Phase Behavior of Binary Polymer Blends Doped with Salt

Dec 20, 2017 - Also, at the same r, the curve becomes strongly asymmetric, with the addition of salt driving up Tcp much more rapidly for smaller amou...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Phase Behavior of Binary Polymer Blends Doped with Salt Shuyi Xie† and Timothy P. Lodge*,†,‡ †

Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: We present cloud point measurements on low molecular weight binary polymer blends doped with salts that exhibit unusual phase behavior. These blends include poly(ethylene-alt-propylene)/poly(ethylene oxide) (PEP/PEO) doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI), NaTFSI, KTFSI, LiClO4, and sodium iodide NaI. The addition of salt dramatically decreases the miscibility of the binary blends and results in an asymmetric cloud point profile. The phase behavior is found to be governed by the concentration of the salt, the size of the anion, and the composition of the polymer mixture. The experimental results are compared with a recent theory, which predicts the effect of ions on the polymer phase diagram by taking into account both ion-induced cross-linking and self-energy effects. Furthermore, the coexistence curve of salt-doped PEP/PEO blends is determined quantitatively by 1H NMR spectroscopy when the volume fraction of PEO is maintained at 0.6. The coexistence curve does not coincide with the cloud point profile, which can be attributed to the effect of the redistribution of ions between the two coexisting phases. In the interest of generality, the cloud point profile of polystyrene/poly(ethylene oxide) (PS/PEO) doped with LiTFSI is also mapped out, in which similar phenomena are observed.

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INTRODUCTION The search for electrolytes that offer strong ion dissociation and enable rapid ion transport for e.g. rechargeable Li-ion batteries has become intense.1 Safety issues arise because of the dendritic growth from the lithium metal anode upon cycling. One effective approach is to replace the conventional liquid electrolyte with ion-containing polymer materials of higher mechanical robustness, with lower flammability, while retaining good ionic conductivity.1−3 However, to optimize the two orthogonal properties, mechanical strength and conductivity, a polymeric system containing at least two components, one dissolving ions and having rapid segmental dynamics and the other having a rigid architecture, is usually required, and a welldefined cocontinuous structure, in particular, is greatly desirable.4−6 To help design a polymeric system with multiple components, understanding the phase behavior is an important first step.7 However, relatively little is known about the phase behavior of such salt-doped systems, either block polymers or homopolymer blends. The phase behavior of neat polymer blends or block polymers can be predicted reasonably well on the basis of mean-field theory, in which the binary interaction parameter (χ) illustrates the interactions between two unlike chain segments. However, while χ can be derived both experimentally and theoretically,8−11 the introduction of ions complicates the system significantly, and a clear picture of the effect of ions on the phase behavior has yet to be established. An early theory suggested that increasing the number of charges on a A−B diblock polymer, where A is neutral and B is a charged block, might suppress the spinodal temperature due to the translational entropy of the counterions.12 Subsequently, © XXXX American Chemical Society

the stability limit of a charged diblock polymer was calculated using the random phase approximation (RPA) and selfconsistent-field theory (SCFT), and the critical χ for microphase separation was found to be larger than that of an ion-free system.13 In contrast to the prediction that ions in block polymers may suppress microphase separation, experiments on poly(methyl methacrylate)-b-poly(oligo oxyethylene methacrylate) (PMMA-b-POEM) doped with lithium trifluoromethanesulfonate showed a higher order−disorder transition temperature (TODT).14 Similarly, research on lithium perchloratedoped PEO-containing triblock polymers showed that salt could selectively partition into the PEO domains and result in larger segregation strength to bring up the TODT and enlarge the domain spacing.15 Russell et al. showed a larger but less temperature-dependent effective Flory−Huggins parameter (χeff) for PS-b-PMMA doped with lithium chloride using small-angle neutron scattering (SANS).16 These findings seem to indicate that salt may induce microphase separation in block polymers, which inspired new theories in the past decade. Wang et al. initiated a systematic theoretical attack on the phase behavior of block polymers and blends in the presence of salt and constructed a preliminary model considering the selfenergy of ions.17 Then with the special case of PS/PEO blends doped with lithium salt, this model was updated considering the tight complexation between Li+ and PEO.18−20 Both theories pointed out that χeff increases with salt concentration, while the Received: October 30, 2017 Revised: November 29, 2017

