Geometric Freezing of In-Situ Fibrillated PP/PA66 Composites via Low-Temperature Injection: Decoupling the Role of Draw Ratio and Compatibilization

Controlling fibril generation and retaining their morphology during secondary thermal processing remain critical challenges for in-situ microfibrillar composites. Herein, we propose a “melt blending–hot drawing–low-temperature injection” cascade strategy to fabricate polypropylene/polyamide 66 (PP/PA66) composites. By decoupling the synergistic effects of draw ratio (λ) and compatibilizer (PP-g-MAH), an optimal fibrillation window was identified (15 wt% PA66, 3 wt% compatibilizer, λ = 9), which balances interfacial tension and viscous drag to form a dense, oriented microfibrillar network. This solid-state network accelerates matrix nucleation (though slightly restricting overall crystallinity) and induces gel-like rheological behavior through severe structural confinement. Crucially, we demonstrate that conventional high-temperature injection (265 °C) triggers Rayleigh instability, causing fibril break-up and mechanical degradation. Conversely, low-temperature injection (210 °C) successfully achieves the “geometric freezing” of the metastable fibril network. Consequently, the optimal composite exhibits maximized static strength (45.3 MPa) and a continuous, significant leap in notch impact toughness (10.16 kJ/m2). This work bridges the gap between flow-induced fibrillation and thermodynamic morphological retention, offering a robust physical mechanism for high-performance polyolefin composites.