Synthesis and Morphology Study of SEBS Triblock Copolymers

Institute, Troy, New York 12180, United States. Macromolecules , Article ASAP. DOI: 10.1021/acs.macromol.7b01848. Publication Date (Web): January ...
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Synthesis and Morphology Study of SEBS Triblock Copolymers Functionalized with Sulfonate and Phosphonate Groups for Proton Exchange Membrane Fuel Cells Bhagyashree Date, Junyoung Han, Sungmin Park, Eun Joo Park, Dongwon Shin, Chang Y. Ryu,* and Chulsung Bae* Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York 12180, United States S Supporting Information *

ABSTRACT: Aromatic rings of poly(styrene-b-(ethylene-r-butylene)-b-styrene) triblock copolymer (SEBS) were functionalized with various acid functional groups for proton exchange membrane (PEM) applications. Three different acid functional groups (fluoroalkylsulfonic acid, arylsulfonic acid, and arylphosphonic acid) were introduced into SEBS via borylation of aromatic C−H bonds and Suzuki coupling reactions. The incorporation of acid side groups selectively into the polystyrene block of SEBS created nanometerscale phase separated morphology composed of hydrophilic and hydrophobic domains in which the morphology structures and interdomanin distances are dependent on the chemical structure of acid side chains. Despite high ion exchange capacity value, the aryl phosphonated polymer (SEBS-P) showed the lowest water uptake and significantly lower proton conductivity compared to sulfonated SEBS PEMs because of the low acidity of phosphonic acid. Compared to aryl sulfonated SEBS-S2, fluoroalkyl sulfonated SEBS-S1 showed better proton conductivity at low relative humidity condition (150 °C), and the resulting alkyl phosphonated polymers tend to be less stable than aryl phosphonated polymers. For direct covalent bond formation of phosphonic acid and aromatic rings without an alkylene spacer, palladium-catalyzed cross-coupling reactions of aryl halides with diethyl phosphite are commonly adopted. Phosphonic acidfunctionalized PEMs from brominated SEBS17 and brominated polysulfone18 were prepared using this C−P bond formation method. A major drawback of this synthetic method is that although bromination of activated aromatic rings gives quantitative yields the subsequent phosphonation with diethyl phosphite is inefficient, thus requiring a high temperature for an extended period of time (e.g., at 120−150 °C for 3−4 days). Recently, Park et al. reported phosphonated styrene− methylbutylene block copolymers using this method and achieved up to 76% phosphonation degree in the polystyrene block.19 Alternatively, phosphonated aromatic polymer can also be synthesized by a route of lithiation and phosphonation.20 Because conventional sulfonation and phosphonation of aromatic polymers require different precursors and reaction conditions, it is difficult to synthesize sulfonated polymers and phosphonated polymers with the same functionalization degree from identical polymer backbone structure and compare the effects of sulfonate vs phosphonate groups on membrane properties and morphology of PEM.15,19 Recently, our group developed a versatile synthetic strategy to postfunctionalize aromatic polymers with polar groups using transition-metalcatalyzed reactions.21−24 We first introduced pinacolboronic ester (Bpin) groups into aromatic rings of polymer chains using iridium-catalyzed borylation of aromatic C−H bonds25−27 and subsequently converted to a variety of polar groups using palladium-catalyzed Suzuki−Miyaura cross-coupling reactions.28 Unlike traditional polymer postfunctionalization, which frequently causes side reactions, this transition-metalcatalyzed synthetic route induces negligible changes in molecular weight and molecular weight distribution.21−24 In our investigation of acid group effect on membrane properties of PEMs, we herein report synthesis of a series of aromatic polymers functionalized with the same concentration of different acid groups (i.e., fluoroalkylsulfonic acid, arylsulfonic acid, and arylphosphonic acid) from the same precursor block copolymer. Because all proton-conducting polymers were derived from the same precursor polymer, they share the same polymer backbone structure and have the same molar concentration and identical distribution of ionic groups. Preparation of such well-defined ionic block copolymers will allow us to investigate fundamental understanding of the effects of ionic side group (e.g., fluoroalkylsulfonic acid, arylsulfonic acid vs phosphonic acid) on morphology structures and membrane properties (e.g., water uptake, proton conductivity) and elucidate the correlations among the structures of ionic side groups, self-aggregation behaviors, and ion transport properties in polymer membranes. We selected SEBS as a precursor polymer for postfunctionalization because (a) the morphology structures of SEBS are well-known, (b) the selective

