Chain Entanglement Determining the Interlamellar Amorphous Thickness in Semicrystalline Polymers

Molecular dynamics simulations are employed to investigate how molecular topology governs the thickness of the interlamellar amorphous layer (La) in semicrystalline polymers. Lamellar crystals with varying initial interlamellar spacing (La0) are seeded in the melt. When La0 falls below twice the radius of gyration of the molecular chain (2Rg), crystal stems exhibit uneven thickening, preferentially extending away from the adjacent lamellae. This leads to a progressive increase of La as the crystal grows. Quantitative analysis reveals significant increases in local entanglement density (Nz) and conformational entropy-derived free energy (Gcon) within the interlamellar amorphous layer, which collectively suppress crystal stem elongation into the constrained layer and direct growth toward the less entangled side. Regardless of the initial La0, systems with identical chain length evolve toward a similar steady-state La. A linear relationship between La and Nz emerges, whose slope corresponds to the entanglement density, which depends solely on the chain length. This reveals that the interlamellar amorphous layer self-adjusts its thickness to maintain a constant entanglement density, which is predetermined by molecular chain length. Our simulations provide molecular-level evidence for how topological entanglements govern the equilibrium morphology of semicrystalline polymers.