Phosphonated Polymers with Fine-Tuned Ion Clustering Behavior

Jan 23, 2018 - We report the controlled synthesis, self-assembly, and ion transport properties of polystyrene bisphosphonate (PSbP) and polystyrene ph...
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Phosphonated Polymers with Fine-Tuned Ion Clustering Behavior: Toward Efficient Proton Conductors Sanghee Jang, Sung Yeon Kim, Ha Young Jung, and Moon Jeong Park* Department of Chemistry, Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, Korea 790-784 S Supporting Information *

ABSTRACT: We report the controlled synthesis, self-assembly, and ion transport properties of polystyrene bisphosphonate (PSbP) and polystyrene phosphonate (PSP) based polymers, revealing that ion clustering in PSbP (characterized by precisely determined phosphonate group location) was markedly suppressed compared to that in PSP despite the 2-fold higher phosphonic acid group concentration in the former. Moreover, confinement of PSbP chains to ordered nanoscale domains in PSbP-based block copolymers offered a platform for creating nearly homogeneous ionic phases with a radically decreased potential barrier to ion conduction. Notably, the decrease in the degree of polymerization of PSbP chain in the block copolymers by half (i.e., the lower acid group contents) led to 2−3 times improved anhydrous conductivity with incorporated ionic liquids, contrary to the results commonly reported for a range of acid-tethered polymers. Our work provides a first-time demonstration of well-defined self-assembled morphologies of bisphosphonate block copolymers, opening a new chapter in the development of highly conductive phosphonated polymers and thus being of importance to the field of polymer electrolytes.

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INTRODUCTION Over the past decades, polymers with tethered protogenic groups have been widely investigated as components of proton exchange membrane fuel cells to address the growing demand for clean energy.1−3 Sulfonated polymers are a representative example of such materials, having been extensively studied in terms of synthesis4,5 and ion transport mechanisms6−8 but commonly suffering from dehydration and chemical degradation9,10 and thus being unsuited for use in fuel cells operated at high temperature.11,12 In view of the above, phosphonic acid-functionalized polymers have attracted much interest as viable alternatives to their sulfonated counterparts owing to high thermal and chemical stability.13,14 The high dielectric constant of phosphonic acid groups with their amphoteric nature is particularly advantageous for achieving anhydrous proton conductivity at elevated temperatures by allowing selfdissociated acid groups to form hydrogen-bond networks.13,15−17 However, owing to the low acidity of phosphonic acid moieties, phosphonated polymers exhibit lower overall conductivity compared to that of their sulfonated analogues.18−20 The above drawback inspired the preparation of highly phosphonated polymers to achieve high proton concentrations and obtain percolated ionic cluster networks to enhance proton conductivity.15,21−24 In this regard, polymerization of phosphonated monomers represents the most desirable approach to these polymers;25−36 however, only few successful studies have been reported due to the high aggregation tendency of the © XXXX American Chemical Society

above monomers. Thus, most phosphonated polymers reported to date were prepared by postmodification of the polymer backbone23,37 and/or side chains15,21,38,39 with phosphonate moieties. Currently, the available synthetic techniques allow phosphonation levels of up to 80 mol % to be achieved,40 although the limited number of synthetic routes, occurrence of uncontrollable side-reactions, and low yields still present challenges to be addressed. There is no doubt that highly phosphonated polymers feature high proton conductivity. Unfortunately, the randomness of the phosphonation reaction does not allow the distance between acid groups and the ion clustering behavior to be precisely controlled, hampering the concrete underpinning of proton transport mechanisms in phosphonated polymers. Recent advances in “precise ionomers”, i.e., those with finetuned locations of functional groups in the polymer backbone,41−44 demonstrate that these materials enable remarkable control over the microstructures of ionic clusters and thus clearly suggest the future research direction. In this context, one should mention the recent works of Jannasch et al.34 and Boutevin et al.,35,36 who have established controlled polymerizations of diethyl vinylphosphonate (anionic polymerization) and dimethyl vinylbenzylphosphonate (radical polymerization) monomers, respectively. Advantageously, such polymers feature a guaranteed phosphonation Received: November 20, 2017 Revised: January 10, 2018

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DOI: 10.1021/acs.macromol.7b02449 Macromolecules XXXX, XXX, XXX−XXX

