Influences of the comonomer type on the crystallization kinetics of high-density polyethylene

In this study, the crystallization kinetics of 1-butene (B-PE) and 1-hexene (H-PE) polyethylene copolymers with varying comonomer contents are investigated, and an in-depth understanding of how chain branching impacts the crystal growth and nucleation is provided. By performing differential scanning calorimetry (DSC), we discern a distinctly slower crystallization rate for B-PEs than for H-PEs at equivalent comonomer contents. In-situ isothermal crystallization with wide-angle X-ray diffraction (WAXD) measurements demonstrates the delayed emergence of the (200) crystallite plane () in the B-PEs, indicating slower lamellar width expansion. Small-angle light scattering (SALS) analysis of the spherulite formation during isothermal crystallization confirms that B-PEs exhibit both a lower spherulite growth rate and nucleation density. These results are likely attributed to the preferential inclusion of 1-butene in the PE crystal, thereby amplifying the crystallization disturbance in the B-PEs. Furthermore, to elucidate these observations, we experimentally determine the thermodynamic parameters. Remarkably, the values of the free energy of the lamellar folded surface (σf) for B-PEs are significantly greater than those of H-PEs. This discrepancy potentially stems from the higher surface entropy because of the denser excluded 1-hexene comonomers on the lamellar folded surface. The lower σf value causes a reduction in the free energy barrier for critical nucleus formation; thus, this facilitates the preferential nucleation and accelerated lamellar development in H-PEs than in B-PEs. Crystallization kinetics of high-density polyethylene copolymers containing 1-butene and 1-hexene were investigated using differential scanning calorimetry (DSC), in-situ wide-angle X-ray diffraction (WAXD), and in-situ small-angle light scattering (SALS). Compared to 1-hexene copolymers, 1-butene copolymers exhibit slower isothermal crystallization, reduced spherulite growth, and higher lamellar surface free energy (σf), highlighting the pronounced impact of comonomer type on nucleation thermodynamics and lamellar development.