Ion and polymer dynamics in polymer electrolytes PPO-LiClO4. I. Insights from NMR line-shape analysis

Abstract

We investigate ion and polymer dynamics in polymer electrolytes PPO-LiClO4 performing 2H and 7Li NMR line-shape analysis. Comparison of temperature dependent 7Li and 2H NMR spectra gives evidence for a coupling of ion and polymer dynamics. 2H NMR spectra for various salt concentrations reveal a strong slowdown of the polymer segmental motion when the salt content is increased. The 2H NMR line shape further indicates that the segmental motion is governed by dynamical heterogeneities. While the width of the distribution of correlation times G(log tau) is moderate for low and high salt content, an extremely broad distribution exists for an intermediate salt concentration of 15:1 PPO-LiClO4. For the latter composition, a weighted superposition of two spectral components, reflecting the fast and the slow polymer segments of the distribution, describes the 2H NMR line shape over a broad temperature range. Analysis of the temperature dependent relative intensity of both spectral components indicates the existence of a continuous rather than a discontinuous distribution G(log tau). Such continuous distribution is consistent with gradual fluctuations of the local salt concentration and, hence, of the local environments of the polymer segments, whereas it is at variance with the existence of large salt-depleted and salt-rich domains featuring fast and slow polymer dynamics, respectively. Finally, for all studied PPO-LiClO4 mixtures, the 2H NMR line shape strongly depends on the echo delay in the applied echo-pulse sequence, indicating that the structural relaxation of the polymer segments involves successive rotational jumps about small angles gamma < 20 degrees .

Interpretation of 1H and 2H spin–lattice relaxation dispersions: Insights from molecular dynamics simulations of polymer melts

“…2 H SLR is, however, different for deuterons in the methyl groups. These results are consistent with findings in experimental work on perdeuterated PPO [33][34][35]. There, it was reported that 2 H SLR occurs in two steps, which are related to deuterons in the methyl groups and the polymer backbone.…”

Solid-state Li NMR with applications to the translational dynamics in ion conductors

“…Since these conditions are not necessarily met for ion dynamics in all polymer electrolytes, it is an open question whether this approach can be applied to other materials. Although the motional narrowing of the 7 Li central line was often used for a study of lithium ionic motion in polymer electrolytes, it was also emphasized that a straightforward interpretation of the experimental results is in general not possible [429,438], see also Section 3.4.2.3. Specifically, it is necessary to consider the fact that the main contribution to the width of the central line results from 7 Li-1 H dipolar couplings.…”

“…By contrast polypropylene oxide (PPO), -[CH 2 CHCH 3 O] n -, is purely amorphous [402,80,[425][426][427][428][429]. To reduce the tendency of PEO towards crystallization, it is possible to crosslink the polymer chains or to synthesize copolymers [420,421,[430][431][432].…”

“…A number of studies [419,425,[427][428][429][430]432,[436][437][438] exploited the motional narrowing of the 7 Li line-shape to investigate lithium dynamics in polymer electrolytes. In most cases, the motional narrowing sets in near T g , i.e., it takes place in the so-called rubbery state.…”

2H and 13C NMR studies on the temperature-dependent water and protein dynamics in hydrated elastin, myoglobin and collagen

“…The former setup is described in Ref. [20]. In the latter setup, a home-built probe was placed in a Konti CryoVac cryostat operating with liquid nitrogen and controlled by a TIC 304 MA CryoVac temperature controller.…”