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δH (ppm) 7.50 (s, 1H), 7.42 (s, 2H), 6.48−6.67 (m, 1H), 5.70−5.76 (d, 1H), 5.30−5.34 (d, 1H). Synthesis of Tetraethyl (5-Vinyl-1,3-phenylene)bisphosphonate (2c) Monomer. DEP (5.5 mL, 40 mmol) and CyNCH3 (9.5 mL, 40 mmol) were added to 2b (2.62 g, 10 mmol) in 80 mL of ethanol containing Pd(OAc)2 (0.22 g, 1.0 mmol) and PPh3 (0.78 g, 3.0 mmol). The mixtures were refluxed with stirring for 48 h under a nitrogen atmosphere. The products were washed with distilled water, dried over MgSO4, and purified by column chromatography (nhexane:ethyl acetate = 3:1) to yield 2.56 g of tetraethyl (5-vinyl-1,3phenylene)bisphosphonate (2c) monomer. 1H and 31P NMR (CDCl3): δH (ppm) 7.99−8.12 (m, 3H), 6.70−6.80 (m, 1H), 5.88− 5.94 (d, 1H), 5.40−5.44 (d, 1H), 4.04−4.25 (m, 8H), 1.32−1.36 (t, 12H). δP (ppm) 16.91. Synthesis of Polystyrene Diethylphosphonate (1c) and Polystyrene 1,3-Tetraethyl Bisphosphonate (2d) Homopolymers. Atom transfer radical polymerization (ATRP) of 1b and 2c monomers was performed in 30 wt % anisole using the CuBr/ PMDETA catalytic system. 1-Bromoethylbenzene was used as an initiator. The mixtures were refluxed under an argon atmosphere at 85 °C. Upon the completion of the reaction, the products were purified by dissolving in DCM and passing through a neutral Al2O3 column to remove catalyst. The products were precipitated three times in ether and dialyzed against methanol using a cellulose dialysis membrane with 1.0 kg/mol molecular weight cutoff (VWR) for 3 days for further purification. Resultant polystyrene phosphonate (1c) and polystyrene 1,3-tetraethyl bisphosphonate (2d) were recovered by vacuum drying at room temperature for a week. 1c 1H and 31P NMR (CDCl3): δH (ppm) 6.07−8.03 (b, n × 4H, −CH2CH(C6H4)), 3.90−4.45 (b, n × 4H, −POCH2−), 0.34−2.49 (b, n × 3H, −CH2CH(C6H4)), 1.18− 1.55 (b, n × 6H, −POCH2CH3). δP (ppm) 18.64. 2d 1H and 31P NMR (CDCl3): δH (ppm) 6.44−8.29 (b, n × 3H, −CH2CH(C6H3)), 3.54−4.30 (b, n × 8H, −POCH2−), 0.52−2.6 (b, n × 3H, −CH2CH(C6H3)), 0.87−1.40 (b, n × 12H, −POCH2CH3). δP (ppm) 16.32. Synthesis of PIB-BIBB Macroinitiator. TEA (4.2 mL, 30 mmol) was added to PIB (3 g, 0.78 mmol) in 40 mL of DMC. The solution was cooled to 0 °C, and BIBB (1.2 mL, 8 mmol) was added dropwise under an argon atmosphere. The mixtures were returned to room temperature and stirred for 12 h. The solution was washed with distilled water to remove the salts, and the excess of BIBB, followed by drying over anhydrous Na2SO4. Vacuum drying of the product at room temperature yielded 2.94 g of PIB-BIBB macroinitiator (3a). 1H NMR (CDCl3): δH (ppm) 3.82−4.01 (m, 2H, −CH2O−), 1.89−1.99 (d, 6H, −COC(CH3)2Br), 1.52 (s, 2H, −CH2CH2O−), 1.39 (s, n × 2H, −CH2C(CH3)2−), 1.08 (s, n × 3H, −CH2C(CH3)2−). Synthesis of Poly(isobutylene-b-styrene diethyl phosphonate) and Poly(isobutylene-b-styrene 1,3-tetraethyl bisphosphonate) Block Copolymers. Phosphonated block copolymers with different molecular weights were obtained by ATRP using the PIB-BIBB macroinitiator in 30 wt % toluene solution with CuCl/ PMTETA catalytic systems at 90 °C under an argon atmosphere. Upon the completion of the reaction, the mixtures were dissolved in DMC and passed through a column of neutral Al2O3. The products were purified by precipitation three times in n-hexane/ether and dialysis against methanol using a cellulose dialysis membrane with 2.0 kg/mol molecular weight cutoff (VWR) for 5 days. Poly(isobutyleneb-styrene diethyl phosphonate) and poly(isobutylene-b-styrene 1,3tetraethyl bisphosphonate) block copolymers were recovered by vacuum drying at room temperature for a week. Poly(isobutylene-bstyrene diethyl phosphonate) 1H and 31P NMR (CDCl3): δH (ppm) 6.07−7.99 (b, m × 4H, −CH2CH(C6H4)), 3.77−4.41 (b, m × 4H, −POCH2−), 0.6−2.38 (b, m × 3H, −CH2CH(C6H4)), 1.2−1.36 (b, m × 6H, −POCH2CH3), 1.39 (s, n × 2H, −CH2C(CH3)2−), 1.08 (s, n × 3H, −CH2C(CH3)2−). δP (ppm) 18.65. Poly(isobutylene-bstyrene 1,3-tetraethyl bisphosphonate) 1H and 31P NMR (CDCl3): δH (ppm) 6.53−8.29 (b, m × 3H, −CH2CH(C6H3)), 3.47−4.43 (b, m × 8H, −POCH2−), 0.26−2.6 (b, m × 3H, −CH2CH(C6H3)), 0.10−1.41 (b, m × 12H, −POCH2CH3), 1.39 (s, n × 2H, −CH2C(CH3)2−), 1.08 (s, n × 3H, −CH2C(CH3)2−). δP (ppm) 16.39.

