Synthesis and Morphology of Semifluorinated Polymeric Ionic Liquids

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Synthesis and Morphology of Semifluorinated Polymeric Ionic Liquids Senbin Chen,⊥,† Alexander Funtan,† Fang Gao,§ Bin Cui,§ Annette Meister,‡ Stuart S. P. Parkin,§ and Wolfgang H. Binder*,†

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⊥

Key Laboratory of Materials Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China † Chair of Macromolecular Chemistry, Faculty of Natural Science II (Chemistry, Physics and Mathematics), and ‡Institute of Chemistry-Biophysical Chemistry, and Institute of Biochemistry and Biotechnology, Martin Luther University Halle-Wittenberg, von-Danckelmann-Platz 4, Halle (Saale) D-06120, Germany § Max Planck Institute for Microstructure Physics, Weinberg 2, Halle (Saale) D-06120, Germany S Supporting Information *

ABSTRACT: Polymeric ionic liquids (POILs) are important materials in the field of ionic liquid gating, requiring the precise synthesis of new POILs with tailored structural variability and defined nanoscaled structure. In the current contribution, using reversible addition−fragmentation chain-transfer polymerization (RAFT) technique, the homopolymerization of three imidazoliumbased acrylates with different counterions is reported, namely 1-[2acryloylethyl]-3-methylimidazolium bis(trifluoromethane)sulfonamide (APMIN(Tf) 2 ), 1-[2-acryloylethyl]-3-methylimidazolium hexafluorophosphate (APMIPF6), and 1-[2-acryloylethyl]-3-methylimidazolium tetrafluoroborate (APMIBF4), to afford the respective poly(ionic liquid)s (POILs). All polymerizations display pseudo-first-order kinetics and a rapid growth of P(APMIN(Tf)2), P(APMIPF6), and P(APMIBF4), yielding homopolymers with controlled molar mass, revealing a strong influence of the counterions on the polymerization rate, increasing in the order of BF4Θ < PF6Θ < N(Tf)2Θ. As a direct determination of molecular weights via size exclusion chromatography (SEC) of the POIL homopolymers from RI and UV detectors was not successful, we developed an alternative strategy to generate accurate, “normal” SEC peaks of POILs using RAFT copolymerization technique: APMIN(Tf)2 was copolymerized with a semifluorinated monomer 2,2,2-trifluoroethyl acrylate (TFEA), allowing to study the influence of the comonomer feeding ratio on the resulting SEC signals of P(APMIN(Tf)2-co-TFEA) copolymers. We found that when the feeding molar ratio of TFEA is adjusted to 0.77, symmetric SEC peaks from the resulting P(APMIN(Tf)2-co-TFEA) copolymers are obtained. Furthermore, copolymerizations of TFEA with the other two IL monomers, APMIPF6 and APMIBF4, are also performed to afford P(APMIPF6-co-TFEA) and P(APMIBF4-co-TFEA) copolymers. Moreover, the propensity of the soobtained POIL random copolymers P(APMIN(Tf)2-co-TFEA), P(APMIPF6-co-TFEA), and P(APMIBF4-co-TFEA) to grow a new block (polypentafluorostyrene, PPFS) is explored, intending to generate the fluorinated POIL triblock copolymers P(APMIN(Tf)2-co-TFEA)-b-PPFS-b-P(APMIN(Tf)2-co-TFEA), P(APMIPF6-co-TFEA)-b-PPFS-b-P(APMIPF6-co-TFEA), and P(APMIBF4-co-TFEA)-b-PPFS-b-P(APMIBF4-co-TFEA), respectively. The morphology and size of such semifluorinated POILs are investigated using transition electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS), revealing the aggregated nanoparticles from P(APMIN(Tf)2-co-TFEA) due to the mesoscale organization of the ionic “multiplets”. Significantly larger and crowded globular objects/aggregates are formed from the chain-extended POIL triblock P(APMIN(Tf)2-co-TFEA)-b-PPFS-b-P(APMIN(Tf)2-co-TFEA) copolymers under the same conditions.

