Crosslinking of Polymers. Molecular Structure and Properties of Sol and Network
Doctoral thesis, 2004
Crosslinking is a well-known procedure for improving a variety of features of polymers, particularly the dimension stability at elevated temperatures. Crosslinking implies the formation of a network, the gel, but there is always a certain portion of the material, the sol, that remains non-crosslinked within the network. Both fractions contribute to the properties of the crosslinked polymer, and the relative importance of the respective fraction depends on the type of polymer and the field of application. In this thesis, the significance of the molecular structure and property of a sol was illustrated by investigating the ability of crosslinked silicone elastomers to contaminate nearby structures, while analyses of the molecular structure of crosslinked polyethylene elucidated the behaviour of the network.
The silicone study showed that the non-crosslinked fraction of a cured silicone elastomer consists mainly of non-volatile, low molecular weight siloxanes with a potential capacity of causing silicone contamination through migration. Gel-content determinations together with molecular weight determinations of the sol clearly showed that the most effective way to minimise migration is to remove the low molecular weight fraction prior to crosslinking. Thermal analyses demonstrated the pronounced temperature sensitivity of the crosslinking process, something which must be considered, as a too low temperature could lead to unnecessary low gel-contents, whereas an elevated temperature can be used as a tool to increase the final curing level.
Analyses of crosslinked polyethylene demonstrated an extensive impact of the molecular weight on the gel-content formed upon crosslinking. Bimodal polyethylene, which always contains a substantial amount of low molecular weight chains, can thereby be prevented from developing high gel-contents upon crosslinking. A polymer network is to a large extent affected by the presence of long chain branches on the polymer chain. The amount and length of the branches influence the network performance mainly by affecting disentanglement of the branches. Coil volume and intramolecular crosslinking are other factors that are governed by the presence of long chain branches.
long chain branchin