not depend linearly on adsorbate coverage, making quanti­tative analysis difficult. Further, the restriction to certain substrates is a serious limitation. Recently it has been reported that the strong enhancement effects by substrates such as silver spill over for a few nanometers into an adjacent phase. Therefore, if silver is coated with a very thin film of, for instance, SiO2, SERS can be observed for molecules adsorbed onto the silica film [77]. Other approaches include the evaporation of silver overlayers to study surfaces of thin and thick films. An example is shown in Fig. 13. In this study the surface segregation in blends of polystyrene with deuterated polystyrene was investi­gated. SERS was used to investigate the interface with a roughened silver substrate, but also to study the surface by means of a thin overlayer. At both interfaces the technique detected an enrichment of the deuterated form after vacuum annealing [78]. The surface segregation was confirmed by TOFSIMS. In another application the specific adsorption of one of the com­ponents of an adhesive system was studied. The results are summarized in Fig. 14 [79]. These various intricate and ingenious sample preparation methods that are currently being developed by several laboratories show great promise for an extension of the types of substrates and materials to which the SERS technique can be applied.

Extremely low-noise integrating multichannel detectors; the very recent introduction of these very special devices has made it possible for the first time to record Raman spectra from monolayers of organic materials (e. g., fatty acids) on substrates that do not give surface enhancement, for instance a water-air inter­face [80]. Conventional Raman spectroscopy may now have become a viable

Figure 13 Surface-enhanced Raman spectra (SERS) obtained from thin films of blends of polystyrene and deuterated polystyrene before (A) and after annealing (B). Samples and conditions as in Fig. 9.

non-UHV surface analysis tool. Advantages of conventional RS over SERS are that the scattering cross-sections are independent of the type of substrate surfaces. Further, the intensity varies more linearly with coverage and the cross-sections do not vary a great deal for different molecules. Hence the con­ventional Raman technique is more quantitative than SERS.

We may thus begin to see more applications of conventional Raman spectroscopy, e. g., of monolayers on well-defined substrate surfaces. On the other hand, SERS will remain a powerful technique in its own right because of its greater sensitivity and surface specificity. It remains practically the only technique available to determine nondestructively and in a non-UHV environment the orientation of molecules in situ in a dense medium, since the contribution to the observed signal by molecules farther than about 50 A away from the interface is negligible compared with the enhanced signal from the interfacial molecules.

(c) High-Resolution Electron Energy Loss Spectroscopy (HREELS). The specific attri­bute of this vibrational technique, compared with the ones discussed above, is that it can provide functional group information on the surface of polymers. In addition, one can study the interactions between these functional groups and thin films of metals evaporated onto these polymers. The technique, which has gone through some recent instrumental developments, is thus important for the furthering of our understanding of metal-polymer adhesion mechanisms.

The basic experiment in HREELS in the backscattering geometry is straightforward [37]. A monochromatized electron beam of 1-10 eV is directed toward the surface and the energy distribution of the reflected electrons is measured in an electron analyzer with a resolution of up to 7 meV. The spectrum consists of the elastic peak and peaks due to energy losses to the sample surface by the excitation of molecular vibrations. If plotted as wave numbers, these vibrations are very similar to those observed in IR techniques. The resolution achievable in this technique is, however, considerably less than in IR, which becomes clear if one considers that 1 meV = 8.066 cm-1, so the spectral resolution in HREELS is of the order of 100 cm-1 (in IR the resolution is typically around 4 cm-1 or better). Detection of crystallinity or other high-resolution details as is possible in IR is therefore currently not achievable in HREELS.

The most meaningful information in this technique is obtained by varying the elec­tron impact energy and the scattering geometry (angles ©,• and ©r) by rotating the sample holder or the electron analyzer.

A major problem that has hindered applications of the technique to bulk polymer surfaces until recently is the surface charging of insulators. At the low incident electron energies used in this technique, the secondary electron emission is high, so that a positive charge develops. Therefore, this problem can be overcome by using low current defocused flood guns of 1-2 keV electrons [81]. Further, although spectra can now be taken of the surface of polymers, these usually show broad peaks or bands, nowhere near the resolution obtained in IR. It is remarkable that heating (or annealing) polymers in vacuum for a few minutes at temperatures in the range 200-250° C sometimes results in a pronounced shar­pening of the peaks. No satisfactory mechanism for this effect has been put forward yet.

The major strength of the technique lies in the fact that vibrational information of organic surfaces can be obtained with absorption bands that are identical to those observed in IR. In this respect the technique bridges a gap between XPS and IR. Information can be obtained on the polymer surface itself or on molecules segregated to or adsorbed on the polymer surface. Aliphatic and aromatic groups can be distin­guished and hydrogen is also detected via functional groups. The adsorption of water on polymer surfaces is also detected easily. On the other hand, because of the high surface sensitivity, it is difficult to prepare clean model surfaces for HREELS studies. The simi­larity between HREELS and IR bands facilitates spectral interpretation immensely. There is no systematic difference between peak positions in the two techniques. The intensities of the bands in the two techniques are vastly different, however, because electron and photon excitation of molecular vibrations seem to follow different selection rules. At present, the excitation mechanism in HREELS is not completely understood, making quantification impossible. No practical theory is available to quantify electron-induced vibrational spec­tra of polymers.

Some polymer surface studies that have been reported recently are the detection of the molecular orientation at polymer film surfaces. For instance, the spectrum of iostactic polystyrene is different from that of the atactic material [82]. The spectra of thin films of poly(methylmethacrylate) (PMMA) cast on Au, Al, or Cu were also different; especially the intensity of the C = O stretching band at 1710 cm-1 varied considerably [83]. Thus HREELS seems to be capable of identifying molecular long-range order in polymeric surfaces.

Other recent and very interesting studies are those in which monolayers of metals have been evaporated onto polymeric substrates. This allowed a conclusion as to the nature of the interaction and the molecular sites that are preferably attacked by the metal atoms. Systems that have been studied recently are Cu-polyimide, Cr-polyimide,

and Al-polyimide [84-86]. Very recently the interfaces formed between vacuum-evapo­rated Al and polyvinyl alcohol, polyacrylic acid, and polyethylene terephthalate (PET) were investigated [87]. Figure 15 shows the HREELS spectrum of PET. The vibrational assignments and their comparison with IR intensities are shown in Table 4. The HREELS spectra observed during PET metallization are shown in Fig. 16. The bands that disappear are those of O-CH2 and C = O, and the new band that forms has been ascribed to the formation of a C-O-Al group and eventually a carboxylate.

In summary, the HREELS vibrational spectroscopy technique appears to be a new and promising highly surface-sensitive tool for the study of interactions between metals and polymers related to adhesion phenomena. Current limitations are the lack of quanti­fication, the lack of knowledge on the sampling depth, the difficulty to prepare clean polymer surfaces, the need for a clean vacuum (as opposed to the optical vibrational techniques), and the lack of suitable fingerprint spectra of clean surfaces of technological interest. However, as mentioned for Raman spectroscopies, we can expect much activity in the near future and the technique may be capable of carving its own niche in the gamut of available surface spectroscopies.