Anisotropy in ionomer membranes represents a powerful interaction for modulating properties such as mechanical moduli, thermal expansions, and small molecule transport, all tunable via controlled processing. We observe uniform hydrophilic channel alignment in three perfluorosulfonate ionomer membrane types, quantified by 2H NMR spectroscopy of absorbed D2O molecules. Our measurements show biaxial or uniaxial in-plane alignment for extruded membranes, but uniaxial through-plane alignment for dispersion-cast membranes, and further demonstrate affine swelling with both water uptake and thermal expansion. In order to correlate alignment data with a quantity relevant to proton transport, we measure the anisotropy of water self-diffusion using pulsed-field-gradient NMR along different membrane directions. Extruded membranes with stronger alignment exhibit 18% faster in-plane diffusion than through-plane diffusion, while diffusion anisotropy is minimal for weakly aligned membranes. These results should lead to a more quantitative understanding of and control over membrane properties via manipulation of molecular order.

What factors influence ionic conductivity in advanced polymer electrolytes? Clearly, chemically or electrically driven transport through ionic polymers depends on numerous phenomena, many of which may not even be known. Ionic polymer features such as crystallinity and ionic aggregate size have been measured using XRD, NMR, and electron microscopy, 1 but this morphological realm requires deeper exploration. If we can more completely assess morphological properties of these electrolytes, we can optimize and tailor them to suit diverse applications in polymer-electrolyte-membrane (PEM) fuel cells, reverseosmosis membranes, and "artificial muscle" polymer actuators.One phenomenon that has received little attention in terms of membrane design is long-range orientational order. Perfluorosulfonate ionomers such as DuPont's Nafion exhibit a phaseseparated structure, where protons conduct via ionically coordinated, water-swollen hydrophilic channels. 1 We present detailed orientational order measurements in two leading Nafion membranes using a simple and quantitative method based on deuterium NMR. We demonstrate that this channel alignment is uniform over these films but has drastically different properties for the two membrane types. If we can quantify and manipulate this order, we should be able to optimize anisotropic conductivity in, e.g., fuel cell membranes by maximizing channel alignment across the film plane.Ionomer-based PEM fuel cells typically operate with 10-40 wt % water uptake to maximize proton conductivity. 2 Several models based on powder XRD scattering curves describe this hydrophilic phase as a network of channels and cavities that grow in size with water uptake. 3-5 Protons conduct in the acidified bath of water contained in the ion-lined channels, which provides a low activation energy environment for transport. Our initial question is, what are the alignment modes of these membrane channels?A powerful method of determining orientational order and its dynamics in polymers lies in the use of deuterium NMR quadrupole couplings. In many cases, doping a simple deuterated probe molecule into an ordered liquid or solid provides easy access to the orientational order parameter S (a.k.a., the Hermann's orientation function) as well as to higher order tensorial orientation properties. 6 This method provides quantitative, although relative, measures of S and other order parameters, and these can be extremely useful in characterizing ordered phase behavior and symmetries. Perfluorosulfonate ionomers present an ideal case for application of this method since they heavily absorb water, which we may replace with D 2 O. Residual quadrupole splittings ∆ν Q of the water deuterons report on S via 7 ∆ν Q ) Q p SP 2 (cos θ)where Q p is the quadrupole coupling constant (∼260 kHz) and S ) 〈P 2 (cos )〉 is the ensemble average over the second Legendre polynomial with the angle between a particular OD bond axis and the alignment axis of the material. θ defines the angle between the material alignment axis and the spectrometer...

Ionic liquid (IL) based polymeric mechanical actuators show promise for providing an important and unprecedented combination of electromechanical and soft material properties. Measurement of ion transport in an ionomer/IL membrane is essential for understanding and designing these ''artificial muscle'' actuators. Furthermore, water forms a ubiquitous part of these hydrophilic membrane systems when used in open air, greatly affecting electrochemical and physical properties including the ion conductivity critical for actuation performance. Here we present the first study quantifying the delicate interplay between water and an IL absorbed in an ionomer membrane as used in ionic polymer actuators. We use 1 H and 19 F NMR diffusometry to investigate cation and anion diffusion of 1-ethyl-3methyl-imidazolium trifluoromethanesulfonate (EMI-Tf), swollen into a perfluorosulfonate ionomer membrane. The EMI cation diffuses faster than the Tf anion in both the neat state and inside the membrane. We quantitatively evaluate the effects of temperature, IL uptake, and water content. A key factor is the co-existing water content, quantified by in situ 1 H NMR, which dramatically accelerates the diffusive motion of the IL. When water content, c H2O , drops from 1 to 0.5 water molecules per EMI-Tf, IL diffusion coefficients decrease by 36-60%. Our experiments provide critical feedback for optimizing actuator performance via novel materials, device development, and control of operation conditions.

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