Crazing, microstructural

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. 

Fig. 14a—c. Craze microstructure in a a thick film (1.2 nm), b a moderately thick film (0.45 lun) and c a very thin film (0.1 tun). From Ref.courtesy Chapman and Hall... 

Craze Microstructure as Revealed by Small-Angle X-Ray Scattering. 84... 

Normally the craze microstructure is not directly visible in the scanning electron microscope. Thus, an etching procedure using oxygen ions was employed to remove the plastically deformed layer at the sample surface. Control measurements on uncrazed samples showed that this procedure does not lead to artifacts. The surfaces were coated with gold to reduce surface charging. 

SAXS has proved to be a very powerful tool for a quantitative analysis of the craze microstructure. This is not surprising since characteristic craze parameters such as the fibril diameters and the interfibrillar spacings frequently fall into the range of 1-5 X 10 nm covered by SAXS. The general theory of SAXS has extensively been treated in the literature (see e.g. Some basic elements of the SAXS theory,... 

In the above Equations x the total area created by the intersection of a plane perpendicular to the cylinder axis and passing through the origin. denotes the interfacial length of the two dimensional cells in this plane. Crazes may be modeled by a system of parallel cylinders. For a realistic description of the craze microstructure, a distribution in fibril diameter, D, must be taken into account. This yields... 

The objective of the following part of this section is to assess the effect of different parameters on the craze microstructure. In these studies the SAXS curve of each sample has been subjected to the data analysis outlined above. 

The craze microstructure of samples stretched to different extension ratios X > X" has been investigated in Figure 19. It may easily be shown that the observed decrease of D with X cannot be attributed to fibril creep. Otherwise, the relation... 

To discuss the effect of other parameters on the craze microstructure an arbitrary reference state with respect to X must be defined. This has been done by taking X = 1.05).". 

The above results suggest the usefulness of a systematic analysis of the craze microstructure by means of light-scattering. In fact, this method could provide valuable information which is not available by SAXS. For instance, the optical anisotropy of the craze fibrils may be derived from the depolarized light-scattering component. 

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 ... 

Studies of craze microstructure and the surrounding displacements of crazes have established that the only parts around a craze that undergo plastic deformation are concentrated into a process zone at the tip of the craze, and into a fringing layer all around the entire craze body. In the process zone craze matter is generated by one of the two processes discussed above, and fibrils are necked down to the final extension ratio. In the fringing layer, additions are made to craze fibrils by drawing polymer out of half space. Outside the idetifiable parts of a craze, the solid polymer remains entirely elastic while inside the craze body the fully drawn fibers carry the required craze tractions purely elastically in their orientation hardened state at the... 

Fig. 14a and b. Crazes in KRO-1 Resin a general distribution of well defined crazes in the KRO-1 microstructure b detailed view of craze microstructure consisting of drawn tufts of PS incorporating some drawn PB. (From Argon et al. courtesy of J. Wiley and Sons)... 

Figure 20 shows a typical craze microstructure in these strained di-block copolymers, that for PS 600/PB 256. Examination revealed that the scale of the craze micro-... 

Fig. la. Brighl-field transmission electron micrograph (TEM) of typical craze microstructure in polytstyrenc-acrylonitriie) PSAN- b Low angle electron diffraction pattern from the fibrils of the craze in a. Note the main-fibril axis lies primarily along s,. the tensile axis direction and the direction norma] to the craze-bulk polymer interface... 

Suppose now that there is a wide spectrum of craze microstructures with different values of the fibril spacing D,. For the crazes with very small and very large Dq the craze interface velocity from Eq. (9) is miniscule, since for these crazes Vctq is small. Clearly there will be a value of D which maximizes Wq, and hence v, and this value is given by... 

To examine craze microstructure, and to study the effect of molecular variables on craze morphology, the method described by Kramer was followed. Samples of polymers were cast in the form of thin films, strained in tension while bonded to carbon-coated grids, and examined in the transmission electron microscope either before or after staining. The TEM observations were made with an Hitachi HU-11 A unit or with a JEOL JEM-IOOCX unit, operating usually at 75-80 kV. Fracture surfaces of many bulk samples were coated with a thin layer of gold-palladium and examined by an Etec scanning electron microscope. 

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