level of 100 mol% and predictable distances between phosphonic acid groups, implying that polymerization of new monomers having two or more phosphonic acid groups at precisely determined positions would present a new paradigm of achieving high proton conductivity of phosphonated polymers based on rational molecular design. Herein, we report a controlled radical polymerization of diethyl (4-vinylphenyl)phosphonate and tetraethyl (5-vinyl-1,3phenylene)bisphosphonate monomers having one and two phosphonate groups, respectively. Notably, block copolymerization of the aforementioned monomers and ionophobic units provided a platform for radically lowering energy barrier to ion conduction by confining the acid groups within well-defined nanoscale domains to form homogeneously dispersed ionic clusters. Moreover, after the addition of imidazolium ionic liquids, bisphosphonate-based polymers exhibited unprecedented advantages over monophosphonate-based ones, i.e., featured 2−3-fold improved conductivity, higher mechanical strength, and suppressed ionic aggregation at a wide range of ionic liquid concentrations.

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EXPERIMENTAL SECTION

Materials. Diethyl phosphite (DEP, 98%), 4-bomostyrene (97%), methyltriphenylphosphonium bromide (CH3PPh3Br, 98%), potassium tert-butoxide solution (KOtBu, 1.0 M in THF), N,N-dicyclohexylmethylamine (CyNCH3, 97%), palladium(II) acetate (Pd(OAc)2, 98%), triphenylphosphine (PPh3, 99%), copper(I) bromide (CuBr, 98%), 1-bromoethylbenzene (97%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%), triethylamine (TEA, ≥99%), α-bromoisobutyryl bromide (BIBB, 98%), copper(I) chloride (CuCl, ≥99%), and bromotrimethylsilane (97%) were purchased from SigmaAldrich and used as-received. 3,5-Dibromobenzaldehyde (>97%) was purchased from TCI. Poly(isobutylene) (PIB, Mn = 3.8 kg/mol) was obtained from Polymer Source. Characterization of Synthesized Polymers. 1H and 31P nuclear magnetic resonance (NMR, Bruker AVB-300) spectra were recorded in deuterated solvents. The solvent proton signal was used as internal standard for the 1H NMR, while external standard (85% H3PO4) was applied for the calibration of 31P NMR. Polydispersity indices of polymers were determined by gel permeation chromatography (GPC, Waters Breeze 2 HPLC) with polystyrene standards in THF. Synthesis of Diethyl (4-Vinylphenyl)phosphonate (1b) Monomer. DEP (4.1 mL, 30 mmol) and CyNCH3 (6.4 mL, 30 mmol) were added to 4-bromostyrene (1a, 1.8 g, 10 mmol) in 80 mL of ethanol containing Pd(OAc)2 (0.15 g, 0.7 mmol) and PPh3 (0.5 g, 2.1 mmol). The mixtures were refluxed with stirring for 48 h under a nitrogen atmosphere. The reaction was terminated by adding distilled water and aqueous phase was extracted with DMC. The obtained product was dried over MgSO4 and purified by column chromatography (n-hexane:ethyl acetate = 5:2) to yield 1.75 g of diethyl (4vinyl)phenylphosphonate (1b) monomer. 1H and 31P NMR (CDCl3): δH (ppm) 7.78−7.86 (m, 2H), 7.52−7.56 (m, 2H), 6.74−6.84 (m, 1H), 5.88−5.94 (d, 1H), 5.41−5.45 (d, 1H), 4.08−4.24 (m, 4H), 1.39−1.65 (t, 6H). δP (ppm) 18.74. Synthesis of 3,5-Dibromostyrene (2b). CH3PPh3Br (3.15 g, 8.8 mmol) was dissolved in 20 mL of anhydrous THF under an argon atmosphere. The solution was cooled to 0 °C, and 8.8 mL of KOtBu solution was added dropwise under stirring. The reactor temperature was raised to room temperature and stirred for 1 h. After cooling the mixtures to −78 °C, 3,5-dibromobenzaldehyde (1.5 g, 5.8 mmol) in 20 mL of THF was added dropwise. The reaction mixtures were returned to room temperature and stirred for 3 h. The solvents in the mixtures were then evaporated, and the aqueous phase was extracted with DCM for three times, followed by washing with distilled water. The product was purified by silica gel column chromatography using n-hexane as eluent to yield 1.08 g of 3,5-dibromostyrene (2b). 1H NMR (CDCl3): B

