Fuel cells based on polymer electrolyte membranes (PEM) show promise as a means of energy conversion for a wide range of applications both in the transportation sector and for stationary power production due to their high charge density and low operating temperatures. While the structure and transport of bulk PEMs for fuel cell applications have been studied extensively, much less is known about these materials at interfaces and under confinement, conditions that are highly relevant in the membrane electrode assembly of a working PEM fuel cell. Using X-ray reflectivity, neutron reflectivity, grazing-incidence small-angle X-ray scattering, quartz crystal microbalance, and polarization-modulation infrared reflection–absorption spectroscopy, we have studied the structure, swelling, water solubility, and water transport kinetics as a function of relative humidity for confined polyelectrolyte films thinner than 222 nm. While the humidity-dependent equilibrium swelling ratio, volumetric water fraction, and effective diffusivity are relatively constant for films thicker than ca. 60 nm, we observe measurable suppressions of these properties in films less than ca. 60 nm. These effects occur at length scales that are relevant to transport (ion and water) in the polyelectrolyte binders found in the catalyst layer of the membrane–electrode assembly (MEA) of a functional fuel cell. The thin film methodology and findings presented here provide a platform to quantify and validate models of interfacial impedance used within the fuel cell community and have the potential to lead to improvements in MEA materials, design, and optimization.

A vibrational analysis has been carried out to analyze the chain conformation of poly(lactic acid) in the crystalline state. In conjunction with a normal-coordinate analysis, the Raman spectrum has yielded data regarding conformational distributions. Raman spectra of different helices were simulated using previously published structures, force constants, and intensity parameters. Some of the chain conformations predicted are inconsistent with the data. Only one of the four 3 1 helical structures predicted by conformational analysis agrees well with experiment. This analysis provides a new understanding regarding the relative probability of a 103 or 31 helix for poly(lactic acid).

Morphological features governing dimensional stability of variously processed poly(lactic acid) have been investigated. The exotherm observed near the glass transition temperature (Tg) is one of the most sensitive indicators reflecting structural changes as a function of temperature and time. A combination of spectroscopic and calorimetric techniques was used to analyze the amorphous chain conformation of deformed PLA samples. We found that nucleation-enhanced crystallization is the major contributor to the exotherm, not shrinkage. Our analysis suggests the existence of two amorphous regions. One is characteristic of the deformed structure. The other is consistent with a relaxed isotropic conformation distribution. Results showed that the existence of deformed amorphous chains is necessary for any fiber shrinkage. But above a critical crystallinity of 40%, a rigid crystalline network prevents shrinkage, even for highly deformed chains. In addition, the previously reported value of tg′t fraction in the amorphous region (f a ) 0.80) was validated in the current study.

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