Mechanistic Switching of Dual Yielding Pathways in Oriented mLLDPE Films via In Situ Synchrotron Radiation

ABSTRACT Thermomechanical stability of semicrystalline polymers during stretching significantly influences the performance of critical components in advanced energy storage systems, such as microporous battery separators, yet the atomic‐to‐microscale mechanisms underlying temperature‐dependent dual yielding remain to be fully clarified. Through in situ synchrotron SAXS/WAXS coupled with controlled uniaxial stretching, we elucidate the hierarchical structural dynamics in metallocene linear low‐density polyethylene (mLLDPE) films at two distinct thermal states: 50°C and 100°C. At 50°C, dual yielding proceeds via a “Sequential Locking” pathway: low‐stability crystalline domains undergo a shear‐driven orthorhombic‐to‐monoclinic phase transformation (first yield). The subsequent strain hardening is structurally correlated with this phase transition, which facilitates stress transfer to high‐stability domains. Conversely, elevating the temperature to 100°C activates a “Decoupling and Unraveling” pathway, where thermal energy suppresses the martensitic transition, redirecting energy dissipation toward thermally activated slip accompanied by partial melting. A Heterogeneous Crystalline Aggregate Model is proposed that integrates crystalline stability hierarchies with an interpenetrating amorphous tie‐chain network. This framework reveals that temperature dictates the stress transfer pathway: initiating a locking phase transformation within crystallites at low temperatures, while triggering a decoupling mechanism at high temperatures where stress is redirected through the amorphous network.