Decoupling Mechanical and Conductive Dynamics of Polymeric Ionic

Nov 15, 2017 - While potential applications of PILs would benefit from materials in which the conductivity and mechanical properties can be independen...
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Decoupling Mechanical and Conductive Dynamics of Polymeric Ionic Liquids via a Trivalent Anion Additive Joshua Bartels,† Gabriel E. Sanoja,†,§ Christopher M. Evans,† Rachel A. Segalman,*,†,‡ and Matthew E. Helgeson*,† †

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

ABSTRACT: The mechanical and conductive properties of a polymeric ionic liquid (PIL) are decoupled through the addition of a fraction of trivalent anions to a chloride single-ion conductor. Trivalent phosphate ions strongly coordinate with polymerbound imidazoliums, producing an increase in both the ionic conductivity and the polymer viscosity. Both the viscosity and the ionic conductivity increase with phosphate content, and the conductivity is superior to that of the neat PIL at larger trivalent anion concentrations. The interaggregate spacing (determined by X-ray scattering), glass transition temperature (measured by calorimetry), and free volume (estimated by rheology) are each sensitive to the presence of trivalent ions but not to changes in the phosphate concentration. Thus, the presence of a fraction of trivalent anions qualitatively changes the structure and interaction of ions, resulting in modified macroscopic properties of the PIL. We hypothesize that this step change in properties upon introducing phosphate ions is due to a densification of ion aggregates by the trivalent ion, which strongly binds to imidazolium ions. This provides a new mechanism for creating PILs with tailored conductive and rheological behavior. segmental dynamics,1,3,21−24 yet this is distinct from developing material engineering rules for independently controlling ion conductivity and mechanical properties, as is the focus of this work. Therefore, new avenues for stiffening and toughening PILs while maintaining ion conductivity are needed for their incorporation into a wide variety of energy storage and conversion devices. One avenue to reinforce polymers involves the physical cross-linking of polymer chains. Prior examples include hierarchical hydrogels that are based on metal−ligand coordination bonds,25,26 polymer−nanoparticle composites formed in solution by the addition of multivalent phosphate ions,27,28 and ionomers that have been ion-exchanged to contain zinc ions.29−31 These strategies have demonstrated the ability to strengthen polymer networks via ionic associations but lead to the formation of strong ion aggregates that hinder diffusive ion motion. Counterions that interact strongly with

1. INTRODUCTION Polymeric ionic liquids (PILs) exhibit novel and tailorable mechanisms of ion conduction because they tether weakly binding ion pairs, such as those characteristic of ionic liquids, into a polymer backbone.1−7 PILs are therefore particularly interesting as solid-state electrolytes in lithium ion batteries8,9 or ion-conducting membranes in fuel cells and solar-fuel generators.10−12 Optimal operation of such electrochemical systems requires a mechanically robust polymer that allows ion transport across physically separated reaction sites.13−17 While potential applications of PILs would benefit from materials in which the conductivity and mechanical properties can be independently tuned, ion conductivity in polymers is normally directly tied to segmental mobility. In the case of polymeric ionic liquids, weak electrostatic interactions provide the ion pairs with plasticizing properties that facilitate chain mobility.18−20 Although the structural diversity of PILs translates into partial coupling of polymer segmental dynamics and ion motion, there is often a trade-off between mechanics and ion conduction. Much progress has been made in decoupling the mechanism of ion conduction from polymer © XXXX American Chemical Society

Received: June 24, 2017 Revised: October 13, 2017

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DOI: 10.1021/acs.macromol.7b01351 Macromolecules XXXX, XXX, XXX−XXX

