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.

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Random copolymers of poly(4‐vinylpyridine) and polyisoprene were synthesized, and subsequently quaternized with 1‐alkylbromides. The number of carbons on the pendant side‐chain of the resultant comb‐shaped polymer, n, ranged from 2–8. The comb‐shaped polymers were crosslinked employing thiol‐ene chemistry to give mechanically robust ion conducting membranes. Analysis by wide and medium‐angle X‐ray scattering show three morphology regimes that are dependent on the number of carbons on the pendant side‐chains. When n = 2, ionomer cluster morphology was dominant, when n = 8 backbone‐backbone morphology was dominant, and when n = 3–6, the membrane showed a coexistence of both ionomer cluster and backbone‐backbone morphologies. Evaluation of the water uptake of the membranes showed a maximum water uptake per cation of 9.5 when n = 5 at 95% relative humidity (RH) and 60°C. Conductivity of the samples characterized by electrochemical impedance spectroscopy showed bromide conductivity as high as 110 mS/cm when n = 3 at 95% RH and 90°C.

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