Specimen preparation method etching

Different specimen types yield a range of results upon ion or plasma etching. Multiphase polymers generally etch differentially, enhancing the contrast. Melt crystallized polymers can be etched to reveal the spherulites. Surface protuberances and particulate fillers can and do form cones or ridges when etched. Oriented semicrystalline polymers, on the other hand, appear to be the most controversial with respect to the resulting surface textures. Clearly, in such cases the specimen should be prepared by other methods for comparison, and control experiments are essential. There are problems in the industrial laboratory that can be solved, in part, by microscopy of surfaces prepared by etching techniques however, these are far fewer than those addressed by other specimen preparation methods. 

Specimen polishing, for example, using successively finer grades of diamond paste, provides an alternative preparation method for surface examination. Filler particles may be subsequently etched, however, this can complicate interpretation of the image produced, since air voids contained in the structure may be indistinguishable from extracted filler. 

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. 

The formation of micelles, or colloidal particles, by block copolymers in organic solvents has been described and reviewed by Price [2881. The molecular weight of polystyrene was estimated from specimens prepared by spraying and evaporation for TEM. Freeze etching a drop of solution rapidly frozen with liquid nitrogen [289] was described (Section 4.9.4) where the solvent was allowed to evaporate and a replica produced of the fracture surface. Another method [290]... 

As shown in Table 5, in the mode I test, the thicknesses of the residual adhesive layer on the failure surfaces were about 250 xm for all the specimens with different surface preparations, which indicated that the failures all occurred in the middle of the adhesive layer in the test regardless of the surface preparation method since the total thickness of the adhesive of the specimens was 0.5 mm. When the phase angle increased as in the asymmetric DCB test with h/H = 0.75, which contains 3% of mode II fracture component, a layer of epoxy film with a thickness of around SO xm was detected on the failure surfaces of all the specimens. Although the failure was still cohesive, the decrease in the film thickness on the metal side of the failure surfaces indicated that the locus of failure shifted toward the interface due to the increase in the mode mixity. On the other hand, because the failure was still cohesive, no significant effect of interface properties on the locus of failure was observed. When the mode mixity increased to 14% as in the asymmetric DCB test with h/H = 0.5, where the mode mixity strongly forced the crack toward the interface, the effect of interface properties on the locus of failure became pronounced. In the specimen with adherends prepared with acetone wipe, a 4-nm-thick epoxy film was detected on the failure surfaces in the specimen with adherends treated with base/acid etch, the film thickness was 12 nm and in the P2 etched specimen, a visible layer of film, which was estimated to be about 100 nm, was observed on the failure surfaces. This increasing trend in the measured film thickness from the failure surfaces suggested that the advanced surface preparation methods enhance adhesion and displace failure from the interface, which also confirmed the indications obtained from the XPS analyses. In the ENF test, a similar trend in the variation of film thickness was observed. 

The purpose of the sample preparation is to obtain a contrast-rich representation of the True structure. If contrast is not naturally present in the specimen, it may be created by staining, i.e. adding a substance with a characteristic colour or high (electron) density selectively to a specific phase. The contrast may also be achieved by selective etching, leaving a topography indicative of the phase structure. It is possible, however, that the preparation method used may produce artificial structures. 

---