13 C Nuclear Magnetic Resonance Investigation of Ethylene Homopolymers,Copolymers and Model Systems. Chemical Shifts and Relaxation Times

Abstract

This is a report on the use of ¹³C NMR spectroscopy to study the structure of polyethylenes, polyethylene-olefin copolymers, and polyethylene model systems, through the observation of chemical shifts and spin-lattice relaxation times of individual carbons. Information is obtained on the type and numbers of side chains of polyethylenes from solution spectra at higher temperatures. The level of detectability of side chains is established at 2 side chain carbons per 1000 skeletal carbons. The observed chemical shifts are accounted for well by standard additivity schemes. The low-density polyethylenes show spectra which are distinctly different from high-density polyethylenes. The ¹³C-determined branch frequencies show the same general trend as the IR results, but it is also argued that the former are more accurate. Relaxation times of the various carbons of the model system C₄₄ paraffin vary from 2.94 s for the backbone methylenes to 8.33 s for the terminal methyls to 11.11 s for α-methyls. Indicative of the behavior of olefin/polyethylene copolymers is the finding that all carbons of the copolymer have spin-lattice relaxation times of about 1.5 s, except for branch methyls which are 6 s. A series of polyethylenes at this high temperature shows relaxation times for the backbone methylene varying from 1.4 to 1.7 s, indicating that at this temperature, branching does not substantially affect the dynamic behavior of polyethylene.