Li+ Complex

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Annealing To Induce Formation of Defects in Polyether/Li+ Complex Crystals − A Way To Significantly Enhance the Crystalline Segmental Mobility Wei Wang, Jin-Ze Wu, Jiachen Wang, Qun Chen, and Ye-Feng Yao* Physics Department & Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials Science, East China Normal University, North Zhongshan Road 3663, 200062 Shanghai, P. R. China Downloaded via BUFFALO STATE on August 2, 2019 at 04:49:11 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: In this work, we demonstrate that a deliberately designed annealing process can induce formation of defects in polyether/Li+ complex crystals. The Li+ concentration diffusion in the complex crystals and the amorphous structures is considered to play a critical role in the formation of crystal defects. The 13C 2D exchange NMR shows that the annealing-induced crystal defects can greatly enhance the segmental mobility in the polyether/Li+ complex crystals. The rate of helical jump motions in the deliberately annealed polyether/Li+ complex crystals is ∼100 times higher than those reported in the literature. This work deepens our understanding of the defect formation in polyether/Li+ complex crystals and has important implications on the ionic conduction enhancement of polyether-based crystalline polymer electrolytes.

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INTRODUCTION Defects have important influences on the chain dynamics in polymer crystals and consequently the macroscopic material properties of polymeric materials.1−4 For example, in polyethylene crystals, the crystal defects can initiate the 180° helical jumps and the diffusion of the whole chain stem,5−7 which is strongly coupled to the crystal melting,8 the lamella thickening,9 and so on. In polyethylene oxide (PEO)/M+ (M+: alkali metal ion, Li+, Na+, and so on) complex crystals, the crystal defects can initiate segmental dynamics and the long-range diffusion of the crystalline stems,10,11 which have been considered as the molecular origin of the M+ transport in the crystals.12−15 For PEO/M+ complex crystals, introducing defects into the crystals seems to provide a way to enhance the chain mobility and the ionic conductivity. Annealing in polymer science is a commonly used thermal treatment wherein the polymeric materials are kept at a certain temperature for a while and then cooled to room temperature.16−22 Via annealing, it is possible for polymeric materials to achieve reduction of the internal stresses and strains introduced during the fabrication, reduction or elimination of defects, and thus improvement of physical properties.23−25 On a molecular level, annealing of polymeric materials often results in a significant reorganization of polymer chains in the different phase structures.26−29 For polymer crystals, annealing-induced chain reorganization often results in a more ordered packing arrangement of polymer chains and in turn an enhanced crystal perfection.30,31 It is often observed that defects in polymer crystals are reduced or even eliminated after annealing. © XXXX American Chemical Society

In this work, we will show that a deliberately designed annealing process can induce defect formation in PEO/Li+ complex crystals. The Li+ concentration diffusion, which can equilibrate the Li+ concentrations in the different phase structures (e.g., the amorphous phases and the crystalline phases), plays a critical role in the defect formation in the polyether/Li+ complex crystals. The crystal defects were probed by monitoring the defect-induced helical jump motions in the crystals. 13C 2D exchange NMR was used to study the defect-induced helical jump motions. The results show that the rate of helical jump motion in the deliberately annealed polyether/Li+ complex crystals is ∼100 times higher than that reported in the literature,11 indicating the presence of a large number of defects in the complex crystals. Our work is of great significance for our understanding of the defect formation in PEO/Li+ complex crystals, which is closely related to the ionic conduction mechanism of crystalline polymer electrolytes.

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EXPERIMENTAL SECTION

Sample Preparation. All the chemical reagents were purchased from Aldrich. Stoichiometric amounts of poly(ethylene oxide)-blockpoly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-bPEO) (Mn = 8.4 × 103) were dissolved in acetonitrile with lithium trifluoromethanesulfonate (LiCF3SO3) according to different molar ratios of ether and propyl ether oxygen atoms to lithium ions. Then, the solution was stirred continuously for 24 h at room temperature Received: June 4, 2019

