Article pubs.acs.org/Macromolecules
Phase Structure and Helical Jump Motion of Poly(ethylene oxide)/ LiCF3SO3 Crystalline Complex: A High-Resolution Solid-State 13C NMR Approach Ling Wei, Qinghua Liu, Yangwen Gao, Yefeng Yao, Bingwen Hu,* and Qun Chen* Physics Department & Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, P. R. China S Supporting Information *
ABSTRACT: By employing solid-state high-resolution 13C nuclear magnetic resonance (NMR), we found that the helical jump motion of crystalline poly(ethylene oxide) (PEO) segments only exists for the PEO3/LiF3SO3 complexes with the molecular weights of PEO larger than 2 × 103 g mol−1, and the helical jump rate increases with increasing the molecular weight of PEO. It is demonstrated that the helical jump rate of crystalline PEO segments depends on the relative content and chain mobility of the amorphous structures for PEO−alkali metal salt complexes. The sufficient amount of amorphous phase is the necessary condition for the helical jump motion to happen, and the chain motion in the amorphous phase might be the driving force for the helical jump motion of the crystalline PEO segments. On the basis of the above recognition, we tend to believe that the helical jump motion is corresponding to the movement of an entire PEO chain embedded in the crystallites.
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INTRODUCTION Solid polymer electrolytes (SPEs) have attracted great research interests since the 1970s due to their potential application in all-solid rechargeable batteries.1−5 Efforts have been focused on improving the ionic conductivity of SPEs at ambient temperatures.6−8 Recently, a series of highly crystallized polymer electrolytes based on poly(ethylene oxide) and alkali metal salts, i.e. PEO/XAsF6 (X = Na, K, and Rb), were found to exhibit remarkable conductivity (1 × 10−6 S cm−1) at room temperature.9 This observation is contrary to the traditional recognition that the ionic conduction can only happen in the amorphous phases of SPEs6,10−12 and motivates the research interests over the conduction mechanism of these PEO−alkali metal salt complexes.9,13−17 By employing high-resolution solid-state 13C 2D-exchange nuclear magnetic resonance (NMR), we demonstrated in our previous work that there exists marked helical jump motion of the crystalline PEO segments in the PEO/LiClO4 system at ambient temperatures.18 This phenomenon was further discovered in other sample systems based on PEO and alkali metal salts, such as PEO/LiCF3SO3,19 PEO/LiAsF6, and PEO/ NaClO4 (not yet reported), implying that it might be universal for PEO−alkali metal salt complexes. In addition, comparing with neat crystalline PEO, the signal of the crystalline PEO segments in the PEO−alkali metal salt complexes shows high resolution.18,19 Such a unique high-resolution nature of 13C NMR spectra provides not only the possibility of understanding the ionic conduction mechanism of crystalline polymer © XXXX American Chemical Society
electrolytes but also the opportunity of in-depth study over the helical jump motion. This contribution focuses on the investigation of the helical jump motion, which is crucial for understanding many fundamental characteristics of semicrystalline polymers.20−29 Helical jump motion can cause chain diffusion between the crystalline phase and the amorphous phase.22,28 Although whether it is induced by the movements of the whole stems or the motions of defects is still controversial,27 it is believed that chain translation through the crystalline phase must be accommodated by the movement of chain segments in the amorphous phase.29 As the continuation of the aforementioned reports, a series of PEO3/LiCF3SO3 complexes with different molecular weights of PEO (including PEO-oligomers, which are also called PEO hereafter) are investigated in the present work by 13C 2Dexchange NMR with cross-polarization/magic angle spinning (CP/MAS) at room temperature. The purpose of this work is to explore the possible structural factors associated with the helical jump motion of the crystalline PEO segments within the crystalline complex and to discuss the mechanism of this specific type of motion on a segmental level by taking advantage of the unique high-resolution nature of the 13C NMR spectra of this sample system. Received: April 1, 2013 Revised: May 21, 2013
