Methylene wagging

The absorption bands of interest in the IR spectrum of PE are in the region 1425-600 cm Crystalline and amorphous phases contribute to vibrations observed within these limits (62). Amorphous methylene wagging vibrations are observed at 1369, 1353, and 1303 cmThe broad peak at 1078 cm has been assigned to the C-S stretching vibration of the amorphous phase. 

The amorphous methylene wagging bands show a temperature behavior which is more amenable to discussion. The 1352 cm" band has been calculated to result from the deformation of the methylene isolated by the GG conformation (55,56). The intensity of this band increased at elevated temperatures relative to the other amorphous wagging modes as a consequence of the higher energy of its conformation (55,56). This was also observed in the present work for the slow-crystallized sample. However, increases for the rapidly quenched systems only occurred after... 

A number of investigations have contributed to the understanding of the vibrational spectra of hydro-carbon-type chain molecules (see Fischmeister, 1975). The distribution of the methylene wagging, twisting, rocking and C-C stretching frequencies in the spectra for the homologous hydrocarbons... 

A revised assignment of the methylene wagging fundamental of the planar polyethylene chain. J. Mol. Spectry. 23, 224—228 (1967). 

It is very useful to measure FTIR difference absorption spectra, where the spectrometer digitally subtracts an appropriate reference spectrum from a sample spectrum [1048,1222-1224, 1989]. To obtain a difference spectrum in polyethylene, the methylene wagging band at 1369/cm is used as the null wavenumber. An example of such an operation is demonstrated in Figure 10.62. 

Fig. 10.62. IR spectra - the region of methylene wagging in a given samples (a) and (b) and difference (a-b) [711]. (Reproduced with permission from [711] published by Elsevier Science Publishers Ltd, 1987.)...

If you use your hands at the end of your extended arms to represent the hydrogens for this motion, you will notice that it is necessary for you (the carbon atom) to wag or rotate your body to accomplish the motion. Hence, the spectroscopists call this mode the methylene wagging mode, Vw(CH2), which has frequencies ranging from 1308-1170 depending on the environment of the methylene unit and it has a different... 

Polyethylene (PE) is the most-studied example of rotational isomerism. The crystalline domains are made up of the all-trans conformer structure. Bands that are characteristic of rotamers in the amorphous phase are also present. The most intense of the amorphous absorptions are the methylene wagging modes at 1303, 1353 and 1369 cm . The TG conformation is correlated with the bands observed at 1303 and 1369 cm . The 1353-cm absorption band is assigned to the wagging of the GG structure. When PE is heated through its melting point, the concentration of the TG and GG conformations increases. However, the concentration of the TG conformation increases well below the melting point, and this increase indicates the formation of localized conformational defects in the crystalline polymer. 

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