Direct-write formation and dissolution of silver nanofilaments in ionic liquid-polymer electrolyte composites

Abstract

Materials with reconfigurable optical properties are candidates for applications such as optical cloaking and wearable sensors. One approach to fabricate these materials is to use external fields to form and dissolve nanoscale conductive channels in well-defined locations within a polymer. In this study, conductive AFM is used to electrochemically form and dissolve nanoscale conductive filaments at spatially distinct points in a polyethylene glycol diacrylate (PEGDA)-based electrolyte blended with varying amounts of ionic liquid (IL) and silver salt. The fastest filament formation and dissolution times are detected in a PEGDA/IL composite that has the largest modulus (several GPa) and the highest polymer crystal fraction. This is unexpected because filament formation and dissolution events are controlled by ion transport, which is typically faster within amorphous regions where polymer mobility is high. Filament kinetics in primarily amorphous and crystalline regions are measured, and two different mechanisms are observed. The formation time distributions show a power-law dependence in the crystalline regions, attributable to hopping-based ion transport, while amorphous regions show a normal distribution. The results indicate that the timescale of filament formation/dissolution is determined by local structure, and suggest that structure could be used to tune the optical properties of the film.

1. Introduction

Solid polymer electrolytes (SPE) contain a salt dissolved in a polymer host. They are candidate materials to replace flammable liquid-phase electrolytes in devices such as rechargeable lithium-ion batteries.^[1,2] In addition, SPEs have recently been used for memory devices based on resistive switching,^[3] in which conductive filaments formed and dissolved in response to an external electric field are used to achieve on and off states typically separated by several orders of magnitude in resistance. The formation of metal filaments through polymer electrolytes has been demonstrated previously for polyethylene oxide (PEO)-based electrolytes,^[4] including our group’s recent report on silver filament formation kinetics through PEO.^[5]

In addition to batteries and memory, SPEs can potentially form the basis for a new class of metamaterials with reconfigurable optical properties. A metamaterial can be formed by embedding metal nanoparticles (NPs) in a well-defined array within a dielectric, in which the optical properties are tuned by adjusting the spacing of the NPs to interact with electromagnetic radiation over a designed wavelength range.^[6,7] In addition to NPs, densely packed nanofilaments in a dielectric can give rise to strong anisotropy in optical properties.^[8] One approach to introduce optical configurability is to selectively form and dissolve conductive filaments within an ordered NP metamaterial (Figure 1(a)). Such metamaterials with dynamically reconfigurable optical properties would represent a major advance in intelligent coatings, nicely complementing achievements in the internet of things (IOT), where local sensors could trigger the intelligent material to change properties in response to a stimulus. However, designing the scaffold for a dynamically tunable metamaterial is quite challenging, because it must allow precise NP positioning, while simultaneously supporting facile motion of conducting ions, which are needed to form and dissolve the nanofilaments. Commonly studied polymer electrolytes, such as high molecular weight PEO-based electrolytes,^[5] are ionically conductive, but the mechanical properties of the solid film make it difficult to position NPs precisely within the polymer.

Figure 1.

Schematic of PEGDA/IL/Ag salt composite. (a) Metamaterial with a lattice of Ag NPs (large gray spheres) embedded in a PEGDA/IL composite (yellow). Some NPs are electrically connected by Ag nanofilaments (small gray spheres). (b) Polymer electrolyte film where conductive AFM controls the electrochemical formation and dissolution of conductive Ag filaments. The polymer chains are represented in a semi-crystalline conformation, i.e. with regions of order and disorder. (c) Magnified view of filament formation between the conductive AFM tip and a sacrificial Ag layer within an amorphous region of the polymer electrolyte. Silver ions, Ag⁺, (black spheres) are reduced to Ag⁰ atoms (gray spheres) at the apex of the growing filament.

