Three sulfonated aromatic polymers with different sequence lengths were studied in order to better understand the relationship between molecular structure, morphology, and properties of proton exchange membranes as a function of relative humidity. A random copolymer with a statistical distribution of sulfonic acid groups had very small domain sizes, whereas an alternating polymer with sulfonic acid groups spaced evenly along the polymer chain was found to have larger, but quite isolated, domains. The multiblock copolymer studied herein showed highly phase-separated hydrophilic and hydrophobic domains, with good long-range connectivity. Scanning force microscopy as a function of relative humidity was used to observe water absorption and swelling of the hydrophilic domains in each of the three membranes. The conductivity, water sorption kinetics, and fuel cell performance, especially at low relative humidity, were found to be highly dependent upon the morphology. The multiblock copolymer outperformed both the random and alternating systems at 100°C and 40% RH fuel cell operating conditions and showed similar performance to Nafion.

The impact of the membrane–electrode interface on fuel cell durability was investigated in polymer electrolyte fuel cells (PEFCs). Cells using disulfonated poly(arylene ether) copolymer (BPSH) membranes exhibited greater performance loss than a cell using Nafion after 700 h of direct methanol fuel cell (DMFC) testing. Additionally, the performance loss and cell resistance within the BPSH family of copolymers increased with increasing degree of disulfonation. Membrane characterization using normalH1 NMR, potentiometric titration, intrinsic viscosity, water uptake, and proton conductivity showed minimal impact from chemical/physical changes. Fuel cell performance degradation scaled well with initial membrane–electrode interfacial resistance, suggesting that the membrane–electrode interface was an important contributor to DMFC durability. These results are of particular interest for alternative proton exchange membranes where interfacial compatibility with electrodes is a critical, unresolved issue.

The performance of pressure-sensitive adhesives (PSAs) depends on the bulk viscoelastic properties of the adhesive material itself and on the surface with which it is placed into contact. In this work, a probe technique was used to quantify the adhesion and to develop a protocol to interrogate these bulk and interfacial effects. Model acrylic emulsion-based PSAs with different acid contents were used. A rate-dependent work of adhesion was obtained from a simple probe-tack test, where the indenter was retracted from the adhesive layer at a fixed rate. For a given adhesive formulation, a universal relationship was obtained between the average stress under the indenter and an effective strain rate. Complementary information was obtained from a set of programed oscillatory tests designed to probe the strain-dependent properties of the adhesive. The results suggest that an adhesive failure criterion based on the stored elastic energy can be utilized to determine the failure strain, defining a critical strain energy that depends on both the adhesive and on the surface with which it is in contact. The adhesive performance is determined by this critical strain energy and by the stress–strain rate relationship for the adhesive. Application of these concepts to adhesive design was demonstrated by the construction of an optimized two-layer adhesive.