In this paper we report on the charging behavior of latex particles in aqueous suspensions. We use static light scattering and acid-base titrations as complementary techniques to observe both effective and bare particle charges. Acid-base titrations'at various ionic strengths provide the pH dependent charging curves. The surface chemical parameters (dissociation constant of the acidic carboxylic groups, total density of ionizable sites and Stem capacitance) are determined from tits of a Stem layer model to the titration data. We find strong evidence that the dissociation of protons is the only specific adsorption process. Effective particle charges are determined by tits of integral equation calculations of the polydisperse static structure factor to the static light scattering data. A generalization of the Poisson-Boltzmann cell model including the dissociation of the acidic surface groups and the autodissociation of water is used to predict effective particle charges from the surface chemical parameters determined by the titration experiments. We find that the light scattering data are best described by a model where a small fraction of the ionizable surface sites are sulfate groups which are completely dissociated at moderate pH. These effective charges are comparable to the predictions by a basic cell model where charge regulation is absent. 0 I994 American Institute of Physics.

The structure and interactions of water species in hydrated Nafion membranes as a function of water content were investigated on the basis of medium-infrared spectral analysis and molecular dynamics (MD) simulations. The spectral decomposition of the FT-IR data in the stretching OH region was performed on different levels of hydration of the sulfate functional groups (lambdaH2O/RSO3- = 2-22). Quantum mechanical calculations of two model systems [perfluoroethanesulfonic acid/(H2O)6 cluster] and a [perfluorobutanesulfonic acid/(H2O)6 crystal] were carried out in order to account for the band assignments of Nafion in the stretching OH region (2500-4000 cm-1). Our findings indicated that the secondary structure of water species in Nafion can be accurately explained in terms of our reactive force field for water. The distinction between "surface" and "bulk" water contributions in Nafion membrane pores is proposed along with a quantitative estimate of the different types of OH groups present in the system. The average pore size was calculated and supported by the spectral results.

Amorphous, atactic polypropylene structures, consisting of 125, 729, and 2197 monomer repeat units folded into periodic cells, were generated to study the effects of simulation size on the transport of small molecules in simulations of amorphous polymers. The diffusion coefficients and solubilities of three particles having different sizes representative of He, Ar, and CO2 are calculated from 4 ns molecular dynamics simulations. A definite system size dependence is observed in the solubilities resulting from a bias against the formation of large cavities in the smaller structures. Surprisingly, this bias does not significantly affect the diffusivities of the penetrants in these structures despite their jumplike diffusive motion. We also find the characteristic length scale for the turnover from the anomalous to the diffusive regime to be insensitive to the simulation size but inversely dependent on penetrant size. This insensitivity to simulation size of the diffusivity and turnover is in contrast to that found for diffusion in systems that are either static or have percolating networks. This difference points to the importance of dynamic coupling between the penetrant motion and the thermal motion of the polymer matrix. A rigorous statistical analysis of different methods of extracting the penetrant tracer diffusivity from the molecular simulations emphasizes the value of using the van Hove correlation function for analyzing penetrant motion.

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To describe the performance curves of a polymer electrolyte membrane fuel cell we present a Bernardi-Verbrugge-like model. The model contains an improved description of the cathode diffusion and reactive regions. This improvement was motivated by the need to correct the behavior of the Bernardi-Verbrugge model at high cell current densities, where concentration overpotentials and flooding phenomena start to appear. To achieve this goal, we derived a new expression for the overpotential of the cathode reactive region. The advantage of having such an expression is twofold: false(ifalse) elimination of strong nonlinearities in the solution scheme (with the result of having a fast and stable numerical code), and false(normaliifalse) clear inclusion of concentration and flooding phenomena. Extensive applications of the model are presented. The comparison between the results of our simulations and experimental data show astonishingly good agreements. © 2002 The Electrochemical Society. All rights reserved.