Plastic Deformation of the γ Phase in Isotactic Polypropylene in Plane-Strain Compression

Abstract

Morphology and deformation behavior of samples of iPP homopolymer containing exclusively γ-modification with only minor traces of R-crystals, obtained by isothermal crystallization at high pressure of 200 MPa, were investigated. It was found that the growth of γ-lamellae is initiated on "seeds" consisting of a spine of single R-lamella and several shorter R-lamellae branching at the angle of 80°. Epitaxial growth of γ-lamellae on (010) faces of R-parent and daughter lamellae of the seed leads to the formation of immature spherulites, which eventually fill completely the sample volume. The plane-strain and uniaxial compression tests demonstrated higher modulus, higher yield stress and flow stress, yet slightly lower ultimate strain of γ-iPP as compared to R-iPP. During plastic deformation numerous fine shear bands, initiated by the interlamellar shear of the amorphous layers start to develop already at the yield point. Their propagation across the sample causes a limited destruction of γ-lamellae oriented perpendicularly to the direction of the band. Destroyed fragments of crystallites transform partially into a smectic phase. No γ-R phase transformation was detected. With increasing strain the shear bands multiply and tilt gradually toward the flow direction. Lamellae, already fragmented within shear bands, undergo kinking and rotation, resulting in the formation of a chevron-like lamellar morphology. Simultaneously, a relatively weak one-component crystalline texture is developed. This texture is described by the orientation of c crystallographic axis along the constrained direction, b axis 10-30°away from the loading direction toward the flow direction, and a axis 10-30°away from FD. Both crystalline texture and lamellae orientation are developed due to the activity of the same deformation mechanismsthe interlamellar slip produced by the shear within interlamellar amorphous layers. Activity of any crystallographic deformation mechanism within crystalline component was not detected. The interlamellar amorphous shear appears the primary deformation mechanism of γ-iPP. The other identified mechanism, γ-smectic phase transformation, plays a minor, supplementary role in the deformation sequence.

Crystallization behavior of isotactic polypropylene induced by competition action of β nucleating agent and high pressure

“…However, this cannot exclude the presence of α-iPP in the sample. Considering the nucleation and growth mechanism of γ-iPP, the most possibility is that the growth of α-iPP is greatly limited to the very initial phase of crystallization process and in the formation of seeds only [32]. In this condition, the content of α-iPP is too small to be detected by WAXD measurement.…”

“…In these spherulites, the γ-lamellae are organized and packed in parallel stacks, spread radially and show only a moderate branching. With the increasing of pressure, the size of the γ-spherulites decreases slightly, which means that the nucleation rate of the sample is enhanced with the aid of high pressure [32]. For iPP/TMB-5 sample, it exhibits the hedritic supermolecular structure of β-iPP after being nonisothermal crystallized under atmospheric pressure.…”

“…So far, at least two crystalline morphologies of γ-iPP have been reported based on the microscopic observations. One is the spherulitic-like aggregates which are observed in samples crystallized under high pressure [27,32] and the other is the form of large "featherlike" structures, apparently caused by massive selfepitaxy processes [25,33,34]. γ-spherulites do not show the cross-hatching feature characteristic for α-spherulites grown under atmospheric pressure, but show only a moderate branching with lamellae packed in parallel stacks and spread radially.…”

Hierarchical structures and phase nucleation and growth during pressure-induced crystallization of polypropylene containing dispersion of nanoclay: The impact on physical and mechanical properties

“…It may be noted that the peak at ∼20 • corresponding to the reflection (1 1 7) of ␥-phase does not overlap with ␣-phase, and thereby confirms nucleation of ␥-phase. While, the observation of ␥-phase at high pressures is not surprising because it has been previously observed [26,[32][33][34], the objective of the work described here is to examine the phase and microstructural evolution induced by nanoparticles and pressure. This enables us to simultaneously compare polypropylene with and without nanoparticles to delineate nanoparticle effects.…”

γ‐Form Transcrystals of Poly(propylene) Induced by Individual Carbon Nanotubes

“…[22,23] Interestingly, it has been reported that g-form iPP has better mechanical performance than aform iPP. [24] Recently, we reported CNT fiber-induced iPP transcrystallization in the bulk. [19] Analogous to traditional carbon fiber-iPP composites, the macroscopic CNT fibers were placed between two iPP thin films and hot pressed to form a sandwich composite.…”