Sample Preparation Etching and Staining

For solution-grown very thin crystals direct examination is possible. However, for bulk materials either thin sections have to be created with the use of a low-temperature microtome or internal surfaces can be exposed through fracture of the material. In both cases the process of sample preparation can induce damage and great care has to be taken that the features identified are not artefacts of the sample preparation method. To enhance the contrast it is sometimes appropriate to etch the material. 

The mildest form of treatment involves solvent etching, which relies on low melting point being more easily dissolved than the bulk of the material. In crystalline materials phase segregation can occur and hence this process can be very useful in outlining spherulite boundary layers and similar vulnerable areas. 

The solvent can be used as part of a polishing process and the creation of smooth surfaces reveals more clearly the underlying morphological features. 

There are essentially two main etching techniques vapour etching for only a few seconds and controlled isothermal treatment with a liquid solvent for a considerably longer time (several hours). A solvent etching temperature (Td) is selected on the basis of the melting point of the segregated species (Tm)  

An alternative method is to use mixtures of concentrated sulfuric acid and potassium permanganate, giving contrast between crystals and amorphous domain that turns out to be applicable to many different polymers, e.g. polyethylene, polypropylene, poly(but-l-ene), polystyrene and poly(aryl ether ketone)s. The strong etchant degrades the amorphous phase more quickly than the crystals and the resulting topography is revealed by heavy metal shadowing. Replicates are prepared which are examined in the electron microscope. 

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