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INTRODUCTION

cross-linked materials/pores). Especially the local restriction of the ionic moieties seems important for the design of functional gate dielectrics,3,5 as the entrapping of the ILs within the micro/nanoscopic domains not only reduces macroscopic diffusion but also reduces potential chemical reactions between

A recently emerging field has described the application of low molecular weight ILs species as a dielectric gating medium within functional field effect transistors (FETs)1−4 and organic thin-film transistors (OTFTs).5 In many of the studies on ionic liquids gating, the ionic liquids are applied via an ionogel,6,7 where the ILs are entrapped within a gelous matrix (e.g., a polymeric matrix,8 wherein the corresponding ILs are distributed in the form of micro/nanodroplets, or via partially © XXXX American Chemical Society

Received: July 30, 2018 Revised: September 29, 2018

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DOI: 10.1021/acs.macromol.8b01624 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Synthetic route to the POIL homopolymers, random, and triblock copolymers.

finally to semifluorinated triblock copolymers. Therefore, the proper molecular design of such POILs was turning into focus, as the precise control of molecular structure is required in controlling new POILs with advanced gating properties. Although there is an accepted repertoire for the synthesis of POILs by conventional free radical polymerization19 and living/controlled polymerization techniques (such as atom transfer radical polymerization (ATRP),20,21 RAFT,22,23 nitroxide-mediated polymerization (NMP), 24 TERP, 25 and CMRP)26 or living carbocationic polymerization,27−29 there still is the need to more precisely introduce comonomers into POILs and also allow a more detailed characterization of the final POILs.14,30−33 In the current contribution we first investigate RAFT homoand blockcopolymerization of three different imidazoliumbased acrylate monomers, varying their counterions: APMIN(Tf)2, APMIPF6, and APMIBF4 (Figure 1). The imidazoliumbased POILs are chosen due to their ability to provide noncrystalline materials with glass transition (Tg) values, which are generally lower than ammonium- and pyridinium-based analogous POILs.10,18,34 We also desired to address different

the interface and the ILs.9 In contrast to low molecular weight ILs, polymeric ionic liquids (POILs)10−14 are polymerized monomeric ionic liquids (ILs). Besides the remarkable properties contributed by the ILs species (such as high ion conductivity, thermal stability, nonflammability, and high heat capacity), poly(ionic liquid)s can display strong microphase separation, explained by the multiplet-cluster formation model previously postulated by Eisenberg,15,16 also observed in solid materials with enhanced mechanical stability, improved processability, durability, and spatial controllability.17 We have recently described the use of polymeric ionic liquids as a direct gating system, where the applied voltage is distorting the underlying crystal lattice of a metal oxide, in turn inducing the transition to a conductive phase.18 Especially control over the polymeric architecture of the used POIL as gating-medium was found to be important, as e.g. homo- and random copolymers allowed gating, whereas triblock copolymers did not induce the desired gating effects. Four imidazolium-based POILs with different topologies have been studied, ranging from the homopolymers with varying counterions, to semifluorinated random copolymers, and B


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Macromolecules Table 1. Properties of the RAFT-Made P(APMIN(Tf)2), P(APMIPF6), and P(APMIBF4) Homopolymers sample P(APMIN(Tf)2) P(APMIN(Tf)2) P(APMIN(Tf)2) P(APMIN(Tf)2) P(APMIN(Tf)2) P(APMIN(Tf)2) P(APMIPF6) 1 P(APMIPF6) 2 P(APMIPF6) 3 P(APMIPF6) 4 P(APMIPF6) 5 P(APMIPF6) 6 P(APMIBF4) 1 P(APMIBF4) 2 P(APMIBF4) 3 P(APMIBF4) 4 P(APMIBF4) 5 P(APMIBF4) 6

1 2 3 4 5 6

CTA

[M]/[CTA]

solvent

time (h)

conva (%)

Mn,thb (g/mol)

Mn,NMRc (g/mol)

DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF

2 4 5 6 9 11 2 4 5 6 9 11 2 4 5 6 9 11

15.4 26.9 34.0 39.1 52.2 60.1 9.9 20.0 26.1 30.5 42.4 49.5 5.3 12.1 14.7 17.9 26.3 31.1

7600 13100 16500 18800 25300 28900 3700 7100 9100 10700 14800 17100 1800 3700 4400 5300 7200 9000

9200 15300 18900 22100 27600 32300 5400 8500 10300 12800 16900 19700 3600 5200 6300 7100 8900 11300

