Understanding the
complex crystallization behavior of isotactic
polypropylene (iPP) in conditions comparable to those found in polymer
processing, where the polymer melt experiences a combination of high
shear rates and elevated pressures, is key for modeling and therefore
predicting the final structure and properties of iPP products. Coupling
a unique experimental setup, capable to apply wall shear rates similar
to those experienced during processing and carefully control the pressure
before and after flow is imposed, with in situ X-ray scattering and
diffraction techniques (SAXS and WAXD) at fast acquisition rates (up
to 30 Hz), a well-defined series of short-term flow experiments are carried out using 16 different combinations of wall
shear rates (ranging from 110 to 440 s–1) and pressures
(100–400 bar). A complete overview on the kinetics of structure
development during and after flow is presented. Information about
shish formation and growth of α-phase parents lamellae from
the shish backbones is extracted from SAXS; the overall apparent crystallinity
evolution, amounts of different phases (α, β, and γ),
and morphologies developing in the shear layer (parent and daughter
lamellae both in α and γ phase) are fully quantified from
the analysis of WAXD data. Both flow rate and pressure were found
to have a significant influence on the nucleation and the growth process
of oriented and isotropic structures. Flow affects shish formation
and the growth of α-parents; pressure acts on relaxation times,
enhancing the effect of flow, and (mainly) on the growth rate of γ-phase.
The remarkably high amount of γ-lamellae found in the oriented
layer strongly indicates the nucleation of γ directly from the
shish backbone. All the observations were conceptually in agreement
with the flow-induced crystallization model framework developed in
our group and represent a unique and valuable data set that will be
used to further validate and implement our numerical modeling, filling
the gap for quantitatively modeling crystallization during complicated
processing operations like injection molding.
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