Micromechanical Deformation Processes Research Articles

Microstructural construction of polymers with improved mechanical properties

Mechanical properties of materials are based on structure or morphology and on the micromechanical processes of deformation and fracture, i.e. on the “micromechanics”. Developments mainly in electron microscopy and scanning force microscopy opened up a wide range of experiments previously impossible, including the in situ study of micromechanical processes. Direct microscopic techniques on the basis of scanning, transmission, and high-voltage electron microscopy are used to study micromechanical properties of different polymer blends of amorphous and semicrystalline polymers. Of particular interest are polymer blends with improved toughness. Blends with styrene–acrylonitrile as matrix polymer and different types of modifier particles, with their diameters ranging between about 100 nm and 1 μm, have been investigated. Examples of blends with a semicrystalline matrix are polypropylene (PP) blends with different amounts of various rubber and modifier particles. New micromechanical mechanisms have been found, which can change the toughness of some acrylonitrile–butadiene–styrene grades or improve low-temperature toughness of PP. The examples show that detailed knowledge of the morphology and micromechanical mechanisms helps to define criteria of improved properties. This “microstructural” analysis and construction of polymeric systems provides a new basis of polymer modification. © 1998 John Wiley & Sons, Ltd.

Influence of morphology on the toughening mechanisms of polypropylene modified with core-shell particles derived from thermoplastic elastomers

The micromechanical deformation processes of impact- modified polypropylene (PP) with core– shell particles derived from PP/EPR (ethylene– propylene– rubber) block copolymers and PP/PA/SEBS-g-MA (polypropy-­lene / polyamide / polystyrene - block - poly(ethene - co - but -1-ene)-block-polystyrene-graft-maleic anhydride) graft copolymers have been investigated by in-situ tensile tests in high-voltage electron microscopy. Morphology studies in transmission electron microscopy show that the morphology of modifier particles is drastically affected by their concentration. It was found that the driving mechanism for the initiation of plastic deformation is a controlled microvoid formation, which is caused by cavitation in the stretched rubber shell under mechanical loading. According to the inherent properties and phase structures of modifier particles, a single or multiple cavitation appears with or without fibril formation at the interface between the modifier particles and the matrix. The Predominant mechanism for the improvement of toughness is the shear yielding of the ligaments of matrix material induced by local stress concentration by the microvoids. © 1998 John Wiley & Sons, Ltd.

Micromechanical deformation processes in toughened and particle filled semicrystalline polymers: Part 2. model representation for micromechanical deformation processes

Toughening mechanisms occurring in various toughened and particle filled semicrystalline polymers have been investigated by several cursive electron microscopic techniques. In the previous work (Part I) it has been shown that the toughening mechanisms involved in different blend systems are influenced decisively by the phase morphology of modifier particles and the phase adhesion between matrix and modifier particles, which in turn lead to different micromechanical deformation processes. Form these results six different schematic models for micromechanical deformation processes are proposed. To simplify the explanation of micromechanical deformation processes it has been described in form of three-stage-mechanism. Finally, the results of micromechanical deformation processes are discussed with the concept of energy dissipation and stress states.

Micromechanical deformation processes in toughened and particle-filled semicrystalline polymers: Part 1. Characterization of deformation processes in dependence on phase morphology

Relationships between morphology and micromechanical deformation processes in various toughened and particle-filled semicrystalline polymers with different types of modifier particles have been investigated by high voltage electron microscopy and scanning electron microscopy using in situ tensile techniques. From the study of phase structure of modifier particles, two morphological standard types are classified: the binary system (homogeneous modifier particles are dispersed in the matrix), and the ternary system (heterogeneous modifier particles are dispersed in the matrix). Taking into account these categories and the phase adhesion between the modifier particles and the matrix, micromechanical deformation processes have been characterized. As initiation sites of plastic deformation the microvoid formation is considered, which plays an important role for the activation of further plastic deformation of matrix material during deformation processes. According to the morphology of modifier particles this microvoid formation is caused by either cavitation in the stretched rubbery shell/inside particles or debonding at the interface between particles and matrix.

