Rocking mode crystalline

The temperature dependences of the IR bands of PE were quite different for the slow-crystallized and quenched samples. This effect was most noticeable for the crystalline rocking modes. The peak heights of the 720 and 731 cm" components of this doublet increased with increasing temperature. This was not as anticipated. The intermolecular forces lessen with thermal expansion. Therefore, the induced dipole moment of the interaction decreases with increasing temperature (45,46). The intensity of a band which is enhanced by the dipole-dipole interaction is expected to decrease. 

The 725 cm" rocking mode doublet is presented in Figure 3. The peaks at 731 and 720 cm for this sample are approximately 15 times the intensity of the amorphous wagging modes. The 720 cm limiting frequency is assigned to long trans sequences in the amorphous and crystalline phases. For these samples, most of the methylene structures are in the crystal, and the band is associated with this phase predomi-... 

In crystalline PE the deformation and rocking modes split and additional bands are seen around 1470 and 731 cm The crystallinity of PE can be determined from the ratio of the 731 to 719 cm bands. Fig. 6 shows the IR spectra of high density polyethylene (HDPE) and LDPE. HOPE is very regular and is about 70 % crystalline. LDPE is more branched and is only about 50 % crystalline. 

Fig. 6 Intensities of rocking modes in the IR spectrum of more crystalline and more amorphous PE.

This crystal field splitting has been observed for the methylene rocking mode at 720 cm and for the methylene bending mode at 1460 cm in spectra of crystalline PE. Although other modes should also exhibit such splitting, their inherent bandwidth prevents the observation of separate components. AVhen PE is melted, the crystal field splitting disappears. Consequently, a measure of the relative intensities of the 720- to 730-cm bands can be used to rank the relative crystallinity of PE samples. 

Quartz is a widely distributed mineral species consisting of silicon dioxide (silica, SiOj) (Table 7.6). It is one of the most common minerals and is found in many varieties with very diverse modes of occurrence. Quartz is a primary constituent of rocks such as granite, quartz, porphyry, and rhyolite. It is also a common constituent in many gneisses (laminate rocks) and crystalline schists (foliated rocks). It is also, in a sense, mobile since by the weathering of silicates, silica passes into solution and is redeposited in cavities, crevices, and along joints of rocks of all types. Thus, it is not... 

Fig. 5.10. Crystal splitting of the 730-720 cm" band, caused by the in-phase (CH2) rock vibration in crystalline long CH2 chain containing molecules. The unit cell shown has sections of two differently oriented CH2 chains, each performing the in-phase CH2 rock vibration. The bold-face CH2 groups all lie in one plane and the chain progression axis is more or less perpendicular to the page. The relative phase of the two differently oriented chains is different for the two unit cell modes. In the 730 cm" mode the two chains move their hydrogens toward and away from each other, and the close approach is shown. In the 720 cm" mode the two chains tend to stay out of each other s way.

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