Plastic Deformation Of Semicrystalline Polymers Research Articles

Analysis of Mullins effect in polyethylene using ultrasonic guided waves

Abstarct Small strain deformations below yielding can cause plastic deformation in semicrystalline polymers by a process similar to what is described for filled rubber-like materials known as the Mullins effect. Inter-lamellae chains contribute predominantly in deformations at this level, and the residual plastic strain can be attributed to permanent damage to the tie chains, affecting the long-term mechanical resistance of a molded part. With little detectable alteration of the polymer crystallinity at this early onset of plastic deformation, the primary characterization method applied to date is cyclic tensile loading, which provides information of stress-softening by monitoring the unloading path or relaxed stress behavior. An alternative method for monitoring the development of Mullins effect is proposed that can examine a molded part by using selected modes based on ultrasonic guided waves analysis. The technique was examined to determine if it could follow this effect induced by cyclic strain-controlled tensile deformations since it does not require sample preparation and could ultimately be applied while a part was in-service. Results for different polyethylene grades agree in trend with relaxed stress values over four cycles for tests of increasing applied tensile strain, demonstrated by an increase in the attenuation of ultrasonic guided waves. The correlation reveals a good promise in applying this method to structural health monitoring of plastic parts, while in use, to follow the initiation and progress of early service damage.

Molecular Alignment in Polyethylene during Cold Drawing Using In-Situ SANS and Raman Spectroscopy

Changes in the crystalline and mesoscale lamellar structure during plastic deformation of semicrystalline polymers have been extensively studied by X-ray diffraction techniques. However, direct measurements of single chain conformations during stretching have not been realized, although they are key to fully understand the structural transitions during cold drawing and their relation with the state of uniaxial stress. We report direct measurements of molecular alignment of a semicrystalline polymer during cold drawing by combining in-situ small-angle neutron scattering (SANS) and polarized Raman spectroscopy. The sample investigated is a linear low-density polyethylene (LLDPE) with density of 918 kg/m3 and melt index of 1.0 g/10 min. A multifaceted protocol consisting of hydrogen–deuterium exchange, followed by fractionation (by molecular weight, MW) and blending of selected deuterated fractions with protonated LLDPE, was used to elucidate, via SANS measurements, the response of the different fractions to...

Analysis of plastic deformation of semi-crystalline polymers during ECAE process using 135° die

In this paper, analysis of plastic deformation of high density polyethylene (HDPE) and polypropylene (PP) during an equal channel angular extrusion (ECAE) process is investigated. The effects of ram speed, number of passes, processing route and temperature are tested experimentally using a 135◦ die. The results show that the pressing force decreases with an increase in the number of passes and reaches a saturation state rapidly for routes A and C compared to routes BA and BC. Furthermore, it is found that the reduced curvature of the extruded samples is obtained by route C, however, the maximum warping is obtained by route A. A slight influence of temperature on the reduction of the warping is observed on the extruded samples. In order to predict the plastic strain inside the extruded samples, an elastic viscoplastic model is identified using compressive tests at different strain rates and coupled with the finite element method (FEM). A good correlation is found between the numerical modeling and experimental findings. FEM results show that the PP samples display a higher level of plastic strain compared to HDPE samples. However, almost the same degree of strain heterogeneity is obtained for both polymers. Finally, in order to reduce the warping and improve the strain homogeneity, a controlled back-pressure with small corner angle is expected to be an adequate solution.

Plastic Deformation Mechanisms of Semicrystalline and Amorphous Polymers.

We use large-scale molecular dynamics simulations to investigate plastic deformation of semicrystalline polymers with randomly nucleated crystallites. The strain-softening regime is dominated by deformation of crystallites via reorientation of chain-folded lamellae toward the tensile axis, fragmentation of largest crystalline domains, and a partial loss of crystallinity. The strain-hardening regime coincides with unfolding of chains and recrystallization as a result of strain-induced chain alignment. These observed deformation mechanisms are consistent with experimental findings. We compare the tensile behavior of semicrystalline polymers with their amorphous counterparts at temperatures above and below the glass transition temperature.

Micromechanics of semicrystalline polymers: Yield kinetics and long‐term failure

AbstractThe process of plastic deformation in semicrystalline polymers is complicated due to the operation of a variety of mechanisms at different levels and is strongly dependent on their underlying microstructure. The objective of this work is to establish a quantitative relation between the microstructure and the mechanical performance of semicrystalline polymers, as characterized by elasto‐viscoplastic deformation. To do that, a micromechanically based constitutive model is used. The model describes the material as an aggregate of two‐phase layered composite inclusions, consisting of crystalline lamellae and amorphous layers. The starting point for adding quantitative abilities to the model, in particular for the yield kinetics, is formed by experimental observations on both the yield kinetics and the time‐to‐failure of polyethylene at different temperatures, which reveal the contribution of two relaxation processes. To predict the thermo‐rheologically complex short‐term and long‐term failure behavior, the crystallographic slip kinetics and the amorphous yield kinetics are re‐evaluated, and the Eyring flow rule is modified by adding a temperature shift function. To enable the prediction of both tension and compression, a non‐Schmid effect is added to the constitutive relation of each slip system. The creep behavior of polyethylene is then simulated directly using the multiscale, micromechanical model, predicting the time‐to‐failure, controlled by plastic deformation. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

The role of dislocations for the plastic deformation of semicrystalline polymers as investigated by multireflection X‐ray line profile analysis