A

DOI: 10.1021/acs.macromol.9b01086 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules and dried in an air-circulating oven for 4 days at 40 °C (313 K) to remove the solvent. The obtained sample was equally divided into two portions. One portion was placed in a vacuum oven at 40 °C (313 K) for 1 week to remove residual solvent. The other portion was annealed in a vacuum oven at 60 °C (333 K) for 1 week. Afterward, the samples were stored in a glovebox. The prepared samples are listed in Table 1.

complex crystals (see Figure S4). The big width of the melting peak indicates that the EO3/LiCF3SO3 complex crystals are not well crystallized. The coexistence of the PEO crystals and the EO3/LiCF3SO3 complex crystals in this sample has been confirmed by the XRD patterns in Figure 1c where the characteristic reflection peaks of the PEO crystal ([120] 2θ = 19°, [032] 2θ = 23°) and the EO3/LiCF3SO3 crystal ([110] 2θ = 12°, [002] 2θ = 20°, [120] 2θ = 21°) are clearly resolved.32 Figure 1b shows the DSC curve of AN-COPEO-1-6. Only one wide melting peak at 143 °C (416 K) is observed. The big width of the melting peak indicates that the annealing process does not result in an improvement in the perfection of the EO3/LiCF3SO3 complex crystals. In the DSC curve, the melting peak of the PEO crystal completely disappears, indicating that the annealing process has completely removed the PEO crystals in this sample. The disappearance of the PEO crystals is further confirmed by the XRD pattern in Figure 1d where only the diffraction peaks of the EO3/LiCF3SO3 crystal can be observed. We have calculated the sample crystallinity from the DSC measurement. The details can be found in the Supporting Information. Table 2 lists the crystallinity of SC-COPEO-1-6

Table 1. Sample Specifications sample

[Li]/[ O]a

preparation

SC-COPEO-1-x AN-COPEO-1-x

1:x 1:x

solution crystallization annealing

a

x = 3, 4.5, 6, 12.

In the following study, four samples, SC-COPEO-1-x, ANCOPEO-1-x (x = 3, 6), were selected for the investigation. For the other samples, the detailed characterization and study can be found in the Supporting Information (see Figures S1 and S2). Differential Scanning Calorimetry. The thermal behavior of complexes was characterized by differential scanning calorimetry on a TA Q2000 instrument. Approximately 3 mg of the sample was sealed in an aluminum pan. The samples were heated from −40 °C (233 K) to 180 °C (453 K), and the heating rate was 10 K/min. We also performed the DSC measurements using different heating rates. The result shows that the heating rate does not result in clear changes in the DSC curve (Figure S3). X-Ray Diffraction Measurements. X-ray diffraction measurements were performed on a Bruker D8 ADVANCE using Cu Kα (1.5406 Å) radiation (35 kV, 25 mA). The diffraction patterns of the samples were recorded from 5 to 60° (2θ) at a speed of 10°/min. The experimental temperature was room temperature. Solid-State NMR Experiments. The 13C solid-state NMR experiments were performed on a Bruker AVANCE III 600 WB spectrometer operating at 600.44 and 150.11 MHz for 1H and 13C, respectively. A 4 mm double resonance probe was used for the experiments. The spin rate was set to 10 kHz. In the 1H−13C cross polarization (CP)/magical angle spinning (MAS) experiments, the CP time was 1 ms. 1H two-pulse phase modulation (TPPM) decoupling was applied during the 13C signal acquisition. The 13C chemical shift was referred to adamantane (38.56 ppm). The samples were sealed into rotors inside an argon atmosphere glovebox before NMR measurements.

Table 2. Crystallinity of SC-COPEO-1-6 and AN-COPEO1-6 crystallinity of SC-COPEO-1-6

crystallinity of AN-COPEO-1-6

31%

38%

and AN-COPEO-1-6. It shows that the amount of EO3/ LiCF3SO3 complex crystals increases after the annealing process. Note that the crystallinity in Table 2 refers to the weight ratio of the EO3/LiCF3SO3 complex crystals over the weight of the whole sample. Figure 2a shows the DSC curve of SC-COPEO-1-3. Only one narrow melting peak at 163 °C (436 K) is observed. This