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EXPERIMENTAL SECTION

Materials. 4,4′-Di-tert-butyl-2,2′-dipyridyl (dtbpy), chloro-1,5cyclooctadiene iridium(I) dimer ([IrCl(COD)]2), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4), [1,1′-bis(diphenylphosphino)ferrocene]dichloropalladium(II), complex with dichloromethane [Pd(dppf)Cl 2 −CH 2 Cl 2 ], bis(dibenzylideneacetone)palladium(0) [Pd2(dba)3], 4-(N,N-dimethylamino)pyridine (DMAP), sodium dithionite (Na2S2O4), 3,5-dimethylphenol, neopentyl alcohol, triethyl phosphite, 4-bromobenzyl bromide, and 4-bromobenzenesulfonyl chloride were reagent grade and used without further purification. Bis(pinacolato)diboron (B2pin2) from Frontier Scientific Co., BrCF2CF2Br from SynQuest Laboratories, chlorine gas from Praxair Inc., and CFC-113 from Aqua Solutions were used as received. SEBS (Mn = 118 kg/mol with Mw/Mn = 1.08; 18 mol % polystyrene repeating unit; 30 wt % polystyrene) was obtained from Sigma-Aldrich Co. and used as received. Anhydrous THF was purchased from Thermo Fisher Scientific Inc. S1 was prepared according the literature procedure.22 1H, 13C, 19F, and 31P NMR spectra were obtained using a Varian 500 MHz NMR spectrometer. FT-IR spectra were recorded from a PerkinElmer Spectrum One instrument. Synthesis of Polymers. Synthesis of S1. S1 was obtained as a yellowish oil using a literature method.21,22 1H NMR (500 MHz, CDCl3, δ): 7.52 (d, J = 8.8 Hz, 2H), 7.13 (d, J = 8.8 Hz, 2H), 6.99 (s, 1H), 6.92 (s, 2H) 2.25 (s, 6H). 19F NMR (500 MHz, CDCl3, δ): −81.4 (t, J = 4.3 Hz, −OCF2), 112.5 (t, J = 4.3 Hz, −CF2SO3). 13C NMR (126 MHz, CDCl3, δ): 150.0, 147.5, 140.4, 133.1, 129.9, 123.8, 120.7, 119.2, 115.8 (tt, 1JCF = 277 Hz, 2JCF = 28.6 Hz), 113.9 (tt, 1JCF = 298 Hz, 2JCF = 39.1 Hz), 21.4. Synthesis of S2. p-Bromobenzenesulfonyl chloride (10.0 g, 39.1 mmol) was dissolved in CH2Cl2 (50 mL) in a 250 mL round-bottom flask. The flask was cooled to 5 °C in an ice bath. To this cooled reaction mixture, pyridine (9.50 mL, 117 mmol, 3 equiv) and neopentyl alcohol (5.18 g, 58.7 mmol, 1.5 equiv) were added in small portions to the flask. The reaction mixture was stirred at room temperature for 18 h and later diluted with ether (50 mL). The reaction solution was washed with 2 M HCl solution (30 mL × 2), saturated NaHCO3 solution (30 mL), and brine (30 mL), dried over Na2SO4, and evaporated using a rotary evaporator. Crude product was recrystallized from hexane to give pure product of S2 as a white crystalline solid (9.38 g; 78% yield). 1H NMR (500 MHz, CDCl3, δ): 0.91 (s, 9H), 3.69 (s, 2H), 7.74 (distorted dd, J = 8.3 Hz, 4H). 13C NMR (126 MHz, CDCl3, δ): 26.1, 31.8, 80.1, 129.1, 129.6, 132.8, 135.4. Synthesis of P. 4-Bromobenzyl bromide (20.0 g, 80.0 mmol) and triethyl phosphite (17.15 mL, 100.0 mmol, 1.25 equiv) were dissolved in 1,4-dioxane (60 mL) and heated to reflux under N2. The reaction mixture was stirred under reflux for 33 h. P was obtained as a clear viscous oil after vacuum distillation (24.4 g, 95% yield). 1H NMR (500 MHz, CDCl3, δ): 7.42 (distorted, dd, J = 8.4, 2H), 7.13 (dd, J = 8.4, J = 2.5, 2H), 3.98−4.04 (m, 4H), 3.07 (d, JP−H = 21.7, 2H), 1.21 (t, J = 7.1, 6H). 13C NMR (126 MHz, CDCl3, δ): 131.7, 131.4, 130.7, 121.0, 62.2 (d, J = 6.5 Hz), 33.2 (d, J = 139 Hz), 16.4 (d, J = 6 Hz). 31P NMR (202 MHz, CDCl3, δ): 25.6 (85% H3PO4 as external reference). Synthesis of Borylated Polymer (SEBS-Bpin). In a nitrogen-filled glovebox, SEBS (6.45 g, 18 mmol styrene unit), B2pin2 (16.0 g, 63 mmol, 3.5 equiv), [IrCl(COD)]2 (633 mg, 1.5 mol % based on the amount of B2pin2), dtbpy (505 mg, 3 mol % based on the amount of B2pin2), THF (30 mL), and a magnetic stirring bar were placed in a 250 mL flask. The reaction mixture was stirred at 80 °C under nitrogen for 12 h. After cooling to room temperature, the reaction mixture was poured into cold methanol. The precipitated polymer was stirred for 1 h and filtered to give a light yellow solid. Another cycle of dissolution in THF and precipitation with cold methanol gave SEBSB

