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Identification of Branches in Low-Density Polyethylenes by Fourier Transform Infrared Spectroscopy

Qualitative and quantitative analysis of short chain branches was carried out on a series of low-density polyethylenes by Fourier transform infrared spectroscopy. The peak position of the methyl symmetrical deformation bands around 1378 cm−1 was found to vary with the type of branches and used for identification of the branch type in low-density polyethylenes. The reciprocals of extinction coefficients of the methyl deformation bands were determined for various branch types on the basis of 13C NMR data. The values changed from 0.39 to 0.76 depending on the type of branch. The methyl rocking bands ranging from 880 to 940 cm−1 were examined using brominated samples. The peak positions also depended on the type of branch. For the methylene rocking bands, the peak at 772.2 cm−1 due to the ethyl branch was clearly observed, but that due to the n-butyl branch was not detected around 745 cm−1, indicating the n-butyl absorption to be located very close to 730 cm−1 rather than 745 cm−1.