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RESULTS AND DISCUSSIONS Annealing Effects on the Phase Structures. Figure 1a shows the DSC curve of SC-COPEO-1-6. Two melting peaks are observed. The narrow peak at 54 °C (327 K) can be assigned to the melting peak of the PEO crystals (see Figure S4), while the wide peak at 144 °C (417 K) can be assigned to the melting peak of polyether/Li+ (i.e., EO3/LiCF3SO3)

Figure 2. DSC curves of (a) SC-COPEO-1-3 and (b) AN-COPEO-13. XRD patterns of (c) SC-COPEO-1-3 and (d) AN-COPEO-1-3. The heating rate in the DSC experiments was 10 K/min. For the XRD experiments, the experimental temperature was room temperature.

melting peak can be assigned to the EO3/LiCF3SO3 complex crystals. Compared with those of SC-COPEO-1-6, the elevated melting point and the reduced width of the melting peak indicate the formation of more perfect EO 3/LiCF3SO3 complex crystals in this sample. Figure 2b shows the DSC curve of AN-COPEO-1-3. It is observed that the DSC curve of AN-COPEO-1-3 is almost the same as that of SC-COPEO-1-3, indicating that the annealing process does not induce clear changes in the phase structures of SC-COPEO-1-3. This is also confirmed by the XRD pattern in Figure 2c,d where the diffraction peaks of the EO3/LiCF3SO3 crystal are almost the same.

Figure 1. DSC curves of (a) SC-COPEO-1-6 and (b) AN-COPEO-16. XRD patterns of (c) SC-COPEO-1-6 and (d) AN-COPEO-1-6. The heating rate in the DSC experiments was 10 K/min. For the XRD experiments, the experimental temperature was room temperature. B


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Macromolecules

peaks are observed in the spectra, indicating the presence of the helical jump motions in these samples. To quantify the jump motion rate, a three-site exchange (e.g., site 1 ↔ site 5 ↔ site 3; see the cartoon picture in Figure 3) has been introduced to model the process. Based on this model, the exchange rate (i.e., the jump rate), k, can be calculated from the ratio between the cross-peak intensity and the diagonal peak intensity by using the following equation:

We have also calculated the crystallinity of SC-COPEO-1-3 and AN-COPEO-1-3 from the DSC measurements (see Table 3). It is shown that AN-COPEO-1-3 has a higher crystallinity Table 3. Crystallinity of SC-COPEO-1-3 and AN-COPEO1-3 crystallinity of SC-COPEO-1-3

crystallinity of AN-COPEO-1-3

52%

67%

5→1 across 5 adiag

than SC-COPEO-1-3, indicating that the annealing process facilitates the formation of the EO3/LiCF3SO3 complex crystals. Probing Helical Jump Motions in the EO3/LiCF3SO3 Complex Crystals. In the literature, the helical jump motions of EO segments in PEO/Li+ complex crystals can be probed by solid-state NMR.11,15,33,34 The same method has been used in this study to probe the helical jump motions in the complex crystals of AN-COPEO-1-6 and AN-COPEO-1-3. Figure 3a exemplarily shows the 13C CP/MAS NMR spectrum of ANCOPEO-1-3. According to the literature,11 the signals in the spectrum can be assigned (see the cartoon picture in Figure 3). Figure 3b,c shows the 2D 13C exchange NMR spectra of ANCOPEO-1-6 and AN-COPEO-1-3 at 293 K. The clear cross

=

5→3 across 5 adiag

=

1 − e−3kt 1 + 2e−3kt

In the above equation, a refers to the intensity of the diagonal/ cross peak, and t is the exchange time. The calculation yields a jump rate of 0.21 s−1 for AN-COPEO-3-1 at 293 K. For ANCOPEO-1-6, the jump rate of the crystalline segments is 61.5 s−1 at the same temperature, which is ∼300 times higher than that of AN-COPEO-1-3 and ∼100 times higher than that reported in the literature.11 The fast helical jump motions in AN-COPEO-1-6 are confirmed by the temperature-dependent 13C CP/MAS spectra in Figure 4. For a comparison, the 13C CP/MAS spectra of

Figure 4. Temperature-dependent 13C CP/MAS spectra of ANCOPEO-1-6 and AN-COPEO-1-3. The experimental temperature ranges from 290 to 320 K.

