Difference spectrum between polymer

The three spectra shown in Figure 10.9 are (a) the reflection-absorption spectrum of a magnetic disk without fluorocarbon lubricant applied, (b) that with fluorocarbon lubricant, and (c) a difference spectrum between the spectra in (b) and (a). Spectrum (a) shows absorption bands due to polymer components present in the magnetic material on an aluminum... 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

Infrared spectra were recorded on the resist film spun onto a silicon wafer using a JASCO IR-810 spectrometer equipped with a JASCO BC-3 beam condenser or a JASCO A-3 spectrometer. In the measurements on the latter spectrometer an uncoated silicon wafer was placed in the reference beam in order to balance the silicon absorption band. The subtraction between the spectra was carried out on a built-in micro-processor attached to the IR-810 spectrometer, and the resulting difference spectrum was used to detect structural changes in the polymer molecule upon exposure. The subtraction technique was also used to balance the silicon absorption band. 

The intense absorption lines of the photoproduct series A to E obtained by UV-irradiation at low temperature are shown in Fig. 4a This spectrum represents a difference spectrum, where the original spectrum of the monomer crystal, certaining a small amount of polymer has been substracted. Therefore only the effect of the UV-irradiation is shown in the Figure. In the same spectra lines b, c and d of a weaker series a to e are also present. For comparison the low temperature optical absorption of the polymer chains is shown in Fig. 4b. In a simple picture one would expect the positions of the optical absorption of the intermediates to be situated between the monomer and polymer absorption. However, lines D and E are below the polymer absorptions. In contrast to the absorptions at room temperature the polymer chain absorptions at low temperatures are split into doublets. This splitting is caused by a structural phase transition The phase transition occurs at 170 K and results in a doubling of the unit cell in alignment with the chain direction. A corresponding doublet structure is also present in all absorptions of the reaction intermediates described in this paper, however, their intensity ratios are less than 1 10 and therefore in most cases a resolution of the weak lines is possible only after very intense UV-irradiation. 

From a so-called wideline spectrum, one may naively think that it is difficult to obtain the relaxation curve of a particular polymer in a blend. This is not true, because in many blends a short T2 is caused by the mobility of the one of the component polymers or that of side-chains. Thus, there is a chance to discriminate between polymers by their different T2 (mobility). For example, Segre et al. [146] observed two T2 decays for PS/PB. The fast-decaying component was attributed to rigid PS. The slow-decaying component shows the presence of two Ti relaxations. These were attributed to the interphase and pure rubbery PB (Model B). Parizel et al. [147] observed that the FID of polyurethane (PU) in a cross-linked PMMA consists of three... 

The two electrons transferred from TDAE to PEDOT-PSS are expected to undope the conjugated polymer chains. Since TDAE diffuses into PEDOT-PSS, long exposures to the electron donor induce changes in the optical properties of the polymer film. Optical absorption experiments on 200 nm thick PEDOT-PSS films coated onto a transparent polyethylene terephthalate (PET) substrate. The pol5mier film was exposed to the TDAE vapor in an inert nitrogen atmosphere and shows the difference in absorption spectrum between a film exposed to TDAE and the pristine PEDOT-PSS layer (Figs. 3.10 and 3.11). The modification of the optical properties and the sheet resistance of the pol5mier layer were recorded versus exposure time. The two absorption features at 550 nm and... 

In the seemingly pure nematic phase there is an interesting difference between the broad line NMR spectra of polymers Pi and P5 (the latter both random and ordered). Below the biphase, in what appears as a homogeneosly birefringent liquid by optical microscopy (on the scale of c. 0.5 xm resolution), the broad line NMR spectrum of polymers P5 displays an I component indicative of fast isotropic motion, while none is observed in polymers Pi. As already mentioned, NMR cannot distinguish between phase and microphase separation and the presence of the isotropic component could be explained by one or more of the following four factors ... 

Quite commonly, the signals from isotopically enriched abnormal units in a polymer are obscured by responses from the many normal units. This problem can be overcome by the use of difference spectroscopy. A polymer is prepared with a reactant, say initiator, enriched with a second polymer is prepared under identical conditions but with the reactant having at only its natural level. The two polymers are examined spectroscopically under precisely the same conditions. The only difference between the two spectra should be the considerable reinforcement of signals from the enriched sites in any of the C-labelled reactant incorporated in the polymer. Subtraction of the unenriched spectrum from the enriched should yield a difference spectrum containing only the responses from the incorporated enriched reactant or C-enriched fragments derived from it. 

In practice, such subtractions may not be perfect because of slight differences between the actual constitutions of the two polymers and also imperfections during the spectroscopic examinations. Another point must be noted. The main polymer signals vanish as a result of the subtraction because they correspond to identical coherent information but spectral noise on the two separate data sets is random and unrelated therefore it will add. For this reason, the signal-to-noise ratio in the difference spectrum must be worse than for signals directly observed in a single measurement. 

The polymerization kinetics for addition polymerization is obtained by the rate of loss of the C=C Raman bands relative to the increase in the C—C single-bond Raman vibrations as the polymerization proceeds. Similar polymerization kinetics can be obtained for other types of polymerization as long as the monomer and the polymer have some bands that are distinctly different. However, oriented polymers require special treatment because of the changes in the spectrum due entirely to orientation as discussed in Section III. Because most fibers and films have been oriented to some degree, the Raman spectra are altered by the degree of orientation, and comonomer quantification becomes difficult. In this subsection, only a few synthetic polymers will be discussed because most of the differences between oriented fibers/films and pellets, chips, or plaques of the same polymer are due to morphology differences which will be discussed in Section III. 

ATR-FTIR technique presents a crucial role in the assessment of stmctural modifications during UV irradiation of polymer films. Authors introduced a difference spectrum C (Fig. 3—reproduced with kind permission from Elsevier— License No. 3723021063514) between initial sample spectrum and that of sample after irradiation [47]. 

Digital subtraction of absorbance spectra is often used to reveal or emphasize subtle differences between two samples. When a polymer is examined before and after a chemical or physical treatment, and the original spectrum is subtracted from the final spectrum, positive absorbances in the difference spectrum reflect the structures that were formed during the chemical or physical treatment, and negative absorbances reflect those structures that were lost. 

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