A Self-organized Network of Nanochannels Enhances Ion Conductivity through Polymer Films

Abstract

The proton-exchange membrane (PEM) is an integral component of solid polymer electrolyte fuel cells. The membrane acts as a separator to prevent mixing of reactant gases and as an electrolyte for transporting protons from anode to cathode. 1 High proton conductivity, mechanical strength, and chemical stability of the membrane are factors that affect fuel cell performance. 2,3 Many PEM materials, including the widely studied Nafion series of membranes, are ionomers that consist of a hydrophobic backbone possessing pendant cation exchange sites such as SO 3 -. According to the Eisenberg-Hird-Moore (EHM) model, 4,5 the ionic sites aggregate to form multiplets. To explain conductivity through Nafion, Gierke proposed a "cluster-network" model in which clusters of ions, separated by a polymer backbone matrix, are connected via short, narrow channels (∼1 nm in diameter). 6,7 However, it has yet to be proven that the formation of cluster networks is necessary to achieve high conductivity in PEMs. That is, for a given ion content, is proton conductivity served better by having the ions form a cluster network rather than having them dispersed homogeneously throughout the membrane?To examine the relationship between structure, morphology, and conductivity, we report the synthesis, film formation, and properties of a novel class of well-defined graft polymers comprising styrene and sodium styrene sulfonate (PS-g-PSSNa). This system was chosen because it enabled control of both the copolymer's ion content and morphology. PS-g-PSSNa was prepared via the stable free radical polymerization of sodium styrene sulfonate (SSNa), 8-10 and the "living" terminus of PSSNa reacted with divinylbenzene (DVB) to yield PSSNa possessing a terminal vinyl group (PSSNa-DVB). The degree of polymerization, molecular weight, and polydispersity of PSSNa were 32, 6.5 × 10 3 , and 1.25, respectively. The resultant macromonomer was emulsion-copolymerized with styrene as illustrated in Scheme 1. PSSNa-DVB inserts into the propagating polystyryl chains (PS), as graft chains, because the DVB terminus is located in the core of the polymerizing micellar particles. By adjusting the feed ratio of PSSNa-DVB to styrene, a series of copolymers with uniform graft chain length and controlled graft density was

Synthesis and Proton Conductivity of Partially Sulfonated Poly([vinylidene difluoride-co-hexafluoropropylene]-b-styrene) Block Copolymers

“…24 In related block copolymer studies, graft copolymers of PS-g-macPSSA show enhanced conductivity compared to that of random copolymers of styrene and styrenesulfonic acid (PS-r-PSSA). [26][27][28][29] More recently, block copolymers based on sulfonated bisphenol A polysulfone and poly(vinylidene difluoride) (PSF-b-PVDF) were observed to exhibit enhanced proton conductivity compared to that of homopolymers of sulfonated bisphenol A polysulfone. 30 In this work, proton-conducting diblock copolymers, sulfonated poly([vinylidene difluoride-co-hexafluoropropylene]-b-styrene) [P(VDF-co-HFP)-b-SPS], incorporating a nonionic fluorous block and a sulfonated ionic block are synthesized according to Scheme 1.…”

Synthesis of crosslinked polystyrene-b-poly(hydroxyethyl methacrylate)-b-poly(styrene sulfonic acid) triblock copolymer for electrolyte membranes

“…This morphology may be responsible for the high proton conductivity of triblock copolymer membranes. 24,25 The thermal stabilities of pristine and crosslinked PS-b-PHEMA-b-PSSA triblock copolymer membranes were investigated by TGA as shown in Figure 7. The first slight weight loss for all membranes was observed around 100 o C, attributed to the loss of adsorbed water by the hygroscopic property of the membrane.…”

Transport and Electro‐Optical Properties in Polymeric Self‐Assembled Systems

“…3.6) [202]. This leads to 5-10-nm wide sulfonated polystyrene channels through the polystyrene matrix which are connected as a continuous ionic network, thus providing conductivity 0.24 S cm -1 with a styrene sulfonic acid loading of 19.1 mol% and which contains only 37 vol.% water.…”

“…3.6 A concept to achieve conducting hydrated ionic poly(styrenesulfonic acid) (PSSA) channels through a polystyrene (PS) matrix using grafting [202].…”