Deformation-Induced Phase Transitions in iPP Polymorphs

Abstract

This detailed study reveals the relation between structural evolution and the mechanical response of α-, β-and γ-iPP. Uni-axial compression experiments, combined with in situ WAXD measurements, allowed for the identification of the evolution phenomena in terms of phase composition. Tensile experiments in combination with SAXS revealed orientation and voiding phenomena, as well as structural evolution in the thickness of the lamellae and amorphous layers. On the level of the crystallographic unit cell, the WAXD experiments provided insight into the early stages of deformation. Moreover, transitions in the crystal phases taking place in the larger deformation range and the orientation of crystal planes were monitored. At all stretching temperatures, the crystallinity decreases upon deformation, and depending on the temperature, different new structures are formed. Stretching at low temperatures leads to crystal destruction and the formation of the oriented mesophase, independent of the initial polymorph. At high temperatures, above Tα c , all polymorphs transform into oriented α-iPP. Small quantities of the initial structures remain present in the material. The compression experiments, where localization phenomena are excluded, show that these transformations take place at similar strains for all polymorphs. For the post yield response, the strain hardening modulus is decisive for the mechanical behavior, as well as for the orientation of lamellae and the evolution of void fraction and dimensions. β-iPP shows by far the most intense voiding in the entire experimental temperature range. The macroscopic localization behavior and strain at which the transition from disk-like void shapes, oriented with the normal in tensile direction, into fibrillar structures takes place is directly correlated with the strain hardening modulus.

Cavitation‐Induced Stress Whitening in Semi‐Crystalline Polymers

“…With increasing the T s , the appearance of whitening phenomenon gradually retarded and finally disappeared as T s increased to 80 °C. Similar behaviors were also reported in stretching other polymers like PE studied by Xiong et al, iPP explored by Caelers et al . and Bao et al., as well as polyvinylidene fluoride investigated by Castagnet et al .…”

Stretching behavior of the butene‐1/ethylene random copolymer: A direct correspondence between triggering of II‐I phase transition and mechanical yielding

“…Conversely, the occurrence of phase transition can also endow the material with various properties. For example, in isotactic polypropylene, the orthorhombic γ‐phase can transform into the monoclinic α‐phase and even disordered mesophase, which leads to ductile performance of materials . For poly(vinylidene fluoride), the phase transition from α‐ into β‐phases increases the polarity and consequently are successfully utilized to purposely prepare piezoelectric materials .…”

“…For example, in isotactic polypropylene, the orthorhombic γ-phase can transform into the monoclinic α-phase and even disordered mesophase, which leads to ductile performance of materials. [28,29] For poly(vinylidene fluoride), the phase transition from αinto β-phases increases the polarity and consequently are successfully utilized to purposely prepare piezoelectric materials. [30][31][32] Obviously, a fundamental understanding of phase transition is of great importance for structure design and material applications.…”

Stretch‐Induced Melting and Recrystallization of Polyethylene‐Plasticizer Film Studied by In Situ X‐Ray Scattering: A Thermodynamic Point of View

“…In yielding zone (0.95 < ε < 1.45), σ ' ( ε ) decreases and reaches 0 MPa at ε E , where the majority part of the deformation is reversible . In strain softening zone (1.45 < ε < 2.2), the stress decreases with strain, which originates from massive destruction of load bearing structures . In strain hardening zone ( ε > 2.2), stress increases again, suggesting the regeneration of load bearing structures .…”

“…As yet, interpretation of the underlying deformation mechanism has been a subject of controversy between crystallographic slip and melting recrystallization . A vital reason for the disputes is the lacking of quantitative structure information on both lattice and lamellae scale during deformation, combination of which proves to be powerful revealing the deformation process . According to the Tresca plasticity criterion, crystallographic slip will be activated when the resolved shear stress in the slip plane reaches a critical value, which is driven by the coupled effect of temperature and external stress .…”