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DOI: 10.1021/acs.macromol.7b02310 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules change in χeff is dependent on polymer composition as well. Similarly, Sing and Olvera de la Cruz have shown that charge correlations play a crucial role in the case of partly charged polymer blends, an effect that is entirely beyond the reach of mean-field theory.21−25 They found that for an otherwise volumetrically symmetric A/B blend the charge correlations induce much greater effective segregation and strong asymmetry of the phase diagram. Interestingly, although there are several recent published experimental results on block polymer plus salt,26−31 there is no consensus on the effect of ions in terms of an effective interaction parameter χeff. Some found that χeff would increase linearly with salt concentration,26,27,30 while others observed a much more complex picture.28,29,31 On the other hand, an experimental picture of the phase behavior of salt-containing polymer blends that can be used to compare with theoretical predictions quantitatively is not available either. To understand the phase behavior of salt-doped binary blends, we focus on PEP/PEO mixtures, in which the ions dissolve in PEO while the other component is nonpolar and hardly dissolves ions. PEO is one of the most commonly used solid polymer electrolytes, and an experimentally determined phase diagram may benchmark comparison between theory and experiment. The concentration of salt is defined by the molar ratio of the cation to the ether oxygen, i.e., r = [Li+]/[EO]. The introduction of salt dramatically increases the cloud point temperature due to the new interactions brought in by the ions. These interactions, which depend on the identity and concentration of the salt, the composition of the blend, and the electric permittivity of polymers, are complicated and make the phase diagram quite asymmetric. Furthermore, the coexistence curve of PEP/PEO blends doped with LiTFSI r = 0.0040 starting with ϕPEO = 0.6 is determined, but does not coincide with the cloud point profile. We hypothesize that with the addition of salt the system can no longer be treated as a binary system due to the different partitioning of ions into the two phases. In addition, the cloud points as well as the coexistence curves are compared quantitatively with a recent model developed by Ren, Nakamura, and Wang,32 and the selfenergy of ions and ion-induced cross-linking effect are systematically analyzed.

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Table 1. Characteristics of Salts and Polymers Mn (g/mol)

component

a,b

PEO PEP (squalane) LiTFSI NaTFSI KTFSI NaI LiClO4

222 423b 287 303 319 150 106

Nc

Đ = Mw/Mn

ρ (g/mL)

3.7 8.7

0.4. As mentioned above, as ϕPEO decreases, the salt may form ion pairs or even precipitate out, thus accounting for the differences in Tcp at r = 0.0016 and ϕPEO < 0.4. More quantitatively, Figure 6 demonstrates the difference in Tcp of LiTFSI and LiClO4-doped PEP/PEO blends due to the