To address this challenge, we have investigated a non-aqueous electrolyte system combining a photo-crosslinkable polymer (polyethylene glycol diacrylate, PEGDA) with an ionic liquid (1-butyl-3-methylimadazolium hexafluorophosphate, [BMIM]PF₆, Figure S1). PEGDA is an ion-conducting polymer^[9] which is also commonly used in biological applications.^[10] PEGDA provides design flexibility, because the material can start at low-viscosity and be converted to a high-viscosity solid simply by exposing the film to light. This design could support the precise positioning of NPs in a liquid-like dielectric, and the NPs could subsequently be locked into place by simple exposure to ultraviolet light. Previous studies showed that as little as 4 wt.% PEGDA blended with an ionic liquid (IL) was sufficient to yield a solid.^[11,12] In addition, ILs are commonly used to increase ionic conductivity in SPEs at room temperature.^[11–13] Here, [BMIM]PF₆ is chosen as the IL, because it has good chemical and thermal stability, negligible vapor pressure, high ionic conductivity (1.8 × 10⁻³ S/cm at 300 K),^[14] and a large electrochemical window (~ 4.7 V).^[15] Thus, PEGDA provides tunable mechanical properties, while [BMIM]PF₆ enhances ionic conductivity which is required for forming and dissolving filaments.

While electrolytes containing both IL and polymer have been reported previously,^{[11–13,16,17]} a comprehensive understanding of how the IL affects the ionic conductivity and mechanical strength of the electrolyte is still developing. Such understanding is especially crucial here, because one might presume that the kinetics of nanofilament formation and dissolution are a function of ionic conductivity, which is also related to the mechanical properties of the host material. But, as we show, fast nanofilament formation occurs in regions of high polymer crystallinity and high modulus, rather than amorphous regions with high ionic conductivity, indicating that our intuitive understanding of polymer/IL systems is incomplete. Although the eventual application of this work is to form and dissolve filaments between metal NPs, functioning as bipolar electrodes within the host material, the present study focuses entirely on filament formation in the PEGDA-IL electrolyte.

2. Results and Discussion

As shown in Figures 1(b) and (c), a conductive AFM tip and a Ag substrate constitute a model system in which the AFM tip and the Ag substrate represent the two electrodes. We use the AFM to characterize the mechanical properties of PEGDA/IL/AgPF₆ electrolyte films, create Ag nanofilaments through the films via electrodeposition, and correlate the filament formation/dissolution kinetics with the mechanical properties of the polymer composite. PeakForce Quantitative Nanomechanical Mapping (PF-QNM) is used to map the Young’s modulus, while a conductive AFM is used to form and dissolve the Ag filaments electrochemically.^[5,18] The location of the AFM tip is controlled over the x-y plane to enable the direct-write fabrication of filaments in a predefined grid pattern. As shown in Figure 1(c), the conductive AFM tip operates as a mobile top electrode, while a Ag thin-film below the electrolyte functions as a sacrificial counter-electrode. Filament growth inside the PEGDA/IL/salt electrolyte is controlled by the polarity and magnitude of the bias applied between the two electrodes.

2.1. Modulus Measurements

Thin films of PEGDA with IL and AgPF₆ were fabricated by spin-coating, and cross-sectional scanning electron micrographs (SEM) show that the thickness of the films is approximately 50 nm (Figure S2), independent of the IL concentration. First, we measure how the IL impacts the mechanical properties of the electrolyte film. Modulus maps of the cross-linked electrolytes over a 5 × 5 μm region and modulus-composition data with varying IL compositions are shown together in Figure 2. Clearly, modulus decreases with increasing IL content. For example, increasing the IL composition by a factor of 3 (10 to 30 wt.%), decreases the average Young’s modulus by one order of magnitude. This trend is predictable, because the IL is a low viscosity liquid compared to the UV-crosslinked polymer. In contrast, adding Ag salt increases the modulus, which is most obvious for the electrolytes with the highest PEGDA concentration, e.g. the modulus nearly doubles by adding 2 mM AgPF₆ to a sample with 90 wt.% PEGDA. The silver cations electrostatically interact with the ether oxygens in the polymer backbone;^[19] the resulting non-covalent interactions decrease the polymer mobility^[20] and therefore increase the modulus. The data show that the modulus of the polymer coating can be tuned more than 10-fold by relatively small adjustments in the PEGDA/IL ratio (ether oxygen to BMIM ratio from 22:1 to 65:1, corresponding to 70/30 and 90/10 PEGDA/IL wt.%). In contrast, increasing the silver salt concentration by one order of magnitude (0.2 to 2.0 mM) increases modulus by at most a factor of two. A complete set of modulus maps for all the electrolytes investigated in the study are provided in Figure S3.

Figure 2.

