Printable, Degradable, and Biocompatible Ion Gels from a Renewable

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Printable, Degradable, and Biocompatible Ion Gels from a Renewable ABA Triblock Polyester and a Low Toxicity Ionic Liquid Boxin Tang,† Deborah K. Schneiderman,‡ Fazel Zare Bidoky,‡ C. Daniel Frisbie,*,† and Timothy P. Lodge*,†,‡ †

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

ABSTRACT: We have designed printable, biocompatible, and degradable ion gels by combining a novel ABA triblock aliphatic polyester, poly(ε-decalactone)-b-poly(DL-lactide)-bpoly(ε-decalactone), and a low toxicity ionic liquid, 1-butyl-1methylpyrrolidinium bistrifluoromethanesulfonylimide ([P14][TFSI]). Due to the favorable compatibility between amorphous poly(DL-lactide) and [P14][TFSI] and the insolubility of the poly(ε-decalactone), the triblock polymer forms self-assembled micellar cross-links similar to thermoplastic elastomers, which ensures similar processing conditions and mechanical robustness during the fabrication of printed electrolyte-gated organic transistor devices. Additionally, the ester backbone in the polymer structure enables efficient hydrolytic degradation of these ion gels compared to those made previously using carbon-backbone polymers.

B

the all-carbon backbone of the polymer employed does not allow for degradability.22,23 Due to the wide variety of ion pair choices, considerable attention has recently been directed to biotechnology and biomedical applications of ILs,24 including drug synthesis,25,26 active pharmaceutical ingredients,27,28 drug delivery,29,30 and antibiotics.31,32 Among the known ILs with excellent electrochemical properties, 1-butyl-1-methylpyrrolidinium bistrifluoromethanesulfonylimide ([P14][TFSI]) shows promise in biological applications because of its significantly lower toxicity compared to imidazolium-based ILs,33,34 its demonstrated low toxicity toward primary human cells (similar to choline-based ILs),35 and its inherent biodegradability.36 Poly(lactide) (PLA) is a renewable and compostable polymer that is now produced industrially with only 15−25% higher cost than petroleum-based counterparts.37,38 Interestingly, PLA demonstrates good miscibility with IL-miscible poly(ethylene oxide) (PEO)39 and poly(methyl methacrylate) (PMMA).40 More recently, poly(L-lactide) (PLLA) was reported to be a gelator for 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonylimide [EMI][TFSI] via partial crystallization.41 Although homogeneous mixtures of PLLA and IL can be achieved at elevated temperatures, polymer crystallization on cooling complicates the phase behavior and can compromise the mechanical properties and optical

lock polymers have proven to be an elegant way to immobilize ionic liquids (IL), room temperature molten salts with excellent dielectric properties.1,2 The resulting soft materials called ion gels or ionogels have been widely used as electrolytes in numerous applications such as electrochemical actuators,3 supercapacitors,4,5 electrocatalysis,6 biosensors,7,8 electrolyte-gated transistors (EGTs),9,10 and electroluminescent11 or electrochromic displays. 12,13 The mechanical flexibility with little reduction in the IL transport or electrical properties, combined with the ease of solution processing, make such materials promising candidates for plastic electronics and especially as gate dielectrics in organic field-effect transistors.14,15 With proper choice of materials, plastic electronics can be used in biological applications due to advantages such as mechanical flexibility, degradability, skin-conformable softness, and large area processability.16−18 In terms of dielectric layers for biocompatible electronics, inorganic dielectrics have been widely employed, but they require extra procedures to ensure compatibility with plastic substrates.16,17 On the other hand, biopolymers such as albumin and peptides have been considered as dielectrics.19 However, the low capacitance of such polymers leads to unacceptably high voltage operation (>10 V) of the resulting transistor devices.20 With regard to biocompatible ion gels, cellulose has been reported as an IL gelator for ion gel dielectrics, but the limited cosolvent choices to solubilize both cellulose and IL provide challenges for printed electronics applications.21 Meanwhile, ion gels that contain biocompatible cholinium ions have been developed, but © XXXX American Chemical Society

