Confined versus Unconfined Crystallization in Block Copolymer/Salt Mixtures Studied by Depolarized Light Scattering

Abstract

Crystallization in an ordered lamellar diblock copolymer/salt mixture, polystyrene-block-poly(ethylene oxide) mixed with lithium bis(trifluoromethanesulfonyl) imide salt (SEO/LiTFSI), has been studied using a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and depolarized light scattering (DPLS). Such materials have applications as electrolyte membranes in solid-state lithium batteries. The grain structure of the electrolyte was controlled by manipulating thermal history. Poly(ethylene oxide) (PEO) crystallization was confined within the microphase separated morphology and did not affect the grain structure in the case of shallow quenches. Deep quenches resulted in unconfined crystallization, where crystal formation does not affect the microphase separated morphology but does alter the grain structure. The difference between the two modes of crystallization can only be detected by DPLS. This knowledge is particularly relevant for nanostructured electrolytes wherein ion transport is a strong function of grain structure.

Reversible Changes in the Grain Structure and Conductivity in a Block Copolymer Electrolyte

“…In other cases, the microphase structure is preserved, but the grain structure is not. We call this case unconfined crystallization . The electrolyte in this study exhibits unconfined crystallization.…”

“…We called this case unconfined crystallization. 47 The electrolyte in this study exhibits an unconfined crystallization.…”

Propagation of Elliptically Polarized Light through Ordered Block Copolymers

“…The block copolymer was mixed with lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) to give a salt concentration ratio of 0.025 Li + ions to monomer units . The order–disorder temperature ( T ODT ) of the sample was experimentally determined by the birefringence method to be 127 ± 2 °C …”

“…19 The order−disorder temperature (T ODT ) of the sample was experimentally determined by the birefringence method to be 127 ± 2 °C. 2 The dried block copolymer/salt mixture was melted into a 1/ 32″ thick Viton rubber spacer with a 3/16″ inner diameter at 120 °C (McMaster Carr). The rubber spacer and melted sample were pressed between two quartz disks until an airtight, bubblefree sample was obtained.…”

“…This grain structure can affect the mechanical, electrical, and optical properties of the material. When the ordered morphology is lamellar or cylindrical, the grains are optically uniaxial and exhibit form birefringence, and the grain structure can be characterized using depolarized light scattering (DPLS). − DPLS complements other BCP characterization methods such as small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and electron microscopy (EM). − Although recent studies have shown the potential usefulness of these techniques to determine micron-scale structural features of block copolymers, they are primarily used to determine the ordered morphology on the length scale of the unit cell rather than grain structure. In contrast, all of the DPLS signals from ordered block copolymers arise from the grain structure, as typical unit cell sizes in block copolymers are much smaller than the wavelength of light.…”