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containing 1-ethylimidazole. The neat reaction mixture was sealed with a rubber septum, purged with dry nitrogen for 30 min, heated to 80 °C, and allowed to react for 24 h as the imidazole quaternization reaction proceeded. The resulting yellow liquid was purified by three repeated precipitations in diethyl ether and dried at 70 °C under vacuum for 24 h. 1H NMR, 13C NMR, and ESI mass spectrometry (Figures S1 and S2) confirm the successful quaternization of the imidazole with no observable formation of the thioether byproduct. Polybutadiene (PBD). The synthesis of butadiene was adapted from literature procedures.33 Separate burets containing cyclohexane and butadiene were connected to a thick-walled glass reactor fitted with Ace threads. The butadiene was connected by a flexible, stainless steel bellows so that the buret could be held in an ice bath until use. The reactor assembly was flame-dried and then cycled between vacuum and positive argon pressure (5 psi) three times. The reactor was finally charged with an argon atmosphere and then isolated from the Schlenk line. Cyclohexane was added, and the temperature equilibrated at 30 °C. Based on the amount of purified butadiene (11.84 g), a quantity of sec-butyllithium (0.238 mL, 1.4 M in cyclohexane) and 1,2dipiperidinoethane (DPE, 0.085 mL) were added sequentially through a gastight syringe. Caution: sec-butyllithium is a pyrophoric and moisturesensitive material and should be handled with appropriate care! Butadiene was poured in through the flexible bellows, and the reaction mixture turned bright yellow due to the formation of polybutadienyl−lithium chain ends. The reaction was allowed to proceed for 8 h. After complete conversion of butadiene, degassed acidified isopropyl alcohol was added to terminate the polymerization. The resulting PBD was precipitated into an excess of methanol/isopropyl alcohol (50:50) and dried in vacuo. 1H NMR reveals a molecular weight of 22 000 g/mol and a regiochemistry of 94% 1,2-polybutadiene, consistent with the monomer-to-initiator and DPE-to-initiator ratios.34 GPC in THF reveals a dispersity of 1.08 relative to polystyrene standards characteristic of polymers synthesized via living anionic polymerization. Polybutadiene-Based Polymeric Ionic Liquid (PBD-Cl). The functionalization of PBD via thiol−ene click chemistry was adapted from literature procedures.35,36 In a round-bottom flask equipped with a Teflon-coated stir bar, PBD (1.0 g), 1-propylthiol-3-ethylimidazolium chloride (1.78 g), 2,2-dimethoxy-2-phenylacetophenone (DMPA, 0.2 g), and THF/methanol (90:10) were added. The solution was sparged with nitrogen for 30 min and then allowed to react for 1 h under UV irradiation (λ = 365 nm). The resulting polymeric ionic liquid was precipitated in acetonitrile, filtered, and dried in vacuo for 8 h to yield an orange solid. 1H NMR reveals that 48% of the butadiene monomers contain an imidazolium pendant cationic moiety. Ion Exchange. The monovalent PIL, PBD-Cl (0.2 g), was dissolved in deionized water/methanol (50:50) by first sonicating in 9 mL of methanol, heating to 35 °C, and then slowly diluting with equal parts of deionized water. Similarly, tribasic sodium phosphate was dissolved in 9 mL of deionized water/methanol (50:50) by first dissolving in deionized water and then diluting with equal parts methanol. Phosphate ion content was controlled by pipetting a controlled volume of stock solution of PO43− in water into the water/methanol solution. The PIL and sodium phosphate solutions were mixed with no precipitation observed and further poured into a regenerated cellulose dialysis tube. The dialysis tube was immersed in 800 mL of stirred DI water/methanol (50:50). The dialysate was exchanged every 8 h, and the conductivity was measured with a Fischer Scientific Accumet XL200 conductivity probe. Ion exchange was considered complete when the conductivity of the dialysate was equal to the conductivity of the pure water/methanol solution (∼1 μS/cm). This observation indicates that all Na+ and excess Cl− ions had been removed. Extensive dialysis leads to the removal of all excess ions, leaving polymer-bound imidazolium ions and corresponding anionic counterions. FTIR measurements of the dialyzed samples confirm the phosphate ions remain bound to the polymer, whereas no phosphate is observed in the dialysate. As such, the phosphate content initially added for each sample is assumed to be the final ion content. 2.2. Characterization. Molecular Characterization. Gel permeation chromatography (GPC) was performed on a Waters instrument

