Polyethylene correlation splitting

Fig. 4.16 The effect of temperature on the correlation splitting of the Raman-active CH2-twisting doublet of polyethylene (a) the sample at room temperature (b) the sample at —196°C (c) the sample at —196°C with increased spectral resolution and expanded...

The British standards BS 2782, Methods 620 A-D [6]. are identical to ISO 1183. ASTM D 792 [7] covers the displacement method and has two procedures, one for the displacement of water and one for that of other liquids. It is not clear why it has been split in this way, and it is notable that there is no mention of using a sinker, nor does it include a pycnometer method. The density gradient method is given in ASTM D 1505 [8] and is very similar to the ISO procedure. There is also an ASTM method for density of polyethylene by means of ultrasound, ASTM D 4883 (9). This works on the principle of measuring sound velocity in the plastic, which correlates to density. The apparatus requires calibrating with reference materials but is claimed to give accuracies of 0.08% or better. The use of the method would mostly be in quality control, and it is questionable whether it should have been standardized. Essentially it describes the use of a commercial in.strumcnt with no apparatus details, not even the frequency used. 

Figure 3-31. Infrared absoiption spectrum of a commercial film of polyethylene before and after subtraction of the spectrum of the defect mode component near 1440 cm . After subtraction of the defect mode the ratio between component a and b of the correlation field splitting approached the ideal value for a setting angle of 42 (see text).

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