Transmission electron crazes

Processes that occur at a size scale larger than the individual chain have been studied using microscopy, mainly transmission electron microscopy (TEM), but optical microscopy has been useful to examine craze shapes. The knowledge of the crazing process obtained by TEM has been ably summarised by Kramer and will not be repeated here [2,3]. At an interface between two polymers a craze often forms within one of the materials, typically the one with lower crazing stress. 

As an indication of the changes in deformation modes that can be produced in ionomers by increase of ion content, consider poly(styrene-co-sodium methacrylate). In ionomers of low ion content, the only observed deformation mode in strained thin films cast from tetra hydrofuran (THF), a nonpolar solvent, is localized crazing. But for ion contents near to or above the critical value of about 6 mol%, both crazing and shear deformation bands have been observed. This is demonstrated in the transmission electron microscope (TEM) scan of Fig. 3 for an ionomer of 8.2 mol% ion content. Somewhat similar deformation patterns have also been observed in a Na-SPS ionomer having an ion content of 7.5 mol%. Clearly, in both of these ionomers, the presence of a... 

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

Transmission electron microscopy (TEM) and birefringence studies of strained and/ or fractured epoxies have revealed more direct experimental evidence that molecular flow can occur in these glasses. Films of DGEBA-DETA ( 11 wt.- % DETA) epoxies, 1 pm thick, were strained directly in the electron microscope and the deformation processes were observed in bright-field TEM 73 110). Coarse craze fibrils yielded in-homogeneously by a process that involved the movement of indeformable 6-9 tan diameter, highly crosslinked molecular domains past one another. The material between such domains yielded and became thinner as plastic flow occurred. 

Fig. 9. Composite transmission electron micrograph of a typical craze formed in polystyrene 26)...

Descriptions of craze thickening are based on the observed crazes at the tip of a stationary crack for creep tests [29,30] and on observations of crazes in thin films by transmission electron microscopy (TEM) or small-angle X-ray scattering (SAXS) [31,32],... 

Transmission electron micrographs of HIPS (a) narrow specimen (W/b = 2) (b) wide specimen (W/b — 14). Stretching direction is perpendicular to the crazes. 

Figure 5. Transmission electron micrograph (21,600X) of craze material produced by ballistic impact in American Cyanamid material with 16% rubber...

The ESCR performance of a resin is not easily modeled. A laboratory technique for the preparation of thin films of HIPS materials for the study of deformation processes by microscopy allows the deformation process to be better understood. The transmission electron microscope (TEM) allows direct visualization of the crazes themselves in thin films. For good contrast between the crazes and the bulk polystyrene, thin, cast films from 0.5 to 2 p,m are required, and also staining of the rubber phase with a heavy atomic species to provide contrast between the rubber and the polystyrene. Another intricacy of this method requires a solution of the HIPS material in a 65 35 methyl ethyl ketone-toluene solution to prevent significant swelling of the rubber particles during the preparation process. 

Stereo-transmission electron microscopy of craze tips has shown that the meniscus instability is the operative craze tip advance mechanism in a wide variety of glassy polymers Figure 4 shows a craze tip in a thin film of a styrene-acrylonitrile copolymer (PSAN). The void fingers are clearly visible. No isolated voids can be... 

A transmission electron micrograph of a craze in a thin film of poly(styrene-acrylo-nitrile), shown in Fig. 1 a, will serve to introduce the principal microstructural features of crazes. The direction of the tensile stress is marked and it can be seen that the craze grows with the primary direction of its fibrils parallel to this tensUe stress and with the interfaces between the craze and the nearly undeformed polymer matrix normal to the stress. Since the overwhelming portion of the experiments to be reviewed here rely on the use of thin film deformation and transmission electron microscopy techniques, a brief review of the general methods of these experiments is in order. 

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... 

The transmission electron microscope (TEM) image from crazes in such films, the negative from which Fig. la was printed, can be analyzed not only to reveal dimensions of the craze but also to yield the local craze fibril volume fraction v by using a microdensitometer to measure the local density of the image, its mass thickness contrast The extension ratio of the craze fibrils, X equals i. ... 

More direct evidence of the chain scission is difficult to obtain. Mills and Donald have showed that crazes in PS and other polymers can be weakly stainal and the stain detected by transmission electron microscopy on healed crazes by any reagents (e.g., OSO4, Br2, I2, Hg(CH3COO)2) which will react with double bonds. 

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. 

Fig. 4a d. Transmission electron micrographs of craze structure formed by the PB phase cavitation ntechanism in several dibtocks a) SBS h) SB6 e) SB8 d) SBIO... 

Transmission electron microscopy of deformed SB diblocks with spherical PB domains showed clear evidence for PB sphere cavitation in the crazed zones In fact, in contr t to the thick crazes of the cylindrical morphology materials, the crazes in the spherical morphology were exceedingly thin, approaching the dimension of the PB sphere diameter. Because of the simplifications offered by the isotropic nature of the spherical morphology, it was possible to develop a precise model for the rate of growth of crazes for this system. 

Fig. 18a, h. Transmission electron micrographs of crazed specimens 5 pm, of ) large partide sample b) ----—small panicle sample... 

The microstructure and micromechanics of isolated crazes have drawn the attention of many investigators (see, for example, the review of Kramer Berger (1990)), probably because of the similarity in shape between crazes and cracks. Kramer Berger (1990) have observed craze microstructure by transmission electron microscopy (TEM) and analysed the stress-strain relationship in and around a craze on the basis of TEM images. They adopted a skilful technique to strain a copper grid coated with a thin film of a polymer to form crazes in the specimen chamber of... 

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