polymer-bound ions are less likely to participate in conduction and result in a lower concentration of mobile ions. By contrast, weakly binding pendant ions, such as those characteristic of ionic liquids, are expected to form temporary associations with multivalent ions that facilitate charge transport by promoting a higher concentration of mobile ions. This relationship between the binding strength, the mobile ion concentration, and the macroscopic ionic conductivity of the material can be subverted by employing two different counterions in a PIL so that one ion promotes strong physical associations while the other ion participates in conduction. Herein, a strategy is presented to control the macroscopic mechanical and conductive properties of a cationic PIL through the ion exchange of a fraction of weakly binding monovalent chloride anions (i.e., Cl−) for more strongly binding trivalent phosphate ions (i.e., PO43−). Although the addition of multivalent ions to ion-containing polymers results in the formation of microscopic physical associations, this does not necessarily translate into stronger electrostatic interactions as these are also influenced by the polarizabilities of the interacting ions.29 This was reported by Weiss et al., who observed that the incorporation of divalent Zn2+ ions into sulfonated polystyrene results in a lower melt viscosity and faster ion relaxation relative to a monovalent Na+ counterpart. It was concluded that the polarizability of ions plays a significant role in the stability and lifetime of ionic associations and therefore in the polymer and ionic relaxations.29,32 Noteworthy, the concentration of multivalent ions investigated in this work is dilute relative to that of the monovalent ion. As such, PO43− ions promote interpolymer associations without dramatically lowering the number of Cl− ions that participate in ion transport and demonstrate the ability to significantly alter the PIL properties with a dilute additive. It is hypothesized that the trivalent anions act as physical cross-linking agents (Figure 1) that competitively bind to tethered imidazoliums and release chlorides, thereby increasing the ionic conductivity while strengthening the polymer mechanical properties.

Figure 1. Left: chemical structure of the imidazolium pendant polybutadiene polymeric ionic liquid (PIL) studied in this work. Right: scheme displaying a simplistic representation of possible ionic associations with a fraction of trivalent anions resulting in interchain associations.

2. MATERIALS AND EXPERIMENTAL DETAILS 2.1. Synthesis. Materials. All materials were used as received from Sigma-Aldrich unless otherwise specified. CDCl3 and methanol-d4 were purchased from Cambridge Isotope Laboratories; methanol and isopropyl alcohol were from BDH Chemicals. Cyclohexane was collected from a commercial J.C. Meyer dry solvent system and used immediately thereafter. Butadiene was dried over calcium hydride, degassed through three freeze−pump−thaw cycles, and then distilled to a flame-dried buret immersed in an ice bath until use. 1-Propylthiol-3-ethylimidazolium Chloride. 3-Chloro-1-propanethiol was added in molar excess (1.2 equiv) to a round-bottom flask B