Average Young’s modulus measured by PF-QNM over a 5 × 5 μm region for samples with different PEGDA/IL/AgPF₆ compositions. Inset left to right. Modulus maps of the surface of cross-linked electrolytes containing 2 mM AgPF₆ at 90/10, 70/30 and 50/50 PEGDA/IL wt.% composition, respectively. Scale bars indicate 1 μm.

2.2. Filament Formation and Dissolution Kinetics

Nanoscale Ag filaments were formed and dissolved by controlling the magnitude and polarity of bias applied between a conductive AFM tip and the Ag sacrificial electrode (Figure 1(b)). To avoid asperities in the data caused by macroscopic inhomogeneities in the spin-coated samples, formation and dissolution events were measured at multiple regions separated by > 100 μm. Within each region, filaments were formed and dissolved at 100 locations in a 6 × 6 μm array, with a pitch of 600 nm. An example of time-dependent current and voltage data showing a representative formation and dissolution event are provided in Figure S4. The algorithm used to collect the initial filament formation and dissolution times at hundreds of different locations used the following protocol: a +2 V positive bias was applied until a filament was created, as indicated by a significant increase in the current, until compliance was reached. After formation, the bias was held for only 2 ms to avoid filament overgrowth. Then, the bias polarity was reversed to −2 V to start dissolution. When the filament was broken, the current decreased abruptly to zero.

At least 700 formation and dissolution events were captured for each sample, and histograms of filament formation and dissolution times are given in Figure 3. A Gaussian (normal) distribution was used to fit all data sets with skewness adjusted to capture the asymmetries. The formation time distributions are normal, Figures 3(a) and (c), while the dissolution events show a log normal distribution, Figures 3(b) and (d). These are the same types of distributions reported for filaments formed in PEO-based electrolytes,^[5] and the different distributions suggest that the underlying formation and dissolution mechanisms are fundamentally different. This difference is reasonable, because it takes longer to form a filament for the first time, i.e., the formation step, than it does to break the filament. Whereas formation requires the movement of many silver ions to form a percolating conduction path, dissolution only requires the oxidation of a few silver atoms into the nearby electrolyte to disconnect the filament.

Figure 3.

Filament formation (a) and dissolution (b) time distributions in electrolytes at 70/30 wt.% PEGDA/IL with 0, 0.2 and 2 mM AgPF₆. Filament formation (c) and dissolution (d) time distributions for electrolytes with 2 mM AgPF₆ at 90/10, 70/30, and 50/50 wt.% PEGDA/IL. The bin width is 1 s for all formation times and 0.4 Ln(ms) for all dissolution times. The data are fit by Gaussian distributions with adjustments in skewness to capture the asymmetric shape.

Both the Ag salt and the IL strongly affect filament formation kinetics, as shown in Figures 3(a) and (c), respectively. The addition of up to 2 mM Ag salt in the 70/30 wt.% PEGDA/IL electrolyte decreases the formation time by as much as 42% (Figure 3(a)) as expected from straightforward electrodeposition kinetics. The shift in formation times indicates that the kinetics of filament formation can be controlled by adjusting the Ag⁺ concentration without varying electrical field strength.

Decreasing the IL concentration from 50 to 10 wt.% with 2 mM AgPF₆ increases the filament formation rate by ~ 6.5 times as shown in Figure 3(c). Faster filament formation with decreasing IL content is unexpected, because the IL is conventionally thought to function as a plasticizer^[21,22] – enhancing polymer chain segmental motion^[23,24] and improving ionic conductivity.^[25,26] In addition, the magnitude of the formation time distribution is related to specific features in the time-dependent current data. Specifically, there are two different types of processes, denoted type-1 and type-2, as shown in Figure 4(a) and (b), respectively. Type-1 formation involves an abrupt increase in current from zero to the compliance current over a narrow (few ms) time window, whereas type-2 formation involves current fluctuations over longer timescales (> 1 s) prior to reaching the compliance current. Whether a data set exhibits type-1 or −2 behavior is distinguished by setting the total number of data points that exceed a user-defined current level. In this study, we choose this cut-off current as 3.8 nA, and classify the data as type-1 formation if the total number of data points for which I ≥ 3.8 nA is less than 4. Conversely, if more than 4 points in the data set include I ≥ 3.8 nA, then the formation is considered type-2. We know type-2 formation is not dielectric breakdown because when the bias is reversed, the current returns to zero. In the case of dielectric breakdown, an irreversible conductive pathway would be created through the dielectric.

Figure 4.

