Structural Origin of Double Yielding: The Critical Role of Crystallite Aggregate Heterogeneity

Abstract

In situ synchrotron radiation wide- and small-angle X-ray scattering has been employed to elucidate the structural origin of the double yielding of the metallocene linear low-density polyethylene films during the tensile deformation in a temperature range from 25 to 110 °C. The lamellar reorientation or the crystal shearing is observed at the first yielding point. The martensitic transformation is observed at the low-temperature region (T ≤ 100 °C), while such a transformation happens at the very beginning of the high-temperature region (T > 100 °C). The melting–recrystallization happens beyond the second yielding point for all measurements. The structural origin of double yielding is interpreted as the heterogeneous distribution of the crystallite aggregate, where two domains with different stabilities are arranged in series or are under equal stress conditions. The origin of the first yielding point is attributed to the lamellar reorientation, fine slip (crystal shearing), and coarse flip (fragmentation or the decreasing crystallinity χc) of a low stable domain, all of which result in the decreasing modulus or stress softening. The appearance of the hardening point is proposed to be related to the onset plastic deformation of the more stable crystallite aggregates. This explication is further supported by the weak and unclear double yielding in the high-temperature region, where the melting of the low stable crystallite and the chain diffusion between amorphous and crystalline domains lead to more homogeneous plastic deformation. The adaptability of this crystallite aggregate heterogeneity model is further extended to the low-density polyethylene case, where the double yielding can be artificially obtained by precise manipulation of the external parameters, that is, flow and temperature.

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