Monomer conversion from 1H NMR. bNumber-average molecular weight was evaluated from the following equation: Mn,th = conv × ([M]/ [CTA]) × mM + mCTA. cDetermined from relative integration of protons from 1H NMR.

a

Figure 2. (A) First-order kinetic plots and (B) Mn,th versus conversion for the homopolymerization of APMIN(Tf)2 (red), APMIPF6 (green), and APMIBF4 (blue) in DMF at 65 °C mediated by DBTTC using AIBN as initiator: [M]/[CTA]]/[AIBN] = 100/1/0.1.

properties in different context.39−41 We expect that fluorinecontaining parts exhibit a strong impact on the morphology of the prepared copolymers, which may be advantageous for the potential applications in ionic liquid gating. The copolymerizations of TFEA with the other two IL monomers, APMIPF6 and APMIBF4, are also performed exploring the controlled manner, affording the expected P(APMIPF 6 -co-TFEA) and P(APMIBF4-co-TFEA) copolymers. At last, their propensity to further grow a new PPFS block is subsequently explored, allowing to generate the fluorinated POIL triblock copolymer: P(APMIN(Tf) 2 -co-TFEA)-b-PPFS-b-P(APMIN(Tf) 2 -coTFEA), P(APMIPF6 -co-TFEA)-b-PPFS-b-P(APMIPF 6-coTFEA), and P(APMIBF4-co-TFEA)-b-PPFS-b-P(APMIBF4co-TFEA) (Figure 1).

architectures of the POILs, as it is known that POIL statistical35 or block copolymers36 can exhibit a different conductivity37 when compared to the respective homopolymers due to their nanoscale morphology,24 which is generally deemed important also for the ionic liquid gating effects.38 We demonstrate that the use of a conventional trithiocarbonate chain transfer agent, S,S-dibenzyl trithiocarbonate (DBTTC), affords pseudo-first-order kinetics of P(APMIN(Tf)2), P(APMIPF6), and P(APMIBF4) homopolymer chains with controlled molar mass in all cases. As a direct SEC characterization of the so-obtained POIL homopolymers using DMF containing 10 mM LiN(Tf)2 salt as eluent was not successful, we alternatively describe RAFT copolymerization techniques to generate the accurate, “normal” SEC peaks of POILs: APMIN(Tf)2 is copolymerized with a semifluorinated monomer, TFEA, to study the influence of comonomers feeding ratio on the resulting SEC signals of the P(APMIN(Tf)2-co-TFEA) copolymers (Figure 1). Our interests in studying fluorine-containing polymers stem from their promising application fields, such oxygen transport and enhanced chemical resistancetwo requirements important for ionic liquid gatingin addition to their advanced

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RESULTS AND DISCUSSION Synthesis of POIL Homopolymers. In this study, RAFT homopolymerization of APMIN(Tf)2 mediated by DBTTC is first performed to gain a preliminary understanding on the polymerization kinetics (Table 1 and Figure 2). Conversion of the APMIN(Tf)2 monomer into the corresponding polymers, including their calculated molar mass (Mn,th) as well as the C


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Macromolecules

Figure 3. 1H NMR spectra of APMIN(Tf)2 monomer (bottom) and P(APMIN(Tf)2) homopolymer (top), recorded in DMSO-d6 at 27 °C.

Table 2. Properties of the RAFT-Made P(APMIN(Tf)2-co-TFEA) Semifluorinated Copolymers by Varying the Comonomer Feeding Ratio sample P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA)

CTA 1 2 3 4 5 6 7

DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC DBTTC

[M]/[CTA] f TFEAa 260 260 260 260 260 260 260

0.47 0.62 0.69 0.77 0.77 0.77 0.77

time (h)

convb (%)

FTFEAc

Mn,thd (g/mol)

Mn,NMRe (g/mol)

Mn,SECf (g/mol)

