Role of Tethered Ion Placement on Polymerized Ionic Liquid Structure

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Role of Tethered Ion Placement on Polymerized Ionic Liquid Structure and Conductivity: Pendant versus Backbone Charge Placement Christopher M. Evans,†,‡ Colin R. Bridges,†,‡ Gabriel E. Sanoja,†,‡,§ Joshua Bartels,†,‡ and Rachel A. Segalman*,†,‡,∥ †

Materials Research Laboratory, ‡Department of Chemical Engineering, and ∥Materials Department, University of California, Santa Barbara, California 93106, United States § Department of Chemical and Biological Engineering, University of California, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: The role of ion placement was systematically investigated in imidazolium bis(trifluoromethane)sulfonimide (ImTFSI) polymerized ionic liquids (PILs) containing pendant charges and charges in the backbone (sometimes called ionenes). The backbone PILs were synthesized via a facile step growth route, and pendant PILs were synthesized via RAFT. Both PILs were designed to have nearly identical charge density, and the conductivity was found to be substantially enhanced in the backbone PIL systems even after accounting for differences in the glass transition temperature (Tg). Wide-angle X-ray scattering (WAXS) revealed an invariance in the location of the amorphous halo between the two systems, while the anion−anion correlation peak was shifted to lower scattering wavevector (q) in the backbone PILs. This indicates an increase in the correlation length of ions and is consistent with charge transport along a more correlated pathway following the polymer backbone. Due to the linear nature of the backbone PILs, crystallization was observed and correlated with changes in conductivity. Upon crystallization, the conductivity dropped, and eventually, two populations of mobile ions were observed and attributed to ions in the amorphous and near-crystallite regions. The present work demonstrates the important role of ion placement on local structure and conductivity as well as the ability of backbone PILs to be used as controllable optical or dielectric materials based on crystallization or processing history. observed experimentally by TEM22 and WAXS16,23−25 and also via simulations.25−27 The simulation work has been helpful in not only confirming cluster formation but also in demonstrating an array of morphologies from spheres to strings of ionic aggregates depending on the degree of neutralization, ion type, and polymer architecture.25−27 A particularly intriguing result from simulations is that placing the charge in the backbone, termed an ionene or backbone PIL, can lead to substantially different aggregate morphologies.25 In fact, it was demonstrated that backbone PILs can form percolated aggregates which span the entire sample geometry when the charges are separated by five or fewer nonionic repeat units. The aggregation morphology should have profound implications on the conductivity of backbone PILs compared to analogous pendant PILs.

Understanding how polymer structure and packing influence ion conductivity is an essential aspect of designing the next generation of polymer electrolytes. Recently, interest in polymer backbones which incorporate bulky ionic-liquid like charges has grown substantially and such materials have been termed polymerized ionic liquids (PILs).1−20 Due to the large, delocalized nature of the charges, weaker electrostatics prevail and afford polymers with substantial conductivities at or near room temperature. In contrast to conventional charged polymers like poly(styrenesulfonate) which are essentially unprocessable in the dry state with >∼20% charged repeat units,21 PILs typically have a charge on every unit and still can possess subambient glass transition temperatures (Tg).16 Much work has been devoted to elucidating the role of charge type and placement on properties such as conductivity, viscosity, and static dielectric constant.1−18 The vast majority of experiments have focused on PILs where the charged group is pendant to the polymer backbone. Due to the nonpolar nature of most backbones, the charges tend to cluster and form ionic aggregates, which have been © 2016 American Chemical Society

Received: July 11, 2016 Accepted: July 13, 2016 Published: July 15, 2016 925

DOI: 10.1021/acsmacrolett.6b00534 ACS Macro Lett. 2016, 5, 925−930

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ACS Macro Letters

Scheme 1. Synthesis of Precisely Spaced Imidazolium Backbone PILs via Step-Growth Polymerization and Pendant PIL Grown via RAFT Polymerization