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Macromolecules using a refractive index detector and Agilent PL gel 5 μm MiniMIX-D column. THF at 35 °C was used as the mobile phase with a flow rate of 1.0 mL/min. Dispersity (Đ) was determined against polystyrene standards (Agilent). NMR spectra were collected on a Varian VNMRS 600 MHz. Polymer molecular weight was determined using 1H NMR end-group analysis. Differential Scanning Calorimetry (DSC). Samples were dried at 80 °C under vacuum for 24 h prior to being hermetically sealed in aluminum pans. DSC measurements were made using a PerkinElmer DSC 8000 calorimeter, and glass transition temperatures (Tg) were evaluated as the temperature at the half-height step in heat capacity of the second heating curve. Rheology. Neat polymer samples were molded into 8 mm diameter disks that were dried under vacuum at 80 °C for 24 h prior to being loaded into a Rheometric Scientific ARES-LS rheometer. Dynamic frequency sweeps were performed from 25 to 100 °C over a range of frequencies from 0.1 to 50 rad/s at a strain amplitude between 0.1 and 1%. Strain sweeps at 10 rad/s verified that the frequency sweeps were performed in the linear viscoelastic regime. FTIR measurements performed before and after rheological measurements confirmed that the samples were stable when exposed to 100 °C in air for 1 h, indicating that no chemical degradation occurred during the data collection. Conductivity. Polymer samples were dried in a vacuum oven at 80 °C for 24 h before being pressed between two circular stainless steel electrodes. The samples were stored under vacuum prior being placed on an INSTEC temperature-controlled stage equipped with a constant dry nitrogen flow. Through-plane dielectric relaxation spectroscopy was performed on a Biologic VSP-300. A sinusoidal voltage of 100 mV was applied in the frequency range 0.1 Hz−3 MHz. The DC conductivities were determined from the frequency-independent regime of the real component of the complex conductivity for each isothermal scan. Reliable values for the static dielectric constant were unattainable from the dielectric experiments due to excessive noise in the permittivity data. X-ray Scattering. Polymer samples were dried and sealed under dry nitrogen prior to being loaded in an in-house small-angle X-ray scattering diffractometer equipped with a XENOCS Genix 50W X-ray microsource with a 800 μm × 800 μm beam size and a 1.54 Å wavelength. Full two-dimensional scattering patterns were collected using a Dectris EIGER R 1M detector (1030 × 1065 pixels). The scattering patterns were azimuthally integrated using Nika version 1.58.37 Wide-angle scattering data were obtained on the same instrument but do not contain information salient to ion aggregation and can be found in the Supporting Information (Figure S4). Smallangle scattering data were also collected at beamline 7.3.3 at the Advanced Light Source at Lawrence Berkeley National Laboratory;38 these data agree with the in-house data and are available in the Supporting Information (Figure S5).

Table 1. Physical Constants of PILs mobile ions nonionic Cl− Cl− and PO43− Cl− and PO43− Cl− and PO43− Cl− and PO43− Cl− and PO43−

[PO43−]a (nm−3)

σDC (S/cm) at 50 °C

η* (Pa·s) at 51 °C and 1 rad/s

f Cl−b (%)

Tg (°C)

0 0 0.0025

100 99

−9 15 28

−6

1.3 × 10 1.4 × 10−9

4.7 × 104 9.8 × 105 1.8 × 107

0.0050

97

33

6.4 × 10−9

2.4 × 107

0.01

95

32

4.6 × 10−7

4.1 × 107

0.02

91

32

2.9 × 10−6

7.2 × 107

0.04

77

34

4.2 × 10−5

9.1 × 107

a

Ion concentrations were determined from the monomer molar volume and stoichiometric ion incorporation. Total charge density remains unchanged. bThe relative chloride fraction of all mobile ions, f Cl− = [Cl−]/([Cl−] + [PO43−]).

single PO43− ion) that allow mixing of the two ions with minimal changes to charge density. The effectiveness of the phosphate ions in forming electrostatic cross-links is determined by two important thermodynamic properties: enthalpy and entropy. From an enthalpic standpoint, the polarizability and valency of phosphate are both higher than those of chloride, and so phosphate is expected to generate stronger and more stable inter- and intrachain electrostatic cross-links upon interacting with tethered imidazolium cations. Aside from this enthalpic stabilization, the trivalent nature of a phosphate ion also promotes phosphate−imidazole associations due to the increase in entropy upon release of bound monovalent ions (i.e., Manning counterion condensation).41,42 The electrostatically induced cross-links formed upon incorporation of phosphate ions induce changes in the macroscopic mechanical properties of the corresponding PILs (Figure 2). The complex viscosity increases with the concentration of PO43− with a scaling experimentally determined from a least-squares regression power-law fit of η* ∼ [PO4−3]0.72. This behavior is reminiscent of physically cross-linked polymers43 and provides evidence for the formation of temporary PIL networks via a