Received: August 4, 2017 Accepted: September 12, 2017

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Scheme 1. (Top) Synthesis of Poly(ε-decalactone)-b-poly(DL-lactide)-b-poly(ε-decalactone) Triblock Polymer and (Bottom) Cartoon Illustration of Ion Gel Formation and Microstructure

a DL diblock was first prepared via sequential ROTEP, followed by a robust coupling reaction with adipoyl chloride using DMAP as a strong nucleophilic catalyst (see Scheme 1).50 The synthesized triblock polymer, denoted as DLD(5−24−5), has a high coupling efficiency of 86% as shown by size exclusion chromatography (see Figure S2 in Supporting Information). The molecular weights of the respective blocks were targeted based on previously reported results for polystyrene-b-poly(ethyl acrylate)-b-polystyrene triblocks51 with optimal mechanical and electrical properties. Ion gels were prepared by using dichloromethane as cosolvent; clear gels containing 10−50 wt % polymer were formed after solvent evaporation. Differential scanning calorimetry (DSC) was performed on the ion gels and on pure IL and polymer, as shown in Figure S3. The ion gels only exhibit a single Tg from the conducting phase, intermediate between Tg(IL) of −84 °C and Tg(L) of 55 °C, which increases monotonically with polymer concentration. These results are consistent with previous ion gel systems, indicative of homogeneity within the conducting phase. As shown in Figure 1, a small-angle X-ray scattering profile of a 10 wt % polymer DLD ion gel also confirms the microphase-separated spherical micelle formation of the BCP in [P14][TFSI]. The intensity I(q) vs scattering wave vector q was fit to a core−shell form factor with a Percus−Yevick hard sphere structure factor,52 which gives a core radius of 12.4 ± 1 nm and a mean micelle− micelle distance of 53 nm calculated from q* of 0.0119 Å−1. The linear viscoelastic properties of an ion gel with 20 wt % polymer are shown in Figure 2. The time−temperature superposition master curves of the dynamic moduli increase gradually with frequency, with G′ ≥ G″ throughout, and with a distinct elastic plateau at low frequency. In contrast to ion gels prepared with high Tg end blocks such as polystyrene, here the Tg of both the microphase-separated end blocks and the heavily plasticized conducting phase are well below 0 °C. Nevertheless,

transparency of the gel. There are no previous studies that document the complete miscibility of amorphous PLA with an IL. Here, we report a solution printable, degradable, and microphase-separated ion gel consisting of a renewable ABA triblock aliphatic polyester, poly(ε-decalactone)-b-poly(DLlactide)-b-poly(ε-decalactone) (DLD) and a low toxicity IL, [P14][TFSI]. Block aliphatic polyesters can be synthesized conveniently via one-pot, sequential ring-opening transesterification polymerization (ROTEP).42 In order to achieve well-defined blocks, undesirable transesterification reactions need to be suppressed, which requires tuning the reaction conditions (e.g., monomer, catalyst/initiator, solvent, and temperature) such that the ester functional groups in the monomer(s) are more reactive toward the propagating end group(s) than the ester groups in the polymer backbone.43,44 For this reason aliphatic lactone monomers (e.g., ε-caprolactone, menthide, and ε-decalactone) are polymerized typically first, and the corresponding polymers (respectively, PCL, poly(menthide), and poly(ε-decalactone)) are used as macroinitiatiors for chain extension with lactide.45,46 Given the higher reactivity of poly(lactone) end groups relative to poly(lactide) end groups, it is possible to achieve high block fidelity with most commonly used transesterification catalysts.47 This synthetic strategy affords block polymers with excellent mechanical properties as the PLA end blocks act as physical cross-links, and the triblock architecture facilitates bridging between individual chains.48 To form an ion gel, however, the IL-solvating block needs to be the midblock. With few exceptions,43,44 the polymerization of a lactone using PLA as the initiating species would lead to transesterification and consequent loss of block fidelity. While PCL can be grown from PLA using low-activity catalysts, the crystallinity of the former and the modest incompatibility between these two blocks make these block polymers unsuitable for use in ion gels.43,46,49 Therefore, in this work, 1084

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Figure 1. Small-angle X-ray scattering of the DLD(5−24−5) triblock polymer at 10 wt % in [P14][TFSI] and fitting using a core−shell form factor and the Percus−Yevick structure factor.