Current vs time data during filament formation at +2 V and dissolution at −2 V for (a) type-1 and (b) type-2 filament formation processes. Inset of (a) in the blue rectangular box is a magnified plot with expanded time axis.

The percentage of type-1 formation events are provided in Table S1 for all the samples in this study. Systems with faster formation kinetics and the highest modulus have a higher percentage of type-1 formation events. For example, the 90/10/2 mM system with average formation time of 7 s has a 75% type-1, while the slower 50/50/2 mM system, with average formation time of 45 s, exhibits only 5% type-1 events. While further studies are needed to confirm the origin of these two distinct types of formation dynamics, they may relate to the physical structure of the filament. For example, type-1 filaments that are more commonly formed in the high modulus electrolyte (90/10/2 mM) may have well-defined structure, whereas type-2 filaments that dominate in lower modulus electrolyte (50/50/2 mM) may be more dendritic, causing current fluctuations as they form and disconnect.

Additional support for a correlation between filament morphology and modulus arises in the 50/50 PEGDA/IL samples. Specifically, for the 0 and 0.2 mM salt concentrations, overgrowth of silver at the surface of the film was observed. Figures S5 shows direct evidence of silver overgrowth via AFM topology and current maps. Furthermore, as shown in Figure S6, formation and dissolution events for the 50/50/0 mM and 0.2 mM systems indicating that >50% of the filaments that form do not dissolve during the timescale of the measurement, which would be expected for overgrown filaments connected at multiple locations. In contrast, no similar surface overgrowth was observed in systems with higher modulus, such as 70/30/0 mM system, as shown in Figure S7. Thus, varying the Ag concentration and PEGDA/IL ratio not only changes the kinetics, but also the morphology of the filaments from well-defined to overgrown.

Similar to filament formation, the Ag salt and IL compositions also affect dissolution kinetics, as shown in Figures 3(b) and (d), respectively. The dissolution distributions are more complex than the formation distributions, showing a bimodal structure with filaments that dissolve quickly (tens of milliseconds, type-A) coexisting with long-lived filaments (tens of seconds, type-B). In addition, systems exhibiting a higher percentage of type-1 formation events also tend to dissolve faster. This provides further support for the interpretation that type-1 filaments have a more well-defined structure than those formed in type-2 events, because more well-defined filaments would require less time to dissolve than dendritic filaments that may need to disconnect at multiple locations.

Figure 3(b) also shows that the bimodal distribution is more prevalent at higher Ag salt concentrations. Similar experiments addressing Ag atomic-scale junction formation and dissolution also show a marked dependence of dissolution time on Ag activity in the surrounding medium.^{[27, 28]} This is sensible because adjusting the Ag salt concentration shifts the equilibrium potential of the system relative to the applied potential. Hence, the dissolution data suggest that a concentration overpotential-limited dissolution process is operating on a small, but non-negligible fraction of the nanofilaments.

2.3. Thermal Measurements

The observation that electrolytes with the highest PEGDA concentration (90 wt.%) and the largest modulus display the fastest filament formation/dissolution kinetics was surprising. It thus motivated using differential scanning calorimetry (DSC) to characterize the polymer films across the entire range of compositions. Heat traces showing the melting temperature (T_m) in the range of 20 to 55 °C are given in Figure 5(a). IL and silver salt suppress PEGDA crystallization as observed by a decrease in area under the melting peak (IL-free DSC traces are provided in Figure S8 for reference). A secondary peak at lower temperature (~ 20 °C) emerges with increasing IL addition, and this feature is particularly apparent in the 0.2 mM samples. The observation of multiple peaks at a specific salt concentration is similar to the behavior of PEO-based electrolytes at their eutectic composition.^[20,29]

Figure 5.

Heat flow vs. temperature for PEGDA/IL/AgPF₆ showing: (a) melting (T_m) features and (b) glass transition (T_g) features. Data are from the second heating scan.