Although pendant PILs are, in general, easier to realize synthetically, backbone PILs may be made via straightforward step growth polymerization of a diamine with a dibromoalkane. This strategy was applied by Long, Winey, and co-workers in a series of papers on ammonium and imidazolium ionenes.28,29 In the case of backbone PILs containing dodecyl spacers between ammonium cations, they observed a massive drop in Tg upon ion exchange from Br (Tg = 69 °C) to bis(trifluoromethane sulfonyl)imide (TFSI; Tg = −20 °C) as well as exceptional thermal stability up to ∼400 °C with the TFSI counterion.29 For both ammonium and imidazolium ionenes, Long and Winey also observed microphase separation on the several nm length scale in addition to the amorphous halo and ionic aggregate described previously.28,29 They reported that imidazolium backbone PILs with Br− crystallize when there is a mixture of dodecane and poly(tetramethylene oxide) spacers.27 Lastly, Wilkes, Winey, and Long synthesized PDMS based backbone PILs and demonstrated a systematic tunability of mechanical properties depending on the ratio of siloxane to xylene spacers between charges.30 Step growth approaches can be used to make precise backbone PILs where the spacer is exactly the same length between all of the charged groups.31,32 The ability to synthesize polymers that incorporate imidazolium cations either directly into the backbone or as a pendant side chain PIL provides a direct probe of the role of charge placement on structure and conductivity in ionic polymers. Here, the placement of the charge in the polymer backbone is shown to substantially improve the conductivity relative to an analogous pendant PIL both on an absolute temperature scale and when normalized to the calorimetric glass transition. The enhancement in conductivity is accompanied by distinct changes in the structure, as probed by WAXS. A longer length scale associated with anion−anion correlations was found for the backbone PILs, indicating a longer range of correlated ion motions. Finally and surprisingly, the ionenes are observed to crystallize, leading to the formation of ionic polymer crystals.

Upon crystallization, the conductivity decreases substantially and dielectric spectroscopy data provides evidence for the existence of two distinct populations of mobile ions. These results provide important insights into the design and development of future ion-conducting polymers and point to crystallization as a potential tool for tuning both mechanical properties and conductivity. Pendant PILs were synthesized as previously described.19 Backbone PILs were synthesized by a facile step-growth method involving the quaternization of both nitrogens on imidazole by dibromohexane. A stoichiometric mixture of imidazole and dibromohexane was dissolved in DMSO (20 wt % solids), along with 1 eq. of sodium bicarbonate, to neutralize HBr released upon quaternization and to drive the reaction forward (Scheme 1). The mixture was allowed to react for 24 h at 80 °C, and the formation of the ionenes was observed both visually as the polymer solidified in the reaction flask as well as via 1H NMR (Figure S1). Both backbone and pendant PILs were converted from Br− to TFSI− counterions by dissolving the polymer in methanol with a 10-fold molar excess of LiTFSI, followed by dialysis for 3 days in a DI water/methanol mixture. Both PILs were characterized by dimethylformamide gel permeation chromatography to determine relative molecular weights (Figure S2). The molecular weight distributions are with respect to PS and we do not have standards for similar charged materials; however, the Mn of 24.2 (pendant PIL) and 22.1 kg/mol (backbone PIL) are similar, and the two materials are expected to have similar solution configurations and column interactions. The low MW portion of the distribution exhibits a sharp cutoff in both cases due to extensive dialysis of materials, where smaller chains can leach out of the tubing. The conductivity of TFSI in the backbone PIL is substanitially higher than that in the pendant PIL on an absolute temperature scale (Figure 1a). Previous work has indicated that normalization by Tg, to account for differences in the polymer segmental motion, can result in a universal scaling of conductivity for ionic liquids in block copolymers,33,34 neat 926

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typical roughly spherical aggregates of pendant PILs.22 A percolated path for ions is anticipated to substantially improve conductivity and is consistent with the present findings. We note that a higher ion diffusion coefficient and conductivity have also been observed in simulations of backbone PILs compared to pendant PILs.37,38 The length scale associated with correlations is larger for the backbone (WAXS peak at q ∼ 0.83 Å−1) than for the pendant (WAXS peak at q ∼ 0.95 Å−1) PILs corresponding to real space length scales of 7.6 to 6.6 Å, respectively. This peak has been previously assigned to anion−anion correlations within ionic clusters in imidazolium TFSI based polymerized ionic liquids, while the higher q peak around 1.35 Å−1 is designated as the amorphous halo.16 The halo is unaffected by the placement of charge in the backbone PIL and pendant PIL systems (Figure 2a).

