Micromechanical processess

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

In spite of the imperfections of the approach, the reversible work of adhesion can be used for the characterization of matrix/filler interactions in particulate filled polymers. Debonding is one of the dominating micromechanical processes in these materials. Stress analysis has shown that debonding stress (a ) depends on the reversible work of adhesion [8], i.e. ... 

Elinck and coworkers were the first to report on the discontinuous nature of DCG, while the Lehigh University research team under the direction of Hertzberg and Manson, discovered the generality of the DCG process in both amorphous and crystalline polymers. Recently, Doll, Konczol and Schinker have used optical interferometry to provide the greatest insist as to the micromechanical processes that underlie the DCG mechanism. 

To study the influence of these structural parameters on the micromechanical mechanisms of toughening, several techniques of electron microscopy were used. Electron microscopic techniques allow investigations not only of detailed morphology but also of the micromechanical processes of deformation and fracture (3, 9-11). 

Figure 3. Electron microscopic techniques used to study micromechanical processes in polymers (a) investigation of fracture surfaces by SEM (b) investigation by TEM of ultrathin sections prepared from deformed and selectively stained bulk material and (c) deformation of samples of different thicknesses (bulk, semithin, and ultrathin)9 using special tensile stages with SEM, HVEM, and TEM. The technique in (c) shows the possibility of conducting in situ deformation tests in the electron microscope.

Characteristics of the Shear Mechanism. The results of the investigation of the micromechanical processes can also be summarized in a three-stage-mechanism (Figure 17). In stage 1, under an external stress, stress concentrations, crK, or stresses increased by superposition of local stress fields are built up at, and between, the rubber particles (as in the systems described in the preceding discussion). At places with a maximum shear-stress component, weak shear bands are formed between the particles at an angle of about 45° to the load direction (see Figures 16 and 17a). 

Until now, these critical values could be determined only experimentally. When they are known, criteria can be defined to optimize the morphology of modified polymers in order to realize maximum toughness. Therefore, the investigation of micromechanical processes is helpful in choosing the appropriate polymeric components and manufacturing procedures. 

Modifications of these basic techniques are necessary, depending on whether study of morphology or of micromechanical processes is the centre of interest. 

Figure 3 Survey of electron microscope methods for investigating micromechanical processes in polymers (a, b, c, see text).

Results of investigating the micromechanical processes by EM are summarized in a three-stage mechanism, described in more detail in [10] ... 

Fig. 1. Correlations between structure, morphology, micromechanical processes, and mechanical properties of polymers.

Fig. 2. Survey of (electron and scanning force) microscopic methods for investigating micromechanical processes in polymers, a indicates the applied tension stress and the arrows indicate area and direction of investigation in the microscope (see text for full details), (a) fracture surfaces— tension test, impact test, broken pieces (b) surfaces of deformed (loaded) bulk samples (c) in situ deformation of thin samples.

Most of these micromechanical processes are highly localized and depend strongly on the local morphology. Therefore, direct imaging by electron microscopy techniques with a high local resolution are of particular importance, and most of our knowledge of rubber toughening arises from the application of such techniques (2,36-38). 

In addition to influences of mechanical load on degradation in polymer chains, unexpected degradation can occur due to the initiation of specific micromechanical processes and mechanisms. Such mechanisms (shear flow, craze formation, cavitation, etc.) are initiated in particular in inhomogeneous polymers (blends, filled and reinforced plastics, etc.) by inhomogeneous stress distributions caused by external mechanical loads. Typical examples for such physical aging processes are ABS materials with an unfavorable size distribution of rubber particles [72],... 

Due to the variety of different structural details that can occur in polymers, there are also a wide variety of nano-/micromechanical processes that can appear under load. These include changes to individual macromolecular segments (on a nanometer scale), localized plastic yielding in the form of crazes or shear bands (at the micrometer scale), up to crack propagation and macroscopic fracture (at the millimeter scale). Therefore, different techniques for studying these processes are required, which are shown in Figure 3.24. 

When going from method 1 to methods 2 and 3, more and more details of the micromechanical processes can be revealed and investigated in their dependence on the real morphology. Analysis of fracture surfaces by SEM (method 1 - microfractography) yields information mainly about the processes of crack initiation and crack propagation up to the final fracture. Particularly, the influence of structural... 

Figure 3.24 Survey of electron microscopic methods for investigating nano-/micromechanical processes in polymers (from [1,2]).

As hierarchical, multiscale and complex properties in nature, as shown inO Fig. 52.1, adhesion is an interdisciplinary subject, which undergoes vast experimental, numerical and theoretical investigations from microscopic to macroscopic levels. As an example, adhesion in micro- and nano-electromechanical systems (MEMS/NEMS) is one of the outstanding issues in this field including the micromechanical process of making and breaking of adhesion contact, the coupling of physical interactions, the trans-scale (nano-micro-macro) mechanisms of adhesion contact, adhesion hysteresis, and new effective ways of adhesion control (Zhao et al. 2003). 

The influence of all of the different structural details on the mechanical properties is determined by micromechanical processes, which appear under the applied loading conditions. Depending on the very different structural details and loading conditions, there is a very large variety of micromechanical processes of deformation and fracture. These processes define the micromechanical properties of a polymeric material or the micromechanics. Therefore, micromechanical properties form the bridge between structure or morphology and ultimate mechanical properties. Improved knowledge of the micromechanical properties allows a... 

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