Craze formation

As with block copolymers, the important parameters are the surface density and length of the copolymer chains. Toughening of the interface may occurs as a result of pull-out or scission of the connector chains, or of fibril or craze formation in matrix. This last mechanism gives the highest fracture toughness, F, and tends to occur at high surface density of chains. 

Induction Period to Number of days to Carbonyl Formation (h) craze formation... 

Table 1 Important parameters of materials used to study craze formation in creep (from [54])...

The treated phenomena and investigations summarized above are at the centre of the topic of this special two-volume edition of the Advances in Polymer Science on Intrinsic molecular mobility and toughness of polymers . However, little has been said about the mutual relations between molecular configuration, polymer morphology, nature, intensity and cooperativity of sub-Tg relaxation mechanisms, and mode of solicitation on the one hand and their effect on disentanglement, craze formation, yield behaviour and ultimate strength on the other. These subjects will be critically evaluated in the subsequent presentations of this Volume. 

It is worth pointing out that the most reliable stress-strain curves are obtained by uniaxial compression or shear, in order to avoid the craze formation that can occur in tensile measurements. 

In the case of BPA-PC, the crazes occurring at high temperatures and showing a MW dependence have led the authors [57] to introduce the mechanism of craze formation by chain disentanglement (CDC). 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

Antioxidant Induction period at 150 °C (h) Days to craze formation... 

Detection of crazes is very difficult yet craze formation should be avoided in critical applications (such as gas pipes). In order to achieve that, one uses the expression critical strain , which is based on the observation that, as long as a certain level of strain is not exceeded, no crazes are formed. It would be convenient if this critical strain would be independent of the applied stress this is, unfortunately, not the case, as illustrated in the bundle of isochrones in Figure 7.23... 

At higher strains, shear zones become broader, become connected with each other and finally cover the whole visible part of the sample, and subsequent deformation proceeds in a homogeneous manner on the level of dimensions of optical microscopy. No craze formation has been recorded. 

FIG. 13.80 Craze formation in the direction perpendicular to the principal stress. 

Electron microscope studies have shown that the toughness of ABS polymers is caused largely by multiple craze formation (1,2). The rubber particles appear both to initiate and to control craze formation, so that impact energy is dissipated in the production of numerous small crazes (3). However, this theory does not exclude the possibility of contributions from other mechanisms. The observation that many ABS polymers tend to neck during a tensile test suggests that shear mechanisms are also significant. 

The first quantitative study of deformation mechanisms in ABS polymers was made by Bucknall and Drinkwater, who used accurate exten-someters to make simultaneous measurements of longitudinal and lateral strains during tensile creep tests (4). Volume strains calculated from these data were used to determine the extent of craze formation, and lateral strains were used to follow shear processes. Thus the tensile deformation was analyzed in terms of the two mechanisms, and the kinetics of each mechanism were studied separately. Bucknall and Drinkwater showed that both crazing and shear processes contribute significantly to the creep of Cycolac T—an ABS emulsion polymer—at room temperature and at relatively low stresses and strain rates. 

The profound effect of water on tree growth that is so widely reported may be expected with the electrokinetic model on the basis of three principal effects. Water filling the crazes as they develop will help prevent their collapse. Water, as a good solvent for ionic species, will be an excellent medium to facilitate entry of surface-active agents, which, by a process similar to that of environmental stress cracking, will advance the void and craze formation caused by the electric field. Water, with its high relative permittivity, will distort and locally enhance electric fields in the neighbourhood of the voids and crazes where it accumulates. Whether one or other of these effects dominate in a particular situation depends on the exact nature of the specimen and its environment. 

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