Journal
ACS NANO
Volume 14, Issue 10, Pages 13754-13764Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsnano.0c06071
Keywords
electrolyte; salt doping; ternary blend; C15 Laves phase; block copolymer; homopolymer; self-assembly
Categories
Funding
- Office of Basic Energy Sciences (BES) of the U.S. Department of Energy (DOE) [DE-FOA-0001664]
- Doctoral Dissertation Fellowship at the University of Minnesota
- National Science Foundation [00039202]
- E.I. DuPont de Nemours Co.
- Dow Chemical Company
- Northwestern University
- U.S. DOE [DE-ACO2-06CH11357]
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Salt-doped A/B/AB ternary polymer blends, wherein an AB copolymer acts as a surfactant to stabilize otherwise incompatible A and B homopolymers, display a wide range of nanostructured morphologies with significant tunability. Among these structures, a bicontinuous microemulsion (B mu E) has been a notable target. Here, we report the surprising appearance of a robust C15 Laves phase, at compositions near where the B mu E has recently been reported, in lithium bis(trifluoromethane) sulfonimide (LiTFSI)-doped low-molar-mass poly(ethylene oxide) (PEO)/polystyrene (PS)/symmetric PS-b-PEO block copolymer blends. The materials were analyzed by a combination of small-angle X-ray scattering (SAXS), H-1 NMR spectroscopy, and impedance spectroscopy. The C15 phase emerges at a high total homopolymer volume fraction phi(H) = 0.8 with a salt composition r = 0.06 (Li+/[EO]) and persists as a coexisting phase across a large area of the isothermal phase diagram with high PS homopolymer compositions. Notably, the structure exhibits a huge unit cell size, a = 121 nm, with an unusually high micelle core volume fraction (f(core) = 0.41) and an unusually low fraction of amphiphile (20%). This unit cell dimension is at least 50% larger than any previously reported C15 phase in soft matter, despite the low molar masses used, unlocking the possibility of copolymer-based photonic crystals without compromising processability. The nanostructured phase evolution from lamellar to hexagonal to C15 along the EO60 isopleth (phi(PEO,homo-LiTFSI)/phi(H) = 0.6) is rationalized as a consequence of asymmetry in the homopolymer solubility limit for each block, which leads to exclusion of PS homopolymer from the PS-b-PEO brush prior to exclusion of the PEO homopolymer, driving increased interfacial curvature and favoring the emergence of the C15 Laves phase.
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