Deformation Behavior Of Semi-crystalline Polymer Research Articles

In-Situ Synchrotron X-ray Study on the Structure Variation of Morphology-Identified Injection-Molded β-Nucleated iPP under Tensile Deformation

The deformation behavior of semi-crystalline polymers is strongly dependent on the morphology formed during processing. In this study, in-situ synchrotron X-ray was firstly used to identify the morphological distributions of injection-molded isotactic polypropylene (iPP) with different concentrations of β-nucleating agent. It was found that under relatively high concentration of β-nucleating agent (i.e., ≥0.03 wt.%), the outer region (skin and shear region) of the iPP was dominated by mainly highly oriented α-phase as well as certain amount γ-phase, while the core region was rich in β-crystals with little if any orientation. The addition of the β-nucleating agent was beneficial for the formation of lamellae with large lamellar stacking distance in the shear layer. Then the synchrotron X-ray was applied to study the structure variation of those morphology-identified samples under tensile deformation. It was found that voids and cavities along the stretching direction existed in the deformed iPP samples and their volume increased with increasing concentration of β-nucleating agent. The increased volume of void and cavity was associated with the β to α phase transition, which mainly occurred at the core region. In addition, upon stretching crystalline fragmentation and rearrangement took place following the formation of thinner lamellae.

Micro-, meso- to macroscopic modeling of deformation behavior of semi-crystalline polymer

To represent the elasto-viscoplastic deformation behavior of semi-crystalline polymer (SCP), which has a complicated hierarchical structure, a micro-, meso- to macroscopic computational model was developed. A laminar composite model and FE-based homogenization method were used to relate the mechanical behaviors in micro- to mesoscopic and meso- to macroscopic scales of the material. Using the proposed model, elasto-viscoplastic deformation behaviors of high density polyethylene under several macroscopic boundary conditions were computationally investigated. Material’s parameters used in the constitutive equations for microstructure of SCP were defined by fitting the calculated stress–strain relation into the experimental data. Then, the effects of deformation mode and crystallinity on deformation behavior of SCP were investigated. Obtained stress–strain relations represented the experimentally observed characteristics such as an anisotropic strain hardening rate depending on the deformation mode and an increase in the Young’s modulus and the flow stress with the crystallinity. Then, the neck propagation process in the uniaxial tension with different strain rate was investigated. The present model represented a development of macroscopic non-uniform deformation characterized by the mechanical response of micro- to mesoscopic deformation.

703 異なる寸法の球晶を有する結晶性高分子材料の引張り変形挙動の数値シミュレーション(OS7.材料の組織・強度に関するマルチスケールアナリシス(1),OS・一般セッション講演)

703 異なる寸法の球晶を有する結晶性高分子材料の引張り変形挙動の数値シミュレーション(OS7.材料の組織・強度に関するマルチスケールアナリシス(1),OS・一般セッション講演)

Modeling of Micro- to Mesoscopic Deformation Behavior of Semi-Crystalline Polymer Based on Laminar Composite Model

Since semi-crystalline polymer has very complex hierarchical structure, it is essential to construct a suitable multiscale mechanical model for the prediction of its mechanical characteristics. For the establishment of the triple scale mechanical model of semi-crystalline polymer, which can reflect the non-uniform deformations in micro-, meso- and macroscopic scales, computationally efficient model to relate structures and deformations in micro- to mesoscopic scales is indispensable. In this paper, to solve the problems, tangent modulus method was introduced into the constitutive equations for crystalline and amorphous phases consisting of the microstructure of semi-crystalline polymer. Furthermore, we proposed a new laminar composite model connecting deformations in micro- to mesoscopic scales based on the equilibrium of stress acting on the interface between the phases and the compatibility of deformation along the interface. Appropriate time steps for stable simulation and validity of the proposed model were examined through the computational simulation of uniaxial tension of single lamellar and spherulite models of high density polyethylene. It has been clarified that the proposed model could predict an elasto-viscoplastic deformation behavior of semi-crystalline polymer in markedly less computational time and memory.

