Thickness reference band

Fourier transform infrared spectroscopy (FTIR) has emerged as a valuable tool for rubber analysis. Siesler monitored the onset, progress and decay of strain-induced crystallization of a sulfur-cured NR during a cyclic experiment. The infrared absorbance of NR at 1126 cm assigned to C-CH3 in-plane deformation vibration is a band sensitive to crystallinity. A thickness reference band was taken to be the 1662 cm one assigned to v(C=C) stretching. The ratio of the absorbance bands at 1126 cm and 1662 cm revealed the reversible nature of strain-induced crystallization of NR. 

It is important to stress that ATR absorbance is strongly affected by the sample/crystal contact. Quantitative results are thus difficult to obtain even if the contact is maintained during the sample rotation that is required to record all four polarized spectra. A reference band that does not show significant dichroism is thus most often used to normalize the polarized absorbances in order to obtain quantitative data. For instance, the 1,410 cm-1 band of PET has often been chosen for that purpose, not only for ATR studies but also for specular reflectance (see below) and even transmission studies when the sample thickness is not uniform. It was shown that an appropriate normalization is possible even if no such reference band is available, by using a combination of two bands with orthogonal dichroism [34]. When performing ATR experiments, one should also make certain that the applied pressure does not create artifacts by affecting the structure of the sample. 

The intensity of the crystalline bands was monitored simultaneously during the crystallization. To correct for changes in density or thickness in the different samples, the intensities were normalized by the reference band. These normalized intensities were plotted vs. log time for each of the blends at the different crystallization temperatures. The curves obtained are sigmoidal in nature and they level off when the final crystallinity is achieved. A typical curve for the normalized intensity of the 848-cm-1 band vs. log time is plotted in Figure 7 for PET. 

The reference band (2940 cm ) was used to compensate for film thickness variation and is independent of cure conditions and photo-oxidation time. An absorption band at 1612 cm-1 was also monitored and is discussed further below. 

A more informative representation of the reversible transition is obtained by plotting the intensity of absorption bands which are characteristic of the stressed and relaxed segments, respectively, as a function of strain. For this prupose the absorbance variations of the 1459 cm and 916cm (a-form) and 1485 an and 960 cm" (P-form) absorption bands were selected in Fig. 25. The absorbances have been corrected for changes in sample thickness wiUi the aid of the aromatic v(C—C) absorption at 1505 cm" as reference band. 

IR spectroscopy has been widely used in the elucidation of PP stereostructure since Natta and co-workers [26-29] first reported the spectrum of the crystalline polymer. Most studies have concentrated on identifying suitable bands to measure tacticity and to correlate with other indices of stereoregularity. The bands apparently associated with isotactic helices absorb at 8.19, 8.56, 10.02, 11.11, 11.89 and 12.36 pm, and of these the bands of 11.89 pm and 10.02 pm have been principally used for the derivation of calibration curves (Figure 6.4). Because it is difficult to prepare films of standard thickness, it is customary to use an internal reference bond, and the absorptions at 6.85, 8.56 and 10.28 pm are used for this purpose. The origin of the reference band at 10.28 pm, which is observable in the melt of isotactic samples [19, 30] as well as in purely atactic material, is disputed [30]. Thus the hand, which has been attributed [30, 31] specifically to the PP head-to-tail sequence of repeating units, has also been associated with short isotactic helices apparently still present in the melt or atactic material [25]. 

The Fourier Transform Infra-Red spectroscopy (FTIR) technique is a valuable method for investigating the kinetics of sorption of organic molecules in polymers, hi further works (Safa and Abbes, 2002 Safa et al., 2007 Zaki et al. 2009), we have used such method to study the sorption rate of different esters by polypropylene. The ester function is characterized by absorption band in regions where the polypropylene does not absorb. The sorption can be evaluated by the ratio of the carbonyl ester (CO) band area at 1747 cm to the area of a characteristic peak of the PP at 841 cm used as a reference band. The polymer samples are cut-out in thin films of 50 pm thickness by using a microtome. These films are then put on a support adapted for the spectroscopic analysis. Figure 5 shows FTIR spectra of a polypropylene sample after contact with 5000 ppm of amyl acetate solution at 23°C after 5 hours, 4 days and 15 days of immersion. The absorption characteristic band of esters at 1747 cm and its growth is clearly observed. 

Internally, muscle fibers are highly organized. Each fiber contains numerous myofibrils — cylindrical structures that also lie parallel to the long axis of the muscle. The myofibrils are composed of thick filaments and thin filaments. It is the arrangement of these filaments that creates alternating light and dark bands observed microscopically along the muscle fiber. Thus, skeletal muscle is also referred to as striated muscle. 

It must be pointed out that a correction is needed to account for the change in thickness upon stretching of the films. For that reason all carbonyl contents have been referred to the thickness of the non-strained film which has been photooxidized at the same temperature for the same period. The 2840 cm-l band, which corresponds to the symmetric C-H stretching of the methylene groups (14) was chosen as reference for thickness correction (other bands can be used, but the choice was decided by the fact that the 2840 cm l band changed in intensity as the strain varied, but its location and shape were not affected by the strain variation.) Thus, all the photooxidation data are given on a relative scale. 

Figure 5 Schematic representation of absorbance of porphyrin compounds in relation to tissue transmittance at various wavelengths (see text). The lowest energy band (Band I) is shown in each case, apart from the porphyrin spectrum (etio type shown) on the left. The transmittance curve refers to a fold of human scrotal sac 0.7 cm thick (Wan, S. Parrish, J. A. Anderson, R. R. Madden, M. Photochem. Photobiol. 1981, 34, 679-681). The broad feature at ca. 500-600 nm is ascribed to haemoglobin (reproduced by permission of the Royal Society of Chemistry from Chem. Soc. Rev. 1995, 24, 19-33).

Scheme VI. Representation of the interface energetics for intrinsic a-Si H at short circuit, dark equilibrium with ferricenium/ferrocene in EtOH, electrolyte solution (left) and under illumination with 632.8-nm light with a load in series in the external circuit (right). The diagrams are adapted from data in Reference 65 for intrinsic a-Si H (1-4-fi thick) on stainless steel first coated with heavily n-doped a-Si H (200-A thick) to ensure an ohmic contact near the bottom of the conduction band. In typical experiments Eredox = 0.4 V vs. SCE.

The frontier between the depletion and the accumulation situations of the space charge layer is defined by the flat band potential. In fact, when the potential is constant all along the thickness of the electrode, the mobile charge (and naturally the fixed charge) distribution is uniform. In the case of the interface of Si electrode with an electrolyte, the corresponding bias potential has to be determined with respect to the reference electrode. The value of the flat band potential Vfb is expected... 

Solutions are handled in cells of 0.1-1 mm thickness. Volumes of 0.1-1 mL of 0.05-10% solutions are required for readily available cells. A compensating cell, containing pure solvent, is placed in the reference beam. The spectrum thus obtained is that of the solute except in those regions in which the solvent absorbs strongly. For example, thick samples of carbon tetrachloride absorb strongly near 800 cm-1 compensation for this band is ineffective since strong absorption prevents any radiation from reaching the detector. 

---