Investigating the Effect of Added Salt on the Chain Dimensions of Poly(ethylene oxide) through Small-Angle Neutron Scattering

Abstract

We have measured the effect of added salt on the chain dimensions of mixtures of poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) in the melt state through small angle neutron scattering (SANS) experiments. Scattering profiles from blends of hydrogenated and deuterated PEO mixed with LiTFSI were measured as a function of salt concentration. Scattering profiles from pure deuterated PEO/LiTFSI mixtures were used for background subtraction purposes. The densities of PEO/LiTFSI mixtures of varying salt concentrations were measured to calculate partial molar monomer volumes of PEO and LiTFSI to account for non-ideal mixing, which turned out to be negligible. Kratky plots of the scattering profiles were used to calculate the salt concentration dependence of statistical segment length. At low salt concentrations, segment length decreases with increasing salt concentration, before increasing with increasing salt concentration in the high salt concentration regime. The Random Phase Approximation was used to predict theoretical scattering profiles from the calculated segment lengths and partial molar volumes; there is excellent agreement between the theoretical and measured scattering profiles at all salt concentrations.There appears to be a correlation between chain dimensions and coordination between lithium ions and EO monomers. The scattering profiles of the pure deuterated PEO/LiTFSI mixtures suggested the presence ofhowed ion clusters of characteristic size of 0.58 6 nm at high salt concentrations. The presence of ion clusters is hypothesized to cause the increase in segment length seen in this salt concentration window.

Segmental Dynamics Measured by Quasi-Elastic Neutron Scattering and Ion Transport in Chemically Distinct Polymer Electrolytes

“…where l is the statistical segment length of the monomer unit. While explicit measurement of statistical segment length using small-angle neutron scattering for PEO as well as for PEO/LiTFSI mixtures has been reported in literature, 21,22 we are not aware of any reports of data for PCEA. It has been shown in Ref 23 that the statistical segment length of a wide variety of polymers is 6 Å when the monomeric reference volume is taken to be 0.1 nm 3 .…”

Comparing Experimental Phase Behavior of Ion-Doped Block Copolymers with Theoretical Predictions Based on Selective Ion Solvation

“…The ionic volumes of Li + and TFSI − are chosen to match experimental measurements of the density of PEO homopolymer melts with varying concentrations of added LiTFSI salts. 35,52 Assuming that the ions can be approximated as hard spheres, the radii of Li + and TFSI − are 1.0 and 3.8Å, respectively, which correspond to ionic volumes of 0.0042 nm 3 and 0.230 nm 3 for Li + and TFSI − , respectively. Out of the parameters in Table 2, the ion solvation radii a i 's are the only parameters that cannot be directly measured.…”

“…x q P x Y r z O W w v G Y m Y P / I D x 9 g n B w J J d < / l a t e x i t > < l a t e x i t s h a 1 _ b a s e 6 4 = " a Z 0 l f e 3 0 X L y F R z 7 m L 6 D V + 7 t e D 5 o = " > A A A B One potential source of inconsistency in f EO is non-ideal mixing of SEO with LiTFSI salts. 52 In our model, the volume of ions is assumed to be constant. However, changes in the partial molar volume of EO monomers with the addition of salt are expected to produce small changes in the parameterized value of f EO .…”

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

“…We note in passing that the volume change of mixing in bulk PEO/LiTFSI mixtures has only recently been measured. 48 We return to the SAXS profiles in Figure 1b. While the TEM, DSC and WAXS data provide an explanation for the disappearance of the second order SAXS peak at 94 and 103 o C, we have no definitive explanation for the appearance of broad third and fourth order SAXS peaks at these temperatures.…”