Đf

3 3 3 0.5 1.5 2.5 3

45.3 44.5 43.3 7.8 17.1 29.7 42.1

0.49 0.65 0.71 0.78 0.78 0.79 0.79

21600 21100 20700 4200 10100 17100 20300

23400 23000 22600 3700 12300 18100 22100

4400 14200 19900 25200

1.39 1.35 1.33 1.32

Initial molar fraction of TFEA in the comonomer mixture. bOverall comonomer conversion from 1H NMR. cMolar fraction of TFEA in the final copolymers, P(APMIN(Tf)2-co-TFEA), calculated from 1H NMR. dNumber-average molecular weight was evaluated from the following equation: Mn,th = (convAPMIN(Tf)2 × ([MAPMIN(Tf)2]/[CTA]) × mAPMIN(Tf)2) + (convTFEA × ([MTFEA]/[CTA]) × mTFEA) + mCTA. eDetermined from relative integration of protons from 1H NMR. fFrom RI signals of SEC in DMF containing 10 mM LiN(Tf)2 (PS calibration). a

experimental ones estimated by 1H NMR from relative integration (Mn,NMR) of the obtained POIL homopolymers, P(APMIN(Tf)2), are given in Table 1. Homopolymerization of APMIN(Tf)2 reveals a controlled behavior, as shown in Figure 2: kinetic studies highlight a linear relationship between − ln(1 − conv) and time (Figure 2A); furthermore the Mn,th of P(APMIN(Tf)2) polymer chains grow linearly over the considered monomer conversion range (Figure 2B). Unfortunately, no visible SEC peaks could be determined from our POIL homopolymers P(APMIN(Tf)2) using various solvents as eluent, such as DMF containing 10 mM LiN(Tf)2, presumably due to the POILs’ aggregation on the column fillers induced by the ionic groups.21 Nevertheless, the good correlation between Mn,th and Mn,NMR (Table 1) confirms the controlled/living character of the polymerizations. Subse-

quently, structural proof is accomplished by comparative 1H and 19F NMR spectroscopy in DMSO-d6. Compared to the 1H NMR spectrum of APMIN(Tf)2 monomer (Figure 3, bottom), the disappearance of peaks from the vinyl group (5.90−6.32 ppm), along with the peaks appearing at a similar chemical shift but with broader signals, and the emergence of aromatic peaks from DBTTC (7.05−7.28 ppm) undoubtedly proves the successful preparation of POIL homopolymers P(APMIN(Tf)2) via the used RAFT technique (Figure 3, top). Characteristic fluorine moieties of the APMIN(Tf)2 monomer and P(APMIN(Tf)2) homopolymer studied via 19F NMR are shown and assigned in Figure S5. Aiming at studying the effect of counterions on the rate of polymerization, N(Tf)2Θ has been replaced with PF6Θ and BF4Θ. Similar to APMIN(Tf)2, linear pseudo-first-order kinetic D


