| Abstract: | Linear polyethylene was crystallized isothermally from the melt. Specimens were removed at different crystallization times and quenched to room temperature. The density, static mechanical properties, and small-angle x-ray scattering (SAXS) behavior of these specimens were measured at room temperature. The density and Young's modulus increased with crystallization time, whereas the upper yield point decreased with crystallization time. SAXS data showed that a zero-angle peak gradually disappeared as crystallization time increased. Concurrently, the breadth of the SAXS peaks, the Bragg angle, and the integrated intensity decreased. Changes in the ratio of second- and first-order peak intensities were also noted. On the basis of the SAXS and density data, it was concluded that a competition exists between the thickening of existing crystals and the creation of new crystallites between the older ones. At relatively low crystallization times, numerous new crystals can form during quenching to room temperature, whereas quenching after prolonged crystallization primarily results in the additional thickening of existing crystals. No change in the density of the amorphous material is found. A model is given whereby the upper yield stress is coupled to these morphological changes through a stress concentration effect caused by a decreased population of chains connecting adjacent crystallites. The tie-chain population change occurs by their elimination as crystallites disappear. |