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dx.doi.org/10.1021/ma400673y | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
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Article
EXPERIMENTAL SECTION
Sample Preparation. Polymer complexes were prepared by solvent casting. All of the chemical reagents were purchased from Aldrich. LiCF3SO3 was dried in a vacuum at 120 °C for 24 h before use. PEO (average Mn = 1 × 103 and 2 × 103 g mol−1, abbreviated as 1K and 2K) was dried in a vacuum at 25 °C for 24 h because of the low melting point (32 °C). PEO (average Mn = 4 × 103, 6 × 103, 1 × 104, 1 × 105, and 5 × 106 g mol−1, abbreviated as 4K, 6K, 10K, 100K, and 5M) was dried in a vacuum at 45 °C for 24 h before use. Seven samples with the molar ratio of ether oxygen atoms to lithium ions of 3:1 were prepared by using LiCF3SO3 and PEO with various molecular weights. Stoichiometric amounts of LiCF3SO3 and PEO were dissolved in acetonitrile and stirred continuously for 24 h at room temperature. The solution was then cast on a Teflon plate, and the solvent was allowed to evaporate gradually in dry air at room temperature. The obtained samples were finally dried in a vacuum for 5 days at 65 °C and kept at least 2 weeks at room temperature before use. Solid-State NMR Experiments. The solid-state NMR experiments were performed on a Bruker AVNCE 600 spectrometer operating at 600.13 MHz for 1H and 150.96 MHz for 13C. The obtained samples were packed into rotors inside a nitrogen atmosphere glovebox. The 13C chemical shifts were determined from the carbonyl carbon signal (176.03 ppm) of glycine relative to tetramethylsilane. A contact time of 100 μs was used for the CP/MAS experiments. The acquisition time was 50 ms, the spin rate was 5 kHz, and the recycle delay was 5 s. The two-pulse phase-modulation (TPPM) decoupling was used during the acquisition.30 The decoupling power was about 42 kHz. The 13C 2D-exchange NMR spectra were taken with 76 t1 increments and 1.20 s mixing time (tm) at 293 K. Measuring time was about 17 h for each spectrum. X-ray Diffraction Measurements. X-ray diffraction measurements were performed with synchrotron beamline 14B at Shanghai Synchrotron Radiation Facility. All of the obtained samples were sandwiched in Mylar membrane in a nitrogen atmosphere glovebox to avoid moisture. The diffraction patterns of the samples were recorded from 3° to 40° (2θ), with a step rate of 0.02°. The diffraction pattern of the background was also collected. The wavelength of the incident monochromatic X-ray is 1.24 Å, and the effective X-ray energy is 10 keV.
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RESULTS AND DISCUSSION Phase Structures of PEO3/LiCF3SO3 Complexes with Different Molecular Weight of PEO. PEO and LiCF3SO3 can form a crystalline complex with the molar ratio of ether oxygen atoms to lithium ions of 3:1 (denoted as PEO 3:LiCF3SO3). The structure of the PEO3:LiCF3 SO3 crystalline complex was solved from powder X-ray diffraction data in 1993 by Bruce et al.31 Figure 1 shows the powder X-ray diffraction patterns of the PEO3/LiCF3SO3 complexes with different molecular weight of PEO (average Mn = 1K, 2K, 4K, 6K, 10K, 100K, and 5M). The peak positions in Figure 1a are consistent with those reported in the previous work,31 indicating the existence of the PEO3:LiCF3SO3 crystalline complex. It can be clearly seen from Figure 1a−g that the peak positions and relative intensities in the X-ray diffraction patterns of the PEO3/LiCF3SO3 complexes with various molecular weights of PEO (average Mn = 1K, 2K, 4K, 6K, 10K, 100K, and 5M) are almost the same. This clearly indicates that the molecular weight of PEO has no influence on the structure of PEO3:LiCF3SO3 crystalline complex. However, the half-widths of the diffraction peaks in Figure 1a,b are narrower than those of the diffraction peaks in Figure 1c−g, indicating higher crystallinities of the 1K-PEO3/LiCF3SO3 and 2K-PEO3/ LiCF3SO3 complexes.32 It must be noted that the half-widths of
Figure 1. X-ray diffraction patterns of the PEO3/LiCF3SO3 complexes with the molecular weights of PEO of (a) 1K, (b) 2K, (c) 4K, (d) 6K, (e) 10K, (f) 100K, and (g) 5M.
