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 180o-lamellar twisting was approximately 2 μm, which is in good agreement with the X-ray-observed values.
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This work was performed at beamline 03XU of SPring-8, Harima, Japan, with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2013A7214, 2013B7262, 2014B7262, 2015B7262, 2016A7212, 2017A7211, 2017B7263, 2018B7263, 2019B7261 and 2020A7225).
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Snapshots of the Simultaneous Measurement System of WAXD and SAXS Patterns Using a Synchrotron X-ray Micro-Beam at BL03XU, SPring-8
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Yamamoto, H., Yoshioka, T., Funaki, K. et al. Synchrotron X-ray-analyzed inner structure of polyethylene spherulites and atomistic simulation of a trigger of the lamellar twisting phenomenon. Polym J 55, 27–43 (2023). https://doi.org/10.1038/s41428-022-00710-3