Osmium tetroxide staining technique

Electron Microscopy The sample preparation was based on Kato s (10) osmium tetroxide staining technique and a two-step sectioning method. The specimens were exposed to O5O4 vapor and cut with a LKB ultratome III to get a 0.1 y slice. The electron micrographs were taken with an AEI 6B and a Phillips 300 transmitting electron microscope with a magnification of 95,000. 

Kato s osmium tetroxide staining technique was employed, which attacks the double bonds of the triglyceride oils. Specimens with cross sections of 0.2 x 0.2 mm and lengths ranging from two to ten... 

The simplest method of polyblending involves equipment such as rolls or extruders, which can effect the mechanical blending of the two polymeric components in the molten state (Matsuo, 1968). High-impact polystyrene (HiPS) is an important example of a polyblend made by this technique. Such materials commonly contain 5-20 % of rubber, usually polybutadiene, dispersed in a polystyrene matrix. As shown in Figure 3.1, electron microscopy studies on specimens stained with osmium tetroxide reveal well-defined, irregular rubber particles (1-10 fim in diameter) dispersed in the polystyrene matrix. The elastomer domains appear dark because the osmium tetroxide stains the elastomer preferentially (see Section 2.4). 

X-ray microanalysis and BEI provide information regarding the elemental composition of dispersed phase particles and also of the contaminants responsible for failure. These techniques are useful for the study of multiphase polymers, if one of the phases contains an element not contained by the other. For example. Price et al. [409] used SEM/EDS to study the interface between two polymers, one of which was PVC, to measure the local composition during interdiffusion. Failure of elastomer compounds used in tank track pads, natural rubber, SBR, EPDM, and 50/50 SBR/ BR was studied as a function of elastomer cure and degradation in service [410]. Andrade et al. [411] used BEI of osmium tetroxide stained PS/ PB to provide images that showed the dispersed and stained PB as a result of atomic number... 

The Smith technique consisted of fixing the smear in osmium tetroxide vapor, immersion in HCI, mordanting in dilute formaldehyde, and staining with aqueous basic fuchsin. The method was said to possess certain advantages over the procedure of Robinow. 

The electron micrographs of the various ERL-4221-CTBN systems were prepared with the osmium tetroxide technique (3). The castings were stained by reaction with osmium tetroxide vapors for 24 hours. Ultrathin specimens, approximately 1000 A thick, were cut with the Reichert OMU2, ultramicrotome equipped with a diamond knife and were stained again for one hour. The osmium tetroxide selectivity stained the rubber phase, while the epoxy remained unaffected, revealing many structural features in the polyphase systems with excellent contrast and... 

The technique was demonstrated with gap junctions of connexin43-tetracysteine that had been pulse chased with FlAsH and ReAsH [11] as described above in Section 8.1.4.2.1. Fixation of the cells and strong illumination of ReAsH in the presence of DAB and 02 gave localized deposition of precipitates that could be stained with osmium tetroxide. Electron microscopy revealed electron dense material only at cell regions that had been stained with ReAsH (Fig. 8.1-5). Higher magnification of cross sections of gap junction plaques showed structures with appropriate dimensions of individual connexons. Photoconverted vesicles of characteristic size trafficking to or from the plaque could also be visualized. 

Several methods of studies have been developed. Osmium-staining technique, pioneered by Kato [1967], is one of the most successful methods for observing crazing in rubber toughened plastics. It depends upon a reaction between osmium tetroxide, OsO, and double bonds in PBD and other unsaturated polymers. However, it is not suitable for saturated rubbers. 

The staining technique for TEM uses a material that absorbs electrons and preferentially attaches itself to or reacts with certain regions of the polymer rather than other regions. Materials frequently used are uranyl acetate and osmium tetroxide. For polyethylene the technique of chloro-sulphonation can be used. In this method, which involves immersing the sample in chlorosulphonic acid, the electron-absorbing material becomes attached to lamellar surfaces, so that lamellae (see section 3.4.2) with their planes parallel to the direction of the electron beam become outlined in black in the micrographs. 