The glass transition temperatures (T_g), shown in Figure 5(b), provide insight on how the IL and silver salt affect polymer mobility, which governs ion mobility in amorphous domains. The addition of IL from 0 to 30 wt.% at low silver salt concentration (0 and 0.2 mM) increases T_g by more than 7 K, showing that it does not plasticize the system. Generally, the data show that when the ion concentration is increased (either by adding IL or silver salt), T_g increases, reaching a maximum value around −50 °C for the concentration ranges measured. However, it is noteworthy to highlight the 0.2 mM salt concentration in the 90/10 system. Although we expect the Ag salt to increase T_g, because electrostatic interactions between cations and ether oxygens decrease segmental mobility of the polymer, the 0.2 mM salt decreases T_g in the 90/10 PEGDA/IL concentration. This anomalous behavior is consistent with the possibility that 0.2 mM is a potential eutectic concentration as mentioned above. A similar T_g minimum at the eutectic has been reported for a PEO:LiClO₄ electrolyte.^[20]

2.4. Relating Filament Kinetics to Polymer Structure

Based on the DSC data and the prevailing view that less crystal structure in polymer electrolytes correlates with faster ion mobility, we would expect the 50/50 PEGDA/IL sample to have the fastest filament formation/dissolution kinetics. In fact, we observe the opposite—the 50/50 PEGDA/IL electrolyte shows the slowest kinetics. The result suggests that the local structure of the polymer - which can be evaluated by AFM - could be important for understanding the kinetics. Regions of highly crystalline versus highly amorphous electrolyte can be differentiated by optical microscopy in the 90/10/2 mM electrolyte (Figure S9). We therefore focused on measuring the mechanical properties the filament kinetics in these two regions. Modulus measurements in Figure 6 show that the primarily crystalline region has an average modulus of 4.8 GPa, nearly an order of magnitude higher than the primarily amorphous region (0.6 GPa). The filament formation kinetics measured in these regions indeed show distinct distributions: the primarily amorphous domain has a right-skewed Gaussian distribution with long formation times (21% of the filaments formed within 5 s), while the primarily crystalline region has a power-law distribution with a larger percentage of fast formation events (52% of filaments formed within 5 s). Indeed, when we compare the kinetics to the 100/0 PEGDA/IL system, which is the most highly crystalline system by a wide margin, we observe similar formation distributions (Figure S10) as the primarily crystalline 90/10/2 mM sample. Thus, we conclude that a power-law distribution in the formation kinetics is associated with large amounts of crystallinity. Moreover, regions of high crystallinity also have the highest percentage of type-1 formation events (Table S1). Therefore, the picture that emerges is that ion transport within primarily amorphous domains is mediated by polymer chain mobility and can be described by drift-diffusion that leads to poorly ordered filaments. In contrast, faster ion transport occurs in crystalline regions likely by a hoping mechanism that gives rise to a power law distribution and well-ordered filaments.

Figure 6.

(a) Formation time distributions in primarily crystalline (red) and amorphous (gray) regions of the 90/10 PEGDA/IL wt.% with 2mM AgPF₆ sample using a 1 s bin width. Insets show modulus maps captured in each domain; (b) Log-log plot of formation time distributions vs. count percentages in primarily crystalline regions for 90/10 PEGDA/IL at 0, 0.2 and 2 mM of AgPF₆. 120 s is the cutoff time for formation, bin width is 5 s.

The data show that some amount of IL is essential to achieve fast and reliable formation events. For example, in the IL-free system, only 72% of the locations tested form filaments within the 120 s window of the measurement (Figure S9). The electrolytes loaded with 10 wt.% IL further show that ion mobility is decoupled from PEGDA chain mobility, and indicate two different formation mechanisms in the two phases (amorphous and crystalline). While it is well understood that ion mobility in the amorphous phase is driven by chain mobility, there is also support for the notion that specific crystalline structures in PEO-based electrolytes can provide faster pathways for ion transport than their amorphous equivalents.^[30–32] Based on the results obtained here, we suggest a similar explanation for the observation that primarily crystalline domains exhibit faster formation kinetics. Specifically, a balance is achieved at ~10 wt.% IL, where conduction through crystalline regions is favored over primarily amorphous regions. In contrast to the crystalline regions, the amorphous regions have strong electrostatic interactions that suppress polymer/ion transport and therefore filament kinetics.

