Chain dimensions of poly(ethylene carbonate/ethylene oxide) copolymer with salt addition studied by SANS

In this study, we investigated polymer chain dimensions in solid polymer electrolytes (SPEs) composed of hydrogenated and deuterated poly(ethylene carbonate/ethylene oxide) (h-/d-P(EC/EO)) copolymers and lithium bis(trifluoromethane)sulfonimide (LiTFSI) at various lithium-ion concentrations. The results were compared with h/d-poly(ethylene oxide) (PEO)/LiTFSI systems, extensively studied for their ionic conduction properties. The scattering intensity I(q) at low scattering vector q (≤ 0.4 Å⁻¹) decreased with increasing lithium-ion concentration. From the low q data, the radius of gyration R_g of the polymer chains in SPE was estimated by the Guinier method. For both PEO and P(EC/EO) systems, with increasing the lithium-ion concentration (i.e., molar ratio of lithium ion to monomer, r_s), R_g(r_s) first decreased and then increased. Furthermore, the normalized R_g(r_s) by R_g(0) exhibited a minimum at r_s ≈ 0.15 for the PEO systems, whereas the P(EC/EO) system displayed a decrease in R_g to a higher r_s (≈ 0.20) followed by a weak re-increase. Previous MD simulations (Y. Doi, et al., J. Phys. Chem. C2022, 126, 20,284) in the P(EC/EO)/LiTFSI systems have reported preferential coordination of carbonyl oxygens to lithium ions and an increase in the coordination number of oxygen atoms (resulting in the disrupted coordination structure) compared to the corresponding PEO systems, which can well explain the results in this study. The above experimental facts in this study are important findings in that they include data on new polymer systems other than PEO, while the number of reports for polymer conformation studies of SPE systems by SANS is still limited.