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Figure 1. Syntheses of (a) PSP and (b) PSbP homopolymers and (c) PIB-b-PSP and PIB-b-PSbP block copolymers. Synthesis of Polystyrene Phosphonate (1d), Polystyrene Bisphosphonate (2e), Poly(isobutylene-b-styrene phosphonate) (3b), and Poly(isobutylene-b-styrene bisphosphonate) (3c). All polymers in phosphoethoxy forms were dissolved in anhydrous chloroform, and bromotrimethylsilane was added at 0 °C. The mixtures were heated to 40 °C and stirred for 36 h. After evaporating all solvents, an excessive amount of methanol was added and stirred for 8 h, yielding the polymers in acid forms. The mixtures were then dialyzed against methanol using a cellulose dialysis membrane with 2.0 kg/mol molecular weight cutoff (VWR) for 5 days. The resultant polymers in acid forms were recovered by vacuum drying at room temperature for a week. 1d 1H and 31P NMR (methanol-d4): δH (ppm) 6.25−8.20 (b, n × 4H, −CH2CH(C6H4)), 0.55−2.66 (b, n × 3H, −CH2CH(C6H4)). δP (ppm) 16.76. 2e 1H and 31 P NMR (methanol-d4): δH (ppm) 6.57−8.40 (b, n × 3H, −CH2CH(C6H3)), 0.73−2.69 (b, n × 3H, −CH2CH(C6H3)). δP (ppm) 14.67. 3b 1H and 31P NMR (methanol-d4/THF-d8): δH (ppm) 6.23−7.29 (b, m × 4H, −CH2CH(C6H4)), 0.60−2.06 (b, m × 3H, −CH2CH(C6H4)), 1.42 (s, n × 2H, −CH2C(CH3)2−), 1.08 (s, n × 3H, −CH2C(CH3)2−). δP (ppm) 16.66. 3c 1H and 31P NMR (methanol-d4/THF-d8): δH (ppm) 6.67−8.19 (b, m × 3H, −CH2CH(C6H3)), 0.44−2.45 (b, m × 3H, −CH2CH(C6H3)), 1.42 (s, n × 2H, −CH2C(CH3)2−), 1.08 (s, n × 3H, −CH2C(CH3)2−). δP (ppm) 14.60. Preparation of Phosphonated Polymer Membranes Containing Ionic Liquids. Imidazole (≥99.5%), 2-methylimidazole (≥95%), 2-ethyl-4-meth ylimidazole ( ≥95%), and b is(trifluoromethane)sulfonimide (≥95%) were purchased from SigmaAldrich. Three different imidazolium ionic liquids were prepared by varying the type of imidazolium cation at a fixed anion. Inside argonfilled glovebox, predetermined amounts of ionic liquids and phosphonated polymers were dissolved in anhydrous THF/methanol mixtures in 70:30 vol % and stirred overnight at room temperature. Membranes were prepared by solvent casting using argon gasket and further exposed to vacuum at 70 °C for a week. In order to rule out the issue of water contamination, all sample preparations were carried out inside the glovebox. Morphology Characterization by Wide- and Small-Angle Xray Scattering (WAXS and SAXS) Experiments. The phosphonated homo- and block copolymers without and with embedded ionic liquids were laminated into a homemade airtight sample cell. Synchrotron WAXS and SAXS measurements on these samples were

performed using the 3C, 4C, and 9A beamlines at the Pohang light source (PLS). The wavelength (λ) of the incident X-ray beam was 0.733 nm (Δλ/λ = 10−4). Three different sample-to-detector distances of 0.2, 0.5, and 3 m were used to investigate both ion clustering behavior and microphase separation of the samples. Conductivity Measurements. Through-plane conductivities of phosphonated polymers with embedded ionic liquids were measured using a home-built two-electrode cell (dimension to load sample: 1.3 cm × 1.3 cm × 0.3 cm). Inside an argon-filled glovebox, ac impedance spectroscopy data were collected using a VersaStat 3 (Princeton Applied Research) operating over a frequency range of 1−100 000 Hz in a temperature window of 25−150 °C. For the Vogel−Tamman− Fulcher (VTF) analysis, the glass transition temperature (Tg) of each polymer was measured using differential scanning calorimetry (DSC, TA Instruments, model Q20) at a heating and cooling rate of 10 °C/ min.

