Chlorosulfonic Acid and Uranyl Acetate

An important staining technique was developed by Kanig [246] for the enhanced contrast of PE, a material that has been a model compound for fundamental polymer studies. Polyethylene crystals cannot be sectioned, nor are they stable in the electron beam, due to radiation damage. The chlorosulfonation procedure cross links, stabilizes, and stains the amorphous material in crystalline polyolefins, permitting ultrathin sectioning and stable EM observation. Chlorosulfonic acid diffuses selectively into the amorphous material in the semicrystalline polymer, increasing the density of the amorphous zone compared with the crystalline material. The treatment stains the surfaces of the 

Chlorosulfonic acid was used to show the lamellar structure in both linear and branched PE and to study the effects of drawing and annealing [197, 247, 248]. Dlugosz et al. [249] examined cryosections of drawn, rolled and annealed, bulk oriented PE that clearly showed a lamellar texture. Lamellae were shown to have two preferred orientations and to be arranged in stacks perpendicular to the draw direction. Experiments using chlorosulfonation on other olefins and polyesters were shown 

A general method for staining PE with chlorosulfonic acid is as follows  

Wash the stained sample in concentrated sulfuric acid and then in water. 

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Chemical fixation or hardening. Bulk samples are treated with chemical agents, mostly with chlorosulfonic acid and uranyl acetate or osmium tetroxide or with ruthenium oxide [7]. Several reactions cause a fixation or a hardening of the material. For the most part, these reactions are highly selective, therefore, a selective staining of structural details takes place as a positive secondary effect (cf. Figure 5). 

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