Kirt A. Page

Confinement-driven increase in ionomer thin-film modulus.

Ion-conductive polymers, or ionomers, are critical materials for a wide range of electrochemical technologies. For optimizing the complex heterogeneous structures in which they occur, there is a need to elucidate the governing structure-property relationships, especially at nanoscale dimensions where interfacial interactions dominate the overall materials response due to confinement effects. It is widely acknowledged that polymer physical behavior can be drastically altered from the bulk when under confinement and the literature is replete with examples thereof. However, there is a deficit in the understanding of ionomers when confined to the nanoscale, although it is apparent from literature that confinement can influence ionomer properties. Herein we show that as one particular ionomer, Nafion, is confined to thin films, there is a drastic increase in the modulus over the bulk value, and we demonstrate that this stiffening can explain previously observed deviations in materials properties such as water transport and uptake upon confinement. Moreover, we provide insight into the underlying confinement-induced stiffening through the application of a simple theoretical framework based on self-consistent micromechanics. This framework can be applied to other polymer systems and assumes that as the polymer is confined the mechanical response becomes dominated by the modulus of individual polymer chains.

Viscoelastic properties of confined polymer films measured via thermal wrinkling

We present a new wrinkling-based measurement technique for quantifying the viscoelastic properties of confined polymer thin films. This approach utilizes real-time laser-light scattering to observe the kinetics of thermally-induced surface wrinkling, which evolves isothermally as a function of annealing time. Specifically, wrinkling is induced by applying a thermal stress to a polystyrene film that is sandwiched between a silicon substrate and an aluminium thin film superstrate. By following the time evolution of the wrinkle wavelength and amplitude, we can infer the rubbery modulus and shear viscosity of the polystyrene film with the aid of a theoretical model.

Disordered nanoparticle interfaces for directed self-assembly

Self-assembly is a promising route for controlling the nanoscale structure and material properties of coatings, yet it remains difficult to control the microstructure of these systems. In particular, self-assembling materials typically have complex and delicate energy landscapes, which are sensitive to defects, making it difficult to control morphology or orientation. We present a simple and robust strategy for modulating the film-substrate interaction, which can bias the self-assembly energy landscape and thus enforce a desired microstructure. The technique uses nanoparticles with tunable surface energy to generate a rough interface with controlled properties. The intentionally disordered interface is tolerant to variation in substrate preparation. We apply this technique to block-copolymer lamellae, and demonstrate a remarkable thickness-dependence of the induced orientation, consistent with theoretical predictions. The simultaneous control of substrate energy and topography enables expression of the vertical lamellae state without rigorous control of the preparation conditions. We measure an 8-fold increase in surface energy tolerance compared to flat substrates.

In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles.

Perfluorinated ionomers, in particular Nafion, are an essential component in hydrogen fuel cells, as both the proton exchange membrane and the binder within the catalyst layer. During normal operation of a hydrogen fuel cell, the ionomer will progressively swell and deswell in response to the changes in hydration, resulting in mechanical fatigue and ultimately failure over time. In this study, we have developed and implemented a cantilever bending technique in order to investigate the swelling-induced stresses in biaxially constrained Nafion thin films. When the deflection of a cantilever beam coated with a polymer film is monitored as it is exposed to varying humidity environments, the swelling induced stress-thickness product of the polymer film is measured. By combining the stress-thickness results with a measurement of the swelling strain as a function of humidity, as measured by quartz crystal microbalance (QCM) and X-ray reflectivity (XR), the swelling stress can be determined. An estimate of the Youngs modulus of thin Nafion films as a function of relative humidity is obtained. The Youngs modulus values indicate orientation of the ionic domains within the polymer films, which were confirmed by grazing incidence small-angle X-ray scattering (GISAXS). This study represents a measurement platform that can be expanded to incorporate novel ionomer systems and fuel cell components to mimic the stress state of a working hydrogen fuel cell.

Electrically stimulated gradients in water and counterion concentrations within electroactive polymer actuators

While ionic polymer metal composites (IPMCs) have been studied for more than 10 years, the specific actuation mechanism is still unclear. In this work, neutron imaging, applied potential atomic force microscopy (APAFM) and current sensing atomic force microscopy (CSAFM) methods are employed to fundamentally investigate the actuation mechanism of this electroactive polymer system. Direct neutron imaging allowed a mapping of the water–counterion concentration gradient profile (i.e., a non-flat optical density profile sloping from the cathode to the anode) across an IPMC cross-section. While the neutron imaging method was capable of visualizing inside an operating IPMC, APAFM–CSAFM characterized changes in the nanoscale morphology and local surface properties due to redistribution of water–counterions under electrical stimulation. In APAFM, the darker, more energy dissipative features disappeared as the applied bias was varied from 0 V to 3 V, indicating that the surface became dehydrated. Surface dehydration undoubtedly supports the concept of proton and water migration to the negatively charged substrate. Water–counterion redistribution was further evidenced by CSAFM. With a negatively charged substrate (a 2 V bias), 2.8 pA of the average current were detected over the perfluorosulfonate ionomer (PFSI) surface in contact with AFM tip, which suggests the depletion of positively charged cations on the surface. On the contrary, a positively charged substrate (a −2 V bias) led to the average current of −90 pA over the PFSI surface in contact with the AFM tip, which indicates the formation of a cation-rich fluid on the top surface of the PFSI membranes. The observed water–counterion redistribution upon electrical stimulation directly supports a hydraulic contribution to the overall mechanism of actuation in IPMCs.

