Polymer band positions

Q are the absorbance and wavenumber, respectively, at the peak (center) of the band, p is the wavenumber, and y is the half width of the band at half height. Liquid band positions ate usually shifted slightly downward from vapor positions. Both band positions and widths of solute spectra are affected by solute—solvent interactions. Spectra of soHd-phase samples are similar to those of Hquids, but intermolecular interactions in soHds can be nonisotropic. In spectra of crystalline samples, vibrational bands tend to be sharper and may spHt in two, and new bands may also appear. If polarized infrared radiation is used, both crystalline samples and stressed amorphous samples (such as a stretched polymer film) show directional effects (28,29). 

The number of turnovers (10) in hydrocarbon production without apparent decrease in activity proved that the reaction was indeed catalytic. The IR spectrum of the recovered resin showed small absorptions due to CpCo(C0)2 5 due to some recar bony la-tion of " CpCo". In addition, a broad distinct band at 1887 cm"l was seen. The identity of the species exhibiting this carbonyl band is still a mystery in particular, the band position does not match that reported for any of the CpCo (CO) 2 -derived di- and trinuclear carbonyls (vide supra). It is tempting to associate this band with seme catalytic intermediate, such as the polymer-bound analogues of CpCo(H)2(C0) and CpCo(HKPh)(CO), but this is pure speculation. 

Such normal vibration analyses have been applied to the spectra of macromolecules to only a limited extent. In the first place, the only structure which has been analyzed in detail is that of the planar zig-zag chain of CHg groups, i.e., polyethylene. Neither substituted planar zig-zag chains nor the helical chain structures characteristic of many polymers [Bunn and Holmes (28)] have been submitted to such a theoretical analysis. In the second place, even for the case of polyethylene the answers are not in all instances unambiguous. Different assumptions as to the nature of the force field, and lack of knowledge of some of the force constants, has led to varying predictions of band positions in the observed spectrum. For the identification of certain modes, viz., those which retain the characteristics of separable group frequencies, such an analysis is not of primary importance, but for knowledge of skeletal frequencies and of interactions... 

The infrared spectrum of a sample of atactic polystyrene is shown in Fig. 15 [Liang and Krimm (114)]. The band positions are given in Table 16, on the left-hand side of the "Infrared column. Raman data for atactic polystyrene [Signer and Weiler (198) Palm (163)] are also included in the table. Polarized spectra of oriented specimens of isotactic polystyrene have recently been published [Tadokoro, Nishiyama, Noza-kura, and Murahashi (227) Tadokoro, Nozakura, Kitazawa, Yasuhara, and Murahashi (222) Takeda, Iimura, Ya-mada, and Imamura (222a) Morero, Man-tica, Ciampelli, and Sianesi (140a)]. The spectrum of the isotactic polymer differs slightly from that of the atactic polymer, and these differences (above 500 cm-1) are shown... 

Because of the strong coloration, depending on their state of oxidation, intrinsically conducting polymers have frequently been studied with SRRS [507, 508]. Molecular vibrations could be assigned based on various approaches. Most frequently band positions of the monomers and of already known oligomers were compared with those of the polymers. Alternatively, band positions were calculated based on an effective conjugation coordinate [509-516]. In a typical example shown in Fig. 5.84, SRR spectra of poly aniline are displayed as a function of electrode potential. 

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 teehnique is so surface sensitive that oxidation of metals can be determined in situ [51] and the packing... 

Otherwise, a similar upshift of 10-15 cm" was observed in the Raman spectra of poly(butadiene)-MWNT composites [67], The CH-ti interactions observed between nanotube and polymer are stronger than that of the n-n interactions observed between nanotube bundles, resulting in a restriction of the C-C bond vibrations and a corresponding upshift of the Raman signal. A 17 cm" upshift in G-band Raman signal of MWNTs embedded in melt-blended polyethylene-MWNT composites and the evolution of a shoulder to this peak were attributed to compressive forces exerted on the MWNTs by polyethylene chains following intercalation into MWNT bundles. So, the proposed compression-induced effect on MWNT Raman G-band position appears to be consistent with the results obtained for rrP3HT-MWNT composites. 

Polymers and rubbers exhibit characteristic band positions that will vary depending upon the molecular structure and the associated or attached chemical groups. Table 12.1 briefly demonstrates the positions for the major chemical groups encountered in analysis of polymer and rubber materials. The accompanying text describes the individual molecular vibrations and band assignments in much greater detail. 

Brandolini et al. [119] have pointed out some drawbacks to the use of FTIR spectroscopy for quantitative analysis of the extent of phosphite and phos-phonite additive degradation in PE. Band positions are not as distinctive as P NMR resonances. Consequently, it is difficult to distinguish degradation products of similar additives, such as A and B of Fig. 9.3 of ref. [1]. Quantitation in FTIR is also not as straightforward as in NMR. To accurately assess the extent of degradation, appropriate standards would have to be developed. Most importantly, however, the FTIR spectrum contains absorbances from the polymer background and other additives which may obscure the peaks of interest. Spectral subtraction of an appropiate reference polymer can obviate some of this concern, if available. Use of a similar, but not identical, polymer can result in artifacts. In the specific case at hand, the polymer sample was pressed as a 0.1 mm film. An appropriate, secondary oxidant-free reference polymer was available so that spectral subtraction could be performed to remove matrix interferences. 

IR absorption bands in amorphous solids are rather broad, with extensive overlap. Direct identification and quantification of the numerous oxidation products, most often of similar chemical structures, in a degraded polymer are difficult. More precise conclusions about the nature of absorbing species can be obtained by selective chemical derivatization. Selective modification of functional groups with reactive gases, such as SF4, NH3, SO2, or NO, results in a shift in absorption band positions, which can then be compared with model compounds to allow for a better chemical assignment of the absorbing species (Table 15.3) [13]. 

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