Specimen preparation method fracture studies

Cellulose nitrate and cellulose acetate (CA) were among the first asymmetric, reverse osmosis membranes to be produced [121]. Plummer et al. [122] described 13 specimen preparation methods for observation of CA membrane structures. They pointed out the lack of contrast in epoxy embedded sections and that one of the best stains, osmium tetroxide, reacts with the polymer. Freeze fractured membranes were found by these authors to be of questionable value. In our experience, if care is taken, SEM study of fractured membranes can provide an informative view of the structure even though some structures collapse, and their sizes cannot be accurately determined. A method found acceptable was ultrathin sectioning of gelatin embedded wet membranes (TEM). The structure of CA membranes was shown by replication [123] and SEM [124]. 

In literature the following methods of preparation of polymer material specimen and studying their morphology are described methods of ultrathin section and films with contrasting of osmium tetroxide (10.11). method of replication of the brittle fractur-ed surface (11.12) oxygen and chemical etch of the polished surface or the fractured surface with the following replication for electron microscopy (11. 

Manual methods of freeze fracture are often useful in providing specimens for study in the SEM. An example of a freeze shattering method was described by Stoffer and Bone [406] for comparison with microtomy results. Polymers immersed in liquid nitrogen were mechanically shattered with a hammer, mounted, vacuum pumped and sputter coated for observation. This simple method is useful if the materials cannot be sectioned. However, fine structural details are not conclusive when specimens are prepared by such methods. 

Several polymer studies have been reported where the specimens were prepared by freeze fracture techniques. A modification of the freeze fracture method was used by Singleton et ah [432] in the preparation of plasticized PVC. The sample was notched, cooled and fractured and then immediately replicated with platinum-carbon. Replicas were stripped after warming to room temperature. The authors noted that the preparation was not highly reproducible, perhaps due to nonuniform cooling of large specimens. The results of such a study must be compared with other characterizations for accurate analysis. 

Several polymer studies have been reported where the specimens were prepared by freeze fracture techniques. A modification of the freeze fracture method was used by Singleton... 

Six HDPE samples were evaluated in this study. Two samples (HDPE-1 and HDPE-2), which have significantly different ESCR properties, were selected for fiirther investigation of the fracture surface. Specimens were prepared for the ESCR bent strip test as described in ASTM D-1693 Type B method. The specimen thickness was 1.905 mm (75 mils) and notch length 20 mm. The bath temperature was 50 C and the IGEPAL solution concentration was 10%. Fundamental properties of the different samples are given in Table 1... 

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