3. RESULTS AND DISCUSSION 3.1. Decoupling of Mechanical and Conductive Processes. The PIL investigated herein has been designed as a model system to study the effect of electrostatically induced multivalent ionic cross-linking on the macroscopic conductive and mechanical properties. The backbone of the polymer was chosen to be polybutadiene in order to provide a low dielectric constant medium that does not solvate ions. Polybutadiene has a low Tg and highly mobile chains that facilitate ion transport. The PIL contains tethered, charge delocalized, aromatic imidazolium rings with intrinsically high electronic polarizability. Consequently, the polarizability of the mobile counterions will be important not only for the energetics of the electrostatic interactions but also on the polymer macroscopic mechanical and ion conducting properties.39,40 Chloride and phosphate ions were selected due to their similar charge densities (i.e., three Cl− ions have a similar volume to a

Figure 2. Complex viscosity of the PBD PILs at 51 °C increases with increasing phosphate content. Least-squares regression power-law fit yields a scaling of η* ∼ [PO4−3]0.72 (gray lines) over a range of frequencies. C

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consistent with that of a physical temporary network where G′ and G′′ follow self-similar power-law behavior at sufficiently low frequencies (G′′ data are shown in Figure 5).46,47 The

trivalent anion additive. These temporary networks have points that break and form on time scales that not only hinder polymer diffusion and provide the material with interesting viscoelastic properties but also affect ion transport in the presence of an electric field such as that existing in electrochemical devices. The presence of electrostatic cross-links resulting from the incorporation of strongly associating trivalent phosphate anions translates into a significant change in the dynamics of polymer chains and the corresponding bulk viscoelastic properties. The response of these PILs to a small-amplitude oscillatory shear (Figure 3) obeys time−temperature superposition and yields a

Figure 3. Linear viscoelasticity of the unmodified PBD (black) and PIL polymers studied. Closed symbols correspond to G′ data, and open symbols display G′′ data for polybutadiene. Incorporation of trivalent phosphate anions (blue) increases the modulus and delays the polymer relaxation compared to the purely monovalent PIL (red). Increasing trivalent anion content further delays the dynamic response. Figure 5. Comparison of the dielectric and rheological response of PILs at 50 °C with only chloride ions (top, red) and with 0.01 nm−3 of phosphate ions (bottom, blue). Although the rheological responses of the two PILs (left axis, symbols) differ significantly, the dielectric responses (right axis, lines) remain similar. The dotted line represents the time scale where ion motion becomes diffusive.

master curve for the storage modulus (G′) regardless of the anion type or concentration and with frequency shift factors (Figure 4) exhibiting the temperature dependence anticipated by Williams, Landel, and Ferry for polymeric systems.44 Whereas polybutadiene exhibits linear viscoelasticity characteristic of an entangled polymer melt,45 the PIL response is

magnitude of the modulus is extremely sensitive to the presence of phosphate ions, as demonstrated by the order of magnitude increase in modulus for the lowest trivalent ion concentration investigated (i.e., 0.0025 nm−3). Increasing the concentration of trivalent anions thereafter only yields a modest increase in the modulus. Even though no terminal relaxation is observed, the associations are presumed to be transient because the PILs creep under uniaxial compression into homogeneous films. Thus, the phosphate strong associations toughen the polymer while still allowing processing in the melt state. Although the viscoelasticity and ionic conductivity of ionconducting polymers are generally coupled due to the dependence of ion motion on polymer segmental relaxation,33 a significant decoupling of the mechanical and conductive properties of the PILs investigated herein is observed upon addition of trivalent phosphate ions. This is best illustrated by comparing the frequency-dependent dielectric (represented by the real component of the complex conductivity, σ′) and viscoelastic spectra (Figure 5). The upturn at high frequency related with diffusive ion motion is insensitive to the presence of phosphate ions. Whereas the purely monovalent PIL (red

Figure 4. Linear viscoelastic master curve shift factors are well described by the WLF equation. Fits are used to obtain the C1 parameter that is inversely related to polymer free volume plotted in Figure 8D. D

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Figure 6. (A) DC conductivity versus inverse temperature for the PILs studied herein. (B) DC conductivity of the ion-conducting PIL as a function of phosphate content. The conductivity of the PILs with trivalent anions added is comparable to the purely monovalent case (dotted line) at PO43− concentrations of 0.01 and 0.02 nm−3.

