Deformation and failure kinetics of iPP polymorphs

Abstract

In this study, the mechanical performance of the different polymorphs of isotactic polypropylene, typically present in iPP crystallized under industrial processing conditions, is assessed. Different preparation strategies were used to obtain samples consisting of almost solely a, b, or c crystals. X-Ray measurements were used to validate that the desired phase was obtained. The intrinsic true stress -true strain response of all individual phases was measured in uniaxial compression at several strain rates (deformation kinetics). Moreover, measurements were performed over a wide temperature range, covering the window in between the glass transition and the melting temperature. The relation between obtained yield stress and the strain rate is described with a modification of the Ree-Eyring model. Differences and similarities in the deformation kinetics of the different phases are presented and discussed. Furthermore, the presence of three deformation processes, acting in parallel, is revealed. The ReeEyring equation enables lifetime prediction for given thermal and mechanical conditions. These predictions were experimentally validated using constant load tests in uniaxial compression.

Designing polymer crystallinity: An industrial perspective

“…high molecular weight fraction and nucleation, including the result on crystallinity, polymorphism and orientation . The mechanical consequences of such selective phase formation are significant …”

“…[75,154,159] The mechanical consequences of such selective phase formation are significant. [131,160] iPP homopolymer HD120MO (MFR 8 g/10 min) at T exp = 135 C, Next to shear, also extension in flow has been shown to be capable of generating first higher nuclei and-at higher stresses-oriented structures both for PP [161,162] and for HDPE. [163] In contrast to the rheological concept of "weak" (shear-dominated) and "strong" (extension-dominated) flows, comparable levels of work input to a system were found to give comparable effects in shear and extension by…”

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 .…”

Structural origin for the strain rate dependence of mechanical response of fluoroelastomer F2314

“…In general, the total stress of TPEs can be divided into elastic and viscous parts, and the strain rate effect is mainly reflected in affecting the viscous stress contribution. The Eyring model, proposed by Eyring and modified by many other researchers, has been utilized to describe the viscous stress adequately, which depends on strain rate whether in semicrystalline polymers, amorphous polymers, or TPEs . In particular, the relationship between yield stress and strain rate has been given accurately by this model .…”