Mechanism of craze formation

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). 

Residual stresses and molecular orientation play an important role in the toughness enhancement of cold-worked plastics, because toughness is primarily based on the mechanics of craze formation and shear band (crazes and flaws) formation. The shear bands determine the fracture mode and toughness of a polymer when subjected to impact loads. The amount of energy dissipated will depend on whether the material surrounding the... 

In principle, it should be possible to extend the adiabatic and isothermal theories that we have developed to polymers in which crazes form. The relevance of crazes to adhesion was suggested by Kambour, in the 1960s, at a Gordon Conference he subsequently published the material that he had presented.(lO) The mechanism of craze formation in bulk polymers has not yet been completely elucidated but it is worthwhile, if speculative, to extend the concepts that have been reported in crazing studies to adhering systems. 

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. 

Shear bands have been observed in polypropylene under compression at low temperature, which is the first observation of this mechanism of deformation in such a highly crystalline polymer. There are previous reports of craze formation in poi)q)ropylene, but not shear bands which are normally associated with glassy polymers. Two discrete sets of shears band at 35° to 42° to the compression axis have beat seen. It appears to be associated with sliding between lamellae and along spherulite boundaries. 

The model has also been found to work well in describing the mechanics of the interface between the semicrystalline polymers polyamide 6 and polypropylene coupled by the in-situ formation of a diblock copolymer at the interface. The toughness in this system was found to vary as E- where E was measured after the sample was fractured (see Fig. 8). The model probably applied to this system because the failure occurred by the formation and breakdown of a primary craze in the polypropylene [14],... 

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. 

Figure 8 shows the SEM images with a low level of strain (50%). It is clear that even with a low-strain level defects are initiated in the sulfur cured system with the formation of large cracks at the boundary layer between the two phases. However, in the peroxide cured system the mechanism of crack initiation is very different. In the latter case the NR-LDPE interface is not the site for crack initiation. In this case, stress due to externally applied strains is distributed throughout the matrix by formation of fine crazes. Furthermore, such crazes are developed in the continuous rubber matrix in a direction... 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

The most desirable property of polycarbonates is their high ductility on impact, relative to other engineering polymers in the unmodified state. There is no consensus on the mechanism of ductility researchers continue to explore this behavior through molecular dynamics studies of chain segment motion during the formation of crazes and propagation of the failure. 

In solid polymers, the energy of impact is dissipated by the formation of crazes or shear bands in the matrix5. Both mechanisms are enhanced by the addition of a second elastomeric phase to the rigid polymer, but not altered in principle. 

The formation of voids in the rubbery phase in HIPS influences its mechanical properties. The formation of voids is believed to facilitate the energy dissipating deformation processes, i.e., crazing and shearing. Crazing and shearing are facilitated under conditions in that the rubber particles can easily cavitate. 

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. 

---