data) exhibits a response characteristic of ion-containing polymers47 with a plateau in the dielectric spectra and a power law in the linear viscoelastic spectra occurring at similar frequencies, the conductive plateaus in σ′ for the phosphatecontaining PILs occur at frequencies well into the plateau of the linear viscoelastic spectra, where polymer motion is hindered. These observations suggest that when trivalent ions are introduced into the PIL the conductivity remains dominated by the motion of chloride ions even though the mechanics of the polymer is strongly influenced by the binding of the phosphate ions. The physical cross-linking of the PILs promotes conduction at sufficient trivalent ion concentrations. This is evident in Figure 6, which illustrates the concentration dependence of the DC conductivities of the phosphate-containing PILs at 50 °C. Increasing the trivalent phosphate content results in an enhancement in conductivity that can be described by a power law with a slope of 3.84. At sufficiently high phosphate concentrations (i.e., >0.01 nm−3), the DC conductivity becomes larger than that of the PIL containing exclusively monovalent anions (red dotted line). This is noteworthy and a consequence of the decoupling of mechanical and conductive dynamics of PILs via a trivalent anion additive, as restrictions on polymer segmental dynamics imposed upon network formation are generally detrimental for ion transport at high concentrations and yield a maximum, as opposed to a minimum, in ionic conductivity. Hence, no universal scaling relationship between temperature and ionic conductivity arises upon normalization by Tg (Figure S3). This decoupling of the bulk mechanical and conductive properties prevents the slower polymer dynamics (due to strong binding between the PO43− and imidazolium ions) from affecting ion transport. The simultaneous enhancement of conducting and mechanical properties at high PO 4 3− concentrations (i.e., >0.01 nm−3) provides a material design strategy for development of mechanically robust ion-conducting polymers without incorporating insulating reinforcing agents (e.g., nanoparticles). To illustrate further, the relationship between the DC conductivity of each PIL to the elastic modulus, both implicitly dependent on the PO43− concentration, is presented (Figure 7). The shape of each curve in Figure 7 arises from a combination of both the VFT behavior48 of the conductivity (Figure 6A) and the WLF behavior44 of the viscoelastic response obtained from fitting the shift factors as a function of temperature (Figure 4). Figure 7 reveals a

Figure 7. Modulus at 0.1 rad/s as a function of DC conductivity for a monovalent PIL (circles) and mixed monovalent and trivalent PILs (squares). Two regions exist: the “reinforced modulus” regime (white) where the modulus is higher than the neat PIL for a given conductivity and the “decreased conductivity” regime (red) where the increase in modulus does not compensate for the decrease in ionic conductivity.

nonmonotonic dependence of the conductive and mechanical properties on phosphate ion concentration, with two distinct regimes of behavior. At trivalent ion concentrations lower than ∼0.01 nm−3, the trivalent anion-containing PIL has an unfavorable coupling (“decreased conductivity”), in which the increased modulus due to ionic cross-linking comes at the expense of the conductivity relative to the purely monovalent ion-containing PIL. This is the expected behavior typically observed in mixed ion-containing polymers; i.e., the addition of more strongly binding ions results in a decrease in polymer mobility, with a corresponding decrease in ionic conductivity.49 By contrast, at trivalent ion contents larger than ∼0.01 nm−3, a favorable decoupling is observed in which the conductivity is enhanced despite the increased modulus due to ionic crosslinking. This reinforces the observation made previously that by having a mixture of counterions, the conduction of chloride ions can be promoted despite the relatively low mobility of phosphate ions. In this way, the “reinforced modulus” region in Figure 7 is reminiscent of the “superionic” regimes described in Walden plots22 where systems exhibit a higher viscosity/ conductivity ratio than that of a reference ionic solution (e.g., KCl). Polymer-bound imidazoliums interact preferentially with phosphate ions. This produces a physically cross-linked E

