Crystallinity in chemically crosslinked low density polyethylenes: 3. Morphology of the XLPE-2 system

Studies of the morphology of low density branched polyethylene crosslinked using 2% dicumyl peroxide have been carried out using transmission electron microscopy of replicas of surfaces etched with permanganic reagent. Under optimal etching conditions detailed information regarding the morphologies of bulk crystallized samples of low crystallinity can be obtained. The morphology of crosslinked polyethylene is compared with that of the original branched polyethylene as a function of crystallization temperature. Branched polyethylene crystallizes in the form of banded spherulites from 88°C to 103°C with a temperature dependent band periodicity. Introduction of a small amount of crosslinking produces profound effects with banded sheaf or bundle-like morphologies resulting. The absence of lamellar banding in the sheaves of the gel fraction of the crosslinked polyethylene is attributed to pre-existent tie-molecules which are an inherent feature of network junctions in a crosslinked system. In the case of a typical unextracted crosslinked polyethylene, the morphology is found to vary with crystallization temperature. D.s.c. studies reveal that the thermal behaviour of XLPE-2 is not too different from that of OPE. The cause of multiple melting behaviour in this class of material is discussed and it is suggested that in addition to other processes of reorganization, hindrance to solid state thickening may be important. A method of monitoring different stages of lamellar melting for semicrystalline polymers was developed involving annealing at elevated temperatures in the presence of chlorosulphonic acid.