The intracellular osmiophilic area of the flavonoid gland can be correlated to flavonoids because of their heavy staining after osmium tetroxide fixation. However, at some stages of the synthetic process, flavonoids are not osmiophilic. Hence, other electron microscopic techniques for polyphenol detection are also used. Ultra-chemical tests with silver salt reactions are the most interesting ones (Eef. 13) Such investigations have been successfully applied, but in many oases are limited by... 

Some aspects of morphology can be observed directly by transmission electron microscopy of stained and ultramicrotomed thin sections. The most successful staining method, developed by Kato, makes use of osmium tetroxide, which attacks the double bonds in diene type polymers. The OSO4 staining technique can also be used with other active groups, such as polyurethanes.Many saturated or nonreactive polymers are not easily studied by transmission electron microscopy, unfortunately, because they cannot be stained. Other aspects of morphology, such as phase continuity and interface characteristics, are best determined by combining chemical and dynamic mechanical spectroscopy methods with electron microscopy. 

When the structure cannot be characterized by optical microscopy, transmission electron microscopy comes to the fore, although scanning electron microscopy and or replica techniques are also often helpful. Most of the work to date utilizes osmium tetroxide as a stain. Samples that cannot be stained by osmium tetroxide or another similar agent often cannot be studied by electron microscopy, because the phases cannot be distinguished. Sometimes, a double bond is deliberately added in small quantities during the synthesis to facilitate staining with osmium tetroxide. This is easy to do in acrylic or styrene based systems with an addition of a trace of butadiene or isoprene. 

Early results with cryomicrotomes were described by Cobbold and Mendelson [80]. Polyurethane elastomer, a blend of crystalline and noncrystalline polymers, showed spherulitic textures after sectioning at about -70°C. Injection molded polypropylene (PP) was also sectioned at about -70°C, while polytetrafluoroethylene (PTFE) was sectioned at much lower temperatures. The authors concluded that the technique, though difficult, had potential. Extruded styrene-butadiene-styrene (SBS) copolymer was prepared by cryosectioning with a diamond knife in liquid air at —85 to —115°C, followed by osmium tetroxide vapor staining for one hour [81]. This method revealed the alternating sequence of the polystyrene and polybutadiene lamellae. Odell et al. [82] prepared extruded triblock copolymer by first chemically hardening the polybutadiene, with osmium tetroxide, followed by cryoultramicrotomy to produce 30 nm thick sections which showed fine structure details. Parallel polystyrene rods were observed in the SBS copolymer. Ultramicrotomy and selective staining with osmium tetroxide was also used in the preparation of a binary blend of PP and thermoplastic rubber [83]. 

Many of the studies of multiphase polymers are conducted on unsaturated rubbers which are adequately stained by osmium tetroxide, which reveals the nature of the dispersed phase domains. Polymers with activated aromatic groups have been selectively stained by reaction with mercuric trifluoroacetate (Section 4.4.8). Hobbs [262] has successfully used this technique to provide contrast in blends of poIy(2,6-dimethyl-l,4-phenylene oxide) and Kraton G (SBS block copolymer). Although this stain is effective in enhancing contrast, a drawback of the method is that the material is not hardened or fixed by the stain. 

Using this technique, hard filler particles in a matrix become more clearly detectable without superposition by brittle fracture edges of the matrix polymer, as illustrated in Fig. 1.36. in this unconventional approach, the fracture surface must be obtained at a temperature at which the matrix is soft (the temperature must be sufficiently high above the glass transition temperature of the matrix) and the inclusions are hard (below the glass transition temperatures of the particles) at the same time. Besides the study of polymer composites, this procedure can also yield successful results for polymer blends, where the rubber particles were hardened by osmium tetroxide treatment. For instance, taking a fracture of an ABS polymer at 130°C, the temperature is well above the glass transition temperature of the SAN matrix, but the stained particles remain hard [12]. 

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