In Situ Visualization of Extensional Flow-Induced Crystallization in Polypropylene under High-Pressure CO₂

The crystallization behavior of semicrystalline polymers under foaming-relevant conditions is among the most critical factors dictating the morphology and properties of any foam product. While cell nucleation and growth are often discussed in the plastics foaming literature, extensional flow-induced crystallization experienced postdie exit during foam stabilization is overlooked. To address this gap, we aim to build a comprehensive understanding of the nonisothermal crystallization behavior under the effects of CO2 pressure, uniaxial extensional flow, and long-chain branching. Two grades of polypropylene (PP) were used: one linear and one long-chain branched. Under quiescent conditions, an increase in CO2 pressure led to a decrease in onset crystallization temperatures, a decrease in crystal growth rates, and an increase in nucleation densities in both resins. Additionally, X-ray diffraction analyses showed the promotion of gamma-crystals under higher CO2 pressures. Upon the introduction of uniaxial extensional flow, both resins nucleated elongated crystals (i.e., cylindrites) due to chain alignment and stretching. These cylindrites were observed via atomic force microscopy and shown to exhibit the classical shish-kebab structure. Interestingly, the long-chain-branched PP (LCB PP) nucleated more cylindrites with higher aspect ratios and exhibited a crystallization rate faster than that of its linear counterpart. Rheological characterization showed that this difference emerged from the longer relaxation behavior of the LCB PP chains. These findings may present value in modeling polymer processes while emphasizing the role of extensional flow-induced crystallization during cell growth and stabilization.