Micromechanical deformation processes in toughened PP/PA/SEBS-g-MA blends prepared by reactive processing

Toughening mechanisms occurring in PP/PA6 (70 vol.-%/30 vol.-%) blends containing various amounts of SEBS-g-MA have been studied by in situ straining in a high voltage electron microscope (HVEM). Variation of volume fraction of compatibilizer (SEBS-g-MA) with constant PA6 content and reactive processing conditions give excellent control over morphology and toughness/stiffness balance. With increasing volume fraction of compatibilizer the blend morphology changes significantly from discrete to cluster to island-like (percolating) structures. Micromechanical deformation processes have been characterized as a function of blend morphologies. Based on experimental results, the major toughening mechanism is the fibrillized cavitation process, and the main energy dissipation during deformation is shear yielding of matrix material triggered by irreversible plastic growth of microvoids caused by the fibrillized cavitation process. Finally, schematic models for micromechanical deformation processes in the various blend morphologies are proposed.

Micromechanical deformation processes in toughened PP/PA/SEBS-g-MA blends prepared by reactive processing

Micromechanical deformation processes in toughened PP/PA/SEBS-g-MA blends prepared by reactive processing

Time resolved simultaneous small- and wide-angle X-ray scattering during polyethylene deformation—II. Cold drawing of linear polyethylene

The cold drawing behaviour of a range of unoriented linear polyethylenes was investigated using the technique of simultaneous small- and wide-angle X-ray scattering during deformation. The influences of molecular weight and thermal history were studied. The role of both variables was found to be highly important since by altering the percentage crystallinity they influenced the relative amounts of inter- and intra-lamellar deformation. The micromechanical deformation processes governing the different regions of the load-extension curve were found to be the same as for cold-drawn ethylene-α-olefin copolymers, demonstrating the fundamental similarity between all types of unoriented polyethylene PE.

Stress distribution around inclusions, interaction, and mechanical properties of particulate‐filled composites

AbstractStress analysis was carried out to determine the stress distribution around particles in particulate‐filled composites. The effect of interacting stress fields was also taken into account. At large filler contents, interacting stress fields compensate for the effect of stress concentration. The solutions were introduced into the Von Mises criterion for yielding. The composition dependence of tensile yield stress was determined by using different boundary conditions and averaging techniques. An analytical expression was derived that predicts particle size and adhesion dependence of the yield stress. The analysis shows that large particles and weak interaction lead to debonding. In the case of strong adhesion, the dominating micromechanical deformation process is the shear yielding of the matrix. In such cases, particle size dependence can be explained with the effect of interfacial interactions, which lead to the formation of an interphase. The dominating deformation mechanism is determined by particle characteristics and adhesion of the components. The predictions of the analysis are in good agreement with experimental observations.

Relationship between morphology and micromechanical toughening mechanisms in modified polypropylenes

Deformation and fracture processes of two types of modified polypropylenes (PPs) were investigated in situ by high voltage electron microscopy (HVEM, 1 MV) and scanning electron microscopy (SEM). One type of PP modified with ethylene–propylene block co-polymer rubber (EPR) showed a variation in particle size and interparticle distance with content of ethylene. In the other series of modified PP, Al2O3 filler particles with concentrations of 10 and 60 wt % were used. The micromechanical toughening mechanism was comparable in both types. Deformation structures in both systems are closely connected with cavitation and void formation: the systems modified with EPR show void formation inside the modifier particles; in the systems modified with Al2O3, debonding occurs at the interface between the particles and the matrix. Additionally, the effect of the morphology of the modifier particles on micromechanical deformation processes was studied. The experiments showed that besides particle size and center-to-center distance between particles, the ratio of center-to-center distance to particle diameter plays an important role. Models (three-stage mechanism) for the micromechanical deformation process are proposed. © 1996 John Wiley & Sons, Inc.

