Reorganization and melting of polyethylene single crystals: Complementary TEM, DSC, and real-time AFM studies

The reorganization and melting of single crystals of linear polyethylene with lozenge and truncated-lozenge habits has been studied in situ during slow continuous heating using atomic force microscopy. The morphological insights obtained are backed up by complementary experiments using transmission electron microscopy and differential scanning calorimetry and have enabled us to identify different stages of melting/recrystallization. Visible melting is preceded by local reorganization within the crystals, the extent of which depends crucially on the thermal history of the sample and can influence subsequent behavior such as the preferential melting of one crystal sector before another. This is followed by distortion of the crystal edges and limited thickening which does not progress into the bulk of the crystal. Next, larger scale melting and recrystallization occurs within the bulk of the crystal, which leads to a progressive increase in melting point. The observed behavior supports a model of local melting and recrystallization rather than solid-state thickening, and overlying layers can behave quite independently. Finally, the thickened crystals melt at a temperature dependent on the thermal history of the sample.