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Figure 8. (A) Small-angle X-ray scattering for the polymers studied. An ion correlation peak is observed for all PILs; the peak shifts to higher q with the addition of trivalent phosphate ion. The low-q upturn is related to the absorbed water content and varies proportionally to the amount of time the sample was exposed to ambient air. The variance in the low-q upturn decreased when sealed samples were examined at the SSRL beamline (Figure S5). (B−D) The ion correlation peak position (B), Tg (C), and the C1 parameter from rheology WLF fits (D) as functions of phosphate content each show the same trend. A large shift occurs with the addition of phosphate ions, but the data are largely insensitive to further changes to trivalent ion concentration.

polymer network, as revealed by an increase in polymer viscosity upon formation of ionic associations containing phosphate anion additives. The strong binding of the imidazolium to phosphates liberates chloride ions to more easily participate in ion transport by either lowering the activation energy for a chloride ion to leave an aggregate or increasing the chloride ion mobility. The overall effect is a simultaneous enhancement of conducting and mechanical properties with increasing phosphate content at concentrations ∼0.01 nm−3. 3.2. Characterization of Associated Ionic Domains. Although the investigation presented herein confirms that the incorporation of phosphate ions increases both the viscosity and conductivity of PILs, these effects are preceded by a 3 orders of magnitude decrease in conductivity upon the initial inclusion of trivalent ion additives. The drop in conductivity is eventually overcome at sufficiently high PO43− concentrations; however, it is important to consider the origin of the initial precipitous decrease in conductivity. One possible explanation is the effect of the phosphate ions on the polymer

nanostructure. The polybutadiene backbone of the PIL has a low dielectric constant such that it does not promote ion solvation or dissociation, leading to ion-rich and ion-poor domains. This has important consequences in both the macroscopic rheological and dielectric response (discussed in section 3.1) as well as in the polymer structure. Small-angle Xray scattering (SAXS) (Figure 8) reveals a peak in the purely monovalent ion-containing PIL (q = 0.15 Å−1) attributed to a correlation length between ion-rich domains, as it is absent in the scattering of neat polybutadiene. Further analysis of the SAXS profiles confirms that the presence of strong phosphate−imidazolium interactions induces a new preferred organization of ionssimilar to the observations of Tirrell et al. where the addition of multivalent ions dramatically changes ionic associations of polyelectrolyte brushes and results in compression of the brush50,51 (Figure 8). Specifically, Figure 8A shows that the peak correlation distance between ion-rich regions decreases in the presence of trivalent phosphate ions. While there may be finer detail structure in these samples, the broadness of the SAXS peak prevents further F

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eventually overcome by phosphate ions liberating mobile ions to participate in conduction.

interpretation. An increase in the position of the ionic correlation peak, corresponding to a change in d-spacing from 5 to 4 nm upon addition of PO43−, demonstrates not only a smaller spacing between aggregates but also a higher number density of aggregates. Under the assumption of ideal mixing between the polymer and the ionic species (i.e., no change in volume), this cannot be attributed to a difference in the size or concentration of the counterions, since the estimated molar volume of a phosphate anion is comparable to that of three chloride ions, and the total ion content is not changed (50 wt % ions and 50 wt % polymer). Since there is only a single broad peak in the scattering, the change in preferred aggregate spacing occurs throughout the entire material, and so one can assume that the ions are mixed among the aggregates. If the size and concentration of anions are not changing, then a change in the morphology (i.e., size or shape) of the ion-rich domains must therefore be the cause of the shift in the scattering peak. It is important to note that the observed peak intensity is affected by differences in scattering length density between ions and is therefore only considered qualitatively. The qualitative shift in PIL structure upon initial addition of phosphate ions evident in SAXS (Figure 8B) is also evident in a number of other properties including Tg (Figure 8C) and the free volume parameter C1 (Figure 8D) obtained from WLF model fits (Figure 4). This provides further insight into the nature of the change in ionic aggregate morphology in the presence of phosphate ions. The increase in Tg with the addition of any phosphate ions suggests that polymer chains have more restricted motion in the presence of trivalent anions, whereas the concentration of trivalent anions does not have a strong effect on Tg (Figure 8C). Polymer chains that are near ionic aggregates have a more restricted segmental mobility.32 As the distance between ion aggregates becomes smaller, this restricted region comprises a larger volume fraction of the system, increasing the bulk Tg as its fraction grows. Initial addition of phosphate ions (∼1% of the Cl− ions are exchanged for PO43− ions) increases Tg from 15 to 28 °C, which then increases to 34 °C as the phosphate content is increased further. The geometrical requirement of three imidazoliums to pack around a single phosphate results in a new preferred aggregate shape and spacing. The new aggregate morphology has smaller and closer packed aggregates that are able to further restrict polymer motion. In principle, it is possible that this new morphology produces a second distinct Tg due to local interactions with phosphate ions. However, DSC data were unable to resolve a second Tg and instead only show the shift in primary Tg indicated previously. As observed previously in Figure 3, ionic associations dominate the rheological response which, with further analysis, suggests that phosphate ions densify the ionic aggregates and result in a lower free volume of the PIL. Specifically, the free volume is often considered to be inversely related to the C1 parameter obtained from fits of the frequency shift factor (aT) of rheology data by the WLF equation.44 C1 increases upon addition of the phosphate ions and changes a small amount with varying phosphate content (Figure 8D). The C1 parameter is not a direct measurement of the free volume, yet it is interesting that it follows the same trend observed in SAXS and Tg. The rheology data support the conclusion that the presence of phosphate ions induces a change in the organization of associated ions. This is also related to the dramatic decrease in conductivity observed at low phosphate concentrations, where the altered ion organization is unfavorable for conduction and is

