Presence of an ultra high molecular
weight (UHMw) fraction in flowing
polymer melts is known to facilitate formation of oriented crystalline
structures significantly. The UHMw fraction manifests itself as a
minor tail in the molar mass distribution and is hardly detectable
in the canonical characterization methods. In this study, alternatively,
we demonstrate how the nonlinear extensional rheology reveals to be
a very sensitive characterization tool for investigating the effect
of the UHMw-tail on the structural ordering mechanism. Samples containing
a UHMw-tail relative to samples without, exhibit a clear increase
in extensional stress that is directly correlated with the crystalline
orientation of the quenched samples. Extensional rheology, particularly,
in combination with linear creep measurements, thus, enables the conformational
evolution of the UHMw-tail to be studied and linked to the enhanced
formation of oriented structures.
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Low-density
polyethylene (LDPE) shows a stress overshoot in start-up
of strong uniaxial extensional flows of constant rate. It is believed
that the overshoot is caused by a contraction of the polymer backbone
due to alignment of the long chain branchesthe consequence
being that the molecular strain of the backbone does not increase
monotonically with the global strain of the melt. In this study we
investigate the semicrystalline morphology of LDPE quenched before,
after, and at the overshoot. We do this by combining filament stretching
rheometry with ex-situ X-ray scattering. It is found that the overshoot
indeed is reflected in the orientation of the crystalline domains
of the quenched filaments. In a broader perspective, we show that
the final crystalline morphology is determined by the stress at quenchnot
the strain at quench. With these findings we confirm that the much
debated overshoot has a physical origin. More importantly, we conclude
that even for complex architectures like branched systems, the crystalline
orientation is determined by the backbone stretch rather than the
global stretch of the material.
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