Rocking mode doublet

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-... 

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. 

Tuot and Lecomte [35] in 1943 gave data on a series of thirty alcohols in which they noted the appearance of a band near 720 cm when at least four adjacent straight-chain methylene groups were present, and Thompson and Torkington [17] assigned the doublet at 721 and 732 cm in polythene to this cause. The vibration has been shown to be due to a rocking mode of the CH2... 

Despite the fact that PE is the simplest polymer in terms of its structural repeat unit, it produces some complex infrared bands. The spectra of PEs are complicated because of the phenomenon of crystal field splitting. As PE has a small repeat unit which packs efficiently, an ethylene unit of one chain will be in close proximity to a unit in the adjacent part of the chain and there is an interaction between these units. Such an interaction results in the donbUng of the normal modes, as one interacts with and perturbs the other unit. For PE, crystal field splitting results in doublets at 734 and 720 cm (CH2 rocking) and 1475 and 1460 cm (CH2 bending). Crystal field splitting will be the more dominant factor in these regions. 

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