Figure 3. (a) Cartoon cross-sectional view of a printed EGT device. (b) Transfer curve of a typical aerosol jet printed EGT made with 20 wt % DLD ion gel, P3HT as semiconductor, PEDOT:PSS as the topgate electrode, and thermally evaporated gold as source and drain electrodes. Inset top: picture of an aerosol jet-printed EGT device. Inset bottom: close-in view of top-gated device structure.

without device shorting. A cartoon illustration is presented in Figure 3a for the cross-section view of all layers in a printed EGT device. This poly(3-hexylthiophene) (P3HT) EGT shows promising characteristics for applications in printed electronics, with a mobility of 1 cm2/V·s, an ON/OFF channel current ratio of 105, low hysteresis, and low operating voltage. Ion gels have been shown to exhibit high reproducibility and low operating voltage (ca. 1 V). This last attribute results primarily from the formation of electrical double layers (EDLs) at the interfaces with the semiconductor and gate electrode, which leads to a huge capacitance (>1 μF/cm2), typically 1000 times higher than traditional dielectrics. As shown in Figure S5, the specific capacitance of the 20 wt % ion gel used in this device fabrication is as high as ∼2 μF/cm2, ensuring low device operating voltage. Since the EDL dominates the net capacitance, the ion gel thickness does not matter, making it a promising dielectric for printed electronics where there is no need for precise control of the gate dielectric thickness and/or where thicker dielectrics are preferred to avoid gate-channel shorting. The channel current, ID, versus gate voltage, VG, relationship in Figure 3b shows a functional, high performance EGT device. When VG is positive, the channel current is as low as 10−9 A, and the channel is in the OFF state; however, as VG sweeps negative, the channel current increases rapidly, reaching 105 times higher (ON state) for VG ≤ 1 V, owing to the high capacitance of the ion gel. In terms of dynamics (i.e., switching speed), DLD ion gels also demonstrate superior performance

Figure 2. Time−temperature superposition (tTS) master curves of the dynamic storage modulus G′ (closed squares) and loss modulus G″ (open circles) for a 20 wt % DLD(5−24−5) ion gel at 2% strain.

the resulting ion gel still demonstrates a favorable lowfrequency mechanical response, owing to the self-assembled micellar cross-links, and retains a modulus of 2.5 kPa even at 160 °C. SAXS measurements further confirm that no longrange ordering occurs for ion gels up to 140 °C, for concentrations ranging from 10 to 50 wt % polymer. Despite the fact that the chemical identities and physicochemical properties of the triblock polymer and ionic liquid differ significantly from previous ion gels,53 the universal nature of micellar physical cross-links endows the new ion gels with similar facile processability compared to previous examples with, e.g., polystyrene end blocks. We have demonstrated this processability by incorporating the DLD ion gel into printed electrolyte-gated transistors (EGTs). An EGT is an excellent device platform for assessing the dielectric performance and processability of ion gels. As shown in the optical microscopic images in the inset to Figure 3b, a 20 wt % polymer ion gel can be successfully aerosol-jet printed. Importantly, its mechanical strength is sufficient to allow top-gated fabrication of EGTs 1085