component, although only present in tiny amounts, can bring significant changes to the free energy of mixing. A related phenomenon occurs in binary polymer mixtures where one component is disperse.54 Beyond the LiTFSI-Doped PEP/PEO System. After mapping out the cloud point and coexistence curve of the LiTFSI-doped PEP/PEO blends, it is of interest to vary the identity of the salt (e.g., Li+, Na+, and K+ as cations and TFSI, I−, and ClO4− as anions) or the polymer component (e.g., polystyrene/poly(ethylene glycol dimethyl ether) (PS/PEO)). First, to systematically explore the role of ion size in this system, four other types of salts, NaTFSI, KTFSI, LiClO4 and NaI, have been investigated at r = 0.0016 and 0.0040, as shown in Figure 5. Note that at r = 0.0016 the cloud points of PEOrich samples (ϕPEO > 0.6) are almost identical and close to the salt-free blends, but for PEO-lean samples, e.g., ϕPEO = 0.3, the Tcps all increase substantially, and the identity of salt matters; the Tcps show a segregation trend as LiClO4 > LiTFSI > KTFSI ≈ NaTFSI > NaI. Note that there are no data for NaI-doped blends with ϕPEO < 0.3, since NaI precipitates during the mixing process. Figure 5b demonstrates the cloud point profiles of PEP/PEO blends with salts at r = 0.0040, in which the cloud points of PEO-rich samples are not sensitive to the salt concentration and type, but as ϕPEO decreases, Tcp increases to different degrees for those five salts. For example, with ϕPEO = 0.6, Tcp of LiClO4, LiTFSI, KTFSI, and NaTFSI is 167, 135, 123, and 126 °C, respectively, and the trend is the similar as observed in ϕPEO = 0.3 samples with r = 0.0016, except for the fact that Tcp of NaI-doped samples are too high to be measured. As noted above, both theory17,18 and experiments26,27,38 anticipate a strong dependence on ion size. The dominant factors that govern the phase behavior of this system are the solvation energy of the anion and the cross-linking effect of the cation. As demonstrated in eq 4, bulky anions contribute to a lower solvation energy; thus, compared with LiClO4 (anion size 0.240 nm), LiTFSI (anion size 0.381 nm)-doped blends are more miscible under the same conditions. This mechanism alone fails to explain the low Tcp of NaI (anion size 0.206 nm)doped blends at r = 0.0016. In fact, NaI has been shown to be more likely to form ion pairs,46 and the reduced “effective” salt concentration may result in a lower Tcp comparing with other salts. With respect to the cross-linking effect, it is assumed that alkali metal ions Li+, Na+, and K+ complex with ether oxygens

Figure 6. Experimental (solid symbols) and calculated (open symbols) Tcp of LiTFSI and LiClO4-doped PEP/PEO blends at r = 0.0040 and m = 5. Lines are drawn to guide the eye.

difference in anion size. The calculated Tcp of LiClO4-doped blends based on the Ren−Nakamura−Wang theory captures the effect of the smaller size of LiClO 4, although it overestimates the increase in Tcp. Considering that the Tcp values were calculated using the same m, and under the assumption that all ions are free, the discrepancy is understandable. Previous researchers found that ion pairs exist in PEO/LiClO4 mixtures, while as temperature increases, there is a greater tendency for the formation of ion pairs.49,50 Moreover, simulation results showed that Li+ may coordinate with oxygens from both PEO and ClO4−.56 The lowered “effective” salt concentration may bring down the predicted Tcp, while the lowered coordination number (m) of Li+ with PEO oxygens may also depress Tcp, as shown in Figure S8a. It is also worth noting that the PEO tends to be enriched in the vicinity of an ion. Thus, the volume-average dielectric constant may not be strictly applicable around an ion. This effect, which is not F

DOI: 10.1021/acs.macromol.7b02310 Macromolecules XXXX, XXX, XXX−XXX

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can be explained; i.e., the experimental Tcp for r = 0.01 coincides with the calculated one for r = 0.005.

considered in the calculation, may also suppress the process of phase separation.17 Besides PEP/PEO blends, the model may be applied to other PEO-containing binary systems if the other component is a non-ion-dissolving polymer. A cloud point profile of PS/PEO blends doped with LiTFSI (see Supporting Information for characterization and preparation details) has been mapped out, as shown in Figure 7. Note that the cloud points of salt-free

