Overlapping of different rearrangement mechanisms upon annealing for solution-crystallized polyethylene

The lamellar rearrangement upon annealing has been studied by a combination of differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) analysis, and transmission electron microscopy (TEM) observation for solution-crystallized samples of a linear polyethylene having a narrow molecular weight distribution. The initial unannealed sample showed the regular stacking of single crystal lamellae of ∼12 nm thickness, but showed double melting peaks, independent of the measured heating rate (HR). Each of the endotherms corresponds to the melting of initial lamellae and that of the crystals reorganized during DSC heating, respectively. The annealing behavior of the sample was significantly affected by both the annealing temperature and the time. A low-temperature annealing at 115°C, which is below the peak temperature of the former endotherm, slowly thickened the whole lamellae without any melting including partial or local lamellae. Increasing the annealing temperature (Ta) induces the other type of lamellar rearrangement mechanism where the lamellae rapidly thicken with the cooperative partial melting upon annealing. This rearrangement causes lamellar doubling behavior, showing a thickening of about twice, which was confirmed on TEM observation of the annealed samples. Because these doubled lamellae increased at the higher Ta, the lateral continuity of the initial lamellar stacking was lost. The DSC results revealed that these two different rearrangement mechanisms competitively coexist upon annealing over 120°C. Their balance predominantly depends on Ta. Correspondingly, the SAXS results indicate that the initial periodicity of the crystal/amorphous arrangement within the lamellar stacking morphology is lost at the middle Ta range of 123°C, where these two types of lamellar reorganization overlap. At the higher Ta range of 126°C, only a doubly rearranged lamellar structure remains, which causes an increase in the SAXS peak intensity.