Sulfonated hydrocarbon polymers

DTA studies revealed a rather strong interaction between water molecules in sulfonated hydrocarbon polymers and their sulfonic acid groups, which leads to high proton conductivities at high temperature and low hiunidity. 

In general, it is known that the proton conductivity does not directly correlate with either water uptake or lEC for any of the polymers applicable in fuel cell technology. Sulfonated hydrocarbon polymers usually have greater water uptake than the perfluorosulfonic polymers in order to achieve the same proton conductivity, which is probably because the acid content in the hydrocarbon polymers is much lower... 

Another typical kind of sulfonated hydrocarbon polymer, sulfonated polysulfone (sPSU), has also been imbibed with triazole [84, 85]. In a doped membrane of a base doping level of 8.3, the pulsed field magnetic gradient (PFG) NMR spectroscopy showed a self-diffusion coefficient of the triazole 4-5 times lower than that... 

To overcome these problems, a number of synthetic IPs have been proposed. Among these alternative IPs, sulfonated hydrocarbon polymers have received sig-niflcant attention, owing to their eost effectiveness, ease of fabrication, tunable stiffness, and good ion-transport properties, whieh result from their controllable monomer composition, especially via the manipulation of the block copolymers. Naturally abundant functional biopolymers such as cellulose derivatives and chitosan have been considered for their high ionic conductivity, environmental friendliness, low cost, and ability to form uniform films. Another solution is to embed functional nanoparticles in a polymer matrix to fabricate a high-performance nanocomposite membrane (Jo et al. 2013). 

Acid-doped sulfonated PBIs (sPBI) have been synthesized by multiple research groups [12,15,40-42,50-54]. Typical approaches to sulfonation include direct sulfonation of the PBl backbone, chemical grafting of functionalized monomers onto the chain, or copolycondensation of sulfonated monomers. The last approach is highly favored, because side reactions can be avoided and degree of sulfonation easily controlled. A recent review by Rikukawa and Sanui [54] thoroughly describes the preparation of sulfonated hydrocarbon polymers and the properties of the sulfonated membranes. 

While lots of PEMs have been developed, the sulfonated hydrocarbon polymer electrolyte (SHCPE) and perfluorocarbon sulfonated ionomer (PFSI) membranes are most widely investigated for LT-PEMFCs and DMFCs, and the PBI and its modified polymer membranes doped with H3PO4 are most widely applied to HT-PEMFCs. In the following sections, we discuss the morphologies and conducting mechanisms of the SHCPEs, PFSIs, and PBI doped with H3PO4 (PBI/H3PO4) membranes. 

The polymer membranes (other than the perfluorinated membranes) are classified into three groups, viz. (a) modified PFSA membranes, (b) alternate sulfonated hydrocarbon polymers and their inorganic composite membranes and (c) acid-base complex membranes. 

In sulfonated hydrocarbon polymers, the hydrocarbon backbones are less hydrophobic and the sulfonic acid functional groups are less acidic and polar. As a result, the water molecules of hydration may be completely dispersed in the nanostructure of the sulfonated hydrocarbon polymers. Both PFSA and sulfonated hydrocarbon membranes have similar water uptakes at low water activities, whereas at high relative humidity (100%) PFSA membranes have a much higher water uptake due to the more polar character of the sulfonic acid functional groups. The sulfonated aromatic polymers have different microstructures from those of PFSA membranes (Fig. 8) (Li et al., 2003). 

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