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Critical Strains for Lamellae Deformation and Cavitation during Uniaxial Stretching of Annealed Isotactic Polypropylene
journal contribution
posted on 2018-08-09, 18:26 authored by Baobao Chang, Konrad Schneider, Fei Xiang, Roland Vogel, Stephan Roth, Gert HeinrichThe lamellae deformation and cavitation
behavior of annealed isotactic
polypropylene during uniaxial stretching are comprehensively investigated
by in situ synchrotron small-angle X-ray scattering
and wide-angle X-ray scattering. We reveal how lamellae deformation
occurs in the time scales of elastic deformation, intralamellar slip,
and melting–recrystallization, separated by three critical
strains which are only rarely influenced by annealing. Strain I (0.1)
marks the end of elastic deformation and the onset of intralamellar
slip, beyond which the crystallinity decreases gradually. Strain II
(0.45) signifies the start of the recrystallization process, from
where the long period in the stretching direction begins to decrease
from its maximum and the polymer chains in the crystal start to orient
along the stretching direction. The energy required for melting arises
from the friction between the fragmented lamellae produced by intralamellar
slip. Strain III (0.95) denotes the end of the recrystallization process.
Beyond the strain of 0.95, the long period and the crystal size remain
nearly unchanged. During further stretching, the newly formed lamellae
serve as the anchoring points for polymer chains in the amorphous
phase. The extension of the polymer chains in between lamellae triggers
the strain hardening behavior. On the other hand, annealing significantly
decreases the critical strain for voids formation and increases the
voids number but restricts the void size. For those samples annealed
at a temperature lower than 90 °C, voids are formed between strain
II and strain III, and voids are oriented in the stretching direction
once they are formed. However, for those samples annealed at a temperature
higher than 105 °C, voids are formed between strain I and strain
II. In this case, voids are initially oriented with their longitudinal
axis perpendicular to the stretching direction and then transferred
along stretching direction via void coalescence. Additionally, the
formation of voids influences neither the critical strains for lamellae
deformation nor the final long period, the orientation of polymer
chains, or the crystal size. The final long period, the orientation
of polymer chains in the crystal, and the crystal size are determined
only by the stretching temperature through melting–recrystallization.