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RESULTS AND DISCUSSION Syntheses of Phosphonated Homopolymers and Block Copolymers. Figure 1 shows the procedures used to polymerize phosphonate monomers into homo- and block copolymers. Diethyl (4-vinylphenyl)phosphonate (1b) and tetraethyl (5-vinyl-1,3-phenylene)bisphosphonate (2c) were prepared by Pd-catalyzed phosphonation of 4-bromostyrene (1a) and 3,5-dibromostyrene (2b), respectively. 2b was prepared from 3,5-dibromobenzaldehyde (2a) by the Wittig reaction. The 1H NMR spectra of 1a, b, and 2a−c are shown in Figures S1 and S2 of the Supporting Information. Radical polymerization of 1b and 2c and subsequent hydrolysis yielded polystyrene phosphonate (1d, hereafter PSP) and polystyrene bisphosphonate (2e, PSbP) homopolymers. Polyisobutylene (PIB) macroinitiator (3a, see 1H NMR and FT-IR spectra in Figure S3) was prepared to produce PIB-bPSP (3b) and PIB-b-PSbP (3c) block copolymers by radical polymerization and subsequent hydrolysis (Figure 1c). Two PIB-b-PSbP block copolymers were specially designed to have the same degree of polymerization or possess an equivalent number of phosphonic acid groups as the PIB-b-PSP block copolymer, which allowed us to systematically control the ion C

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1.19 for PSP and PSbP hompolymers, respectively, while those of PIB-b-PSP and PIB-b-PSbP block copolymers were 1.2, being determined by that of the PIB macroinitiator. Representative GPC traces of PIB-BIBB, PIB65-b-PSP30, PIB65-b-PSbP16, and PIB65-b-PSP28 block copolymers are shown in Figure 2c, and the molecular characteristics of the synthesized phosphonated polymers are listed in Table 1. The volume fractions of PSP and PSbP (ϕPSP and ϕPSbP) in block copolymers were calculated using pure component densities based on van der Waals volumes of repeating units (ChemAxon modules were employed). Ion Clustering and Microphase Separation. The ion clustering and microphase separation of phosphonated homoand block copolymers were investigated in the presence and absence of embedded ionic liquids. A set of protic ionic liquids was prepared by combining different imidazolium cations (imidazolium (hereafter Im), 2-methylimidazolium (2MIm), and 2-ethyl-4-methylimidazolium (2E4MIm)) with the bis(trifluoromethane)sulfonimide (TFSI) anion. First, we studied the morphologies of neat polymers. Figure 3a shows wide-angle X-ray scattering (WAXS) profiles of neat PSP and PSbP homopolymers, measured at 25 °C, which remained unchanged in the temperature window of interest. For both PSP and PSbP, a peak at q value of 3.9 nm−1 (corresponding to a spacing of 1.6 nm) was observed, indicating the occurrence of scattering between ionic aggregates. However, the ion clustering peak of PSbP was considerably weaker than that of PSP, and an additional broad peak was detected for PSbP at higher q of ∼7.2 nm−1 (corresponding to a spacing of 0.87 nm). This finding implied that ion clustering in PSbP with a 2-fold higher concentration of phosphonic acid groups than PSP was suppressed and rather yielded smaller dispersed aggregates, as schematically illustrated in the right panels of Figure 3a. It should be noted here that the distance between phosphonic acid groups in PSbP phases was calculated as 0.55 nm. In the high q region, a narrow single peak centered at 14.3 nm−1 was observed for PSbP, whereas a broad peak at 12.6 nm−1 was observed for PSP. Given that this length scale corresponds to the local amorphous packing of the polymer backbone, one can expect the PSbP chains to be more correlated and close-packed despite having a larger van der Waals volume of the repeating unit (197 Å3 for PSbP and 153 Å3 for PSP). The ionomer peaks of PIB-b-PSP and PIB-b-PSbP block copolymers were significantly weaker than those of their homopolymer analogues (Figure 3b). Intriguingly, PIB-b-PSbP block copolymers consistently exhibited restrained ion cluster formation, compared to PIB-b-PSP, as indicated by the orange shade. It is particularly noteworthy that the ionomer peak of PIB-b-PSbP16 (with short PSbP chains) appeared to be the weakest, which suggested that nanoconfinement and chain

exchange capacity (IEC) and local acid group concentration of phosphonated block copolymers. Successful syntheses of 1d, 2e, 3b, and 3c avoiding selfcondensation of P−OH groups were confirmed by 1H and 31P NMR spectroscopy. Figures 2a and 2b show representative assigned 1H and 31P NMR spectra of 3c before and after hydrolysis, respectively. Data for 1d, 2e, and 3b are shown in Figures S4 and S5.

Figure 2. (a) 1H and (b) 31P NMR spectra of PIB-b-PSbP (ester form in CDCl3 and acid form in methanol-d4/THF-d8). (c) GPC traces of PIB-BIBB, PIB65-b-PSP30, PIB65-b-PSbP28, and PIB65-b-PSbP16. The molecular weight of PIB-BIBB was determined as 3.8 kg/mol.

