Fibrils shear deformation

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... 

In a subsequent investigation, with Roos and Kampschreur (1989), Northolt extended the modified series model to include viscoelasticity. For that an additional assumption was made, viz. that the relaxation process is confined solely to shear deformation of adjacent chains. The modified series model maybe applied to well-oriented fibres having a small plastic deformation (or set). In particular it explains the part of the tensile curve beyond the yield stress in which the orientation process of the fibrils takes place. The main factor governing this process is the modulus for shear, gd, between adjacent chains. At high deformation frequencies yd attains its maximum value, ydo at lower frequencies or longer times the viscoelasticity lowers the value of gd, and it becomes a function of time or frequency. Northolt s relations, that directly follow from his theoretical model for well-oriented fibres, are in perfect agreement with the experimental data if acceptable values for the elastic parameters are substituted. 

As entanglement density increases, the craze stress rises, shear deformation becomes more favorable, the extension ratio of craze fibrils and of shear deformation zones reduce, and average fatigue life and resistance to fatigue crack propagation are enhanced. 

In thin film studies, however, Kramer and co-workers have observed the reverse to be true, that crazing is preferred at higher temperatures, while shear deformation zones are observed at lower tefnperatures. This diametrically opposite behavior may have its origins in the different type of craze fibril formation that may occur in the bulk vs in the thin films. Preliminary observations indicate that chain scission may-dominate in bulk craze fibril formation, leading to weaker crazes at lower temperatures, whereas disentanglement processes may dominate in the thin films, leading to stronger crazes at lower temperatures... 

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.

Figure 5. Schematic model of shear deformation of fibrils (a and bj and the concurrent unfolding of interfibrillar tie molecules (c and d)...

Basically the same reasoning applies to the shear deformation of fibrils. The much longer microfibrils are less easily displaced in a shear mode. Hence, the increase in the fraction of interfibrillar tie molecules per amorphous layer is substantially smaller in linear polyethylene than in nylon 6. At a total draw ratio of 15-20, the total fraction of taut tie molecules seems to reach saturation, as is demonstrated by nitric acid treatment of highly drawn polyethylene fibers (23). 

As a consequence of shear deformation of fibrils during plastic deformation of fibrous structure one obtains a large fraction of interfibrillar tie molecules connecting adjacent microfibrils. The effect is larger the smaller the draw ratio X at which the transformation of spherulitic into fibrous structure is completed because the length of the fibrils is proportional to A and the lateral dimensions to The almost fully... 

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). 

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