Pyrolysis–gas chromatographic analysis of chain branching in polyethylene

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Abstract
A system with flash pyrolysis and gas chromatography (PGC) which includes in‐line hydrogenation and packed columns is described. Under optimal conditions of temperature programming and flow rate, a high degree of resolution was obtained over a broad range of fragment lengths. The system has been applied to the analysis of side groups and long branches in polyethylene. It was found that, for low concentration of branches, yield and distribution of the isoalkanes is very sensitive to the type of polymerization used to form the polyethylene. A great number of the single‐branched isoalkanes have been identified by comparing their retention time with the scale of respective boiling points and by pyrolyzing well characterized ethylene‐propylene copolymers. The pyrograms of the ethylene‐propylene copolymers demonstrated that a CC bond in an α‐ or β‐position to a tertiary carbon cleaves about two times more readily than a CC bond in a linear chain. It was further found that the branch length of the side chain is indicated by the identity of the fragments produced, and the probability of formation of these fragments is directly related to the possibility of a cyclic decomposition mechanism. From the high yields of 3‐ and 5‐methylalkanes it was possible to detect about 20 ethyl and 10 n‐butyl branches per thousand in a low‐density polyethylene. Thus, pyrolysisgas chromatography has to be considered seriously as a method for the determination of branches, even when in present in low concentration (Ziegler‐type polyethylene).