The number-average molecular weights (Mn) of PSP and PSbP homopolymers were determined by end-group analysis of 1 H NMR data. For block copolymers, we first obtained Mn values of ester forms by integrating 1H NMR peaks at 4.1/1.2 ppm (ethylphosphonate) and 1.4/1.1 ppm (PIB-BIBB with Mn = 3.8 kg/mol). Based on these values, the molecular weights of hydrolyzed samples were calculated. Hereafter, block copolymers are labeled using the degree of polymerization of each block as a subscript, e.g., PIB65-b-PSP30. The polydispersity index (PDI) of each polymer was determined by GPC, yielding the PDI values of 1.07 and Table 1. Materials Used in the Present Study

a

polymer

MW (kg/mol)

degree of polymerization

IEC (mmol/g)

ϕPSP or ϕPSbP

morphology

d-spacing (nm)

PSP PSbP PIB65-b-PSP30 PIB65-b-PSbP16 PIB65-b-PSbP28

3.3 3.5 3.8−5.5 3.8−4.3 3.8−7.5

18 13 65−30 65−16 65−28

5.45 7.43 3.23 3.95 4.96

1 1 0.458 0.362 0.479

sphere HEXa HEXa

23.4 19.5 26.1

HEX: hexagonally packed cylinder. D

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Figure 3. WAXS profiles of phosphonated polymers in the presence and absence of ionic liquids: (a) neat homopolymers, (b) neat block copolymers, (c) [2E4MIm][TFSI]-containing homopolymers, and (d) [2E4MIm][TFSI]-containing block copolymers. Schematic drawings in the right panels of (a) illustrate ion clustering in PSP and PSbP. Scattering profiles of block copolymers in (b) and (d) are vertically offset for clarity. λ values in (c, d) indicate local concentrations of ionic liquid. Characteristic peaks are indicated by ↓,▼, and ▽ symbols.

particularly noteworthy, indicating that homogeneous ionic phases with restrained ionic aggregation were present even after the addition of the ionic liquid. It should be noted here that the results shown in Figure 3 differ from those of sulfonated polymers reported in the literature. Typically, an increase in acid group concentration (by increasing sulfonation level47 or by attaching multiple sulfonic acid groups to the backbone48) expedited ion aggregation owing to strong dipolar interactions among sulfonate moieties. We investigated the microphase separation of phosphonated block copolymers by small-angle X-ray scattering (SAXS). Figure 4 shows the SAXS profiles of PIB65-b-PSP30, PIB65-bPSbP28, and PIB65-b-PSbP16 block copolymers measured at 25 °C, with these profiles being temperature-insensitive up to 180 °C. Both PIB65-b-PSbP16 and PIB65-b-PSbP28 showed ordered hexagonally packed cylindrical (HEX) morphology, as indicated by a series of Bragg peaks (↓ and ▼) at q*,√3q*,√4q*,√7q*,√9q*,√11q*,√12q*, and √16q*, with q* = 2π/d100 (d100 = 19.5 and 26.1 nm, respectively). Note that the development of HEX morphology in nearly symmetric block copolymer has been proven experimentally49 and theoretically50 in the literature. On the contrary, the Bragg peaks observed at the q ratio of 1:1.5:2 (▽) indicated illdefined spherical packing for PIB65-b-PSP30 with a d-spacing of 23.4 nm. The inset of Figure 4 shows that the three block copolymers featured dissimilar domain sizes. Notably, whereas PIB 65 -b-PSbP 16 and PIB 65 -b-PSbP 28 showed HEX morphologies, the thicknesses of their PSbP cylinders were markedly different. Given that both block

stretching in PSbP-based block copolymers inhibit ion agglomeration. The presence of closed-packed PSbP chains (amorphous halo at ∼14 nm−1) was also confirmed for PIB-bPSbP16 and PIB-b-PSbP28 block copolymers (see Figure S6 for more details). The peak at 10.4 nm−1 (spacing = 0.60 nm) observed for all three block copolymers (indicated by the gray shade in Figure 3b) was ascribed to the local packing of the PIB backbone. In the presence of embedded ionic liquids, pronounced ion clustering was observed in PSP samples, as indicated by the sharp intense peak at q = 3.9 nm−1 (Figure 3c). On the contrary, the ion cluster peaks remained weak for PSbP analogues. The λ values in Figure 3c indicate the local ionic liquid concentration, defined as the ionic liquid to phosphonic acid group molar ratio in the polymer (λ ≡ [ionic liquid]/ [−PO3H2]). For both samples, we also observed scattering intensity changes in mid-q values (corresponding to spacings of 0.60−0.72 nm), as indicated by an inverted filled triangle, with these changes becoming more pronounced with increasing ionic liquid concentration. This behavior was attributed to anion correlation in the studied samples.45,46 Moreover, it should be noted that although we only show representative data obtained for [2E4MIm][TFSI], ion clustering was not significantly affected by the type of ionic liquid. Figure 3d shows scattering data for PIB65-b-PSP30, PIB65-bPSbP16, and PIB65-b-PSbP28 block copolymers containing [2E4MIm][TFSI] (λ = 0.5), displaying the appearance of anion correlation in mid-q values (inverted filled triangle) and ionic aggregation at q = 3.9 nm−1. The almost negligible ion clustering in ionic liquid−containing PIB65-b-PSbP16 is E