4. CONCLUSIONS The mechanical properties of an ion-containing polymeric ionic liquid (PIL) can be controllably altered through exchange of monovalent anions (chloride) with a trivalent anion additive (phosphate). This produces significant changes in both the modulus and conductivity of the PIL for very small degrees of ion substitution. First, trivalent anions act as physical crosslinkers of polymer-bound cationic groups, resulting in a delay of the mechanical relaxation and an increase in the viscosity and elastic modulus with increasing trivalent anion content. Second, the ionic conductivity exhibits a nonmonotonic dependence on phosphate concentration, and at phosphate concentrations above 0.02 nm−3 the PIL has a higher conductivity than that of the neat PIL. The ability to achieve such significant modification in properties with relatively mild ionic substitution is attributed to two effects. First, the presence of trivalent anions (even at small concentrations) has a strong effect on ionic interactions and aggregation in the PIL. This effect appears as a nearly discontinuous change in a number of properties when extremely low concentrations of trivalent ions are introduced, including the length scales of ion−ion correlations, the Tg of the PIL, the ionic DC conductivity, and the free volume as probed by rheological shift factors. Second, strong imidazolium−phosphate interactions enhance the delocalization of chloride ions, producing a highly mobile population of conductive ions that increases as the phosphate content increases. Most importantly, a wide range of conditions for which the mechanical properties and ionic conductivity of the PIL are both simultaneously enhanced, in contrast to other strategies for mechanical reinforcement that present a trade-off between stiffness and ion conductivity. This decoupling of the mechanical and conductive responses of the PIL from the simple addition and ion exchange of trivalent anions presents a new route for tailoring the macroscopic properties of ionconducting PILs.

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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.7b01351. NMR and mass spectra of 1-propylthiol-3-ethylimidazolium chloride; polymer conductivity data normalized to Tg; SAXS and WAXS spectra for the polymers studied (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (M.E.H.). *E-mail [email protected] (R.A.S.). ORCID

Christopher M. Evans: 0000-0003-0668-2500 Rachel A. Segalman: 0000-0002-4292-5103 Matthew E. Helgeson: 0000-0001-9384-4023 Notes

The authors declare no competing financial interest. G

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ACKNOWLEDGMENTS This work was supported by the MRSEC Program of the National Science Foundation under Award No. DMR 1720256 as part of IRG2. We also gratefully acknowledge the AFSOR MURI program under FA9550-12-1 for financial support for the synthesis portion of this project. This research used resources of the Advanced Light Source beamline 7.3.3, which is a DOE Office of Science User Facility under Contract DEAC02-05CH11231.

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