Double Yielding of mLLDPE: Effects of Temperature and Strain‐Rate and Insights Into Dumbbell Specimen Deformation and Self‐Heating

ABSTRACT Double yielding, a distinctive tensile behavior of semicrystalline polymers (SCPs), was investigated in metallocene‐catalyzed linear low‐density polyethylene (mLLDPE) with narrow molecular weight distribution and uniform short‐chain branching. During tension, the dumbbell specimens underwent four stages—quasi‐linear elasticity, first yield and softening, second yield and softening, and rubber‐like deformation—where the surface deformation evolved from nearly uniform strain to localized necking, accompanied by surface temperature rise and self‐heating. Digital image correlation (DIC) and infrared thermography revealed pronounced strain localization during the second yield, with local strain exceeding 600% and ~2°C heating under near‐adiabatic conditions. Yield stresses exhibited temperature and strain‐rate dependencies consistent with linear and Ree–Eyring models, while modulus and hardening–softening coefficients varied systematically with external conditions. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and ‐ray diffraction (XRD) were employed to characterize the surface morphology and crystallinity evolution at various tensile stages, thereby elucidating the deformation mechanism underlying the double‐yielding behavior. A novel application of DSC peak deconvolution quantitatively distinguished high‐ and low‐stability crystalline domains, elucidating their sequential transformation during double yielding. These results provide quantitative evidence linking crystalline heterogeneity to the mechanical response of mLLDPE and offer a basis for performance optimization in SCPs.