the diffraction peaks in Figure 1a are even narrower than those of the diffraction peaks in Figure 1b, implying a higher crystallinity and a larger average grain size of the 1K-PEO3/ LiCF3SO3 complex.33 The crystallinities of the PEO3/LiCF3SO3 complexes with various molecular weights of PEO (average Mn = 1K, 2K, 4K, 6K, 10K, 100K, and 5M) were measured by 1H wide-line NMR.34,35 The 1H wide-line NMR spectra of the complexes consist of a narrow peak overlapping on a broad one. Because the intense chain mobility of the amorphous phase has averaged out 1H−1H dipolar interactions, the 1H wide-line NMR signal of this phase appears as a narrow peak. The 1H signal of the crystalline phase is rather broad because that the PEO chain B
dx.doi.org/10.1021/ma400673y | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
Figure 2. 13C 2D-exchange spectra of the PEO3/LiCF3SO3 complexes with the molecular weights of PEO of (a) 1K, (b) 2K, (c) 4K, (d) 6K, (e) 10K, (f) 100K, and (g) 5M. The spectra are acquired with tm of 1.20 s at 293 K. The 13C CP/MAS spectrum of each complex is plotted on the top of the corresponding 13C 2D-exchange spectrum with a contact time of 100 μs. The inset is the schematic diagram of the helical jump motions between the corresponding carbon atoms. The structure of the PEO3:LiCF3SO3 crystalline complex is also shown: PEO chain adopts a helical conformation (green: carbon; red: oxygen); lithium cation (purple sphere) is coordinated by three ether oxygen atoms. The anion groups are not shown.
is believed that the jump motion can cause chain diffusion between the crystalline phase and the amorphous phase.22,28 Despite extensive studies on the helical jump motion, whether it is induced by the movements of the whole stems or the motions of defects remains controversial.27 To study this jump motion of the crystalline PEO segments in PEO−alkali metal salt complexes is of special significance because of the possible correlation between the metal ion transportation and the helical jump motion within the crystalline complex. The helical jump motion has been widely studied by 13C 2D-exchange NMR.18,19,21−24,28 The principle can be described as follows: if the chemical shift of a spin changes to another site during a given mixing time, denoted as tm, it will give a pair of crosspeaks in the 2D-exchange spectrum with two chemical shift axes. The intensity of the cross-peak represents the quantity of exchange during tm. The high-resolution nature of the 13C NMR spectra of these PEO−alkali metal salt complexes allows us to monitor the jump motion on the monomer level. Figure 2 is the 13C CP/MAS and 2D-exchange spectra of the PEO3/LiCF3SO3 complexes with various molecular weights of PEO (average Mn = 1K, 2K 4K, 6K, 10K, 100K, and 5M). The 13 C CP/MAS spectrum of each complex is plotted on the top of the corresponding 13C 2D-exchange spectrum. The resolution of the 13C CP/MAS spectrum of neat PEO at room temperature is poor due to the inefficient heteronuclear dipolar decoupling as a result of the interference between the molecular motion and 1H dipolar decoupling at room temperature.45 However, in the PEO−alkali metal salt complexes, the mobility of the crystalline PEO segments is slowed down by the coordination between the ether oxygen atoms and the lithium ions, which also leads to efficient heteronuclear dipolar decoupling and then the high-resolution feature of the NMR spectra of the crystalline PEO segments. In
mobility of this phase is restricted by the crystal lattice compared with that of the amorphous phase. Therefore, the narrow and broad peaks in 1H wide-line NMR spectra are corresponding to the amorphous and crystalline phases, respectively. Their relative contents and subsequently the crystallinities can be obtained quantitatively by peak deconvolution. In addition, the line width of the amorphous signal in the 1H wide-line NMR spectra can be used to determine the dynamics of the amorphous phase, which will be discussed later. The crystallinities of the 1K-PEO3/LiCF3SO3 complex (92%) and 2K-PEO3/LiCF3SO3 complex (90%) are very high, which are also supported by the X-ray diffraction results as shown in Figure 1a,b. For the PEO3/LiCF3SO3 complexes with the molecular weights of PEO of 4K, 6K, and 10K, the crystallinities are 82%, 80%, and 81%, which are almost the same. The crystallinity of the 100 K-PEO3/LiCF3SO3 complex decreases to 76%. For the 5M-PEO3/LiCF3SO3 complex, the crystallinity is only 66%. These results suggest that although the structure of the PEO3:LiCF3SO3 crystalline complex does not vary with the molecular weight of PEO, the phase structures as reflected in the crystallinities of the PEO3/LiCF3SO3 complexes change greatly with the variation of the molecular weight of PEO. It is well-known that PEO crystallizes as lamellae with extended chains when the molecular weight is low (ca.