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Figure 5. SAXS profiles of PIB65-b-PSbP28, PIB65-b-PSP30, and PIB65b-PSbP16 block copolymers with embedded [2E4MIm][TFSI] (λ = 0.5), measured at 25 °C and vertically offset for clarity. The schematic drawings the right panel display the self-assembled morphologies of polymers containing ionic liquids. Bragg peaks (↓ and ▼) at q*, 2q*, 3q*, 4q*, 5q*, 6q*, 7q* and at q*,√3q*,√4q*are indicated in the figure.

Figure 4. SAXS profiles of PIB65-b-PSbP28, PIB65-b-PSP30, and PIB65b-PSbP16 block copolymers, measured at 25 °C. The scattering profiles are offset vertically for clarity. The domain sizes of block copolymers are shown in the inset plot. The schematic drawings in the right panels display the self-assembled morphologies and ion clustering behaviors of each sample. Bragg peaks (↓ and ▼) at q*,√3q*,√4q*,√7q*,√9q*,√11q*,√12q*,√16q* and at 1q*:1.5 q*:2 q* (▽) are indicated in the figure.

Although the Bragg peaks of PIB65-b-PSP30 with [2E4MIm][TFSI] at q ratio of 1:√3:√4 (▽) indicated ordered HEX morphology, the major discrepancy between PIB-b-PSP and PIB-b-PSbP was again the lack of ordering, as indicated by the broad and indistinct high-order reflections of PIB65-b-PSP30 (Figure 5). This poor ordering remained unchanged in a wide λ range of 0−1.0 (data not shown here). It should be noted here that while we only discussed subset of data obtained with [2E4MIm][TFSI], the results obtained for [Im][TFSI] and [2MIm][TFSI] led to the same conclusion, except for the fact that ionic liquids with long alkyl substituents on the imidazole ring, i.e., [2E4MIm], induced better ordering18 (see SAXS data set in Figure S7). Anhydrous Ionic Conductivity of Phosphonated Polymers with Embedded Ionic Liquids. In the next step, we investigated the anhydrous ionic conductivity of ionic liquid-containing phosphonated polymers. Figure 6a shows temperature-dependent conductivities of PSP and PSbP homopolymers with fixed [Im][TFSI] and [2E4MIm][TFSI] loadings of 50 wt % in the temperature range of 80−150 °C. Notably, the use of [2E4MIm][TFSI] resulted in higher conductivities than that of [Im][TFSI], which was ascribed to the lower glass transition temperatures (Tgs) of the former membranes and the favorable hydrophobic interactions of [2E4MIm] with PSP and PSbP chains.18 Importantly, PSbP samples exhibited over 2-fold higher conductivity than PSP analogues, regardless of the type of ionic liquid. Considering the fact that the Tg of PSbP is 20 °C higher than that of PSP (145 vs 120 °C, DSC data not shown here), the observed conductivity difference becomes even more pronounced, which can be explained by the higher acid group content of PSbP (IEC = 7.43 mmol/g) compared to that of PSP (IEC = 5.45 mmol/g). The solid lines in Figure 6a indicate fits obtained using the Vogel−Tammann−Fulcher (VTF)

copolymers composed of the same PIB block, the PSbP cylinders with small diameter in PIB65-b-PSbP16 were inferred to provide more confined environments, inhibiting ion aggregation. The insets of Figure 4 depict the self-assembled morphologies observed for PIB65-b-PSbP28, PIB65-b-PSP30, and PIB65-b-PSbP16 block copolymers by reflecting the dissimilar ion clustering behaviors (as revealed from Figure 3). The ill-defined morphology of PIB65-b-PSP30 with a high phosphonation level of 100% bears resemblance to the disruption of ordered morphology commonly observed for sulfonated block copolymers with increasing sulfonation levels, which has been rationalized by ion agglomeration.51,52 With this in mind, the long-range ordered HEX morphology of PIB65-b-PSbP28 and PIB65-b-PSbP16 block copolymers is not trivial in view of the 2-fold higher phosphonate group content (i.e., phosphonation level of 200%) of PSbP blocks. The suppressed ion aggregation in PSbP phases of PIB-b-PSbP block copolymers should be connected to this observation. It should be noted that the synthesis of bisphosphonate block copolymers has not been previously reported, with this work therefore providing a first-time account of well-defined nanoscale morphologies of highly phosphonated polymers. The addition of ionic liquids to phosphonated block copolymers resulted in intriguing morphological changes and enhanced microphase separation. As shown in Figure 5, welldefined lamellar (LAM) structures were observed for both PIB 65 -b-PSbP 16 and PIB 65 -b-PSbP 28 with embedded [2E4MIm][TFSI] at λ = 0.5, as shown by a series of Bragg peaks (↓ and ▼) at q*, 2q*, 3q*, 4q*, 5q*, 6q*, 7q*, ... (q* = 2π/d100), along with increases in domain size. This behavior indicated the selective swelling of PSbP phases with incorporated ionic liquid due to favorable intermolecular interactions among ionic moieties. F