AN-COPEO-1-3 acquired using the same condition are also shown in Figure 4. With increasing temperature, the signals in the spectra of AN-COPEO-1-6 show a clear gradual decrease in the signal resolution. At 320 K, the signals almost merge together and look like a very wide peak. Because the melting point of the EO3/LiCF3SO3 complex crystals is well above 320 K, the change in the signal resolution cannot be attributed to the crystal melting. In the literature, it is known that motions in the slow intermediate exchange regime can cause the signal broadening.36 As the exchange rate is increased toward the crossover point, the distinct peaks will move together and finally merge. We thus attribute the signal broadening in the temperature-dependent 13C NMR spectra of AN-COPEO-1-6 to the motional modulation of the signal line shape. In contrast, the signals in the spectra of AN-COPEO-1-3 are almost constant in the temperature range from 290 to 320 K, indicating the very slow jump motions in the crystals of this sample. In the literature, the origin of the jump motions in polymer crystals is strongly coupled to the crystal defects.11−15 The significantly enhanced jump motions in the complex crystals of AN-COPEO-1-6 thus indicate the presence of a high amount of crystal defects in this sample. Comparison between ANCOPEO-1-6 and AN-COPEO-1-3 shows that these two samples have similar chemical natures (prepared using the

Figure 3. (a) The 13C CP/MAS NMR spectrum of AN-COPEO-1-3. (b) The 13C 2D exchange spectrum of AN-COPEO-1-6. The exchange time was 10 ms. (c) The 13C 2D exchange spectrum of AN-COPEO-1-3. The exchange time was 1.2 s. The experimental temperature was 293 K. More 13C 2D-exchange spectra of these samples are given in the Supporting Information (Figure S5). In order to quantify the exchange process, one-dimensional slices were extracted from the 13C 2D-exchange spectra at 71.2 ppm. The extracted slices are shown in Figure 3b,c. The signal decomposition was achieved by using the DMFIT software.35 A cartoon picture is shown in Figure 3a to illustrate the signal exchanges from the helical jump motions among sites 1, 3, and 5 (e.g., site 1 ↔ site 5 ↔ site 3). C


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ence between the complex crystals and the surrounding amorphous structures. Therefore, different from SC-COPEO1-3, during the annealing process of SC-COPEO-1-6, it is very likely that there will be a big Li+ concentration difference between the complex crystals and the surrounding amorphous structures. To minimize the Gibbs free energy induced by the Li+ concentration difference, the Li+ ions in the complex crystals will have a large tendency to diffuse from the complex crystals into the surrounding phase structures. The relatively high temperature during the annealing process may provide the thermal energy for the diffusion. As the Li+ ions in the complex crystals diffuse into the surrounding amorphous structures, the defects are introduced into the complex crystals. Note that in most cases, polymer crystallization consists of complicated dynamic and kinetic processes. For the copolymer/Li+ complex systems in this work, the formation of the different phase structures (i.e., the EO3/LiCF3SO3 complex crystals, the PEO crystals, and the EO3/LiCF3SO3 complex amorphous phase) strongly depends on the preparation conditions. The DSC results show that the annealing process has resulted in the increase of the amount of complex crystals. This strongly indicates that the annealing condition can facilitate the formation of EO3/LiCF3SO3 complex crystals in the samples. For AN-COPEO-1-6, the EO 3 /LiCF 3 SO 3 complex crystals have a relatively high Li+ concentration where the complex amorphous structures likely have a low Li+ concentration. The more EO3/LiCF3SO3 complex crystals there are, the less the Li+ concentration in the amorphous structures. The formation of more EO3/LiCF3SO3 complex crystals thus may further intensify the Li+ concentration difference between the complex crystals and the amorphous structures. Therefore, the Li+ concentration diffusion, driving the Li+ in the complex crystals into the surrounding amorphous structures, most likely provides a plausible mechanism to balance the different Li+ concentrations in the different phase structures and in turn minimize the local Gibbs free energy.

