Structure craze microstructure

As the craze microstructure is intrinsically discrete rather than continuous, the connection between the variables in the cohesive surface model and molecular characteristics, such as molecular weight, entanglement density or, in more general terms, molecular mobility, is expected to emerge from discrete analyses like the spring network model in [52,53] or from molecular dynamics as in [49,50]. Such a connection is currently under development between the critical craze thickness and the characteristics of the fibril structure, and similar developments are expected for the description of the craze kinetics on the basis of molecular dynamics calculations. 

In Eq. (10) that gives the toughness in dilatational plasticity the factors C and A are dependent on craze microstructure and will not vary significantly. The stress and temperature dependence of the craze velocity while quite determinate in the interface convolution process of craze matter production will also be quite sensitive to micro-structural detail of phase distribution in block copolymers. The appUed stress = Y ... 

For ionomer samples with low ion. content (less than 5 mol %), only crazes are formed. Figure 24 shows a typical TEM picture of a craze in a deformed thin film of an ionomer with low ion content. This can be compared with the craze structure of starting PS (Fig. 12b). Also, in Fig. 25 two views of the craze microstructure in PS (Fig. 25a and b) are compared with corresponding views (Fig. 25c and d) of the craze structure of the ionomer containing 4.8 mol % ion content. These micrographs show typical structural features of crazes of glassy polymers a) a midrib of lower fibril... 

In analogy to crazing in amorphous glassy polymers, the above difference in ceaze structure between these crystalline polymers may be attributed to molecular structure, e.g., chain entanglements etc. To date, there have been no studies on the craze microstructure of crystalline polymers in relation to their chain entanglements although this is now well understood in amorphous polymers. ss.tq)... 

Recently, Dettenmaier and Kausch have observed an intrinsic craze phenomenon in bisphenol-A polycarbonate (PC), drawn to high stresses and strains in a temperature region close to the glass transition temperature, T. This type of crazing is not only initiated under extremely well defined conditions which reflect specific intrinsic properties of the polymer but also produces numerous crazes of a very regular fibrillar structure. These crazes were called crazes II in order to distinguish them from the extrinsic type of craze, called craze I. As shown by the schematic representation in Figure 1, a detailed quantitative analysis of intrinsic crazes in terms of craze initiation and microstructure was possible. The basis of this analysis and the results obtained are reviewed in this article. 

Much attention has been focused on the microstructure of crazes in PC 102,105 -112) in order to understand basic craze mechanisms such as craze initiation, growth and break down. Crazes I in PC, which are frequently produced in the presence of crazing agents, consist of approximately 50% voids and 50% fibrils, with fibril diameters generally in the range of 20-50 nm. Since the plastic deformation of virtually undeformed matrix material into the fibrillar craze structure occurs at approximately constant volume, the extension ratio of craze I fibrils, Xf , is given by... 

The structural analysis of intrinsic crazes in PC has been carried out by SAXS. The fibrillar microstructure of these crazes gives rise to pronounced scattering effects which enable a detailed analysis of the craze structure in terms of both the voliune fraction of craze fibrils and of the fibril diameter. This analysis showed that the microstructure of intrinsic and extrinsic crazes is largely different. There exists some evidence that the distinct microstructure primarily reflects the different stress-strain states of the matrix at craze initiation. Further investigations are necessary to answer... 

Looking at the structure of these crack tip plastic zones in more detail, it is found that the individual crazes are less straight compared to the low temperature crazes (Fig. 21). This indicates a more pronounced influence of the crystalline microstructure on craze formation. Figure 21a and b demonstrate for fine spherulitic, highly isotactic PP the interaction between the crazes and the microstructural features. Most of the... 

The contour of the loaded and unloaded craze was investigated by interference optics (Doll Kdnczdl, 1990 Kdnczdl et al, 1990). Additionally the structure of the crazes was investigated by high-voltage TEM. Sections of about 1 pm were microtomed for TEM observations. Structural inspection of the craze zone was done by TEM. The fibrillar structure of a craze zone in SAN is shown in Fig. 3.14. The microstructure of this craze created in a CT sample is similar to the structure investigated in situ in strained semithin sections (Michler, 1990). 

An analogous microstructure can be produced in amorphous polymers by crazing them. Figure 2b shows the similarity of the craze structure to the crystalline morphology. We have reported that extensively crazed high-impact polystyrene (HIPS) exhibits hard elastic behavior, as the loading cycle illustrated in Figure 3 shows [10,11]. Hence, hard elastic behavior is associated with a bulk-microfibril composite structure... 

The microstructure observed for thick films shows fibrils, about 4-10 nm in diameter for polystyrene, in agreement with SAXS measurements on the crazes in the bulk polymer. Very thin films of polystyrene (100 nm) show modification in the craze structure as there is no plastic restraint normal to the film [397]. Deformation zones have also been studied in polycarbonate, polystyrene-acrylonitrile and other polymers [398]. Crazes in thermosets can be studied in thin films spun onto NaCl substrates which can be washed away when the film has been cured. Mass thickness measurements are difficult to make in radiation sensitive materials that is why most TEM work has been done on polystyrene and least on PMMA. After developing the techniques described above for TEM Donald and Kramer [398] applied similar methods in optical microscopy to study radiation sensitive materials and the kinetics and growth of deformation zones. Thin films were strained on grids in situ in a reflecting OM. Change of interference color, which depends on the film thickness, was a very sensitive method for observing film deformation. 

Microstructure of crazes in thin films by transmission electron microscope. In thin PS films e (craze initiation) =1% Fibrils of 25-50 nm are a common feature of the microstructure of crazes in cast and bulky films. Major fibrils diameter = 20-30 nm. Minor fibrils connecting major fibrils have a diameter >10 nm and they tend to orient normally to major fibrils. At large total deformation at low e, there is a gradual transition from coarse to fine micro structure of crazes. Beahan, Bevis Hull (1151 PS... 

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