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With the optimized copolymerization condition in hand the kinetic studies of P(APMIN(Tf)2-co-TFEA) are subsequently conducted keeping the initial molar fraction of TFEA in the comonomers mixture equal to 0.77, which thus yielded the “normal” symmetrical SEC trace in all cases (Table 2 and Figure 4). Consistent with a living/controlled copolymerization process, linear first-order kinetic plots have been obtained (Figure 5A), along with the Mn,th growing linearly over the considered comonomer conversion range (Figure 5B). Again, the “normal” symmetrical SEC traces and low dispersity (Đ = Mw/Mn) values have been obtained in all studied conversions (Figure 5C and Table 2). More importantly, the molar masses obtained from the SEC analysis (Mn,SEC) fit very well with the theoretical values (Mn,th) and the experimental ones from the 1 H NMR analysis (Mn,NMR), thus proving the reliability of this RAFT copolymerization methodology for the SEC characterization of POILs (Table 2). The structure analysis of the semifluorinated POIL copolymer P(APMIN(Tf)2-co-TFEA) has been subsequently performed via 1H and 19F NMR spectroscopy in DMSO-d6 at 27 °C (Figure 6). The characteristic protons of DBTTC, P(APMIN(Tf)2), and PTFEA segments (Figure 6A), as well as the characteristic fluorines of P(APMIN(Tf)2) and PTFEA segments (Figure 6B), have been visualized and fully assigned. Encouraged by the successful copolymerization of TFEA with APMIN(Tf)2 using DBTTC, the RAFT copolymerization of TFEA with the other two IL monomers, APMIPF6 and APMIBF4, is subsequently performed keeping the feeding TFEA molar ratio in the comonomers mixture equal to 0.77, to afford P(APMIPF6-co-TFEA) and P(APMIBF4-co-TFEA). Polymer structure analysis is then performed by 1H, 19F, 31P, and 14B NMR spectroscopy in DMF-d7 (Figures S12 and S13). Consistent with a controlled copolymerization process, the experimental molar masses obtained from 1H NMR (Mn,NMR) analysis fit very well with the theoretical values (Mn,th, see the Supporting Information). Synthesis of POIL Triblock Copolymers. The propensity of the so-obtained POIL random copolymer to grow a new block is subsequently explored. P(APMIN(Tf)2-co-TFEA) 7 is used as macroRAFT agent and then chain-extended with PFS (Figure 1), which has been proven to efficiently mediate the polymerization of a new block, evidenced by kinetic studies (Figure S5A,B), and the progressive shift of the SEC traces toward higher molar masses compared to the starting P(APMIN(Tf)2-co-TFEA) 7 (Figure S5C). Control and livingness of the chain extension polymerization of PFS via the macro-RAFT agent are further proven by the good agreement between theoretical molar mass (Mn,th) and experimental ones (Mn,NMR and Mn,SEC, Table 3). Polymer end-group analysis is subsequently performed by 1 H and 19F NMR spectroscopy (Figure 7). Because of the poor solubility of the PPFS polymer in DMSO, DMF has been employed for the NMR studies of our POIL triblock copolymers, as all three segments (P(APMIN(Tf)2), PTFEA, and the new PPFS block) are sufficiently soluble in this solvent. In case of 1H NMR, the only peaks of PPFS are the aliphatic signals appearing around 2.5 ppm, which are unfortunately overlapping with the characteristic aliphatic protons of P(APMIN(Tf)2) and PTFEA segments (1.6−2.8 ppm, Figure 7A). Thus, aiming at identifying each segment and clearly determining the Mn,NMR of our POIL triblock copolymers, 19F NMR using DMF-d7 as solvent has been conducted, as seen in Figure 7B, visualizing and identifying the

plots are observed for the homopolymerization of both APMIPF6 and APMIBF4 (Figure 2A), along with the Mn,th growing linearly over the considered monomer conversion range (Figure 2B). The excellent agreement between Mn,NMR and Mn,th also confirms the controlled/living character of the homopolymerization (Table 1). Importantly, by means of comparison, we found that P(APMIN(Tf)2) under equivalent conditions gives the highest rate of polymerization, followed by P(APMIPF6) and P(APMIBF4), the latter displaying the lowest polymerization rate (Figure 2A,B). The characteristic protons, fluorines, phosphorus moieties, or boron of the APMIPF6, APMIBF4, P(APMIPF6), or P(APMIBF4), respectively, are fully studied via NMR which are shown and assigned in Figures S6−S11. Synthesis of the POIL Random Copolymers. In an effort to facilitate the SEC characterization and visualize the SEC signals of our POILs, we subsequently have studied the RAFT copolymerization of APMIN(Tf)2 with TFEA varying the comonomer feeding ratio (Figure 1). To avoid the bimodal molar mass distributions frequently observed in the RAFT polymerization of acrylates at high monomer conversions and high molar masses,42 the polymerizations are purposely designed with maximum comonomer conversion of ∼45%, and molar masses of ∼20000 g/mol. The comonomer feeding ratio, overall comonomer conversion, comonomer final ratio, and their molar mass (Mn,th, Mn,NMR, and Mn,SEC) of the semifluorinated POIL random copolymers, P(APMIN(Tf)2-coTFEA), are given in Table 2. The so-obtained semifluorinated POIL copolymers P(APMIN(Tf)2-co-TFEA) subsequently are analyzed by SEC in DMF containing 10 mM LiN(Tf)2, yielding strongly unsymmetrical signals (Figure 4, blue curve),

Figure 4. Normalized RI signals of SEC traces for the copolymerization of APMIN(Tf)2 with TFEA varying the comonomer ratios in DMF at 65 °C, mediated by DBTTC using AIBN as initiator: [M]/ [CTA]/[AIBN] = 260/1/0.1.

