Polyethylene crystal field splitting

The most significant difference between the two spectra is in the 750-700 cm region, where CH2 rocking bands are observed. There is a doublet in each spectrum, but the relative intensity of the component peaks of the doublets in the two spectra are notably different. Both spectra show a doublet with peaks at 734 and 720 cm, but spectrum (b) shows a relatively more intense band at 734 cm when compared to spectrum (a). This is evidence of more crystal field splitting for sample (b). High-density polyethylene (HOPE) has a closer packing than low-density polyethylene (LDPE), and so will show more crystal field splitting. Hence, spectrum (b) must represent HDPE, while spectrum (a) represents LDPE. 

Figure 3-8. Infrared absorption spectrum of highly crystalline polyethylene where crystal field splitting is shown in the 1450 and 720 cm range due to crystal field splitting (crystaUinity bands).

Beginning in 1968,Tasumi and Krimm (103) undertook a series of experiments using a mixed crystal infrared spectroscopy technique. Mixed single crystals of protonated and deuterated polymer were made by precipitation from dilute solution. The characteristic crystal field splitting in the infrared spectrum was measured and analyzed to determine the relative locations of the chain stems of one molecule, usually the deuterated portion, in the crystal lattice. The main experiments involved blending protonated and deuterated polyethylenes (104-106). 

In order to confirm the anisotropy due to the a-hydrogen, Salovey et al. [9] and Shimada et al. [10] studied the patterns of the ESR spectra from irradiated solution grown crystals of polyethylene. The crystal c-axis was oriented perpendicular to the plane of the sample while the a- and b-axes were randomly oriented in the plane as shown in Fig. 7.8. Six- and ten-line spectra were observed when the c-axis of the crystal was set to be parallel (Fig. 7.9(a)) and perpendicular (Fig. 7.9(b)) to the direction of the applied magnetic field, respectively. From these results, the anisotropic hyperfine splitting due to the a-hydrogen Ay = 0.75 mT, Ax = 1.72 mT and Az = 3.70 mT were determined and were related to the molecular orientation of the crystal. The x-, y- and z-axes coincide with the directions of the p-orbital, the (Ca )— Ha bond, and the main chain axis, respectively, as shown in Fig. 7.10. 

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