DOI: 10.1021/acs.macromol.7b01848 Macromolecules XXXX, XXX, XXX−XXX

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room temperature, followed by immersing in deionized water for 1 day (water was changed several times). Membrane Casting of SEBS-S2e and SEBS-Pe. A 3 wt % toluene solution of the polymer in ester forms was prepared with gentle heating. A film was cast on a Teflon plate, which was subsequently dried inside a closed chamber at room temperature for 8 h and at 70 °C for an additional 8 h. The Teflon plate was cooled to room temperature and immersed into cold water for 12 h, after which the membrane was peeled off. The detached membrane was dried at 70 °C for 8 h to give a tough and flexible film with thickness in the range 40− 50 μm. Synthesis of SEBS-S2. Thermolysis of neopentyl group was carried out by heating membrane of SEBS-S2e, which was obtained on a Teflon plate from the above solvent casting method, at 155−160 °C under a continuous flow of N2 in a convection oven for 1 h. At the end of the thermolysis process, we observed that the color of the membrane changed to light brown, and the membrane size was also reduced. FT-IR and IEC measurements confirmed successful removal of the neopentyl protecting group. Synthesis of SEBS-P. SEBS-Pe membrane (0.30 g) was placed in 50 mL of 12 M HCl solution. After heating the solution until it reflux for 18 h, the polymer membrane was collected by filtration and washed with water repeatedly until its pH became neutral. The membrane was dried under vacuum at 70 °C for 12 h. Measurement. Ion Exchange Capacity (IEC). The calculated IECs of acid-functionalized SEBS ionomers were estimated from the mol % of ester protected form of SEBS-S1e, -S2e, and -Pe in 1H NMR spectra. Experimental IECs were determined using the titration method. Acid membranes were equilibrated in 2 M NaCl solution at room temperature for 24 h before titration. The protons released into the aqueous solution were titrated with 0.025 M NaOH solution using phenolphthalein as an indicator. Experimental IEC values of acidic polymer membranes were calculated according to the equation

