Synchrotron X-ray-analyzed inner structure of polyethylene spherulites and atomistic simulation of a trigger of the lamellar twisting phenomenon

To clarify the details of the inner structure of polyethylene spherulites and the mechanism of the lamellar twisting phenomenon, we performed simultaneous step-scan measurements of wide-angle and small-angle X-ray scattering by using a synchrotron X-ray beam of μm size as well as atomistic simulation of the lamellar plate. Assorted new structural information has been extracted: (i) the chain axis is tilted by 15~22° from the normal to the lamellar plane, correcting the previously reported evaluation; (ii) the a- and b-axial lengths of the unit cell change periodically along the radical direction; and (iii) the lamellar twisting pitch is longer in the central part of the spherulite and adopts a constant shorter value in the equilibrated outer part. Second, in association with these experimental results, molecular mechanics calculations were performed to confirm the role of folded chain parts as a trigger of the lamellar twisting phenomenon. The calculated tilting angle of the chain stems was approximately 13°^, and the calculated helical pitch for 180^o-lamellar twisting was approximately 2 μm, which is in good agreement with the X-ray-observed values.