same copolymer and Li salts) and the same thermal treatment procedure. The only difference between the two samples lies on the different doping amount of Li salts. What follows next is to find how the different amounts of Li salts can cause the different amounts of crystal defects in AN-COPEO-1-6 and AN-COPEO-1-3. Li+ Concentration Diffusion between Crystals and Amorphous Regions during Annealing. The DSC results demonstrate that the annealing process can result in a significant reorganization of polymer chains in the polyether/ Li+ complexes. Accompanying the reorganization of polymer chains, the Li+ ions in the samples most probably will have a redistribution in the different phase structures. The driving force of the redistribution of Li+ ions in the sample is likely to be the Li+ concentration difference in the different phase structures. This provides the basis for the following discussion about how the annealing process can induce defects in the EO3/LiCF3SO3 complex crystals. According to the preparation procedure, SC-COPEO-1-3 has a doping [Li]/[O] molar ratio of 1:3. In the crystal structure of this sample, the [Li]/[O] molar ratio is 1:3. Therefore, the [Li]/[O] molar ratio in the amorphous phase should also be 1: 3. Namely, there is no Li+ concentration difference between the crystals and the amorphous phase (see Figure 5a). Thus, the presence of the Li+ concentration diffusion between the crystals and the amorphous phase during the annealing process is not expected.

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CONCLUSIONS In this work, we demonstrate that the amount of defects in the polyether/Li+ complex crystals can be significantly increased by applying the deliberately designed annealing process on the sample having a proper Li+ concentration. The formation of the crystal defects is strongly associated with the Li + concentration difference between the different phase structures in the samples. We propose that the Li+ concentration diffusion, which can equilibrate the Li+ concentrations in the different phase structures, could drive the Li+ ions out of the polyether/Li+ complex crystals (i.e., the highly concentrated components) to the surrounding phase structures (i.e., the low-concentration components) and in turn induce the defects in the polyether/Li+ complex crystals. We used the 13C 2D exchange NMR to probe the defect-induced helical jump motions in the complex crystals. The results show that the rate of helical jump motions in the deliberately annealed polyether/ Li+ complex crystals is ∼300 times higher than that of the polyether/Li+ complex crystals having much fewer defects and ∼100 times higher than that reported in the literature.16 The observations in this work deepen our understanding of the defect formation mechanism in polyether/Li+ complex crystals. Because the presence of a high amount of crystal defects can greatly facilitate the chain motions in the polyether/Li+ complex crystals, this work demonstrates a way to enhance

Figure 5. Illustration of the annealing-induced changes in the structure and dynamics in (a) SC-COPEO-1-3 and (b) SC-COPEO1-6.

For SC-COPEO-1-6, the DSC and X-ray diffraction data indicate that this sample consists of both the complex crystals and pure PEO crystals. Because the doping ratio of this sample is 1:6 while the [Li]/[O] ratio in the complex crystals is 1:3, the averaged [Li]/[O] ratio of the overall other phase structures but excluding the complex crystals should be far below 1:6. Therefore, it is highly possible that there will be a large Li+ concentration difference between the complex crystals and the surrounding phase structures (e.g., the complex amorphous phase, the PEO crystals, and so on; see Figure 5b). When annealing the samples at 60 °C (333 K), which is below the melting temperature of the PEO3/LiCF3SO3 crystal but well above the melting temperature of the PEO crystals, the complex crystals will remain in the samples, whereas the PEO crystals will melt. Note that the PEO melts are free of Li+ ions. The PEO melts, if they penetrate into the complex amorphous structures, will further dilute the Li+ concentration in the complex amorphous, increasing the Li+ concentration differD


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the segmental mobility in the polyether/Li+ complex crystals, which has important implications on the ionic conduction enhancement of polyether-based crystalline polymer electrolytes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01086. Crystallinity calculation, DSC curves, XRD patterns, 2D13C exchange NMR spectra, and helical jump motion rates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ye-Feng Yao: 0000-0002-6274-2048 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC (21574043) and the Fundamental Research Funds for the Central Universities.

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