Mechanical stability counterion effects

Counterion effects (5). Experiments which have been carried out using lithium, sodium, potassium, ammonium and morpholinium laurates have shown that the effects of these five laurates upon mechanical and chemical stability are broadly similar, although such differences as are observed are statistically significant. 

The results for effects upon mechanical stability are summarised in Table II. That lithium laurate behaves similarly to, say, potassium laurate is perhaps surprising, in that it is known that a lithium salt is mor ffective in reducing the mechanical stability of natural rubber -ftian is the corresponding potassium salt (6.). The inference has been drawn that the counterion of the car-boxylate soap has a negligible effect upon the ability of the soap to enhance mechanical stability, relative to the effect of the anion, at least for those cations for which specific adsorption effects are absent. 

Table II shows that morpholinium laurate is markedly less effective in enhancing mechanical stability than are the other laurates which have been investigated. This is attributed to specific counterion adsorption, with a consequent reduction of the effective surface potential at the rubber-water interface.

It is important to point out that our investigation of counterion effects in carboxylate soaps has so far been concerned almost exclusively with laurate soaps. Laurate soaps were chosen partly because they are generally convenient to handle in that many of them are readily soluble in water to give solutions of low viscosity, and partly because, as has been shown above, laurate soaps are very effective in enhancing the mechanical and chemical stability of natural rubber latex. It must therefore be borne in mind that the conclusions which have been drawn from this investigation concerning effects attributable to counterion variation in laurate soaps may not be generally valid for carboxylate soaps as a family. 

Table II Effects of added laurate soaps of various counterions upon mechanical stability of natural rubber latex (5)...

Some data are also available for the effect of the counterion of a dodecyl sulphate upon its ability to enhance the mechanical stability of natural rubber latex. As in the case of the laurates the lithium, sodium, potassium and ammonium salts are similar in behaviour, but the morpholinium salt is slightly less effective. Again, the latter effect is attributed to specific adsorption of the morpholinium cation. Calcium and magnesium dodecyl sulphates are also effective in enhancing mechanical stability, their abilities being similar to that of morpholinium dodecyl sulphate. 

Some interesting results have recently become available for the effects of a range of n-alkyl triethyl ammonium bromides upon the mechanical stability of natural rubber latex. The number of carbon atoms in the alkyl group varied from 6 to 18. Figure 6 summarises the results. It is usually believed that the addition of cationic surfactants to an anionic latex such as natural rubber latex invariably leads to a reduction in colloid stability, the effect being attributed to adsorption of the cations with consequent partial neutralisation of the particle charge and reduction of the counterion cloud surrounding the particles. 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

In any case, the thermal stability of electrochemically prepared polypyrrole can be improved by ion exchange of the dopant counterion [90,95]. The degree of stabilization achieved depends on the counterion (a greater effect is observed for sulfate and bisulfate anions), temperature, and duration of treatment. Although the mechanism for improved stability is not yet clear, it is apparent that the original polymer microstructure is important. 

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