Bpin as a white solid, which was subsequently dried under vacuum at 80 °C for 8 h. Based on analysis of 1H NMR spectrum, 100% of PS block (i.e., 18 mol % of SEBS repeating unit) was borylated. Synthesis of SEBS-S1e. SEBS-Bpin (500 mg of Bpin-functionalized SEBS, 1.03 mmol of boron), S1 (1.18 g, 2.58 mmol, 2.5 equiv), and K3PO4 (656 mg, 3.09 mmol, 3 equiv) were placed in a 50 mL flask. Inside a nitrogen-filled glovebox, Pd(dppf)Cl2−CH2Cl2 (33.6 mg, 0.03 mmol, 3 mol %) and THF (10 mL) were added to the flask. The flask was removed from the glovebox, and deionized water (1 mL) was added to the solution using a syringe. The solution was stirred at 65 °C for 10 h, cooled to room temperature, diluted with chloroform (50 mL), and filtered through a short pad of silica gel to remove the palladium catalyst and salt residue. The filtrate was concentrated to about 4 mL, and cold methanol was slowly added to precipitate polymer product. Another cycle of dissolution in chloroform and precipitation with cold methanol provided 0.65 g of 3,5-dimethylphenol protected sulfonate polymer SEBS-S1e as a white solid. 1H NMR (500 MHz, CDCl3): δ 6.00−8.20 (aromatic H), 2.30 (CH3 of 3,5dimethylphenol), 1.26 (CH2 of polymer backbone), 0.83 (CH3 of polymer backbone). 19F NMR (470 MHz, CDCl3): −80.6 (−OCF2) and −112 (−CF2SO3). Synthesis of SEBS-S2e. SEBS-Bpin (2.50 g of Bpin-functionalized SEBS, 5.14 mmol of boron), S2 (4.72 g, 15.4 mmol, 3 equiv), and K3PO4 (3.28 g, 15.4 mmol, 3 equiv) were placed in a 100 mL flask. Inside a nitrogen-filled glovebox, Pd(dppf)Cl2−CH2Cl2 (126 mg, 0.150 mmol, 3 mol %) and THF (50 mL) were added to the flask. The flask was removed from the glovebox, and deionized water (5 mL) was added to the solution using a syringe. The solution was stirred at 80 °C for 8 h, cooled to room temperature, diluted with chloroform (50 mL), and filtered through a short pad of silica gel. The filtrate was concentrated to about 30 mL, and cold methanol was slowly added to precipitate polymer product. Another cycle of dissolution in chloroform and precipitation with cold methanol provided 2.94 g of neopentyl-protected sulfonated polymer SEBS-S2e as a white solid. 1H NMR (500 MHz, CDCl3, δ): 6.00−8.20 (aromatic H), 3.75 (CH2 of neopentyl), 1.26 (CH2 of polymer backbone), 0.92 (CH3 of neopentyl), 0.83 (CH3 of polymer backbone). Synthesis of SEBS-Pe. SEBS-Bpin (3.23 g of Bpin-functionalized SEBS, 6.64 mmol of boron), P (6.12 g, 19.9 mmol, 3 equiv), and K3PO4 (4.23 g, 19.9 mmol, 3 equiv) were placed in a 100 mL flask. Inside a nitrogen-filled glovebox, Pd(dppf)Cl2−CH2Cl2 (325 mg, 0.398 mmol, 6 mol %) and THF (50 mL) were added to the flask. The flask was removed from the glovebox, and deionized water (5 mL) was added using a syringe. The solution was stirred at 80 °C for 12 h, cooled to room temperature, and filtered through a short pad of silica gel. The filtrate was precipitated by adding water (250 mL). The precipitated polymer was initially a dark colored sticky solid, and additional stirring in water over 12 h reduced its stickiness. An additional two cycles of dissolution in THF and precipitation with water provided 4.51 g of SEBS-Pe. 1H NMR (500 MHz, CDCl3, δ): 6.0−7.7 (aromatic H), 4.0 (−CH2P(O)(OCH2CH3)2), 3.1 (−CH2P(O)(OCH2CH3)2), 1.26 (CH2 of SEBS backbone and −P(O)(OCH2CH3)2), 0.83 (CH3 of SEBS backbone). Membrane Casting and Removal of Ester Protecting Groups To Generate Acid-Functionalized Polymers. Synthesis of SEBSS1. SEBS-S1e (0.14 g, 0.19 mmol fluoroalkyl sulfonate) was dissolved in THF (8 mL) with gentle heating and a mixture of NaOH (0.4 g, 1.4 mmol, 30 equiv), and H2O (20 μL) was added. The resulting solution was stirred at 80 °C for 6 h. After cooling to room temperature, volatile solvent was removed using a rotary evaporator, and the resulting solution was poured into a mixture of methanol and water (3:1). Sodium sulfonate form of SEBS-S1 was obtained as a dark colored solid. Membrane Casting of SEBS-S1. The sodium sulfonate form of SEBS-S1 was dissolved in a mixture of DMSO and dichloroethane after repeated cycles of heating and vortexing in a glass vial. This solution was cast on a Teflon plate and dried at 40 °C under a positive air flow for 24 h and then at 80 °C under vacuum for 12 h. The acid form of SEBS-S1 was obtained by immersing the membrane in 1 M H2SO4 for 2 days (during which time the solution was changed every day) at

