Ionomers carboxylated polystyrene

Sulfonate vs Methacrylate vs Carboxylate. A comparison study (19) of the melt viscosity values of polystyrene ionomers, either randomly carboxylated or sulfonated in the para position of a benzene ring, showed that the melt viscosity values at 220°C of the two ionomers were significantly different. For example, at 2 mol% of ions, the melt viscosity of sodium sulfonated polystyrene ionomer was more than 2 orders of magnitude higher than that of the sodimn carboxylated polystyrene ionomer. 

Comparison of the dynamic mechanical properties of the p-carboxylated polystyrene ionomers and those of the P(S-co-MANa) ionomers shows a more subtle difference. For the p-carboxylate ionomers the ion pairs are farther from the poljuner backbone than for the P(S-co-MANa), and thus form larger multiplets (102,103). Again, since the size of multiplets is smaller for the P(S-co-MANa), the total number of multiplets is larger, which results in the smaller amount of reduced mobility regions for the carboxylated polystyrene system, at a comparable ion content. As a result, the ionic modulus should be lower in the p-carboxylate system than in the methacrylate system. 

In the case of the p-sulfonated polystyrene ionomer, the ion pairs are at the same distance from the backbone as in thep-carboxylated polystyrene ionomer. As was mentioned above, however, the ionic interaction in the sulfonated ionomer is stronger than that in the carboxylated ionomer. Thns, at the identical ion content, the modulus-temperature curves show longer ionic platean, and the volume fraction of reduced mobility regions for the p-sulfonated ionomer is smaller than for the p-carboxylated ionomer (103), leading to less clnstering for the p-snlfonated ionomers. 

Figure 4. FT-IR spectra of (A) polystyrene/poly(isobutyl methacrylate) diblock copolymer and (B) carboxylated block ionomer obtained after hydrolysis with K0-.

The perfluorinated, carboxylated and sulfonated ionomer membranes form the ionic clusters of a few nm in size, as in the case of the hydrocarbon-based ionomers such as polyethylene,polystyrene and polybutadiene(9). The ionic clusters strongly affect physical properties of the membranes, e.g., the swelling behavior of the membranes (amount of water uptaken by the membranes, W and... 

To be informative, it is desirable that the comparisons of these two different technologies be based on identical polymer backbones, having identical molecular weights, and having comparable levels of ionic functionality present. In addition, it is the purpose of these studies to make such comparisons with the same metal cation and thereby quantify, insofar as possible, the nature of the ionic interactions that exist. To do this, ionomers were prepared based on a polystyrene (PS) hydrocarbon backbone into which the ionic functionality was incorporated. PS was selected as the backbone because of the relative ease of functionalization and the relative freedom of side reactions during the sulfonation or carboxylation reactions. The polymers prepared were designed to come as close as possible in terms of ionic functionality for both sulfonate and carboxylate ionomers over a range of ionic contents. 

The preparation of ionomers involves either the copolymerization of a functionalized monomer with an olefinic unsaturated monomer or direct functionalization of a preformed polymer. Typically, free-radical copolymerization of ethylene, styrene, or other a-olefins with acrylic acid or methacrylic acid results in carboxyl-containing ionomers. The copolymer, available as a free acid, is then neutralized partially to a desired degree with metal hydroxides, acetates, or similar salts. The second route for the preparation of ionomers involves modification of a preformed polymer. For example, sulfonated polystyrene is obtained by direct sulfonation of polystyrene in a homogeneous solution followed by neutralization of the acid to the desired level. Some commercially available ionomers are listed in Table 15.17. 

Eisenbach C D, Hofmann J and MacKnight W J (1994) Dynamic-mechanical and spectroscopic study of ionomer blends based on carboxylated or sulfonated flexible polystyrene and rigid poly(di-acetylenes) with functional side groups. Macromolecules 27 3162-3165. 

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