Permanganate etching method

Permanganic acid, a weaker acid than nitric acid, has been found to selectively remove the amorphous regions of polyolefins and many other crystalline polymers to reveal their internal lamellar organization. It was developed and has been applied by Olley and Bassett [328-330] and others [251, 252, 331] as a complementary method to chlorosulfonation. The original permanganate etching method involved the use of... 

Sutton and Vaughan [280] described a new method of preparation using permanganic etching of specimens which are difficult to handle for geometrical reasons. Polypyrrole p-toluenesulfo-nate films embedded in acrylic or epoxy resins are etched with a number of different reagents... 

Propylene based random copolymers with co-units of ethylene, 1-butene, 1-hexene, and 1-octene were studied after rapid and isothermal crystallization by AFM of etched film surfaces and ultramicrotomed plaques. To enhance contrast, the surfaces were permanganate etched by the method described above, using 1% w/v KMn04 for 10 min. Comparison of the structures were made by OM, AFM, and SAXS [366]. 

As predicted in the first edition, there has continued to be progress in the interim in further development and utilization of the permanganate method for a variety of polymers. It is important to understand that the correct recipe for etching is very dependent on your own composition of the polymer and on its morphology. Care must be taken to identify the appropriate reference and then do controlled experiments on your own materials. Bashir et al. [277] for instance conducted a comparative study... 

A modification of the procedure has also been developed for PEEK. Olley, Bassett and Blundell [279] found that a 2% w/v solution of potassium permanganate in a mixture of 4 volumes of orthophosphoric acid and 1 volume of water was effective. Samples were etched at room temperature for 50 min, then etching was stopped by adding the reagent to twice the volume of hydrogen peroxide solution followed by washing and replication. The method clearly revealed the spherulitic detail in PEEK [279]. 

It is now known that a wide array of polymers can be etched using potassium permanganate [273] although some care must be taken to limit the effect of artifacts. The list includes linear and branched PE, PP, PS, poly(4-methylpentene-l), poly (butene-1), PVF2, PEEK, PET and various copolymers such as EPDM terpolymers [273]. More recent work has shown that even liquid crystalline polymers can be etched by a variation of this method. Controls and complementary microscopy are essential to ensure that the experimentalist is not led astray imaging artifacts, hills and valleys or nussing fine structure, lost in the wash baths. 

The lamellar structure is best studied using TEM. Let us take polyethylene. Two methods of preparation are commonly used staining with chlorosulphonic acid/uranyl acetate and etching with permanganic acid. 

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