Structural Aspects of Peroxide Crosslinking of Polyethylene
Despite major developments in the ways of polymerising ethylene (the processes as well as the catalysts) the use of polyethylene is still restricted in several applications due to its low melting point. This shortcoming can in some areas be overcome by the introduction of crosslinks between the polymer chains.
This thesis discusses some recent insights in how crosslinks are introduced and utilised in the network structure. Vinyl groups, especially those of pendant type, have been found to have a tremendous effect on the amount of gel formed and on the network density reached. For these studies, two new ethylene copolymers, poly(ethylene-co-1,9-decadiene) and poly(ethylene-co-divinylsiloxane), were used. The vinyl groups were found to be rapidly consumed during the crosslinking reaction, most probably in a polymerisation reaction. Allylic hydrogens were found not to be needed for the reaction of vinyl groups. Reacted vinyl groups contribute to the network structure as chemical crosslinking points. Deeper studies of the network structure revealed that maximum one third of the existing network points present are of chemical nature (i.e. originating from macroradical combination or from a reacted vinyl group). The remaining two thirds consist of entanglements that become trapped to different degrees by the chemical crosslinks. The creation of intermolecular crosslinking points proved to govern the macroscopic network build-up. Thus, the topography of the polymer coil has a significant effect on the network formation. The reference LDPE used in this work was found to occupy only 15% of the volume of a linear polymer having the same average molar mass. This leads to that a large fraction of the network points, both chemical and physical, will be of intramolecular type thereby not contributing to the macroscopic network build-up in an efficient way.
The here presented insights can aid in further development of polyethylene resins, especially those designed for use in crosslinked applications.