Tensile yield in poly(4‐methyl pentene‐1)

Abstract

Uniaxial tension tests to the yield point were performed on a crystalline polymer, poly(4‐methyl pentene‐1) (PMP) as a function of temperature from 21° to 200°C at a strain rate of 2 min−1. After testing, the specimens showed considerable stress whitening as a result of microvoid formation. Yield energy was found to be a linear function of temperature extrapolating to zero at the melting point (240°C). Thus, the behavior of this crystalline polymer is similar to that of glassy polymers, but with the melting temperature, rather than the glass transition temperature, as the reference point. The ratio of thermal to mechanical energy input to produce yielding is an order of magnitude smaller for PMP than it is for glassy polymers. The ratio of yield stress to Young's modulus is about 0.02, which is typical for polymers. Yield stress is a linear function of log strain rate, which implies that yielding can be described as a segmental flow rate process in which the applied stress biases the activation energy. The activation volume is on the order of 20 monomer unit volumes and increases as the temperature increases. The activation energy is 19 kcal/mol.

Tensile yield of isotactic polypropylene in terms of a lamellar-cluster model

“…in which ⌬T ϭ T m Ϫ T. This empirical relation was already explained by Hartmann and Cole 35 on the basis of the assumption that the mechanical energy available to overcome the activation barrier to yielding can be related to its thermal energy C p ⌬T, in which C p is the heat capacity and is the density. In this view, the coefficient K in eq 11 involves the C p term.…”

“…35 The value of U y can be estimated from the area under the nominal stress-strain plot from the origin to the stress drop. It is likely that U y corresponds to the elastic-strain energy U* required for the lamellae to be fragmented.…”

Influence of Morphological Factors on Tensile Properties in the Pre-yield Region of Isotactic Polypropylenes

“…In this work, the yield stress y was defined as a maximum stress in the stress-strain curve and the Young's modulus E Y was evaluated from the initial slope. The yield energy U y , which is defined as the mechanical energy dissipated for yielding to take place, 12 was here defined as the area under the stress-strain plot from the origin to the stress drop. It is likely that U y corresponds to the elastic-strain energy required for the crystalline lamellae constructing spherulites to be fragmented.…”

Morphology and mechanical properties of transcrystalline isotactic polypropylene

“…5. This work focused on mechanical parameters such as elastic modulus and yield energy, which is defined as the energy dissipated for yielding to take place [18]. The yield energy was estimated from the area under the stress-strain plot from the origin to the yield maximum point.…”