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CONCLUSIONS The phase behavior of LiTFSI-doped PEP/PEO blends has been studied by cloud point measurements and the composition of the coexisting phases determined by NMR. The thermodynamics of mixing is modified significantly by the introduction of salt, which was reflected by the elevated Tcp and the resulting asymmetry of the cloud point profile. In addition, the coexistence curve was found to deviate from the cloud point profile, which was mainly due to the uneven distribution of ions between the two coexisting phases. The theory of Ren, Nakamura, and Wang,32 which accounts for the self-energy (or solvation energy) of the anion and the cation-induced cross-linking effect, successfully explains the unusual cloud point profiles as well as the discrepancy between cloud point and coexistence curves. This approach has been extended to other salts and to the PS/PEO system. The observations are in agreement with the concept that the miscibility is governed by the size of the anion, as proposed in both experimental and theoretical studies.17,27 However, while the qualitative trend of Tcp in PS/PEO blends could be captured by the Ren−Nakamura−Wang model, Tcp values are consistently overestimated, which we tentatively attribute to possible ion pair formation and the inaccurate dielectric constant of the blend. The phase behavior of ion-containing polymer systems, whether polymer blends or block polymers, is far from fully understood since there are so many interactions involved which are not easily separated. These interactions include, but are not limited to, those between polymer chains (χ), increased translational entropy of anions, solvation penalty of ions,18−20,39,60 reduced polymer conformational entropy because of cation−polymer interactions,18,32 and ion−ion correlations.21,23,25,61,62 This work may serve as a benchmark for both experimental design of polyelectrolyte systems and theoretical research aims to systematically incorporate more effects mentioned above.

Figure 7. Experimental (solid symbols) and calculated (open symbols) cloud point profiles of PS/PEO doped with LiTFSI. Lines are drawn to guide the eye. The shaded green and red areas are inaccessible temperature regions. Parameters: εPS = 4, εPEO = 7.5, and m = 4.

blends are not measurable since all blends are miscible even at room temperature. The Flory−Huggins interaction parameter (χ) of PS/PEO blends has been determined by SANS before; i.e., for deuterated polystyrene (2.15 kg/mol)/poly(ethylene oxide) dimethyl ether (2 kg/mol) blend, χ = 37.5/T − 0.03457 and χ = 0.09 at room temperature. Since N for PS and PEO involved in this work are 10 and 8.7, respectively, this estimated χ justifies the miscibility of the blends at room temperature. Similar to PEP/PEO blends, Tcp shifts to much higher temperatures as salt is added; for example, Tcp of ϕPEO = 0.6 samples increases in order from 24 °C through 104 to 218 °C as the salt concentration increases from 0.01 through 0.02 to 0.04. This trend qualitatively agrees with results on hydroxylterminated PS/PEO blends doped with LiTFSI, except for the fact that the hydroxy groups make the salt-free blend even less miscible.33 On the other hand, while the UCST-type phase behavior persists, the phase diagram becomes quite asymmetric, which is also observed in PEP/PEO blends. The calculated Tcp profile based on Wang’s theory qualitatively agrees with the experimental trend but overestimates the immiscibility of all blends. As discussed before, there may be several reasons for the higher calculated Tcp. First, εPS and εPEO may not represent the true value of the system, and a mismatch in ε might result in a huge difference in the cloud point profile (see Figure S8b). In fact, although εPS ≈ 4 was used in the calculation, this is somewhat larger than a commonly accepted value for PS, εPS ≈ 2.6. A higher dielectric response is due to the chain connectivity, as demonstrated by both the theoretical and computational studies of ion-containing polymer blends.58,59 Second, the fraction of ion pairs may not be negligible in the system. In fact, if neglecting the influence of ion pair on the phase behavior and only considering free ions at a fraction of 0.5, the discrepancy between experimental and theoretical data

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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.7b02310. MALDI-MS and 1H NMR spectra, differential scanning calorimetry (DSC) thermograms, cloud point and coexistence curve analysis, additional model calculation details, and sample preparation and characterization methods (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.P.L.). ORCID

Shuyi Xie: 0000-0001-7966-1239 Timothy P. Lodge: 0000-0001-5916-8834 Notes

The authors declare no competing financial interest. G

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ACKNOWLEDGMENTS This work was supported by the Office of Basic Energy Sciences (BES) of the U.S. Department of Energy (DoE), under Contract DE-FOA-0001664. We thank Zhen-Gang Wang (Caltech), Frank S. Bates, and Qile P. Chen for helpful discussions on the model and critically reading the manuscript.

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DOI: 10.1021/acs.macromol.7b02310 Macromolecules XXXX, XXX, XXX−XXX