Doctoral student Yu Luo and colleagues in our team, under the joint supervision of Researcher Liangbin Li, Professor Fengmei Su, and Professor Kunpeng Cui, investigated the challenges of α/β crystal-form selection and microstructure regulation during industrial-scale isotactic polypropylene (iPP) film casting. By combining a custom-built industrial-scale steel-belt in situ casting apparatus with high-time-resolution synchrotron X-ray scattering, the team directly observed, under realistic industrial casting conditions, the preferential direct nucleation of the metastable β-phase without epitaxial growth on α-crystal templates. The study revealed the coupled effects of stretching ratio (SR), belt temperature (Tbelt), and the through-thickness temperature gradient (Tgra) in governing the competitive nucleation and growth of α- and β-crystals, providing new mechanistic insights and research strategies for precise crystal-form control and performance enhancement of industrial polypropylene films. The related work was published in Macromolecules.
In this study, the team integrated a custom-built steel-belt casting apparatus with synchrotron wide-angle X-ray scattering (WAXS), enabling in situ monitoring of microstructural evolution from the extruded melt to the fully solidified film while independently controlling SR and Tbelt. By systematically varying SR from 1.398 to 29.5 and Tbelt from 103 to 143 °C, the team obtained detailed information on the microscopic evolution of the crystalline structure in polypropylene cast films and observed that the metastable β-phase could nucleate earlier than the α-phase. Combined temperature simulations and experiments showed that within a processing window characterized by low SR (1.398–3.18), temperatures of 100–140 °C, and a moderate temperature gradient of Tgra ≤ 225 °C/mm, the β(300) diffraction peak appeared before the α(110)/α(040) peaks. This result demonstrates that the β-phase can nucleate directly and preferentially without relying on epitaxial growth on α-crystal templates. Increasing SR progressively favored α-phase nucleation, whereas increasing Tbelt promoted β-phase formation up to 133 °C. The total crystallinity remained approximately 65%, while the α/β ratio changed systematically. FTIR mapping further confirmed that the β-phase content on the side of the film adjacent to the steel belt was significantly higher than that at the upper surface. Based on these observations, the team established a processing-window map describing the competitive nucleation and growth of α- and β-crystals during industrial film casting and quantitatively constructed relationships among SR, Tbelt, Tgra, and the final crystalline-phase composition.
This work revealed the direct preferential nucleation of β-crystals under industrial-scale casting conditions, demonstrating that β-phase formation does not necessarily require epitaxial growth on α-crystal templates. The findings further highlight the importance of realistic processing conditions for uncovering previously unresolved crystallization pathways. The mechanistic framework established in this study can be used to optimize the crystalline structure and microstructure of iPP cast films through regulation of stretching ratio, belt temperature, and temperature gradient, providing experimental evidence and new research strategies for structural control and industrial manufacturing of high-performance polymer films.
This work was supported by the National Natural Science Foundation of China (U2430213) and a project of the Chinese Academy of Sciences (JZHKY-PY-2021-04). The authors also acknowledge the support of the BL10U1 beamline at the Shanghai Synchrotron Radiation Facility.
Luo, Y.; Cheng, S.; Liu, S.; Wei, X.; Zhu, J.; Li, L.; Su, F.; Cui, K. Competition of Nucleation and Growth between α- and β-Crystals during Industrial-Scale Isotactic Polypropylene Film Casting Revealed by Synchrotron Radiation X-ray Scattering. Macromolecules 2026, 59(12), 6786–6799.
Paper Link: https://doi.org/10.1021/acs.macromol.6c01070