which progressively turn to symmetrical shapes (Figure 4, black curve) depending on the different comonomer ratios (Figure 4). We hypothesize that the presence of the TFEA monomer counterbalances the effect of ionic groups in the POIL copolymers. Interestingly, when feeding a molar fraction of TFEA in the copolymers P(APMIN(Tf)2-co-TFEA) of ∼0.77, a symmetrically shaped SEC peak is observed (Figure 4, black curve), which is the first time, to the best of our knowledge, that by employing a second monomer into POILs a “normal” symmetrical peak in the SEC characterizations is enabled. E


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Figure 5. (A) Pseudo-first-order kinetic plots, (B) Mn,th versus conversion, and (C) evolution of the normalized RI signals for the copolymerization of APMIN(Tf)2 with TFEA in DMF at 65 °C. In all cases, [M]/[CTA]/[AIBN] = 260/1/0.1.

Figure 6. 1H (A) and 19F (B) NMR spectra of P(APMIN(Tf)2-co-TFEA), recorded in DMSO-d6 at 27 °C.

characteristic fluorines of P(APMIN(Tf)2), PTFEA, and new PPFS blocks. In a similar manner, the chain extension of P(APMIPF6-coTFEA) and P(APMIBF4-co-TFEA) with PFS is subsequently carried under the same conditions to afford the POIL triblock copolymers, P(APMIPF6-co-TFEA)-b-PPFS-b-P(APMIPF6-coTFEA) and P(APMIBF4-co-TFEA)-b-PPFS-b-P(APMIBF4-co-

TFEA), respectively. The POILs triblock copolymer structure analysis is then performed by 1H and 19F NMR spectroscopy in DMF-d7 (Figures S15 and S16, respectively). Again, consistent with a controlled polymerization process, the experimental molar masses obtained from 1 H NMR (Mn,NMR) analysis fit very well with theoretical values (Mn,th, see the Supporting Information). F


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Macromolecules Table 3. Properties of the Obtained POIL Triblocks: P(APMIN(Tf)2-co-TFEA)-b-PPFS-b-P(APMIN(Tf)2-co-TFEA) sample POIL POIL POIL POIL

triblock triblock triblock triblock

macroRAFT agent 1 2 3 4

P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA) P(APMIN(Tf)2-co-TFEA)

7 7 7 7

[M]/[CTA]

conva (%)

Mn,thb (g/mol)

Mn,NMRc (g/mol)

Mn,SECd (g/mol)

Đd

DPne

300 300 300 300

10.1 28.3 39.9 50.2

26100 36600 43500 49400

29300 37100 45700 52400

32100 40100 48200 55300

1.41 1.40 1.38 1.39

37 76 122 156

Monomer conversion from 1H NMR. bNumber-average molecular weight was evaluated from the following equation: Mn,th = conv × ([M]/ [CTA]) × mM + mCTA. cDetermined from relative integration of fluorines from 19F NMR. dFrom RI signals of SEC in DMF containing 10 mM LiN(Tf)2 (PS calibration). eDPn stands for the degree of polymerization of PFS.

a

Figure 7. 1H (A) and 19F (B) NMR spectra of POIL triblock 4, recorded in DMF-d7 at 27 °C.

Based on the mesoscale organization of ionic “multiplets”,28 the formation of nanostructures consisting of ionic clusters within the obtained POILs in solution is expected. Therefore, the morphologies of P(APMIN(Tf)2-co-TFEA) 7 and POIL triblock 4 were first investigated via TEM (Figure 8A,D): nanoparticles with diameter (d) of 10−20 nm are formed from P(APMIN(Tf)2-co-TFEA) 7 (Figure 8A), while POIL triblock 4 demonstrates larger and crowded globular nanoparticles (d = 40−70 nm) under the same experimental conditions (Figure 8D). Subsequently, AFM is applied to visualize the shape of the POILs aggregates (Figure 8B,C,E,F). Again, nanoparticles are obtained from P(APMIN(Tf)2-co-TFEA) 7 (d = 30−45 nm, Figure 8B,C). In contrast, different structures have been observed from POIL triblock 4 under the same conditions, showing much larger and “mountain-like” objects/aggregates (Figure 8E,F). In comparison to the TEM images (Figure 8A,D), larger sizes and slightly different structures are obtained