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Figure 6. Temperature-dependent through-plane conductivities of (a, b) ionic liquid-containing phosphonated homopolymers and (c, d) phosphonated block copolymers. The solid lines in (a−d) indicate VTF fits.

equation,53 with the potential barriers to ion conduction of ionic liquid-containing PSbP samples being as low as 138 K for [Im][TFSI] and 170 K for [2E4MIm][TFSI], whereas the respective values for PSP analogues equaled 179 and 230 K. For a better understanding of the dissimilar ionic conductivities of ionic liquid-containing PSP and PSbP samples, Figure 6b shows another set of conductivity data, acquired at the same local concentration of ionic liquid (λ = 0.5). In this case, the differences between PSP and PSbP sample conductivities further increased (up to 4-fold), allowing us to undoubtedly conclude that the efficiency of ion transport in PSbP samples was markedly higher than that in their PSP analogues. Moreover, VTF fits indicated that PSbP samples featured low potential barriers for ion conduction (111 K with [Im][TFSI] and 194 K with [2E4MIm][TFSI]) as compared to those of PSP samples (159 K with [Im][TFSI] and 268 K with [2E4MIm][TFSI]). Thus, the suppression of ion clustering to yield dispersed clusters at smaller length scales ( PIB65-b-PSbP28 > PIB65-b-PSP30. This trend was not affected by the type of ionic liquid, with representative data obtained for [Im][TFSI] shown in Figure 6c (loading = 50 wt %) and Figure 6d (λ = 0.5). The lowest conductivity observed for PIB65-b-PSP30 membranes suggested their lowest ion transport efficiency. Thus, it was inferred that the formation of long-range hydrogen bond networks in PSbP-based polymers plays a key role in improving ion transport. We paid particular attention to the results obtained for PIB65b-PSbP16 samples, which exhibited the highest conductivity despite their IEC being roughly 20% lower than that of PIB65-bPSbP28, concluding that high proton concentration is not responsible for the improved conductivity of PIB65-b-PSbP16 G

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samples. The narrow PSbP domain of PIB65-b-PSbP16 with excluded ion aggregation should be responsible for the facilitated fast ion transport. Furthermore, block copolymer samples featured significantly lower ion conduction barriers than homopolymer ones, i.e., the barriers determined from VTF fits (shown as solid lines in Figure 6c) equaled 78, 85, and 63 K for PIB65-b-PSP30, PIB65-b-PSbP28, and PIB65-b-PSbP16 samples containing 50 wt % [Im][TFSI], respectively. This result sets clear rules of designing ion aggregation-free phosphonated polymers with well-defined phosphonated phases to achieve improved ionic conductivity.

CONCLUSIONS Herein, we propose rules for designing phosphonated polymers with improved ionic conductivity, with the salient points of this work summarized as follows. 1. A successful first-time synthesis of bisphosphonate monomer-based block copolymers displaying long-range ordered self-assembled morphologies was demonstrated. 2. A new approach of avoiding ion aggregation in phosphonated polymers was established, featuring the confinement of bisphosphonate polymer chains to few-nanometer domains. 3. The addition of imidazolium ionic liquids to the synthesized polymers resulted in the formation of virtually homogeneous ionic phases with suppressed ion clustering for bisphosphonate-based block copolymers. This behavior was ascribed to the radically lower potential barrier to ion conduction of the latter copolymers, their 2−4-fold improved anhydrous conductivity, and the stabilization of ordered phases upon attenuation of dipole−dipole interactions among phosphonic acid groups. 4. In view of the fact that the Tg of polystyrene bisphosphonate is >20 °C higher than that of polystyrene phosphonate, our work has established a way of improving both the conductivity and mechanical strength of phosphonated polymers. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02449. Characterization of materials by 1H NMR, 31P NMR, FT-IR, GPC, WAXS, and SAXS; Figures S1−S7 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.J.P.). ORCID

Moon Jeong Park: 0000-0003-3280-6714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. NRF-2017R1A2B3004763), by the Korea government (Ministry of Trade, Industry and Energy) (No. 10067135), and by the Korea government (MSIP) (No. NRF2017R1A5A1015365). H

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