Pendant side‐chain sterics against electrostatic forces: Influencing short‐range ordering in random polyelectrolytes

Abstract

The length of pendant side chains in charged, random, comb-shaped polymers dictates the nature of their short-range ordering. Random copolymers, and terpolymer, of 4-vinylpyridine (4VP), styrene, and isoprene were synthesized and subsequently fully quaternized with 1-alkylbromides having varying number of carbons on the alkyl group ranging from 2 to 8. Evaluation by wide angle X-ray scattering revealed that dipole-dipole attraction facilitates the formation of ionomer cluster morphology in samples with two carbons on the pendant side chain, whereas for samples with four or more carbons on the pendant side chains, side-chain sterics was dominant resulting in periodic backbone spacing.Copolymers with isoprene, having flexible backbones, favor the formation of ionomer cluster morphology while styrene copolymers having rigid backbones disfavor the formation of ionomer clusters. An "in-line" dipole model was developed to predict the separation distance at which both ionomer cluster and backbonebackbone morphologies could coexist.Another short-range ordering that can be observed in WAXS for random polyelectrolytes is the periodic spacing between adjacent polymer backbones. Polyelectrolytes with pendant side chains, for example, poly(1-n-alkyl-3-vinylimidazoliumbromide), show this kind of morphology. 18 The characteristic spacing obtained from the backbone-backbone morphology has similar dimensions to the cluster-cluster spacing present in the ionomer cluster morphology. However, this short-range order is a consequence of the separation between adjacent polymer backbones. The length of the pendant side chains dictates the spacing between adjacent backbones by sterically repelling other pendant side chains on adjacent polymer backbones. 19 The spacing between adjacent backbones, the so-called backbone-backbone spacing, increases with increasing length of the pendant side chains Additional Supporting Information may be found in the online version of this article.

Optimization of anionic conductivity through the coexistence of ionomer cluster and backbone‐backbone morphologies in anion exchange membranes

“…5,22,23,[43][44][45] Thus, shifting the ionomer peak to lower Q values. Unlike the WAXS diffractograms for the P4VP-r-PI_CnBr series, which show ionomer feature in the WAXS region, 19,33 the diffractograms of the XP4VP-r-PI_CnBr series show no ionomer peak features. However, the diffractograms of the crosslinked membranes show an upturn at lower Q values (see Figure 2).…”

“…There were also no differences between the backbonebackbone spacing before or after crosslinking. 19,33 This suggest that crosslinking between the pendant double bonds of adjacent polymer chains had no effect on backbone-backbone spacing. Although crosslinking did not change the backbone-backbone spacing, crosslinking; however, influenced the LVDW feature in the WAXS diffractograms of the membranes.…”

“…Before crosslinking, P4VP-r-PI_C4Br WAXS diffractograms show no distinct LVDW peak or ionomer cluster peak at lower values of Q but rather a broad feature peak. 19,33 This broad peak suggests an overlap between the LVDW and ionomer peaks. After crosslinking, XP4VP-r-PI_C4Br WAXS diffractograms show a distinct, LVDW peak, corresponding to a distinct backbone-backbone spacing (see Figure S1).…”

“…Based on 1 H NMR data obtained for P4VP-r-PI_CnBr, the pendant vinyl group is estimated to be approximately 10 mol% of the isoprene component. 19,33 In a typical crosslinking protocol, Scheme 1, 100 mg (0.054 mmoles of vinyl group) of P4VP-r-PI_C3Br is dissolved in methanol, and 3.03 mg (0.0135 mmoles) of IRGACURE and 23 μL (0.11 mmoles) of 1,10 decanedithiol is added to the solution. Two-fold excess of dithiol crosslinkers were used to ensure that all pendant vinyl groups reacted.…”

“…4,14,1514,[16][17][18] Optimization of ionic conductivity via structure-property correlations, especially in regards to the evolution of the different morphologies that can be achieved in charged random copolymers, will be explored in this work. 19 Fabricating AEMs with high ion exchange capacity (the number of ions per dry mass of membrane) will generally result in materials with low mechanical integrity. 13 Coughlin et al copolymerized 4-vinylbenzylchloride with isoprene, 5,20 and the resulting random copolymers were quaternized with trimethylamine and crosslinked to obtain mechanically robust, ion conducting anion exchange membranes.…”