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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.

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.

Block copolymer/lithium salt mixtures are promising materials for lithium battery electrolytes. The growth of ordered lamellar grains after a block copolymer electrolyte was quenched from the disordered state to the ordered state was studied by depolarized light scattering. Three quench depths below the order-to-disorder transition temperature were studied: 6, 12, and 24 °C. Regardless of quench depth, elongated ellipsoidal grains with aspect ratios between six and eight were formed during the initial stage of order formation. This was followed by a rapid reduction in aspect ratio; at long times, isotropic grains with aspect ratios in the vicinity of unity were obtained. Unusual grain growth kinetics were observed at all quench depths: (1) The average grain volume decreased with time after the early stage of grain growth. To our knowledge, a decrease in grain size has never been observed before in any quenched block copolymer system. (2) The volume fraction occupied by ordered grains of the shallowest quenched sample (quench depth of 6 °C) was significantly less than unity even after waiting times approaching 400 min. This is consistent with recent theoretical and experimental work indicating the presence of a coexistence window between ordered and disordered phases due to the partitioning of the salt into the ordered domains. At quench depths of 12 and 24 °C, which are outside the coexistence window, the grain volume fraction increases monotonically with time, and ordered grains occupy the entire sample at long times. © 2014 American Chemical Society.