Infrared spectroscopic studies on polyethylene, 1. The measurement of low levels of chain branching

The problems encountered in the infrared spectroscopic estimation of the low levels of chain branching present in high density polyethylene, using the compensation method, are discussed systematically. The precision to be expected has been determined and it is shown that, if possible, a correction should be applied for methyl groups present at the chain ends. This is feasible with some types of polymer but more difficult with others. The compensating wedge in the reference beam of the spectrometer should be made of polymethylene rather than a high molecular weight polyethylene. The ultimate sensitivity of the manual compensation method is about one methyl branch per two thousand carbon atoms. However, when this is supplemented by computer smoothing and off‐line computation of average transients (C.A.T.), a significant improvement is obtained in the signal to noise ratio, and branching levels down to 0,1 methyl groups per thousand carbon atoms are readily measured.