Fibrillated crazes

As one example, in thin films of Na or K salts of PS-based ionomers cast from a nonpolar solvent, THF, shear deformation is only present when the ion content is near to or above the critical ion content of about 6 mol% and the TEM scan of Fig. 3, for a sample of 8.2 mol% demonstrates this but, for a THF-cast sample of a divalent Ca-salt of an SPS ionomer, having only an ion content of 4.1 mol%, both shear deformation zones and crazes are developed upon tensile straining in contrast to only crazing for the monovalent K-salt. This is evident from the TEM scans of Fig. 5. For the Ca-salt, one sees both an unfibrillated shear deformation zone, and, within this zone, a typical fibrillated craze. The Ca-salt also develops a much more extended rubbery plateau region than Na or K salts in storage modulus versus temperature curves and this is another indication that a stronger and more stable ionic network is present when divalent ions replace monovalent ones. Still another indication that the presence of divalent counterions can enhance mechanical properties comes from... 

We note from the outset that crazing, which is a form of cavitational localization of deformation, can be viewed as a form of transformation plasticity made possible by the long chain molecular nature of the material and the natural molecular entanglements that give rise to well-defined cavitational transformation strains. Therefore, we have called craze plasticity also dilatational plasticity. Thus, if well managed to avoid fracture in the fibrilated craze matter, crazing can be an attractive mechanism of inelastic deformation and a source of toughness. 

The same mechanism can appear in ABS polymers. Besides the formation of the fibrillated crazes, and depending on the matrix and local stress state, a homogeneous plastic deformation between particles, comparable to the appearance of homogeneous crazes in SAN (12, 13), is also possible (Figure 6). The homogeneous deformation in ABS is associated with cavitation inside the rubber particles. In general, this mechanism precedes the formation of the fibrillated crazes. 

Case a stress-induced formation of fibrillated crazes. The weak rubber particles act as stress concentrators. Crazes are formed starting from the particle-matrix interface around the equatorial region of particles. The voids inside the crazes initiate a stress concentration at the craze tip, which propagates together with the propagating craze therefore, the crazes reproduce the stress state necessary for their propagation. Cavitation inside the rubber particles is not necessary, but it enables a higher stress concentration and easier deformation of the particles. 

Case b stress-induced formation of homogeneous crazes. The stress concentration at the particles causes homogeneous crazes to start at the particle-matrix interfaces. Propagation of these crazes into the matrix is accomplished by an increase of volume, which arises from cavitation inside the particles (the possible mechanism of cavitation inside the originally homogeneous crazes is unlikely). Therefore, these crazes are closely connected to the cavitated rubber particles—they cannot propagate for distances as long as those of the fibrillated crazes—and appear mainly between particles. 

Figure 19. Schematic representation of the three different toughening mechanisms in dispersed systems, where the assumed loading direction is vertical (a) induced formation of fibrillated crazes (i.e., with microvoids in them) at the equatorial zones of rubber particles (b) induced formation of homogeneous crazes at cavitated particles and (c) induced formation of shear deformation between cavitated particles.

If fibrillated crazes (case a) coexist with homogeneous deformation (cases b or c), the homogeneous mechanisms and rubber-particle cavitation precede the formation of crazes. 

Adhering and Wearing Systems The Fibril/Craze Model. We may apply the concepts just described, to an adhering system, either for a thermoplastic polymer adhering to another solid, or for the case where two solids are glued together by a thermoplastic polymer. This requires that we postulate a fracture front progressing either at some distance from the polymer-solid interface, or else at the interface itself. The former corresponds to "cohesive failure" of the polymer, and need not concern us, as it has already been discussed by the senior author (24). 

The crack growth condition of Eq. (9.15) can be used A craze fails when its opening displacement reaches a critical value. Flowever, this does not explain the failure mechanism. It could be by failure of the entanglement network in the craze fibrils. Crazes in some polymers fail at their midplanes, and in other polymers at the bulk-craze interface. For viscoelastic materials, in which both the craze stress and the Young s modulus vary with the strain rate, Eq. (9.19) predicts that the crack tip opening displacement is no longer proportional to the stress intensity factor. Figure 9.11 shows that... 