Using Indentation to Quantify Transport Properties of Nanophase-Segregated Polymer Thin Films

Indentation of hydrated Nafion thin films reveals that both the in-plane diffusivity of water and the intrinsic permeability of the phase-segregated network decrease dramatically with decreasing film thickness. Using pore-network theory, this decrease in diffusivity is attributed to both an increase in ionic-domain heterogeneity and a reduction in ionic-domain connectivity upon confinement.

A robust and high-throughput measurement platform for monomer reactivity ratios from surface-initiated polymerization

This article describes a robust approach to measure monomer reactivity ratios from surface-initiated copolymerization, by measuring composition of statistical copolymer brush surfaces using X-ray photoelectron spectroscopy. Statistical copolymer brushes were prepared from various monomer feeds by surface-initiated radical copolymerization at room temperature under ultraviolet (UV) irradiation. The copolymer brush composition data were fit to the terminal copolymerization kinetic model resulting in point estimates for the monomer reactivity ratios that are in good agreement with values measured under bulk reaction conditions. Additionally, a high-throughput approach was demonstrated to measure reactivity ratios using a single substrate exhibiting a gradient in copolymer brush composition. This high-throughput approach significantly reduces the time and effort required to generate reliable and reproducible point estimates of reactivity ratios, and these values are in good agreement with values obtained from both the discrete sample surface measurements and classical bulk analytical methods.

Neutron Techniques as a Probe of Structure, Dynamics, and Transport in Polyelectrolyte Membranes

Polyelectrolyte membranes (PEMs) have been employed as solid electrolytes in fuel-cell technologies as early as the 1950s, when they were used in NASA’s Gemini program. However, PEM materials have only gained wide-spread attention in the last two decades due to advancements in membrane electrode-assembly (MEA) formation and the synthesis of new and interesting materials. Over the past several decades, various neutron techniques have played an instrumental role in measuring the structure and transport properties of PEMs in order to develop a deeper understanding of structure-property and performance relationships in PEM materials for fuel-cell applications.

Influence of Electrostatic Interactions on Chain Dynamics and Morphological Development in Perfluorosulfonate Ionomer Membranes

Several high temperature methods of processing Nafion® have been developed using various alkylammonium ion forms of the ionomer, and the choice of counterion has been shown to have a significant effect on the thermal and mechanical properties of this material. In particular, it has been shown that neutralization gives rises to two high-temperature mechanical relaxations as observed in dynamic mechanical analysis (DMA). While several studies in the literature have attempted to explain the molecular origins of these mechanical relaxations, the assignments were based primarily on limited DMA results and have at times been contradictory. The study presented here is a fundamental investigation into the molecular origins of the thermally induced morphological relaxations and dynamics of alkylammonium forms of Nafion® membranes as studied by variable temperature small-angle x-ray scattering (SAXS) and solid-state 19 F NMR spectroscopy. The intensity of the small-angle ionomer peak at ca. q = 2 nm –1 was monitored as a function of temperature for each alkylammonium neutralized sample in unoriented and oriented states. In the case of the oriented samples, the degree of anisotropic scattering from the oriented ionomer morphology was quantified using the Hermanns orientation function and monitored as a function of temperature. Changes in intensity of the ionomer peak and the Hermanns parameter as a function of temperature were shown to correlate well with relaxations observed in DMA. Several variable temperature solid-state 19 F NMR techniques (including spin diffusion, side-band analysis and T 1ρ experiments) were used to investigate the dynamics of the Nafion® chains. Side band analysis indicated that the side-chain is more mobile than the main chain and that the mobility is greatly affected by the size of the counterion. Changes in side-band intensity as a function of temperature were shown to correlate well with DMA data. Results from T 1ρ experiments show strong counterion dependence and suggest coupled main- and side-chain motions. A two-component relaxation process was also observed for the main-chain fluorines. The results of the NMR investigations, along with the SAXS data, have led to the development of a more detailed description of the dynamics of Nafion® and the molecular origins of the mechanical relaxations. With this information, the continuing goal to determine how the strength of the electrostatic interactions in perfluorosulfonate ionomers affects the chain dynamics and developing morphology may be realized for the purpose of controlling the morphology to create more efficient ionomeric membrane materials.