PEROXIDE CROSS-LINKING OF LINEAR LOW-DENSITY POLYETHYLENES WITH HOMOGENEOUS DISTRIBUTION OF SHORT-CHAIN BRANCHING

Extraction and swelling experiments were carried out in order to measure the crosslinkability by dicumyl peroxide (DCP) of new homogeneous linear low-density polyethylenes (LLDPEs) with different molecular weight, branch content, and length of branches. Considering that the tertiary radicals are most probable macroradicals, our results indicate that tertiary carbons in LLDPEs could be either sites for scission reaction or effective coupling, depending upon degree of branching and to a lesser extent on degree of crosslinking. In the range of branching between 16 and n CH3/1000 C (n = 31 for low DCP concentration and decreases with DCP content approaching the value of 23 for the range above ca. 6 DCP moles per number average PE mole), the tertiary carbons seem to be rather the sites for effective coupling. In this range of branching an increase of crosslinkability with branch concentration and molecular weight of PE is due to the increase of crosslinking efficiency, but the scission probability is very low. At branch content exceeding the value of n CH3/1000 C, the dominant reaction on tertiary carbons is scission, leading to a small decrease of crosslinkability with increasing branch content. The possible mechanisms favoring tertiary carbons for either coupling or scission are discussed. (C) 1995 John Wiley & Sons, Inc.