Abstract
Herein, a systematic study where the macromolecular architectures of poly(styrene-block-2-vinyl pyridine) block copolymer electrolytes (BCE) are varied and their activity coefficients and ionic conductivities are compared and rationalized versus a random copolymer electrolyte (RCE) of the same repeat unit chemistry. By performing quartz crystal microbalance, ion-sorption, and ionic conductivity measurements of the thin film copolymer electrolytes, it is found that the RCE has higher ionic activity coefficients. This observation is ascribed to the fact that the ionic groups in the RCE are more spaced out, reducing the overall chain charge density. However, the ionic conductivity of the BCE is 50% higher and 17% higher after the conductivity is normalized by their ion exchange capacity values on a volumetric basis. This is attributed to the presence of percolated pathways in the BCE. To complement the experimental findings, molecular dynamics (MD) simulations showed that the BCE has larger water cluster sizes, rotational dynamics, and diffusion coefficients, which are contributing factors to the higher ionic conductivity of the BCE variant. The findings herein motivate the design of new polymer electrolyte chemistries that exploit the advantages of both RCEs and BCEs.
Highlights
Macromolecular architectures of polymer electrolytes used in ion-exchange membranes (IEMs) have a profound impact on ionic conductivity[1] and other transport properties such as permselectivity and osmotic drag.[2,3] Ionic conductivity dictates ohmic resistances in electrochemical separations and contributes to the overall energy efficiency of these units.[4,5] Permselectivity, on the other hand, in uences the current utilization in electrochemical separation units.[3]
In our previous work,[31] we reported a multitude of techniques, such as a quartz crystal microbalance (QCM), environmental grazing incidence small-angle X-ray scattering (GI-SAXS) and molecular dynamics (MD) simulations, for probing ionic activity in a model thin lm block copolymer electrolyte composed of poly(styrene-block-2 vinyl pyridine-co-n-methyl pyridinium iodide) (PSbP2VP/nitroxide mediated polymerization (NMP)+IÀ)
By using QCM, GI-SAXS, io-sorption experiments, MD simulations, and conductivity measurements, we show that the random copolymer electrolyte (RCE) has higher ionic activity coefficients while the block copolymer electrolytes (BCE) demonstrate higher ionic conductivity
Summary
Macromolecular architectures of polymer electrolytes used in ion-exchange membranes (IEMs) have a profound impact on ionic conductivity[1] and other transport properties such as permselectivity and osmotic drag.[2,3] Ionic conductivity dictates ohmic resistances in electrochemical separations and contributes to the overall energy efficiency of these units.[4,5] Permselectivity, on the other hand, in uences the current utilization in electrochemical separation units.[3]. A subset of polymeric materials that has received signi cant attention includes block copolymer electrolytes (BCEs)[11,12] as their percolated pathways of ionic domains ameliorate ionic conductivity and the non-ionic domains foster mechanical properties and curtail excess swelling. Several studies[1,11,13,14] exist comparing the ionic conductivity of random/ amorphous polymer electrolytes (RCEs) and microphase separated BCEs (as well as aligned and anti-aligned ionic domains),[12] there is a lack of studies dedicated to the ionic activity differences within these materials with systematically varied macromolecular architectures – especially when the repeat unit chemistries are the same. Ionic activity is important because this thermodynamic property strongly in uences selectivity and ionic transport properties.[15,16,17,18]
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