The selection of the dominant deformation mechanism in the matrix depends not only on the properties of this matrix material but also on the test temperature, strain rate, as well as the size, shape, and internal morphology of the rubber particles (BucknaU 1977, 1997, 2000 Michler 2005 Michler and Balta-Calleja 2012 Michler and Starke 1996). The properties of the matrix material, defined by its chemical structure and composition, determine not rally the type of the local yield zones and plastic deformation mechanisms active but also the critical parameters for toughening. In amorphous polymers which tend to form fibrillated crazes upon deformation, the particle diameter, D, is of primary importance. Several authors postulated that in some other amorphous and semiciystalline polymers with the dominant formation of dUatational shear bands or extensive shear yielding, the other critical parameter can be the interparticle distance (ID) (the thickness of the matrix ligaments between particles) rather than the particle diameter. 

The fibrillar structure of the crazes is t5 ical of PS. Another glassy polymer is styrene-acrylonitrile (SAN) copolymer with a PS content of usually about 74%. In this material the dominant deformation structures are homogeneously deformed zones, but in many cases they coexist with the fibrillated crazes (2,29). Depending on the loading conditions (stress state, loading velocity, temperature), the deformation character can be shifted from one to another. Another typical glassy polymer is PMMA with a t5 ical appearance of homogeneously deformed crazes at room temperature (29). 

Fibrillated crazes are usually thinner than the homogeneous deformation bands. Therefore, the whole content of pol5uneric material, which is plastically deformed, is smaller in the crazes and larger in the shear bands or deformation zones. The result is shown macroscopically in the unusual effect of a decrease of toughness and an embrittlement with increasing temperature. 

If the particle volume content is in average above 10%, a remarkable superposition of the individual particle stress fields appears. The result is a higher stress concentration and the more intense initiation of local matrix yielding in the form of fibrillated crazes, homogeneous crazes, or shear yielding. 

The typical micromechanical behavior of this material at room temperature is shown in Figure 10. The lower magnification in Figure 10a shows a dense pattern of cavitated and elongated rubber particles and plastically deformed matrix material between the particles. The matrix deformation occurs mainly in the form of homogeneous shear deformation zones and a small number of short and relatively thin fibrillated crazes (40). 

Fig. 14. Deformation structures of PP impact-modified with EPDM at different test temperatures (a) RT — cavitated particles as well as adjacent matrix strands are strongly plastically deformed by shear yielding (b) —40°C—coexistence of shear yielding and crazing (c) —40°C — fibrillated crazes between elongated particles (thin sections, deformed and investigated by hvem (a) and tern (b and c)). From Reference 1.

If solvents penetrate a. polymer and weaken the intermolecular bonds, they also ease the formation of crazes. On the one hand, the solvent reduces the surface energy, facilitating the formation of free surfaces needed to initiate and propagate crazes. On the other hand, the reduced bond strength reduces the force needed to draw chain molecules out of the bulk material to form fibrils. Crazes formed by this mechanism may,... 

Deformation zones (also called homogeneous crazes) possess the same orientation as fibrillated crazes, but they do not contain voids, do not show volume increase, and are the result of homogeneous deformation processes with gliding of macro-molecular segments (see Figs. 1.49 and 1.50 in PC in Part 11). 

Ductile behavior can be studied using a modification of Method 2, a bending experiment see Fig. 1.53. A bent specimen is illustrated in Fig. 1.53(a), and a result of deformed rubber-toughened PVC is shown in a TEM micrograph in Fig. 1.53(b). The sample from the loaded area is a chemically stained thin section prepared by ultramicrotomy, showing fibrillated crazes between the acrylic rubber particles (both stained dark). 

Scheme (b) in Fig. 1.6 shows the transition from the precraze into the fibrillated craze in two directions the A direction from above and the B direction in cross section. An increase of the applied load P causes additional strain in the precraze domains, rupture of individual overstressed chain segments, and fibrillation of the polymer strands between the voids, transforming the structure of individual domains and nanovoids (a closed-cell structure) into the structure of fibrils and interconnected voids (an open-cell structure). The average distance of fibrils is, therefore, correlated to the distance between domains of the craze nuclei in the precraze and, consequently, to the mesh distance of the entanglement network in Fig. 1.2. 

The fibrillated crazes grow through the continuous stretching of new material at the craze boundaries surface drawing or pull-out mechanism). The situation at a craze/bulk interface is illustrated in Fig. 1.7. Stretching of the fibrils occurs up to an elongation that depends on the parameters f and d of the entanglement network. The transition zone (active zone g) forms the craze interphase with a characteristic thickness g. 

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