Chlorosulfonic acid staining

Staining permits TEM observation of the dispersed phases in multiphase blends. Osmium tetroxide is the most commonly used stain for this application, while other stains have more limited application. Detailed fine structure of polymers is also made visible by staining. For example, chlorosulfonic acid staining enhances the lamellar texture of PE [105]. There are cases where a stain has been associated with a specific functional group of polymers. A specific stain for nylon, developed by Reimschuessel and Prevor-sek [106], showed the sizes of the macrofibrils and microfibrils of nylon. Fibers were immersed in 10% aqueous solution of SnCl2 for 10 min at 100°C, rinsed, placed in NH4OH solution to convert the tin chloride to insoluble SnO and then embedded for ultrathin sectioning. 

Fig. 4.15 TEM micrograph of a chlorosulfonic acid stained linear polyethylene crystallized isothermally from the melt reveals the electron dense interlamellar surfaces typical of polyethylene.

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). 

Peterlin et al. [157] described the use of iodine vapor, at 60°C (24 h) to reveal the structure in PE fibrils drawn from solution grown single crystals. Andrews et al. [78] extended this method to prestaining a block of material prior to ultrathin sectioning. They showed the contrast enhancement of PE spherulites by action of the iodine. However, the iodine dissipates upon standing in air and is known to vaporize in the vacuum of the electron microscope. The chlorosulfonic acid method (Section 4.4.4) has replaced iodine staining of polyethylene. 

Figure 14. TEM images for (a) DPNR-graft-PS-l. 5, (b) DPNR-gra/t-PS-4.5 (c) DPNR-gra/f-PS-5.5 and (d) DPNR-graff-PS-5.5 sulfonated with 0.8 N chlorosulfonic acid 30 °C for 5 h. The ultra thin sections of about 100 nm in thickness for (a) DPNR-graft-PS-l. 5, (b) DPNR-gra/t-PS-4.5 (c) DPNR-gra/f-PS-5.5 were stained with OsO at room temperature for 3 min, in which gloomy domains represent natural rubber and bright domains represent the PS. On the other hand, the ultra thin section of about 100 nm in thickness for (d) sulfonated DPNR-gra/f-PS-5.5 was stained with RuO for 1 min, in which bright domains represent natural rubber and gloomy domains represent the PS.

The sulfonation of polyethylene membranes was examined under various experimental conditions and the results showed that sulfonation greatly enhanced the water permeability of the resultant membranes. A lower concentration of chlorosulfonic acid at low temperature was preferable, so that only the surface and inner walls of the polymer were sulfonated and the mechanical properties of the membrane were not damaged. The electromicroscopy of polythene is facilitated by staining the polymer by immersing pieces of the polymer in chlorosulfonic acid at 60 °C for several hours. Chlorosulfonated polyethylene rubbers are useful for specific purposes, e.g. as ozone-resistant hoses. ... 

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... 

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

Figure 4.20. A representative TEM image of a chemically cross linked UHMWPE, used as an implant material in artificial joints, prepared by staining with chlorosulfonic acid and poststaining in 2% uranyl acetate [261], shows numerous lamellae. (From Kurtz [261] used with permission.)...

Figure 4 Sample of polyethylene crystallized at 130 C and stained with chlorosulfonic acid, showing ridged lamellae...

Staining was also applied (120°C) to the crystallization of PE [121,122]. The chlorosulfonation method has been applied to bulk polymers and high modulus fibers [123-125]. Smook et al. [125] studied the fracture process of ultrahigh strength PE fibers and used the method in a unique application to show the nature of the kink bands present. Highly oriented ultrahigh molecular weight PE fibers were shown to be preferentially attacked at these kink bands when exposed to the acid for 45 min at 80°C. 

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