Figure 1. Ionic conductivity of backbone (red diamonds) and pendant (green triangles) PILs with TFSI counterions indicates a substantial enhancement in conductivity for the backbone PIL on both the (a) unnormalized and (b) Tg-normalized temperature scales. This enhancement is attributed to a difference in local ordering due to changes in the charge placement on the polymer.

ionic liquids,35 and many other classes of disordered ionic solids.36 The movement of the imidazolium and TFSI anions from a pendant position to the backbone produces a large shift in the Tg of the materials from 16 (pendant PIL) to −35 °C (backbone PIL). While this may be partially attributable to hydrogen bonding between the acrylamide nitrogen and the carbonyl groups of the pendant PIL, which are entirely absent in the backbone PIL, an analogous TFSI and imidazolium pendant PIL linked via an ethyl group to an acrylic polymer backbone (rather than acrylamide) had a reported Tg of 15 °C, indicating a minor role of acrylamide H-bonding.16 The enhanced conductivity in the backbone PIL persists even when normalized to T-Tg (Figure 1b). The same trends are preserved on a T/Tg plot as well (Figure S3). Differences in conductivity grow upon cooling of the material and are almost 2 orders of magnitude higher in the backbone PIL at Tg. The backbone PIL has a weaker temperature dependence and is consistent with more efficient packing of the material, which is then less affected by changes in temperature. It is unclear at present precisely how the temperature-dependent conductivity in the two PILs is quantitatively related to the temperature dependence of ion spacings, dissociation of ion pairs or clusters, the static dielectric constant, or possibly other factors. The enhancement in conductivity of the backbone PIL upon cooling is likely partially due to a more correlated path of charge transport along the polymer backbone because the charges are fixed along the contour of the chain. In the case of the pendant PIL, two charges on neighboring monomers could be adjacent to one another or could be pointing in opposite directions from the backbone. Additionally, simulations25 have indicated that the structure of ionic aggregates in ionenes should be percolated throughout the sample rather than the

Figure 2. WAXS data of the (a) backbone (black curve) and pendant (red dashed curve) containing the TFSI counterion. The amorphous halo is unchanged but the anion−anion peak is shifted to lower q indicating longer range correlations. (b) Upon crystallization at room temperature, sharp diffraction peaks (green curve) emerge in the backbone PIL which are distinct from those observed in analogous small molecule ionic liquid crystals.39 (c) SAXS data on the backbone PIL reveals a correlation peak at 0.066 Å−1 that is only present in the crystalline material. Amorphous PIL, both pendant and backbone, show no SAXS peak (Figure S4).

Previous simulations work on backbone PILs and pendant PILs indicated that, for a given spacer between charged groups, the relative intensities of scattering peaks would change but little to no shift in the wavevector for anion−anion correlations was reported.25 In the present system, it is worth noting that the spacing between charges in the backbone PIL is exactly six carbons, while the pendant PIL has a spacing of two carbons between side chains that have four carbons between the backbone and imidazolium. Including the carbonyl oxygen, there are seven atoms (C, N, and O) per imidazolium in the pendant PILs and six atoms (C only) per imidazolium in the backbone PILs. This corresponds to ion exchange capacities (IECs) of 2.49 mmol/g for the backbone and 2.41 mmol/g for 927

