Sulfonated styrene-based monomers

There have been few synthetic reports employing these monomers beyond the Ballard work, most likely as a result of presumed high cost and monomer availability. However, the performance and stability demonstrated by these materials in fuel cells may spur further developments in this area. The above-reported copolymers are believed to be random systems both in the chemical composition of the copolymer backbone and with regard to sulfonic acid attachment. Novel methods have been developed for the controlled polymerization of styrene-based monomers to form block copolymers. If one could create block systems with trifluorostyrene monomers, new morphologies and PEM properties with adequate stability in fuel cell systems might be possible, but the mechanical behavior would need to be demonstrated. 

Presently, two commercial (or semicommercial) PEMs are based on styrene or styrene-like monomers BAM from Ballard, and Dais Analytic s sul-fonated styrene—ethylene—butylene—styrene (SEBS) membrane. Ballard Advanced Materials Corporation introduced a styrenic membrane based on a novel family of sulfonated copolymers incorporating a,/3,l3-trifluorostyrene and substituted a,y3, -trifluoro-styrene comonomers. These are registered as BAM membranes, and their general formula is given in Figure 3. 

Typically, carboxylate ionomers are prepared by direct copolymerization of acrylic or methacrylic acid with ethylene, styrene or similar comonomers by free radical copolymerization (65). More recently, a number of copolymerizations involving sulfonated monomers have been described. For example, Weiss et al. (66-69) prepared ionomers by a free-radical, emulsion copolymerization of sodium sulfonated styrene with butadiene or styrene. Similarly, Allen et al. (70) copolymerized n-butyl acrylate with salts of sulfonated styrene. The ionomers prepared by this route, however, were reported to be "blocky" with regard to the incorporation of the sulfonated styrene monomer. Salamone et al. (71-76) prepared ionomers based on the copolymerization of a neutral monomer, such as styrene, methyl methacrylate, or n-butyl acrylate, with a cationic-anionic monomer pair, 3-methacrylamidopropyl-trimethylammonium 2-acrylamlde-2-methylpropane sulfonate. 

Typically, the oil phase contained 78% monomer/co-monomer, 8% divinyl benzene (cross-linking agent), and 14% non-ionic surfactant Span 80 (Sorbitan monooleate), while the aqueous phase contained 1% potassium persulfate as the initiator. In most cases studied here, monomer is styrene and when elasticity of the polymer is required, 2-ethylhexyl acrylate (2EHA) was used (styrene/2EHA ratio is 1 4). Whenever additives/fillers are placed in the aqueous phase their amounts are stated as weight percent while the phase volume of the aqueous phase remains constant. In some cases, the aqueous phase contains 0.5% hydroxyapatite and 15% phosphoric acid which is used to dissolve the hydroxyapatite, or alternatively, the aqueous phase may contain varying amounts of water-soluble polymer, such as polyethylene glycol or polyethylene oxide. If the styrene-based PHP is to be sulfonated to obtain ionic-hydrophilic foam, the pre-dispersion of sulfuric acid within the pores is useful, if not essential, and in that case, acids (typically 10%) can be used as the internal phaseP . ... 

The first type of hydrocarbon membrane for fuel cell applications was the sulfonated polystyrene-divinylbenzene co-polymer membranes equipped for the power source in NASA s Gemini space flights, but the sulfonated polystyrene had low chemical stability for long-term applications, because the proton on the tertiary carbons and benzylic bonds are easily dissociated in an oxygen environment forming hydroperoxide radicals. Since a styrene monomer is easily co-polymerized with other vinyl monomers via radical polymerization methods, various styrenic polymers were researched intensively. Two commercial polystyrene-based/related membranes are available BAM (Ballard), and Dais Analytic s sulfonated styrene-ethylene-butylene-styrene (SEBS) membrane. Dais membranes are produced using... 

Fig. 5 Property map for styrene/DVB grafted and sulfonated membranes based on 25- rm FEP tmd ETFE. Influence of the (a) degree of grafting and (b) extent of crosslinking on global membrtme characteristics. The crosslinker content refers to the DVB concentration with respect to total monomer content (styrene + DVB) in the grafting solution. White areas near the center denote the range of optimum composition...

A variety of ionomers have been described in the research literature, including copolymers of a) styrene with acrylic acid, b) ethyl acrylate with methacrylic acid, and (c) ethylene with methacrylic acid. A relatively recent development has been that of fluorinated sulfonate ionomers known as Nafions, a trade name of the Du Pont company. These ionomers have the general structure illustrated (10.1) and are used commercially as membranes. These ionomers are made by copolymerisation of the hydrocarbon or fluorocarbon monomers with minor amounts of the appropriate acid or ester. Copolymerisation is followed by either neutralisation or hydrolysis with a base, a process that may be carried out either in solution or in the melt. 

Therefore, RAFT polymerization is also a powerful technique to functionalize both MWCNTs and SWCNTs. Conventional monomers, such as NIPAAm, A-(2-hydroxypropyl)methacrylamide (HPMAm), DMAEMA, acrylic acid, 3-[A-(3-methacrylamidopropyl)- A, A-dimethyl] ammoniopropane sulfonate (MDMAS), styrene,maleic anhydride and A -vinylcarbazole (macroCTA-6) were used in RAFT polymerization in the presence of a macroCTA or a CNT-based RAFT agent. However, it is noteworthy that a monomer always needs a specific CTA to achieve better control over the polymerization. In other words, a CTA cannot work well for all monomers. This should be considered in the molecular design to functionalize CNTs by the RAFT technique. 

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