AbstractWide angle X‐ray experiments evaluated by recent developments of X‐ray line profile analysis allow for the detection of the presence of dislocations as well as to determine their density in crystalline materials. The application to semicrystalline polymers not only provides information on the crystal size and the dislocations but—in combination with in situ deformation—also information on the evolution of these microstructural parameters. Investigations on cold rolled and on uniaxially compressed samples of α‐phase isotactic polypropylene (α‐iPP) as well as poly(3‐hydroxybutyrate) (P3HB) are presented. The synchrotron experiments reveal a dislocation governed deformation process in α‐iPP. P3HB, however, deforms by a process not including dislocation generation. Here, microcracking and strain localization in the amorphous phase seem to be the predominant deformation mechanisms. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Elasticity of syndiotactic polypropylene: Insights from temperature and time dependence

The origin of the unusual and puzzling elasticity of drawn sPP samples was investigated. The mechanism responsible of the elasticity was studied for drawn samples characterized by a very simple structural organization, where there are no involvements of crystallographic modifications with different chain conformation. The elastic behavior of the drawn samples, valued through the hysteresis cycles, was determined at different temperatures. At room temperature the samples show remarkable elastic properties, whereas decreasing the temperature the elastic behavior becomes worse and worse, disappearing at temperatures lower than 0 °C. The elasticity also disappears in drawn samples after a long aging under tension. Thermomechanical and structural investigations, as well as shrinkage as a function of temperature and aging at room temperature of the fixed drawn samples, support the idea that the elastic behavior of sPP can be explained by the model of the plastic deformation of semicrystalline polymers. The interpretation is based on the presence of “tie” molecules axially connecting the crystals in the oriented samples, whose extension, chain conformation and/or crystallization determine the retractive stress of the oriented sample, as well as other mechanical properties. We show that many experiments on the drawn samples, either fixed or relaxed, are strictly connected to the morphology of the drawn sample, derived by the transition between the lamellar initial and the fibrillar final structure. By applying the model we can answer the questions derived from the experimental facts not yet well clarified, giving a new insight into the interesting elasticity of sPP.

Electrical breakdown and ultrahigh electrical energy density in poly(vinylidene fluoride-hexafluoropropylene) copolymer

This paper investigates the electrical breakdown of a polar fluoropolymer, poly(vinylidene fluoride-hexafluoropropylene) which exhibits an exceptionally high discharged electrical energy density (>25 J/cm3). It is shown that above room temperature, the breakdown strength decreases with temperature. It is further shown that such a temperature dependence of breakdown strength is consistent with the electromechanical breakdown model by taking into consideration of the plastic deformation of semicrystalline polymers.

Dislocation approach to the plastic deformation of semicrystalline polymers: Kinetic aspects for polyethylene and polypropylene*

AbstractThe plasticity of semicrystalline polymers is analyzed in the framework of Young's dislocation model under the assumption of nucleation of screw dislocations from the lateral surface of the crystalline lamellae. It is proposed that the driving force for the nucleation and propagation across the crystal width of these screw dislocations relies on chain twist defects that migrate along the chains stems and allow a step‐by‐step translation of the stems through the crystal thickness. Such defects are identified as thermally activated conformational defects responsible for the so‐called crystalline relaxation. Dislocation kinetic equations are derived. Plastic flow rates attainable by dislocation motion in polyethylene and polypropylene are assessed with frequency–temperature data of the crystalline relaxation. Comparisons are made with experimental strain rates that enable homogeneous plastic deformation. In addition to temperature, the crystal lamellar thickness, which is a basic factor of the plastic flow stress in Young's dislocation model, is a major factor in dislocation kinetics through its influence on chain twist activation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 593–601, 2002; DOI 10.1002/polb.10118

On the micromechanisms of plastic deformation in semicrystalline polymers

Many different molecular processes that cause plastic deformation in polymers can be investigated by transmission electron microscopy (TEM). A short review of those deformation mechanisms is given in this paper using model morphologies and correlating them to the corresponding mechanical behavior.

Mobility and relaxation of amorphous chains in drawn polypropylene: 2H-NMR study

Mobility and orientation of amorphous chains in cold drawn films of isotactic polypropylene (PP) are studied by 2H-NMR as a function of the annealing temperature. It is shown that the deformation is not affine. The relaxation of amorphous chains in materials annealed with free and clamped ends appears respectively at T*≈Tg+50°C and at Tαc≈80°C. Above these transition temperatures, corresponding to the onset of the chain mobility in the amorphous and crystalline states, the orientation of amorphous chains decreases linearly with the temperature. These reports show that the 2H-NMR technique provides important information on the plastic deformation of semi-crystalline polymers.

Rate Dependent Deformation of Semi-Crystalline Polypropylene Near Room Temperature

We examine the strain rate dependent, large plastic deformation in isotropic semi-crystalline polypropylene at room temperature. Constant strain rate uniaxial compression tests on cylindrical polypropylene specimens show very little true strain softening under quasi-static conditions. At high strain rates very large amounts (38 percent) of apparent strain softening accompanied by temperature rises are recorded. We examine the capability of a recently proposed constitutive model of plastic deformation in semi-crystalline polymers to predict this behavior. We neglect the contribution of the amorphous phase to the plastic deformation response and include the effects of adiabatic heating at high strain rates. Attention is focused on the ability to predict rate dependent yielding, strain softening, strain hardening, and adiabatic temperature rises with this approach. Comparison of simulations and experimental results show good agreement and provide insight into the merits of using a polycrystalline modeling assumption versus incorporating the amorphous contribution. Discrepancies between experiments and model predictions are explained in terms of expectations associated with neglecting the amorphous deformation.

On the plastic deformation of fibre self-reinforced polymers

The plastic deformation of semi-crystalline polymers containing needle crystals has been investigated. Stress-strain curves exhibit two linear regions, the elastic (Hookeian) range and a second linear plastic part. The onset of the plastic part depends on pre-straining and the slopeα = dσ/de of the plastic part is dependent on the morphology of the samples. An interpretation is given in terms of the shear-lag theory of discontinuous uniaxially oriented fibres.