Synthesis and thermal stability of novel anion exchange resins with spacer chains

Abstract

Spacer-modified anion exchange resins were prepared by suspension copolymerization of v-bromoalkylstyrenes or v-bromoalkyloxymethylstyrenes with 2-8 mol % of divinylbenzene, followed by quaternization with trimethylamine. The thermal stability of the spacer-modified anion exchangers of the OH form was examined by standing the resins in deionized water at 100-140ЊC for 30-90 days. The anion exchangers with alkylene chains such as butylene or heptylene groups between the benzene ring and the quaternary nitrogen exhibited higher thermal stability compared with commercial, strongly basic anion exchangers with benzylic ammonium groups. The thermal stability of the exchangers with butyleneoxymethylene or hexyleneoxymethylene spacers was also higher than that of the commercial exchangers. The exchanger with the propyleneoxymethylene spacer, however, had less stability than did the commercial ones. The decreased stability of this spacer-modified exchanger is due to the accelerated degradation of the spacer chain via Hofmann elimination. The excellent stability of the anion exchangers with alkylene or alkyleneoxymethylene spacers, except propyleneoxymethylene, results from the structure of the exchangers, where there are no reactive benzylic carbons, which are attached directly to the quaternary nitrogen.

Highly Stable, Anion Conductive, Comb-Shaped Copolymers for Alkaline Fuel Cells

“…The improved stability of these types of cations has been supported by computational calculations using model compounds which revealed that attachment of quaternary ammonium groups to the polymer backbone having an alkyl spacer of >3 carbon atoms can lead to improved alkaline stability of the cation. 39,40 Recently, Hibbs and co-workers reported a similar stability result for a poly(phenylene) backbone with trimetylalkylammonium cations attached by a hexamethylene spacer. 41 Although the comb-shaped PPO membranes showed promising properties as AEMs, poor mechanical properties were observed for high IEC (>2.0 meq/g) membranes probably due to the poor compatibility between long alkyl side chain and PPO backbone.…”

“…It is assumed that the degradation of these materials was a result of typical direct nucleophilic substitution and/or β-hydrogen Hofmann elimination that occurs for quaternary ammonium moieties where the Hofmann elimination might also provide a pathway for the observed cross-linking and insolubility of the CyDx samples. 39,40 The bicarbonate conductivities of comb-shaped CyDx membranes remainted at ∼80% of their initial values after the test of 2000 h while the conductivity of the BTMA20 membane decreased rapidly to ∼40% of the initial value after only 80 h (Figure 9a). This degradation behavior suggested that the comb-shaped membranes with one long alkyl side chain have much better alkaline stability than the BTMA membranes, and have sufficient long-term stability for AEM fuel cell applications.…”

“…This finding is supported by a recent computational study which predicts that n-alkyltrimethylammonium groups are more stable than BTMA when n > 3. 39,40 Although there are two β-hydrogen atoms around the quaternary ammounium groups in comb-shaped membranes in which the Hofmann elimination is probably the preferred decomposition pathway, the steric hindrance of the alkyl chains may hinder Hofmann elimination and protect the ammonium cations from being attacked by OH − .…”

Development of imidazolium-type alkaline anion exchange membranes for fuel cell application

“…This poor stability in alkaline conditions derives from the strong nucleophilicity of the OHanions, which induces displacement (S N2 , or via ylide intermediates) and Hofmann elimination reactions (predominantly the latter when ß-hydrogens present). [18][19][20][21] With the aim of obtaining AAEMs with enhanced stability, alternative cationic head-groups including guanidinium, 22,23 stabilized phosphonium, 24,25 and imidazolium, [26][27][28][29][30] have been recently investigated. Zhang and co-workers reported a new class of AAEMs containing guanidinium head-groups, which maintained ionic conductivity even after immersion in aqueous NaOH (1 mol dm -3 ) for a week at room temperature; this suggested good alkaline stability.…”

Study on Preferential Selectivity of Nuclear Grade Resin Indion-223 towards some Bivalent Ions

“…As a result organic ion exchange resins are developed extensively [8][9][10][11] and various aspects of ion exchange technologies have been continuously studied to improve the efficiency and economy in various technological applications [5,6,12,13]. Development and synthesis of new organic ion exchange resins is usually followed by their characterization .…”