Specimen preparation method microtomy methods

Specimens for AEM should be on the order of 20-100nm thick and should accurately represent the features which are to be analyzed. In general, these requirements are often difficult to achieve simultaneously, and various specimen preparation methods must be used to approach the ideal specimen. For catalyst specimens, three main specimen preparation methods can be used depending on the catalyst material, the form of the catalyst, and the information desired. These are grinding and dispersing, microtomy, and ion-beam thinning. 

The aforementioned methods of specimen preparation, except microtomy, are regarded as an important part of metallography. These methods are also used for non-metallic materials... 

Specimen preparation for TEM generally in-voles the formation of a thin film of the material less than 100 nm thick. The methods used for this preparation depend upon the nature of the polymer and its physical form. In the case of thick or bulk specimens, microtomy is generally used. In the case of solutions, powders or particulates, simpler methods can provide a thin, dispersed form of the material. Three types of simple preparations will be described later in this section dispersion, disintegration and film casting. The more complex methods such as microtomy, replication, etching and staining will be described in other sections of this chapter. 

A major difficulty in plasma and ion etching is that textures, such as steps, cones and holes, can be produced which are artifacts and which do not reveal the microstructure. Of all the methods of specimen preparation, etching is the most prone to such artifacts and thus image interpretation is very difficult. Etching preparations are useful for comparison with structures formed during other specimen preparation processes, especially microtomy. Such complementary studies are essential to the determination of the true polymer structure. 

Table 4.4 includes functional groups and polymers and their respective etchants. Chemical etching, such as with solvents and acids, and ion and plasma etching are conducted in order to reveal selectively structures in polymers that may not be observed directly. In all these methods, interpretation of the structures formed can be more difficult than specimen preparation. Accordingly, the etching methods are best used to complement other methods, such as microtomy, fractography and staining. Controls are essential to any experiment of this type, but, with care, the structures of semicrystalline polymers and polymer blends may be observed. 

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. 

The major structural unit of interest in emulsions, microemulsions, colloids and latexes is the particle. It is well known that the particle shape, size and distribution of a latex controls the properties and end use applications. Many latexes are manufactured with a controlled and sometimes monodisperse distribution of particle sizes. Polymer liquids, in the form of emulsions and adhesives, are wet and sticky, and therefore specimen preparation for electron microscopy is very difficxilt. As a result of the importance of the determination of particle size distribution, microscopy techniques have focused on specimen preparations which do not alter this distribution or which alter it as little as possible. Methods have included special cryotechniques (Section 4.9), staining-fixation methods (Section 4.4), microtomy (Section 4.3) and some simple methods (Section 4.1) such as dropping a solution onto a specimen holder. This section is meant to provide a brief survey of the types of microscopy applications which have been foimd useful in the evaluation of emulsions and latexes. 

Optical, scanning and transmission electron micrographs of a commercial cellulose acetate asymmetric membrane are shown in Fig. 5.26. Each view provides a different perspective on the membrane structure while, together, they give the complete structural model. Specimen preparation for OM and TEM cross sections was by microtomy of embedded membrane strips using a method developed to limit structural collapse (Section 4.3.4). An optical micrograph (Fig. 5.26A)... 

Ultramicrotomy is basically the same type of method as microtomy for preparing soft specimens for light microscopy. However, ultramicrotomy can be used to section a specimen to the 100 nm scale. It is commonly used to prepare polymeric or biological TEM specimens. 

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