Crosslinking of bimodal polyethylene

Two bimodal polyethylenes, differing only in polymerisation order, were investigated with respect to crosslinking behaviour and network properties. The crosslinked materials were examined using size exclusion chromatography (SEC), gel-content measurements, and calculations of the network density. Dynamic mechanical analyses in the melt were performed to monitor the crosslinking and to provide another measure of the network density. The experiments were performed to investigate any potential influence of the polymerisation order on the crosslinking as well as to study the network formation in the crosslinked polymers. The bimodal polyethylenes were also compared to two monomodal polyethylenes representing the short chain branched, high molecular weight fraction, and the linear, low molecular weight fraction, respectively. The SEC measurements clearly showed how the crosslinking starts with the consumption of the high molecular weight fraction. The gel-content measurement showed the importance of a high molecular weight material for the gel formation. The network density calculations demonstrated how long chains can give rise to apparent networks which are mainly due to chain entanglements. The experiments showed that the polymerisation order for the bimodal polyethylenes has no effect on the crosslinking.