IEC (mequiv/g) = MNaOH × VNaOH/Wdry Water Uptake. In order to measure water uptake, the membrane in acid form was dried under vacuum at 60 °C for 48 h to obtain dry weight (Wdry). The membrane was then immersed in deionized water at 60 °C for 24 h. Excess water on the membrane surface was quickly removed with a tissue paper before the weight of wet membrane (Wwet) was obtained. The water uptake was calculated using the equation WU = [(Wwet − Wdry ]/Wdry ] × 100% Thermal Stability. The thermal stability of ionic polymers was evaluated by TGA (Q500 analyzer from TA Instruments). After the membrane samples were dried at 70 °C for 12 h, the TGA data were collected from 100 to 800 °C under nitrogen at a heating rate of 10 °C/min. Proton Conductivity. To measure proton conductivity of SEBS ionomers, the membrane in acid form was immersed in deionized water for at least 24 h. In-plane proton conductivity was measured using a four-point electrode method with a BT-512 membrane conductivity test system (BekkTech LLC). Proton conductivity was measured by changing relative humidity (RH) from 30% to 100% at 60 °C. RH control started at 70% RH, stabilized for 2 h, and decreased to 30% RH at a rate of 10% RH/20 min, and data were collected from the cycle of 30% to 100% RH. The proton conductivity was calculated according to the following equation:

σ (mS/cm) =

L R×W×T

where L is the distance between the two inner platinum wires (0.425 cm), R is the resistance of the membrane, and W and T are the width and the thickness of the membrane in centimeters, respectively. Transmission Electron Microscopy (TEM). Direct morphological analysis of SEBS ionomers with different acid functional groups was performed by TEM. Ultrathin specimens (∼60 nm) of epoxyembedded samples were prepared at −60 °C by cryo-ultramicrotome C

