Competition of Nucleation and Growth between α- and β-Crystals during Industrial-Scale Isotactic Polypropylene Film Casting Revealed by Synchrotron Radiation X-ray Scattering

Film casting is a predominant industrial method for manufacturing polymer films, whose end-use properties are critically governed by microstructure and morphology. Understanding the polymer crystallization under realistic processing conditions remains a challenge because most studies rely on simplified laboratory-scale models that cannot capture the complexity of industrial operations. This study bridges this gap by integrating an in situ high time-resolved synchrotron X-ray scattering technique with a custom-designed industrial-scale steel belt casting line, enabling continuous structural monitoring from the melt to the fully solidified film under independently controlled stretching ratio (SR) and belt temperature ( T belt ). Under low SR (1.398 – 3.18), the 100 –140 °C temperature window and moderate temperature gradients ( T gra ⩽ 225 °C/mm), we observe direct and preferential nucleation of the metastable β-phase without epitaxial growth on α-templates, which is a phenomenon not captured in conventional model experiments. This direct β-nucleation occurs within a specific processing window (100 –140 °C) and demonstrates that coupled flow and thermal fields can kinetically override thermodynamic preference. Increasing SR progressively favors α-phase nucleation, while raising T belt promotes β-phase formation up to 133 °C. Total crystallinity remains invariant at ∼ 65%, but the α/β ratio changes systematically, and a through-thickness phase gradient is confirmed by FTIR mapping. These results establish a mechanistic framework for phase selection in industrial film casting and highlight the necessity of realistic processing conditions for discovering new crystallization pathways.