Abstract
The microstructural origin of the double yield points of metallocene linear low-density polyethylene (mLLDPE) precursor films has been studied with the assistance of the synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS). It has been shown that the microstructural origin of the double yield points is highly related to the initial orientation of the original precursor film. For less oriented mLLDPE precursor films, the rearrangement of lamellae and the appearance of the monoclinic phase are the microstructural origins of the first yield point. In comparison, for the highly-oriented mLLDPE precursor film, only the orthorhombic-monoclinic phase transition appears at the first yield point. The melting-recrystallization and the formation of the fibrillary structure happen beyond the second yield point for all studied mLLDPE precursor films. Finally, the detailed microstructural evolution roadmaps of mLLDPE precursor films under uniaxial tensile deformation have been established, which might serve as a guide for processing high-performance polymer films by post-stretching.
Highlights
The vast majority of polymer films or fibers undergo external stretching at the initial stage of processing, such as the extrusion-casting and spinning [1,2]
Several interesting findings are summarized: (i) For less oriented precursor films like PE-61, the first yield point is associated with the rotation of the lamellae stacks and polymorphic transition, while the second one is related to the lamellar fragmentation and melting-recrystallization; (ii) for the highly-oriented precursor film PE-87, the first yield point is characterized by the polymorphic transition without the rotation of lamellae, while the second one is closely related to the lamellar fragmentation and melting-recrystallization, which is the same as less oriented precursor films
The mechanical properties, especially the double yield behaviors of metallocene linear low-density polyethylene (mLLDPE) precursor films were coupled with the orientation, slippage, and fragmentation of crystalline lamellae, as well as the polymorphic transition and melting-recrystallization behavior
Summary
The vast majority of polymer films or fibers undergo external stretching at the initial stage of processing, such as the extrusion-casting and spinning [1,2]. Pre-stretched polymer films or fibers usually exhibit superior properties, such as the substantially enhanced tensile strength along the oriented direction. One typical case is the extremely high modulus of polymer fiber due to the highly-oriented chains along the spinning direction [3,4,5]. Post-stretching processing is usually indispensable for polymer films or fibers with specific applications [6,7]. The ultimate properties of polymer films are closely related to the microstructural evolution of both crystalline and amorphous domains during stretching [10,11,12,13,14]
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