DOI: 10.1021/acsmacrolett.6b00534 ACS Macro Lett. 2016, 5, 925−930

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ACS Macro Letters the pendant PIL. Thus, the overall charge density is almost identical in the two systems and the charge placement is the primary factor in determining the WAXS structure and conductivity. The shift to lower q for anion correlations in the backbone PIL is consistent with the concept of the fixed charges being correlated over longer length scales along the polymer backbone. The linear nature of the backbone PILs allows these materials to crystallize, while the pendant PILs do not. After melting the backbone PIL to collect the amorphous data in Figure 2, the material was quenched to room temperature and allowed to crystallize overnight, leading to the intense diffraction pattern in Figure 2b. This pattern is strikingly different from the diffraction pattern of small molecule ionic liquids such as hexyl methyl imidazolium TSFI,39 the closest molecular mimic of the present polymeric system. In the SAXS region, the crystalline backbone PIL reveals a correlation peak at q ∼ 0.066 Å−1 corresponding to d ∼ 9.5 nm (Figure 2c). This lowest q peak is sometimes termed the ionic aggregate peak,16,25,40 which may provide information on both the aggregate sizes as well as their spacing. This SAXS peak is absent in both the amorphous pendant and backbone PILs (Figure S4), indicating that these longer range correlations break up when they are not confined within the crystal. Due to the placement of charge in the polymer backbone, the ions must be incorporated in the crystal. The ability to create ionic polymer crystals has exciting implications for future materials, which possess tunable conductivity, optical, and dielectric properties, depending on the processing and crystallization history. The crystallization of the backbone PIL leads to a clear melting phenomena after holding overnight at 20 °C (Figure 3a). The fact that crystallization occurs is remarkable considering the difficulty associated with placing a large ion (∼7.9 Å length41) within the polymer crystal. In the crystallized sample, there are three major peaks due to melting, a small shoulder below the melting peaks and a glass transition at −35 °C. This behavior is qualitatively similar to that observed in many semicrystalline polymers, including polyethylene terephthalate42,43 and PEEK.44 The small shoulder peak is generally assigned as a rigid amorphous fraction due to an interfacial layer or tie chains between the fully crystalline and fully amorphous regions which has an elevated Tg due to the constraints of being connected to immobile crystalline material. The lowest temperature melting endotherm has been associated with secondary crystallization of more defective crystals and is typically found ∼10 °C above on the crystallization temperature (Tc).42 The two highest temperature melting peaks are associated with melt-recrystallization, similar to that described previously in literature,45 where smaller crystals melt and then reform into bigger, less defective crystals leading to the highest temperature peak. In the case of backbone PILs, the Tg is unambiguously assigned by comparison with the freshly melted sample. The slight dip after this peak is due to cold crystallization, and modulated DSC scans reveal that this dip is absent in the reversible, equilibrium heat flow trace (Figure S5b). Separation between the meltrecrystallization peaks decreases with increasing scan rates (Figure S5a), and ultimately, the peaks merge at a rate of 40 °C/min, supporting the assignment of melt-recrystallization. The highest temperature melting peak shifts to higher temperature with decreasing scan rate (Figure S5), as the crystals have more time to perfect. Polarized optical microscopy

Figure 3. (a) Differential scanning calorimetry of the backbone PIL crystallized overnight at 20 °C (red curve) reveals a multiple melting peaks and a weak Tg at −35 °C. After melting (black curve), a second heating reveals only a Tg, which is much more pronounced in the absence of crystallinity. Heat flow axis is in 0.5 W/g increments. (b) Polarized optical microscopy shows the spherulitic morphology.

reveals the geometry of the crystallites, which appear to be spherulites (Figure 3b). A large shift in conductivity accompanies crystallization of the backbone PILs (Figure 4a). All of the conductivity data from Figure 1 was collected in the amorphous state by melting the polymer and collecting data on cooling over the course of ∼1 h. After melting, the material is held at 0 °C and the plateau in conductivity shifts to lower values. Additionally, a low frequency second plateau appears in the conductivity data, which eventually becomes a new single plateau after ∼600 min. Clear evidence for the two modes is observed in the tan delta data. With time, the high frequency mode is suppressed and converted into the low frequency mode. A simple interpretation of this data would be the existence of amorphous and near crystallite regions, which have substantially different time scales for ion transport and mobility. In summary, the role of charge placement in polymerized ionic liquids was explored by synthesizing precise backbone and pendant PILs based on imidazolium TFSI. The conductivity was substantially enhanced in the backbone PIL, even after normalization to Tg, which is attributed to longer range ionic correlations in the material. This enhanced correlation was also evidenced by a shift in the anion correlation peak to lower q. Interestingly, the backbone PILs are not only more conductive but are capable of crystallization. They form highly ordered structures, distinct from those of the analogous small molecule ILs, which has a direct impact on the conductivity. The ability to tune the conductivity and mechanical properties via controlling crystallization and processing history gives unprecedented control over these ionic polymers. In particular, these materials should find use not only in energy and electrochemical applications but also as tunable optical and dielectric materials based on processing and thermal history. 928

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Figure 4. Crystallization during dielectric spectroscopy experiments of the backbone PIL at 0 °C reveals (a) the primary conductivity plateau decreases with crystallization and a second plateau emerges at low frequency. This low frequency process is more clearly observed in (b) tan delta plots where the low frequency process emerges and shifts to higher frequency with time. The two modes indicate two distinct populations of mobile ions attributed to amorphous (high frequency) and near-crystallite (low frequency) regions. Time points for the data are 0 (black dots), 308 (blue dots), 418 (green dots), 528 (red dots), and 638 min (purple dots).

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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/acsmacrolett.6b00534. Additional supporting figures and experimental details (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MRSEC Program of the National Science Foundation at the UCSB Materials Research Laboratory (MRL) under Award DMR 1121053. X-ray scattering was performed at the Advanced Light Source (ALS), supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (Contract No. DE-AC02-05CH11231).

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DOI: 10.1021/acsmacrolett.6b00534 ACS Macro Lett. 2016, 5, 925−930