Tensile Yielding and Hardening in Semi-Crystalline Polymers: Synergistic Effects of Amorphous Entangled Network and Tie-Chain Network

The tensile deformation behavior of semicrystalline polymers is governed by both the entanglement density within the amorphous regions and the density of tie chains interconnecting the adjacent crystalline lamellae. To overcome the masking effects of complex shear yielding typical in conventional spherulitic samples, this study employs an aliphatic polyketone as a model system to prepare a series of samples with uniaxial lamellar orientation and crystallinity maintained within a narrow range via a melt-drawing process. The effects of entanglement density in the amorphous phase and the areal density of tie chains on tensile behavior were systematically investigated. The results demonstrate that the mechanical response in the large-strain regime is primarily controlled by entanglements in the amorphous phase. An increase in the effective entanglement density of the amorphous regions reduces the yield strain and markedly enhances strain hardening. Elevating the drawing temperature promotes disentanglement within the amorphous network, thereby weakening the strain-hardening effect, with an associated activation energy of approximately 10.21 kJ/mol that is independent of the initial sample state. Cyclic tensile tests revealed that the effective entanglement density in the amorphous regions governs the critical strain for tensile-induced fibrillation. During yielding, stress transfer is mediated by a crystalline network of lamellar clusters interconnected by tie-chain molecules, combined with a densely entangled amorphous network. The yield strength scales with the areal density of tie chains, reflecting the densification of the tie-chain network. The deformation process initiates with the expansion of interlamellar amorphous regions and the stretching of the tie-chain network. Once this network attains its maximum extension, lamellae undergo bending deformation, ultimately triggering yield. From a micromechanical perspective, this study clarifies the pivotal role of the amorphous entanglement network in the yielding, strain hardening, and fibrillation of semicrystalline polymers, establishing a quantitative structure–property relationship between the entanglement state of the amorphous phase and the macroscopic mechanical performance.