DOI: 10.1021/acs.macromol.7b01848 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules using PTX (RMC) with a diamond knife. The aromatic ring of styrene block in the membrane was selectively stained with vapor of RuO4 (0.5% aqueous solution) for about 10 min at room temperature to enhance the electron density contrast. TEM was operated using a JEM-2010 (JEOL) with accelerating voltage of 200 kV. Small-Angle X-ray Scattering (SAXS). To characterize the membrane morphology of SEBS ionomers with different acid functional groups, SAXS experiments were performed at different drying conditions. First, to emulate the environment for the membrane morphology development at high relative humidity conditions, “hydrated” SEBS ionomer membrane samples were prepared as following: after drying the membrane samples overnight at ambient conditions they were soaked in water for an hour to fully hydrate before they were subject to SAXS experiments. The SAXS experiments were performed in air at 5-ID beamline at the Advanced Photon Source (Argonne National Laboratory, Argonne, IL). The typical operating conditions were set as follows: a wavelength of λ = 0.7293 Å−1, a beam size of 0.5 × 1 mm2, a sample-to-detector distance of 8.503 m, and atmosphere condition. The SAXS profiles were recorded by a Rayonix CCD area detector with dimensions of 170 × 170 mm2. Second, to emulate the environment for the membrane morphology development at low relative humidity conditions, “dried” SEBS ionomer membrane samples were also prepared by drying the membranes at room temperature for a month, and the SAXS experiments were conducted in vacuum using a Bruker Nanostar-U instrument equipped with a turbo (rotating anode) X-ray source with the following condition: a wavelength of λ = 1.5418 Å−1, a beam size of 0.2 × 2 mm2, a sample-to-detector distance of 1.055 m, vacuum condition. The SAXS profiles were recorded by a HI-STAR area detector with dimensions of 230 × 230 mm2.

Scheme 1. Synthesis of SEBS Ionomers Functionalized with Different Acid Groupsa

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RESULTS AND DISCUSSION Synthesis and Characterization of SEBS Ionomers. The synthetic strategy is based on sequential reactions of (a) the iridium-catalyzed borylation of aromatic C−H bonds of SEBS, (b) the palladium-catalyzed Suzuki cross-coupling reactions of the borylated SEBS with phenyl bromides that contain sulfonated or phosphonated precursor groups (S1, S2, and P), and (c) removal of the protecting group from the sulfonates or phosphonate in the polymer (Scheme 1). Synthesis of Sulfonated and Phosphonated Phenyl Bromides. To introduce sulfonate and phosphonate groups to SEBS via Suzuki coupling reactions, we prepared two different phenyl bromides functionalized with sulfonate groups (fluoroalkyl sulfonated S1 and aryl sulfonated S2) and a phenyl bromide functionalized with phosphonate group (P) (see Scheme 2). The synthesis of 3,5-dimethylphenol protected fluoroalkyl sulfonate S1 was previously reported by our group.22 For an aryl sulfonate precursor, we prepared a neopentyl protected p-bromobenzenesulfonate S2 by following procedure previously reported in the literature.29 As discussed later, this neopentyl protecting group has an advantage of thermoinduced generation of sulfonic acid group on polymer. Nucleophilic substitution reaction of p-bromobenzenesulfonyl chloride with neopentyl alcohol in the presence of a mild organic base such as pyridine gave S2 in 78% yield. Phosphonated phenyl bromide P was synthesized by reacting 4-bromobenzyl bromide with triethyl phosphite under typical Arbuzov reaction conditions. Ethyl bromide side product was removed by vacuum distillation, giving P as a clear viscous oil in 95% yield. In the 1H NMR spectrum, a doublet at 3.07 ppm was observed for the coupling of phosphorus and benzylic CH2 (3J = 21.7 Hz), and a multiplet at 4.01 ppm was observed for OCH2CH3. A single resonance at 25.6 ppm in 31P NMR also confirmed successful synthesis of P. Both 1H and 31P NMR spectra of P matched well with the previous reported

a Reagents and conditions: (i) B2pin2 (3.5 equiv), [IrCl(COD)]2 (1.5 mol %), dtbpy (3 mol %), THF, 80 °C, 12 h; (ii) S1, S2, or P (3 equiv), Pd(dppf)Cl2−CH2Cl2, K3PO4 (3 equiv), THF/H2O (10:1), 80 °C, 12 h; (iii) NaOH (30 equiv), THF/H2O (100:1), 80 °C, 6 h; (iv) thermolysis: 1 h, 155−160 °C, N2; (v) 12 M HCl, 18 h, reflux.

