Knowledge Bank

Polyethylenes are hydrocarbon resins prepared by the polymerization of ethylene. By making progressive changes in reaction conditions a series of polyethylenes of increasing molecular weight has been produced. Structural variations in this series have been determined by infrared spectrophotometry. Methyl contents of polyethylenes can be measured at 3.38 or 7.25 microns. From the methyl contents of the resins studied, it was found that the degree of branching decreased with increasing molecular weight. The absorptions at 6.08 microns, and in the 10.0 to 11.3 micron region, revealed that the three types of double bond groups present, i.e., $RHC=CH2,RHC=CHR$, and $RRC=CH2$, decrease in concentration with increasing molecular weight. $Richards1$ (J. Appl. Chem. 1951, 1, 370) has stated that the relative intensity of absorption at 13.7 microns increases as the crystalline content of the resin increases. For the series of resins reported on here, the relative intensity of this absorption increases with increasing molecular weight. Measurements with polarized infrared radiation showed that cold drawing a polyethylene film produces a decrease in the relative intensity at 13.7 microns whether the plane of polarization is parallel or at right angles to the direction of stretch.