Micromechanical deformations in particulate filled thermoplastics: volume strain measurements

Volume strain measurements were carried out on PP composites containing different CaCO3 fillers. During deformation, a volume increase was detected which could be divided into two linear sections as a function of elongation. Comparison of data with existing theories has shown that in the first part, mostly elastic deformation takes place and the slope can be related to the Poisson's ratio of the composite. Scanning electron microscopy revealed that in the second stage, the dominating micromechanical deformation process is debonding. Void formation is initiated at a certain stress which approximately corresponds to the yield stress of the composites, but data in the literature and model calculations indicate that separation of the matrix/filler interface may start at lower stresses. Initiation stress depends on the particle size of the filler and on interfacial interactions. The rate of volume increase has non-linear dependence on the volume fraction of the filler. Volume strain measurements reflect micromechanical deformations well, but further study is needed to explain contradictions between experimental results and theoretical predictions.

Electron Microscopy in Polymer Science

Abstract In many fields of research in science, engineering, and medicine, electron microscopy as a method for directly imaging submicroscopic structures has become increasingly important in recent decades. Electron microscopy (EM) includes several different techniques: conventional transmission electron microscopy (TEM), high-resolution electron microscopy (HREM), highvoltage electron microscopy (HVEM), scanning electron microscopy (SEM), analytical electron microscopy (AEM), emission electron microscopy (EEM), and others. In the past the central aim of using electron microscopy was structure determination, but recently it has been of growing importance for also investigating different processes, i.e., changes in materials by interaction with several influential factors (e.g., heat, electric or mechanic fields, mechanical loading). Of particular interest is the study of the micromechanical processes of deformation and fracture. Therefore, electron microscopy is a very powerful tool for materials science. The present review reports on some capabilities and limitation of the application of electron microscopy to solid polymers. In Section II the techniques of electron microscopy are briefly reviewed, followed by a section that describes the main methods of specimen preparation of solid polymeric materials. In Section IV the results of several applications of electron microscopy are discussed to reveal the morphology as well as the micromechanical deformation processes of several polymeric materials.

The effect of grain-size on the stress and velocity fields ahead of a crack in a material which deforms by Coble creep

The stress and velocity fields ahead of a grain-boundary crack in a material which deforms by Coble creep are investigated for a series of grain sizes using the numerical analysis technique established by Cocks [1–3]. The numerical results are compared with the linear viscous fields obtained by the boundary element method and the finite element method. It is demonstrated that a detailed knowledge of the micromechanical processes of material transport and deformation are required to determine the stress and displacement fields for the grains which immediately surround the crack tip. But the response of the surrounding material can be adequately described from a conventional linear viscous analysis of the problem.

Correlations between molecular weight, morphology and micromechanical deformation processes of polyethylenes

The influence of molecular weight on morphology and micromechanical deformation processes of polyethylene has been studied by conventional transmission and high-voltage electron microscopy. Fractions with very narrow molecular weight distributions and commercial samples of high-density polyethylene in the range of molecular weight 104 to 1.6·106 have been studied. With increasing molecular weight there is a change in the morphology from sheaf-like structures and banded spherulites, to small bundles of parallel lamellae or randomly distributed lamellae. For molecular weights greater than 105, the thickness of the lamellae increases more slowly than the thickness of the interlamellar, amorphous layers.

Formation of crazes in polymer blends

AbstractHigh‐voltage electron microscopy was used to study the micromechanical processes of deformation directly on thin deformed samples of rubber‐modified, high‐impact polymers. In these polymers the microprocesses are closely connected with the initiation and formation of crazes. Craze formation with its effects on the fracture toughness are discussed in dependence on several important morphological factors, particularly on the rubber volume content, particle diameter, and particle diameter distribution.