Mercuric trifluoroacetate staining

Trimmed and faced blocks are stained by immersion in a 10% solution of HgO in trifluor-oacetic acid for 10-60 min. Samples are washed in a dilute solution of trifluoroacetic acid followed by distilled water. Poly(phenylene oxide) (PPO) appears to have a higher mercury uptake than polystyrene in a bonded laminate of the two films [6]. Blends made by coextrusion of Kraton G and PPO show dispersed particles in a matrix. Kraton G is the lighter contrast polymer as it seems to take up less of the stain than the PPO. In summary, mercuric trifluoroacetate staining has been shown for several polymers where the dispersed phase particles are differentiated by this stain. The method has limited application. 

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. 

The structural elucidation of multiphase polymers is a continuing problem as, at times, the few commonly used staining reagents are not satisfactory. Osmium tetroxide has the widest range of applicability, but there are polymers that cannot be differentiated by this stain. Mercuric trifluoroacetate has been described for the staining of several polymers [154] polystyrene, poly(2,6-dimethyl-l,4-phenylene oxide) and saturated styrene-butadiene-styrene block copolymers (Kraton G, trademark. Shell Oil Company). 

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