from AFM (Figure 8B,C,E,F). One reason may be that spincoating could lead to a potential elongation or flattening effect on the nanoparticles in the case of AFM studies; moreover, the difference between Figure 8A,C might due to the limited lateral tip resolution of AFM,43 which is known to be ∼20 nm or larger and is therefore more prominent when small features are measured. Compared to the nanoparticle morphology from POIL triblock 4 measured by TEM (Figure 8D), the much larger and “mountain-like” objects/aggregates from AFM (Figure 8F) probably due to the interconnection/agglomerations of several particles. Dynamic light scattering (DLS) is further used to investigate the size of P(APMIN(Tf)2-coTFEA) 7 and POIL triblock 4 in DMF under the same concentration (2 g L−1), revealing nanoparticles with an average hydrodynamic diameter (Dh) of 32 and 125 nm obtained from P(APMIN(Tf)2-co-TFEA) 7 and POIL triblock 4, respectively (Figure S13). Compared to the results (d = 10− G


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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.8b01624.

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Materials, characterization methods, detailed synthesis of IL monomers, RAFT polymerization of POIL homopoplymers, random copolymers and triblock copolymers, supporting NMR spectra, DLS and DSC results (PDF)

AUTHOR INFORMATION

Corresponding Author

*(W.H.B.) Phone +49 345 55 25930; fax +49 345 55 27392; email [email protected].

Figure 8. TEM (A, D), AFM height (B, E), and phase (C, F) images of P(APMIN(Tf)2-co-TFEA) 7 (top row) or POIL triblock 4 (bottom row).

ORCID

Wolfgang H. Binder: 0000-0003-3834-5445 Notes

The authors declare no competing financial interest.

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20 and 40−70 nm, respectively) from TEM, the higher Dh values determined via DLS can be ascribed to the solvated form of the nanoparticles, while TEM reflects the size of materials in their dried, solvent-free state.44 Moreover, the thermal analysis by means of DSC (differential scanning calorimeter) is then carried out, as shown in Figure S1718 proving the microphase separation for the POIL random and triblock copolymers.

ACKNOWLEDGMENTS S.C., A.F., and W.H.B. gratefully acknowledge the financial support from the German Science Foundation, DFG-project A03 in the SFB TRR 102. We thank Ms. Anja Marinow for useful discussions on ionic liquids.

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REFERENCES

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CONCLUSIONS In this study the RAFT homopolymerization of three imidazolium-based acrylates with different counterions has been successfully exploited for the preparation of POILs in a controlled manner. The effect of counterions on the RAFT polymerization rate is also examined, following an order under the same experimental conditions: BF4Θ < PF6Θ < N(Tf)2Θ. Aiming at analyzing molecular weights and Đ values of the POILs, further RAFT copolymerization of APMIN(Tf)2 with a fluorinated monomer (TFEA) in various comonomer feeding ratios has been investigated. We have found that when the feeding molar fraction of TFEA in the copolymers P(APMIN(Tf)2-co-TFEA) is adjusted to 0.77, useful and symmetrical SEC peaks are obtained, in turn allowing to determine molecular weights and Đ values of the final copolymers. The copolymerizations of the other two IL monomers, APMIPF6 and APMIBF4, with TFEA are also performed to afford P(APMIPF6-co-TFEA) and P(APMIBF4-co-TFEA) copolymers. The propensity of so-obtained POIL random copolymers P(APMIN(Tf)2-co-TFEA), P(APMIPF6-co-TFEA), and P(APMIBF4-co-TFEA) to grow a new block (polypentafluorostyrene, PPFS) is explored, intending to generate the fluorinated POIL triblock copolymers. At last, the morphology and size of such semifluorinated POILs are investigated using transition electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS), revealing the aggregated nanoparticles from P(APMIN(Tf)2-co-TFEA) due to the mesoscale organization of the ionic “multiplets”. Significantly larger and crowded globular objects/aggregates are formed from the chain-extended POIL triblock copolymers P(APMIN(Tf) 2 -co-TFEA)-b-PPFS-b-P(APMIN(Tf) 2 -coTFEA) under the same conditions. H


Article

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Synthesis and Morphology of Semifluorinated Polymeric Ionic Liquids

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