Polymethylene and the structure of polyethylene: Study of short‐chain branching, its nature and effects

Abstract

Model compounds in the form of polymethylenes, linear and branched, have been prepared and used to develop methods of measuring the numbers and lengths of branches in polymethylene. The polymethylenes used for calibration work contain deuterated butyl branches; the numbers of branches present in the polymers hava been established by two independent methods: direct assay for deuterium and mass spectrometric analysis of the irradiation products. Using this series a new infrared tecnique has been calibrated absolutely to measure total number of methyl group and also butyl branches specifically. The infrared technique is a compensation method, using solid specimens in a double‐beam spectrometer, in which the compensating material (linear polymethylene) is devoid of the structural features, namely branches, to be measured. Using the same technique and reference samples of ethyl‐branched polymers (Hydropol), a method of estimating ethyl branch concentration has been developed. A study of the gaseous products of irradiation of the butyl‐branched polymethylenes, using mass spectrometric anlysis, has shown the extent to which this technique might provide a quantitative means of identifying and estimating branches in such polymers; it has also greatly strengthened the evidence of earlier workers that low density polymethylenes contain primarily ethyl and butyl branches. The same conclusion, in more quantitative terms, is drawn from an infrared study of various polymethylene‐specimens: practically all the branches observed can be accounted can for as ethyls and butyls present in the ratio of approximately 2:1. Some high density polyethylenes contain a few branches, which appear to be ethyl groups. The effects of the numbers and lengths of branches on degree of crystallinity, crystalline melting point, density, rate of oxidation, and dynamic mechanical properties are illustrated and discussed. The status of theories concerning the mechanism of formation of short side chanis in polymethylene is considered.