Amorphous band intensities

This picture was found to be consistent with the comparison of Raman spectra and optical gap of a-C H films deposited by RFPECVD, with increasing self-bias [41], It was found that both, the band intensity ratio /d//g and the peak position (DQ increased upon increasing self-bias potential. At the same time, a decrease on the optical gap was observed. Within the cluster model for the electronic structure of amorphous carbon films, a decrease in the optical gap is expected for the increase of the sp -carbon clusters size. From this, one can admit that in a-C H films, the modifications mentioned earlier in the Raman spectra really correspond to an increase in the graphitic clusters size. 

In some cases crystalline polymers show additional absorption bands in the infrared spectrum, as in polyethylene ( crystalline band at 730 cm amorphous band at 1300 cm" ) and polystyrene (bands at 982,1318, and 1368 cm" ). By determining the intensity of these bands it is possible to follow in a simple way the changes of degree of crystallinity caused, for example, by heating or by changes in the conditions of preparation. 

A routine method for determining relative crystallinity based on the amorphous bands in the spectrum has proved more rapid and precise than the x-ray method. In practice, the ratio of the 778 cm-1 (12.85 ft) and 2367 cm-1 (4.22 ft) band intensities is measured. Use of a ratio eliminates the thickness measurement and increases precision to about 1% at 50% crystallinity and considerably better at higher levels. A density measurement and an infrared crystallinity determination when combined give an estimate of the fraction of microvoids which can occur in molded specimens of polytetrafluoroethylene. The density of a sample is predicted on the basis of its crystallinity as measured by the infrared method and the difference between this density and the actual density measured by displacement in water is a measure of the microvoid content. This determination is precise to about 0,2% voids by volume. By the use of confirmatory infrared measurements, it is possible to check the possibility that the presence of a substantial percentage of voids may have led to erroneous indications of the molecular weight in the standard specific gravity test discussed earlier. 

An early example of Raman mapping by Breitenbach et al. [52] showed that when crystalline ibuprofen is formulated in a hot melt extrudate the API changes to the amorphous form. Ibuprofen is a sparingly water-soluble compound, so this formulation provides a route to better bioavailability via the more soluble amorphous form. Using confocal Raman mapping the form of the API was determined at the time of manufacture and under stress conditions and was used to assess the stability of the amorphous form. These studies also showed that the API was homogeneously distributed throughout the formulation based on the relative band intensities of the amorphous API and a formulation excipient, polyvinylpyrrolidone (PVP). 

Specific interactions between PCL and PVC are clearly indicated. In the solid state (Figure 5.9a) the spectrum of neat PCL indicates the presence of crystalline (1724 cm 1) and amorphous (1737 cm"1) bands. At mole ratios up to 2 1 of PVC to PCL, the spectra indicate that in the solid state the blends consist of crystalline and amorphous phases. As the PVC concentration increases, a parallel increase of the intensity of the amorphous band is observed. Moreover, the frequency shifts observed for both the crystalline and amorphous bands as a function of the composition of the blend suggests that specific interactions between the two polymers occur. No shift is observed in the carbonyl stretching vibration of PPL/PVC blends, in the molten state or in the solid state over the entire range of compositions and the two polymers are incompatible [28]. 

The most noticeable effect during the in situ Raman studies was the near complete disappearance of the disorder-induced D band after oxidation (Fig. 12.3c). These results show that for the DWCNT sample, the D band originates mainly from amorphous carbon present in the sample and not from defects in the wall structure of the nanotubes. While the concentration of defects probably increases during the oxidation, disordered carbon and the associated D band disappear completely. However, it is well known that only metallic CNTs contribute to the D band intensity [57]. Therefore, the absence of any Raman signal... 

In situ Raman spectroscopy analysis of isothermal and nonisothermal oxidation of DWCNTs in air showed a decrease in the intensity of the D band starting around 370°C, followed by complete D band elimination at 440°C. The oxidation process produced the purest CNTs ever reported, which were free of amorphous carbon and highly defective tubes, while the removal of amorphous material was not accompanied by tube damage. In situ Raman measurements allowed us to determine the different contributions to the D band feature and show the relationship between D band, G band, and RBM Raman modes in the Raman spectra of DWCNTs upon heating. The described approach thus provides an efficient purification method for DWCNTs and SWCNTs, which is also selective to tube diameter and chirality. While oxidation of MWCNTs did not significantly decrease the D band intensity below 450°C, oxidation in air can be an effective route to control the number of... 

The intensities of the vinyl absorption bands increase as the temperature is lowered. The crystalline bands at 1050 and 1176 cm exhibit a much narrower bandwidth at lower temperatures. Only slight changes in the amorphous bands are observed with temperatme. However, differences between slow-crystallized and quenched samples are apparent... 

Additional information is available by subtracting spectra taken at different temperatures for the same sample. Since the position of the film was not altered as the temperatures varied, a 1 1 subtraction is a systematic method to illustrate the thermal eflFects. The spectra at the two temperature extremes for a slow-crystallized sample are subtracted in Figure 20. The 909 and 990 cm vinyl bands narrow in width, increase in intensity, and shift to slightly higher frequency as the temperature is decreased. The crystalline absorptions at 1050 and 1176 cm shift to lower frequency and sharpen considerably. The 1303 cm amorphous absorption shifts its maximum to 1300 cm and possibly increases in intensity. The 1353 cm" band remains fixed in position. The most intense amorphous band, 1369 cmmoves to 1371 cm at 78 K and has an intensity increase. These results are shown clearly in the difference spectrum. Similar results are obtained for the isopentane-quenched sample before and after annealing. 

One of the attributes of Raman spectroscopy is the ability to discern different crystalline (or amorphous) phases having the same stoichiometry. Figure 10 depicts several Raman spectra of mixed anatase/ruti1e phase films of Ti02 sputter deposited on silica. The phase composition can easily be discerned from measured band intensities. In fact, trace amounts of anatase in rutile films (.1 wt%) can be determined from the magnitude of the 143 cm"l anatase feature, which is a factor of ten more intense than vibrational bands intrinsic to other phases of Ti02. 

Thermodynamic considerations suggest that such transformation phenomena are thermally controlled. In fact, amorphous phase coatings spontaneously crystallize above some critical temperature. The transformation is manifested in the Raman spectrum by dramatic band intensity increases and marked band narrowing (20). When metastable crystalline treated, recrystallization temperature phase occurs as the temperature-dependent spectra shown in Figure 13, which depicts the irreversible transformation of anatase to rutile in a thin titania film. 

Marcilla and Beltr studied the evolution of PVC-plastidzer mixtores by FTtK dnring heating. When the plasticizer spectnun was subtracted fiom the plastisol one, the resultant spectral difference was nearly the same as the spectrum of pure PVC (some modifications observed depended on the plasticizer type). Figure 9.7 shows the results for DBP, DOP and DIDP. The more compatible plasticizer (DBP > DOP > DIDP) cansed greater modifications in the PVC spectrum (DIDP difference spectrum nearly matches that of pure resin). Moreover, it was observed that the relative intensity of the crystalline bands of PVC (1427 and 637 cm ) decreases as compared to the amorphous bands (at 1435 and 616 cm, respectively) with increasing compatibility between the resin and the plasticizer. 

---