Ionomers glass transition temperature

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

A TMS-2 Thermomechanical Analyzer (Perkin-Elmer) was used to determine the glass transition temperatures of t e ionomer pseudo-IPNs at temperatures ranging from -100°C to +100°C and 0.01 mm of penetration range, 80g of penetrating weight, and a heating rate of 10°C/min. 

Figure 3> Effect of the composition in the ionomer and nonionomer IPN on the glass transition temperature.

Note 2 Typically, an ionomer exhibits two glass transition temperatures (Tg), one for the nonpolar matrix and the other for clusters. 

Figure 2. Glass transition temperature as a function of

Of the microphase-structure dependent physical properties of ionomers, perhaps the most widely studied are glass transition temperatures, (Tg), and dynamic mechanical response. The contribution of the Coulombic forces acting at the ionic sites to the cohesive forces of a number of ionomeric materials has been treated by Eisenberg and coworkers (7). In cases in which the interionic cohesive force must be overcome in order for the cooperative relaxation to occur at Tg, this temperature varies with the magnitude of the force. For materials in which other relaxations are forced to occur at Tg, the correlation is less direct. 

Novel sulfonated and carboxylated ionomers having "blocky" structures were synthesized via two completely different methods. Sulfonated ionomers were prepared by a fairly complex emulsion copolymerization of n-butyl acrylate and sulfonated styrene (Na or K salt) using a water soluble initiator system. Carboxylated ionomers were obtained by the hydrolysis of styrene-isobutyl-methacrylate block copolymers which have been produced by carefully controlled living anionic polymerization. Characterization of these materials showed the formation of novel ionomeric structures with dramatic improvements in the modulus-temperature behavior and also, in some cases, the stress-strain properties. However no change was observed in the glass transition temperature (DSC) of the ionomers when compared with their non-ionic counterparts, which is a strong indication of the formation of blocky structures. 

As can be seen from the last column of Table II, DSC studies did not indicate any change in the glass transition temperature of the polyacrylates due to the presence and/or concentration of the ionogenic monomer (SST) incorporated. We have observed the same behavior in styrene, methyl acrylate and other systems (26). Other workers have also reported similar results in ionomers synthesized by the direct reactions of various acrylic acid salts and covalent vinyl monomers (27). This indicates either the simultaneous homopolymerization of both monomers or a "block copolymerization. 

It should be emphasized that in the Nafions, as in other polymers, and especially ionomers, the glass transition temperature can be strongly influenced by the thermal history and the moisture content of the polymer. Furthermore, in the present case, some decomposition can be seen at ca. 190°C in the acid samples, which show considerably lower thermal stability than is observed in the salts. These results are consistent with those reported earlier by Yeo and Eisenberg (31), based on weight loss in thermogravimetric studies. This feature appears to be a common phenomenon in sulfonated systems for example, in the sulfonated polysulfones, improved thermal stability is also observed in the neutralized materials (2). 

Figure 3. The individual stress relaxation curves and master curve reduced to the a glass transition temperature for Nafion acid, compared with those of polystyrene and two styrene ionomers reduced to their glass transition temperatures (31).

In addition to the more conventional plasticizers, ionic plasticizers are also used. This is specific case of plasticization which acts on different segments of ionomers. lonomers are composed of polymeric chains which have certain number of ionic groups. The ionic groups that are distinct from the nonpolar polymer chain form aggregates known as mul-tiplets. The multiplets are ionic crosslinks surrounded by matrix of polymer chains. At a certain ionic content, the restricted mobility of ionic crosslinks becomes the significant factor in determining the glass transition temperature of the ionomer. 

Figure 11.13. Glass transition temperatures of matrix and clusters of polyphenylene oxide ionomer plasticized with variable quantities of dioctyl phthalate. [Adapted, by permission, from Hee-Seok Kim Joon-Seop Kim Jin-Wook Shin Young-Kwan Lee, Polym. J. (Japan), 31, No.3, 1999, p.306-8.]...

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