Finite element simulation of deformation behavior of semi-crystalline polymers with multi-spherulitic mesostructure

Elasto-viscoplastic deformation behavior of semi-crystalline polymer with multi-spherulitic mesostructure was investigated using large deformation finite element homogenization method. Multi-spherulitic mesostructure was modeled in representation volume element (RVE) by Voronoi polygons, and the deformation behavior during tension in different directions was numerically investigated. Mesoscopic stress strain relations were almost similar for different mesoscopic structures except for a quarter spherulite mesoscopic models, and its anisotropy decreased with increase in the number of spherulites in RVE. Although anisotropy in mesoscopic stress strain relation disappeared for enough number of spherulites, highly heterogeneous distribution of local strain and stress was observed in each spherulite. Distribution of lamellar orientation in the spherulite caused the heterogeneity and played an important role in the local deformation field.

A Microscopic Insight into the Deformation Behavior of Semicrystalline Polymers: The Role of Phase Transitions

The deformation behavior of semicrystalline polymers associated with polymorphic transformations under tensile deformation is discussed in the case of syndiotactic polypropylene. We report a phase diagram of this polymer where the regions of stability of the different polymorphic forms are defined as a function of the degree of stereoregularity and deformation. The values of critical strain corresponding to the structural transformations depend on the stereoregularity that affects the relative stability of the involved polymorphic forms and the state of the entangled amorphous phase.

Micro-to Mesoscopic Deformation Behavior of Spherulite of Semi-Crystalline Polymer

In the present study, we clarify the micro- to mesoscopic deformation behavior of semi-crystalline polymer by employing large-deformation finite element homogenization method. 3D structure of lamellae in the spherulite of high density polyethylene is modeled, and tensile/compressive deformation behavior was investigated. A series of computational simulation clarified the difference in degree of strain hardening of tension and compression due to different directional orientation hardening. The localized deformation appears depending on the initial distribution of lamellar direction and interaction with surroundings in the mesoscopic area. These mesoscopic deformations are closely related to such microscopic deformation behavior as the rotation of molecular chains which is characterized by the stretch direction and principal slip system in the crystalline phase. Furthermore, interlamellar shear and normal strain in the amorphous phase generally depend on the direction of initial lamellar normal, however, marked scattering is observed due to the interaction with surroundings.

Computational characterization of micro- to mesoscopic deformation behavior of semicrystalline polymers

In the present study, we clarify the micro- to mesoscopic deformation behavior of semicrystalline polymers by the finite element homogenization method. The crystalline plasticity theory using a penalty method for the inextensibility constraint in the chain direction and the nonaffine molecular chain network theory were used to the representation of the deformation behavior of crystalline and amorphous phases, respectively, in the composite microstructure of a semicrystalline polymer. Various directional tensions are applied to the two-dimensional plane-strain unit cell model of a composite microstructure. The results reveal a highly anisotropic deformation behavior caused by the rotation of the chain direction and lamella interface, which depends on tensile direction and manifests as substantial hardening/softening in the early stage of deformation. The mesoscopic structure of a semicrystalline polymer was modeled using a voronoi polygon comprised of composite microstructures with different lamella interface directions. The initial isotropy of the response of the mesoscopic scale was verified. Due to their interaction with the surrounding grains, the individual grains of the mesoscopic scale show a conservative response as compared with that of the unit cell, and a very nonuniform response depending on the location of the respective grain is observed; these are typical of the mesoscopic response of semicrystalline polymers.

Deformation Behavior of Semi-crystalline Polymer in Mesoscopic Region under Different Stress States

In the present study, we clarify the deformation behavior of semi-crystalline polymer in microto mesoscopic region by using large deformation finite element homogenization method. The crystalline plasticity theory using a penalty method for the inextensibility constraint in the chain direction and the nonaffine molecular chain network theory were used to the representation of the deformation behavior of the crystalline and amorphous phases, respectively, in the composite microstructure of the semi-crystalline polymer. The deformation behaviors of High Density Polyethylene (HDPE) in micro- to mesoscopic region under mesoscopically uniform plane strain tension, compression and shearing were investigated. The results revealed that the microstructure of semicrystalline polymer exhibits highly anisotropic deformation behavior including strain softening or substantial hardening. Furthermore, mesoscopically localized deformation zone intersepting several grains appears in the direction of maximum shear stress due to the interactions with surroundings.

Independent slip systems and the deformation behavior of semicrystalline polymers

Independent slip systems and the deformation behavior of semicrystalline polymers