Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A4: Polymers and Energy Applications |
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Sponsoring Units: DPOLY Chair: Nitash Balsara, University of California, Berkeley Room: 306/307 |
Monday, March 16, 2009 8:00AM - 8:36AM |
A4.00001: Ion solvation and its effects on the miscibility of binary polymer blends Invited Speaker: We study the effects of adding salt ions on the miscibility of a binary blend of polymers having different dielectric constants. The competition between the preference of the ions to be solvated by the component of the higher dielectric constant and the entropic tendency for the ions to be distributed uniformly results in non-trivial effects on the miscibility. We first study the thermodynamics of the polymer blend-ion mixture using a simple Born model in a uniform dielectric medium of the average composition of the polymer blend. We then study the effect of local enrichment of the higher dielectric constant polymer near the ion. We find that when the dielectric constants of the polymers are both low, adding salt decreases the miscibility, while when the dielectric constants of the polymers are both high, the addition of salt enhances the miscibility. When the blend consists of a high dielectric constant polymer and a low dielectric constant polymer, miscibility is decreased if the low dielectric constant component is the majority and is increased if the high dielectric constant component is the majority. The effect becomes significant at ion concentrations corresponding to an order of one ion per polymer chain. The quantitative change in the effective $\chi$ parameter depends on the functional form of the composition dependence of the dielectric constant of the mixture. We also illustrate the difference between fixed ion concentration and fixed chemical potential of the ions. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 9:12AM |
A4.00002: The Nanostructure of Nafion for Fuel-Cell Membranes from Small-Angle Scattering and NMR Analysis Invited Speaker: We have investigated the long contentious nanometer-scale structure of the Nafion ionomer used in proton exchange membranes of H$_{2}$/O$_{2}$ fuel cells. Using a simple algorithm based on 3D numerical Fourier transformation, we have quantitatively simulated previously published small-angle scattering data of hydrated Nafion. The characteristic ''ionomer peak'' arises from long, parallel but otherwise randomly packed water channels surrounded by the partially hydrophilic sidebranches, forming inverted-micelle cylinders. The channels are stabilized by the considerable stiffness of the Nafion backbones, detected by $^{13}$C and $^{19}$F NMR. An upper limit of 300 nm to the persistence length of the water channels has been estimated from $^{2}$H NMR of $^{2}$H$_{2}$O in the channels. At 20 vol{\%} water, the water channels have diameters between 1.8 and 3.5 nm, with a 2.4-nm average. The hydration-induced changes in small-angle scattering patterns and in the surface-to-volume ratio have also been analyzed in quantitative detail. Nafion crystallites ($\sim $10 vol{\%}), which form physical crosslinks crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross sections of $\sim $(5 nm)$^{2}$. Simulations for a dozen other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data. The water-channel model is the first without constrictions of $\sim $1.2 nm diameter; it can explain important features of Nafion, including fast diffusion of water and protons through Nafion and its persistence at low temperatures. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A4.00003: New Materials and Approaches for Solution-Processed Organic Solar Cells Invited Speaker: Organic solar cells have been proposed as low-cost and sustainable alternatives for power generation. To realize the low cost aspects of organic solar cells, conventional vacuum deposition technologies are to be replaced with solution processing. Our group has focused on the development of solution processable conductive polymers. Conductive polymers, like polyaniline, are generally doped with small-molecule acids. Though highly conductive, such materials are not processable. To overcome this intractability, polymer-acid dopants have replaced small-molecule acids. While the introduction of polymer acids render the conductive polymer solution processable, such gains in processability are accompanied by reduced conductivities. With a post-processing solvent-annealing treatment, however, we have been able to dramatically improve the electrical properties of polymer-acid doped conductive polymers; these polymers make efficient anodes in organic solar cells. To further improve the efficiencies of organic solar cells, we have introduced fractional amounts of additives within the active layer of the device. Depending on the hydrophobicity of the additives, they preferentially segregate into the electron donor phase, effectively enhancing phase separation between the electron donor and electron acceptor. This change in morphology increases charge separation; we see a two-fold increase in the short-circuit current in such devices over those without additives in the active layer. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A4.00004: Charge Transport and Storage within Radical Redox Polymers as Electroactive Materials in Energy Devices Invited Speaker: |
Monday, March 16, 2009 10:24AM - 11:00AM |
A4.00005: Polymers as active components in harnessing solar energy Invited Speaker: In the last couple of decades molecular and polymeric photovoltaic cells have attracted considerable attention as a possible low cost alternative to conventional semiconductor solar cells. While considerable advances in improving device efficiencies have been made, significant challenges in developing efficient, reliable and low cost solar cells using polymers as an active component remain. Some of these advances and challenges as well as on going efforts to mass manufacture solar cells modules will be discussed. [Preview Abstract] |
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