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ACS Macro Letters compared to the widely used SMS ion gels.9,11 The Tg of PDLLA is 50 °C lower than that of PMMA, facilitating ion transport. Consequently, as shown in Figure S7, the reduction in ionic conductivity as polymer concentration increases in DLD ion gels is much less compared with SMS ion gels. This provides more freedom in formulation design to balance mechanical and electrical properties of the ion gels for optimal device processing and use. Degradable electronics have attracted much attention, to address emerging issues such as electronic waste in the environment and to facilitate development of implantable or wearable medical devices.19 While [P14][TFSI] has shown promise with respect to biocompatibility35 and degradability,36 aliphatic polyesters can be derived from renewable resources, and ester backbones can undergo various kinds of degradation pathways such as bio-, enzymatic-, and hydrolytic degradation.54,55 To assess their hydrolytic degradability under accelerated conditions, a DLD ion gel and a previously studied polystyrene-b-poly(ethyl acrylate)-b-polystyrene (SEAS) ion gel,51 each with 20 wt % polymer, were combined in separate vials with 0.1 g of gel in 5 mL of 0.1 mol/L NaOH solution under stirring at 60 °C.56 By the end of the seventh day, it is evident from the picture in Figure 4a that the SEAS solution

after hydrolysis increases dramatically, indicating a significant drop in molecular weight (Figure 4d). With a higher concentration of ester linkages along the backbone, PDLLA is more likely to degrade compared to the more hydrophobic poly(ε-decalactone) end blocks. While [P14][TFSI] does not hydrolytically degrade readily on its own, the low toxicity and biodegradable nature35,36 of the IL are favorable features for either biocompatible or disposable electronics applications. In summary, we have successfully demonstrated a new ion gel system by combining a renewable ABA triblock aliphatic polyester with a low toxicity ionic liquid. The triblock architecture presents a challenge to synthesis via sequential ROTEP, which was conveniently resolved by efficient coupling of a monofunctional AB diblock precursor. The self-assembled microstructure of the PDLLA midblock reveals an appealing compatibility between this polymer and the ionic liquid. Despite the significant chemical differences between the DLD and previous ion gel forming polymers, the self-assembled micellar cross-links ensure similar processability, as exemplified by the fabrication of high performance EGT devices. Additionally, DLD ion gels showed promising hydrolytic degradation. With abundant choices of both the constituent ions for ILs and monomers for aliphatic polyesters that open up a large variety of possible combinations, block aliphatic polyester ion gels may prove to be desirable in various applications such as biocompatible electronics.

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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.7b00582. NMR, SEC, and DSC traces, impedance and ionic conductivity data, diblock polymer−ionic liquid solution, and triblock ion gel rheology data and SAXS data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.D.F). *E-mail: [email protected] (T.P.L). ORCID Figure 4. (a) SEAS ion gel in NaOH solution at day 7. (b) DLD ion gel in NaOH solution at day 7. (c) Size exclusion chromatograms of SEAS ion gels before and after NaOH solution treatment. (d) Size exclusion chromatograms of DLD ion gels before and after NaOH solution treatment.

Timothy P. Lodge: 0000-0001-5916-8834 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Air Force Office of Scientific Research under Grant FA9550-12-1-0067. CDF thanks the NSF (ECCS-1407473) and the Office of Naval Research Multi-University Research Initiative (N00014-11-10690) for support of this work. The authors would like to thank Professor Marc Hillmyer, Angelika Neitzel and Liangliang Gu for helpful discussions and suggestions. Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by Northwestern University, E.I. DuPont de Nemours & Co., and The Dow Chemical Company. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under

retains a significant amount of solid, while for DLD, the solution is transparent with a negligible amount of sticky solid adhering to the stir bar (Figure 4b). Note that neither polymer is water-soluble, so that any degradation reaction should occur primarily at the solid−liquid interface. The remaining solids (without purification) and original ion gel polymer samples were each dissolved in THF and examined by SEC to compare molecular weights before and after degradation. As expected, the SEAS ion gel undergoes little or no degradation of the hydrocarbon backbone structure, indicated by minimal change in the elution time for the two samples (Figure 4c). One possible route for chemical attack for SEAS occurs on the ester side chains, which may undergo hydrolysis reactions, as suggested by the significantly lower solubility of the remaining solid in THF. In the case of DLD, the elution time for remnants 1086

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Contract No. DE-AC02-06CH11357. Data were collected using an instrument funded by the National Science Foundation under Award Number 0960140.

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DOI: 10.1021/acsmacrolett.7b00582 ACS Macro Lett. 2017, 6, 1083−1088