46 research outputs found
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Characterization of a Block Copolymer with a Wide Distribution of Grain Sizes
Block copolymer/lithium salt mixtures are an emerging class of lithium battery electrolytes. Previous studies have shown that the ionic conductivity of these materials is a sensitive function of grain size. Both depolarized light scattering (DPLS) and small-angle X-ray scattering (SAXS) have proven to be effective techniques for elucidating the grain structure of block copolymer (BCP) materials. DPLS is particularly useful for the characterization of samples with grain sizes larger than 1 μm, whereas SAXS is particularly well suited for samples with grain sizes smaller than 0.1 μm. We present the results of both DPLS and SAXS measurements of grain structure in a BCP/lithium salt mixture that was annealed after being initially prepared by freeze-drying from solution. The combination of the two techniques demonstrates that our sample is characterized by an extremely wide distribution of grain sizes. In particular, the sample has a large population of small sub-micrometer-sized grains that cannot be detected optically. A bimodal grain distribution model is presented to support this interpretation of the experimental data. The presence of both large grains and regions of undetectable small grains was confirmed by polarized optical microscopy (POM). Two-wavelength DPLS measurements provide an additional approach for characterizing block copolymer samples with a broad distribution of grain sizes
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Characterization of a Block Copolymer with a Wide Distribution of Grain Sizes
Block copolymer/lithium salt mixtures are an emerging class of lithium battery electrolytes. Previous studies have shown that the ionic conductivity of these materials is a sensitive function of grain size. Both depolarized light scattering (DPLS) and small-angle X-ray scattering (SAXS) have proven to be effective techniques for elucidating the grain structure of block copolymer (BCP) materials. DPLS is particularly useful for the characterization of samples with grain sizes larger than 1 μm, whereas SAXS is particularly well suited for samples with grain sizes smaller than 0.1 μm. We present the results of both DPLS and SAXS measurements of grain structure in a BCP/lithium salt mixture that was annealed after being initially prepared by freeze-drying from solution. The combination of the two techniques demonstrates that our sample is characterized by an extremely wide distribution of grain sizes. In particular, the sample has a large population of small sub-micrometer-sized grains that cannot be detected optically. A bimodal grain distribution model is presented to support this interpretation of the experimental data. The presence of both large grains and regions of undetectable small grains was confirmed by polarized optical microscopy (POM). Two-wavelength DPLS measurements provide an additional approach for characterizing block copolymer samples with a broad distribution of grain sizes
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Depolarized Scattering from Block Copolymer Grains Using Circularly Polarized Light
The grain structure of ordered block copolymer materials affects their viscoelastic, adhesive, optical, and electrical properties. Depolarized light scattering has proven to be an important method for characterizing this grain structure. In this paper, we use both theory and experiments to demonstrate the relationship between grain structure and depolarized light scattering from ordered block copolymer samples performed with crossed circular polarizers. We model the sample assuming it comprises randomly oriented ellipsoidal grains with optic axes coincident with the ellipsoid axes. We show that the scattering pattern obtained using circularly polarized (CP) light is azimuthally symmetric, in contrast to that obtained using linearly polarized (LP) light which exhibits 4-fold angular modulation. The integrated scattered power in the CP case is twice as large as that in the LP case. By simultaneously fitting CP and LP light scattering data, we obtain robust measures of parameters that characterize grain structures. In addition, CP light scattering can, in principle, be used to characterize nonrandom grain orientation distributions
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Grain growth kinetics of A(n)B(n) star block copolymers in supercritical. carbon dioxide
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Effect of grain size on the ionic conductivity of a block copolymer electrolyte
A systematic study of the dependence of ionic conductivity on the grain size of a lamellar block copolymer electrolyte was performed. A freeze-dried mixture of poly(styrene)-block-poly(ethylene oxide) and lithium bis(trifluoromethylsulfonyl)imide salt was heated in steps from 29 to 116 °C and then cooled back to 29 °C with an annealing time ranging from 30 to 60 min at each temperature. Grain structure and ionic conductivity during these steps were quantified by in situ small-angle X-ray scattering and ac impedance spectroscopy, respectively. Conductivity depends both on grain structure and temperature. A normalization scheme to decouple the dependence of conductivity on temperature and grain structure is described. Ionic conductivity at a given temperature was found to decrease by a factor of 5.2 ± 0.9 as the SAXS measure of grain size increased from 13 to 88 nm. The fact that in the system studied, large, well-formed lamellar grains are less conducting than poorly defined, small grains suggests a new approach for optimizing the transport properties of block copolymer electrolytes. Further work is necessary to confirm the generality of this finding. © 2014 American Chemical Society
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Phase behavior of a block copolymer/salt mixture through the order-to-disorder transition
Mixtures of block copolymers and lithium salts are promising candidates for lithium battery electrolytes. Structural changes that occur during the order-to-disorder transition (ODT) in a diblock copolymer/salt mixture were characterized by small-angle X-ray scattering (SAXS). In salt-free block copolymers, the ODT is sharp, and the domain size of the ordered phase decreases with increasing temperature. In contrast, the ODT of the diblock copolymer/salt mixture examined here occurs gradually over an 11 °C temperature window, and the domain size of the ordered phase is a nonmonotonic function of temperature. We present an approach to estimate the fraction of the ordered phase in the 11 °C window where ordered and disordered phases coexist. The domain spacing of the ordered phase increases with increasing temperature in the coexistence window. Both findings are consistent with the selective partitioning of salt into the ordered domains, as predicted by Nakamura et al. (ACS Macro Lett. 2013, 2, 478-481). © 2014 American Chemical Society