3. Conclusions

We developed a UV-crosslinkable electrolyte consisting of PEGDA/IL and silver salt where direct-write electrodeposition of silver nanofilaments is produced by conductive AFM with possible applications as flexible films with designer optical properties. By tuning the IL and salt concentration, the mechanical strength was varied from hundreds of MPa to a few GPa in modulus. Silver nanofilament formation and dissolution events were correlated to local structure, showing that while the addition of Ag salt in up to a concentration of 2 mM improves filament kinetics, the addition of IL beyond 10 wt.% slows the kinetics. Moreover, surprisingly, the electrolyte with the largest polymer crystal fraction and the best mechanical strength shows the fastest filament kinetics. The results indicate that ion mobility in this system is likely governed more by the local polymer structure than by PEGDA chain mobility, thus creating an opportunity to tune the filament kinetics by tuning the structure of the polymer. If the kinetics can be controlled reproducibly by local morphology, one could envision a system where both the timescale and the spatial location for reconfiguring optical properties could be tuned, offering a novel approach to smart coating materials with, for example, reconfigurable optical properties.

4. Experimental Section

Reagents.

Poly(ethylene glycol) diacrylate (PEGDA, M_n = 2,000), 1-butyl-3-methylimadazolium hexafluorophosphate ([BMIM]PF₆, ≥ 98.5%), 2-Hydroxy-2-methylpropiophenone (HMPP, 97%), silver hexafluorophosphate (AgPF₆, 99.99%) and anhydrous acetonitrile (ACN, 99.8%) were purchased form Sigma-Aldrich. All reagents were used as received without further purification.

Sample preparation for AFM characterization.

Silicon wafers (University Wafer, P/Boron, 500 ± 15 μm) were cleaned by sonication in acetone followed by a 2-propanol rinse and nitrogen (N₂) blow dry. 5/100 nm of Ti/Ag was deposited by electron-beam evaporation (Plassys, MEB 550s) at 5 × 10⁻⁷ mbar base pressure. The following steps were completed inside an argon-filled glovebox (Mbraun, MB-200B) where O₂ and H₂O < 1 ppm. AgPF₆, BMIMPF₆, and PEGDA were dissolved in ACN and combined to prepare a total of 9 samples with PEGDA/BMIMPF₆ compositions of 90/10, 70/30 and 50/50 wt.% at 0, 0.2, and 2 mM AgPF₆. In all 9 samples, the PEGDA concentration was 1 wt.% together with 0.02 wt.% of HMPP (photoinitiator). The polymer electrolytes were spin-coated on the Ag-coated silicon substrate at 4000 rpm for 30 s, and annealed at 80 °C for 2 mins to drive-off ACN. Samples were photo-crosslinked with a UV lamp (UVP Compact UV Lamp, λ = 365 nm, P = 1.3 mW/cm² at 3 inches) at a working distance of 1.25 cm for 1 h.

C-AFM characterization.

A Bruker Dimension Icon AFM coupled with conductive AFM probe (SCM-PIT-V2) was operated in contact mode. A custom script was used to modulate the location and potential applied to the tip, and has been reported by our group previously;^[5] additional details of the tip automation are provided in the SI. Electrical contact was made between the AFM stage and the Ag layer of the substrate using copper tape (76555A711, McMaster-Carr). A formation voltage of +2 V, dissolution voltage of −2 V, and a rest voltage of 0 V relative to ground were used. Formation and dissolution current thresholds were chosen as +4 nA and −0.5 nA, respectively. The compliance current of the instrument is ca. 5 nA at a current sensitivity of 1 nA/V, which is selected by the user.

PF-QNM measurement.

A Bruker Dimension Icon AFM was operated at PF-QNM mode (PeakForce Quantitative Nanomechanical Property Mapping). Different types of AFM probes were used for the measurement of 9 samples based on their working ranges for sample modulus with details provided in the SI section 3. All probes were calibrated for their deflection sensitivity, spring constant and tip radius before each measurement. The force applied to the electrolyte was then correlated with the surface indentation to give a quantitative measurement of its mechanical properties.

DSC sample preparation and measurement.

Each DSC sample was prepared inside the glovebox. PEGDA, BMIMPF₆, and AgPF₆ were dissolved in ACN and drop-cast onto Teflon, where the solvent evaporated. Approximately 10 mg of the resulting films were hermetically sealed in an aluminum DSC pan. In addition to the 9 samples described above, 3 additional control samples containing silver salt without ionic liquid were measured (100/0 PEGDA/IL wt.% with 0, 0.2, and 2 mM AgPF₆). Measurements were made on a Pyris DSC 6000 calibrated with an indium standard. To measure T_g, T_c, and T_m, samples were heated to 80 °C to erase thermal history, cooled to −70 °C at 3 °C/min and heated to 80 °C at 5 °C/min.