INVESTIGATIONS OF THE CRYSTALLIZATION OF POLYETHYLENE BY MEANS OF SIMULTANEOUS SMALL-ANGLE AND WIDE-ANGLE X-RAY-SCATTERING

The isothermal crystallization of linear polyethylene fractions was studied by means of simultaneous measurements of wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) employing synchrotron radiation. From WAXS the overall degree of crystallinity x(c) was determined. From SAXS, the scattering power Q was evaluated and compared with several different models for the crystallizing system. When the scattering arises solely from the supermolecular structures, such as spherulites, Q is proportional to x(s) . x(cL) . (1 - x(cL)), where x(s) is the fraction of material transformed into such structures and x(cL) is the degree of crystallinity within these structures. By comparing x(c) with Q it was possible to separate the primary and secondary crystallization and to show that secondary crystallization takes place within the lamellar stacks where the crystals become thicker and the amorphous layers become thinner. The process of recrystallization during annealing of crystalline samples was studied in the same way. Other models were treated in a similar manner.