Polyethylene reflection spectrum

Figure 3-32. Multiple internal reflection spectrum in the CHi-bending region at different angle of incidence of a 56 pm-thick film of commercial polyethylene. The spectrum shows the increase of the fraction of conformationally irregular material by increasing penetration depth.

The above-mentioned bands are found at the same positions in the transmission spectrum of polyethylene, indicating that the recorded specular-reflection spectrum can indeed be converted, by using the KK relations, into a spectrum that is essentially the same as the absorbance spectrum derived from a transmission measurement. However, several bands observed in the region of 2000-1700 cm in the (v) spectmm of Figure 8.5 are not expected for polyethylene. A more careful examination is needed to clarify their origin. 

If the dirt or contaminant spots can be dissolved in a low boiling-point solvent, solvent extraction (removal) may be an effective method for collecting the sample. If the base plate is a plastic material, the solvent must be carefully selected to ensure that none of the polymer support is dissolved. The solution obtained may then be slowly dripped from a syringe onto a thin film, for example, of polyethylene or polytetrafluoroethylene, and after the solvent has evaporated completely, the remaining powder can then be made into a KBr disk. If the solution is dropped directly onto an infrared-transparent window or a metal plate, the spot often tends to spread to form a circle of the sample after evaporation of the solvent, making the sample then not suitable for either the transmission or transmission-reflection measurement method. If the solution is dropped into a small hole (for example, with a diameter of 1 mm and a depth of 3 mm drilled into a metal support) filled with KBr powder, a diffuse-reflection spectrum may be measured from this after complete evaporation of the solvent. 

For infrared measurements, cells are commonly constructed of NaCI or KBr. For the 400 to 50 cm 1 far-infrared region, polyethylene is a transparent window. Solid samples are commonly ground to a fine powder, which can be added to mineral oil (a viscous hydrocarbon also called Nujol) to give a dispersion that is called a mull and is pressed between two KBr plates. The analyte spectrum is obscured in a few regions in which the mineral oil absorbs infrared radiation. Alternatively, a 1 wt% mixture of solid sample with KBr can be ground to a fine powder and pressed into a translucent pellet at a pressure of —60 MPa (600 bar). Solids and powders can also be examined by diffuse reflectance, in which reflected infrared radiation, instead of transmitted infrared radiation, is observed. Wavelengths absorbed by the sample are not reflected as well as other wavelengths. This technique is sensitive only to the surface of the sample. 

Fig. 29. The ESCA valence band spectra of (a) upper surface of green leaf (b) upper surface of yellow-green leaf and (c) lower surface of yellow-green leaf. The heights of key spectral features reflect relative quantification, (d) Most of these spectral features are present in the spectrum of a representative hydrocarbon, low-density polyethylene [35]. Alteration of a hydrocarbon by adding extra links to the (C—C) chain or introducing branching or multiple bonds blunts the peaks and valley features, as seen in (aMc). On the other hand, carbon-oxygen bonds produce O (2s)-dominated peaks at ca. 27.5 eV if C—O—C species are present (first arrow) and/or add a shoulder at ca. 24 eV if >C=0 species are present (second arrow). Reprinted with permission from T. L. Barr, S. Seal, S. E. Hardcastle, M. A. Maclauran, L. M. Chen, and J. Klinowski, Bull. Pol. Acad. Sci. 45, 1 (1997).

The reason for the consistent overprediction of the multiplication factor for these assemblies is not clear. It Is felt that the U cross sections are unlikely candidates for the source of the discrepancy. This conjecture is based upon the accurate prediction of the critical mass for Assembly No. 1 of the Thermionic Critical Experiment, which is a bare Oralloy-polyethylene assembly (NH/N U a 0.8) whose spectrum wha closely tailored to that of a thick BeO-reflected core. i... 

For the accentuation of these small differences in the spectra of the stressed and unstressed polymer the absorbance subtraction technique has proved particularly useful. In Fig. 3 this is illustrated with reference to the 972.5 cm absorption band of the v(0—CH2) skeletal vibration of polyethylene terephthalate. Fig. 3 a shows the shape of this absorption band for the unstressed and stressed (300MN/m ) polymer. In the difference spectrum (see Fig. 3 b) the shift of the peak maximum toward lower wavenumbers and the low-frequency tailing are reflected by a pronounced asymmetrical dispersion-shaped profile. 

At temperatures above their Tg, the resonance spectrum of noncrystalline polybutadiene (PB) (Fig. 8a) is clearly different from that of the semicrystalline polyethylene (Fig. 8c). Amorphous PB exhibits a narrow Lorentzian line shape with a width of 0.2 G. In contrast, the PE spectrum comprises two components, ie, narrow and broad line shapes. When the spectra of semicrystalline polymers are recorded in the glassy state (Figs. 8b and 8d), only abroad component is observed. This indicates that the line shape corresponds to molecular mobility and the line width reflects a correlation (or relaxation) time. Therefore, the broad and narrow components of semicrystalbne PE (Fig. 8c) are related to protons of methylene groups in rigid and mobile (amorphous) environments, respectively. On the basis of this, it was proposed that the degree of crystallinity could be determined by resolving the area of the broad component (rigid phase) from the spectrum. 

The sharper spectrum of the radicals in UPEC shown in Fig. 5.1c. reflects a much more mobile character of the radical site in UPEC compared to the radical site in the bulk polyethylene. A similar phenomenon is also shown in Fig. 5.3. i.e., in the case of UPEC, two kinds of hyperfine splitting due to P-protons at low temperature become merged into one kind at a certain temperature (barrier temperature). The barrier temperature for bulk materials, on the other hand, is much higher than that of UPEC. 

---