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Effect of Flowing Preformed Spherulites on Shear-Induced Melt Crystallization Behaviors of Isotactic Polypropylene
journal contribution
posted on 2018-02-20, 19:20 authored by Junyang Wang, Xuehui Wang, Qiaojiao Wang, Cui Xu, Zhigang WangThe effects of flowing
preformed spherulites on shear-induced melt crystallization behaviors
of isotactic polypropylene (iPP) had been investigated by using a
Linkam shearing cell coupled with an optical microscope, atomic force
microscope, and rheometer. By quenching iPP melt to 130 °C for
a short time, preformed spherulites were intentionally obtained. While
iPP films were heated up to temperatures below the nominal melting
point of iPP (164 °C), various shear conditions were applied,
and then the crystalline morphologies during crystallization at the
isothermal crystallization temperature of 135 °C were traced
by alternately using phase contrast optical microscopy and polarized
optical microscopy modes. Two distinct highly orientated thread- and
cylinder-like crystalline morphologies were induced by shearing with
preformed spherulites present, which were dependent on the applied
shear rate and temperature. The distinct crystalline morphologies
were attributed to relative movements between shear flowing melt and
preformed spherulites. Atomic force microscopy was used to reveal
the crystal aggregates of the two typical morphologies, indicating
that the former was composed of highly packed orientated crystal nuclei,
while the latter was composed of loosed packed random crystal nuclei,
for which the thin crystal lamellae grew perpendicularly to crystal
nuclei, leading to large widths for the cylinders as seen by an optical
microscope. Rheological measurements indicated that preformed spherulites
were isolated in the iPP films, which could obviously retard the stress
relaxation of iPP films at the lower shear temperature.