Shear deformation zone

Studies of PMMA-based ionomers also demonstrate the influence of thermal treatment on deformation modes (16). For Na salts of PMMA-based ionomers of 6 and 12 mol% that were cast from DMF, only crazes were observed on straining. However, after an additional heat treatment (48 h at 160°C), which also removes any DMF solvent that is present, shear deformation zones are induced. Hence, the ionic cluster phase, which was destroyed by the polar solvent, has been restored by the heat treatment. 

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... 

The combined effects of a divalent Ca counterion and thermal treatment can be seen from studies of PMMA-based ionomers [16]. In thin films of Ca-salts of this ionomer cast from methylene chloride, and having an ion content of only 0.8 mol%, the only observed deformation was a series of long, localized crazes, similar to those seen in the PMMA homopolymer. When the ionomer samples were subject to an additional heat treatment (8 h at 100°C), the induced crazes were shorter in length and shear deformation zones were present. This behavior implies that the heat treatment enhanced the formation of ionic aggregates and increased the entanglement strand density. The deformation pattern attained is rather similar to that of Na salts having an ion content of about 6 mol% hence, substitution of divalent Ca for monovalent Na permits comparable deformation modes, including some shear, to be obtained at much lower ion contents. 

Depending on the polymer chemical structure and MW and on the deformation conditions (temperature and strain rate), two types of deformation heterogeneities are observed crazes and shear deformation zones. 

For these reasons, PMMA and its maleimide and glutarimide copolymers represent very suitable materials for investigating the effect of the chemical structure and of the solid state molecular motions on the plastic deformation, the occurrence of the various micro-mechanisms of deformation (chain scission crazes, shear deformation zones, chain disentanglement crazes), as well as the fracture behaviour. 

In order to understand fracture behaviour, it is important to analyse the types of deformation micromechanisms undergone under strain chain scission craze (CSC), shear deformation zone (SDZ), chain disentanglement craze (CDC) and the temperature range over which each one occurs. Furthermore, it is worth wondering whether these micromechanisms are related to /i transition motions. 

Fig. 29 Typical example of shear deformation zone. Thin film of PMMA at 53 °C and a strain rate of 2 x 10-3 s 1...

Shear deformation zones appear in all the CMIMx copolymers, except CMIM25, above a given temperature, denoted T 2. These SDZs are similar to... 

It illustrates, in the case of a bulk fracture, the way crazes and shear deformation zones interact... 

For the higher MW BPA-PC(31), for which an increase of toughness with increasing temperature is observed, an additional mechanism is involved a contribution of shear deformation zones (SDZ). Indeed, as shown from defer-... 

Fig. 17. TEM image of a plane stress shear deformation zone in a thin film of PPO. From Ref. courtesy Butterworth and Co., Ltd.

Several means of doing this have been proposed. One method is by copolymerization. For example, Donald and Kramer have shown that if styrene is polymerized with increasing amounts of acrylonitrile (AN), the 1 values reduce from about 400 A for PS to 270 A in PSAN (24% AN) and to 180 A in PSAN (66% AN) and, in tests made on thin films, increasing evidence of shear deformation zones was found as the AN content increased. It has also been noted, in tests carried out in our laboratory... 

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... 

Characteristic stress associated with shear deformation zone Yield stress... 

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