Effect of block composition on the morphology, hydration, and transport properties of sulfonated PS‐b‐PEGPEM‐b‐PS

Abstract

This work discusses the effect of block composition on the properties of proton conducting polymer membranes. A homopolymer and two block copolymers were synthesized using atom transfer radical polymerization. The homopolymer poly(ethylene glycol phenyl ether methacrylate) (PEGPEM) was used as a bifunctional macroinitiator. Polystyrene (PS), was added to both sides of PEGPEM (A) with two different percentages of PS (B) (i.e., 18 and 31%). These copolymers, BAB 18, BAB 31 and the homopolymer A, were completely sulfonated (SA, SBAB 18 and SBAB 31). The resulting polymers produced different water absorption values and transport properties for direct methanol fuel cell (DMFC) applications. The nanostructure and morphology of the casted membranes were studied using small-angle X-ray scattering and atomic force microscopy. The results revealed that all six membranes exhibited a disordered phase-segregated morphology, which changed on sulfonation into small-interconnected ionic domains. Normalized DMFC selectivities (proton conductivity over methanol permeability divided by the respective values for Nafion V R ) were calculated and ranged from 1.16 (SBAB 31) to 15.30 (BAB 18), indicating that the performance of these materials can be comparable or better than Nafion V R . Transport property results also suggest that chemistry (block nature and composition), morphology and water content play a critical role in the transport mechanism of protons and methanol. For example, the percentage of B in BAB 18 provides shorter interstitial ionic distances and sufficient water content to produce high proton conductivity, while maintaining low methanol permeability in a multi-ionic proton exchange membrane.

Synthesis and characterization of novel phosphonated and sulfonated poly(styrene–isobutylene–styrene) for fuel cell and protective clothing applications

“…Also, SIBS‐g‐PVPA SO 3 H follows a similar behavior to sulfonated SIBS at a high sulfonation level (i.e., 92 mole %) since the water uptake, IEC, σ , and P MeOH are reduced possibly due to a morphological change caused by the additional ionic interconnections of the SO 3 H groups . μ eff is related to the ionic channel tortuosity . As observed in Figure , the property increases with temperature and sulfonic content as only SIBS‐g‐PVPA SO 3 H allowed proper proton mobility at 80 °C.…”

“…The grafting of copolymers via ATRP have been previously studied . In recent years, ATRP has been employed to prepare a variety of polymers for diverse applications as its one of the most powerful reversible‐deactivation radical polymerization (RDRP) techniques, mostly due to its simplicity, polydispersity and molecular weight controllability, and monomer, initiator, catalyst, ligand, and solvent versatility, among others . In this study, PVPA was selected as the phosphonated polymer to be grafted to chloromethylated SIBS because the material contains PO 3 H 2 groups directly attached to a flexible main chain, providing chain mobility to the grafts.…”

Influence of carboxylated and phosphonated single‐walled carbon nanotubes on the transport properties of sulfonated poly(styrene‐isobutylene‐styrene) membranes

“…Table summarizes the scattering vectors ( q Bragg ) and Bragg's interstitial distances ( d Bragg ) for the membranes in both states. The first ionomer peak is the most relevant because it corresponds to the largest ionic domain; the slope after this peak is usually used to understand membrane morphology . q Bragg decreases as the sulfonation level increases, indicating an increase in the crystalline domains of the polymer in the dry state and that the sulfonic groups rearrange to form larger nanostructures .…”

“…, Tables ) are also not excessive and thus, do not favor the formation of ionic crosslinks that detrimentally affect the transport of protons (as in the case of SIBS 90). Crosslinking of the ionic domains of SIBS 90 increases the tortuosity of the pathways, decreasing proton mobility, and thus, proton conduction . The high swelling ratio (Table ) and low mechanical strength (Table ) of these membranes could explain the increase in tortuosity that affects their proton transport (i.e., dilution or collapse of the pathways, respectively).…”

“…The first ionomer peak is the most relevant because it corresponds to the largest ionic domain; the slope after this peak is usually used to understand membrane morphology. 53 q Bragg decreases as the sulfonation level increases, indicating an increase in the crystalline domains of the polymer in the dry state and that the sulfonic groups rearrange to form larger nanostructures. 25 Transmission electron microscopy (TEM) results from other studies have showed that these nanostructures form ionic nanochannels.…”