spectroscopic data of the phosphonate ester by Sørensen.30 Likewise, purity of all acid precursors (S1, S2, and P) was confirmed by NMR spectroscopies (1H, 13C, 19F, and 31P). Synthesis of Borylated SEBS. SEBS is a triblock copolymer composed of PS end blocks and PEB middle block. Selective functionalization of PS block of SEBS was achieved by the iridium-catalyzed borylation of aromatic C−H bonds. After the reaction, the integral area at 1.25 ppm was increased due to the signal from four CH3 groups of Bpin while the integral area of aromatic region at 6.0−8.2 ppm was decreased due to the substitution of aromatic C−H bonds to C−B bonds. We calculated the molar concentration of the Bpin group in borylated polymer (SEBS-Bpin) by comparing the increase in integral values of PEB block at 0.9−2.4 ppm (peak b of Figure 1b) with reference to the methyl group of polybutylene at 0.83 ppm (peak a of Figure 1a) because no C−H borylation should occur at saturated Csp3−H bonds of PEB block. On the basis of the 1H NMR data, we confirmed almost all PS repeating unit was borylated (i.e., 18 mol % of total repeating unit of SEBS). Next we proceeded to carry out Suzuki coupling reactions of the borylated SEBS with S1, S2, and P to synthesize the corresponding acid precursor polymers (SEBS-S1e, SEBS-S2e, and SEBS-Pe, respectively, of Scheme 1 where the suffix “e” indicates they are in ester form). D

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on the integral ratio of the CH3 of PEB block in the polymer chain (labeled as a at 0.83 ppm in Figure 1a) and the CH3 of the 3,5-dimethylphenol structure (labeled as c in Figure 1c). The molar concentration of fluoroalkyl sulfonate ester group matched well with that of Bpin in the borylated precursor polymer. Successful incorporation of fluoroalkyl sulfonate group was further confirmed by two distinct resonances at −80.3 ppm (−OCF2−) and −112.1 ppm (−CF2SO3−) in the 19 F NMR spectrum. The C−H borylation reaction of polystyrene is known to give a mixture of meta- and parasubstituted (approximately 1:1 ratio) borylated polymers.24 After reaction of SEBS-Bpin with S1 by Suzuki coupling, the resulting polymers SEBS-S1e contain extra benzene rings that are derived from the aryl bromide. The above two factors (i.e., formation of regioisomers and incorporation of additional aromatic rings) attributed to broadening of the proton signal in the aromatic region (6.0−8.0 ppm). Basic hydrolysis of SEBS-S1 removed the 3,5-dimethylphenol protecting group and generated sodium sulfonate form of SEBS-S1 which was subsequently acidified with 1 M H2SO4. Unfortunately, ionic form polymers of all acid-functionalized SEBSs including SEBS-S1 have very poor solubility in organic solvents, preventing their analysis by NMR spectroscopy. Thus, confirmation of successful ionic acid form of the polymers was validated by FT-IR spectroscopy and titrated IECs. Figure 2 compares the FT-IR spectra of SEBS-S1e and SEBS-S1, before and after basic hydrolysis. The absorption bands of symmetric trisubstituted benzene ring at 1615, 850, and 700 cm−1 and the C−H asymmetric bending of aromatic CH3 at 1479 cm−1 disappeared completely, and a new broad absorption at 3500 cm−1 for O−H stretching of water was observed due to hygroscopic nature of ionic polymer, indicating successful removal of the 3,5-dimethylphenol structure. SEBS-S2e and SEBS-S2. For synthesis of SEBS-S2, we had to adopt a new protecting group because the 3,5-dimethylphenoxy group could not be completely removed by basic treatment from aryl sulfonated SEBS. Thus, we chose neopentyl protecting group for SEBS-S2 because neopentyl sulfonated groups can be thermally cleaved to generate arylsulfonic acids.31,32 After the Suzuki coupling reaction with S2, the resulting polymer SEBS-S2e showed reduced integral area of proton resonance at 1.26 ppm as a result of substitution of the Bpin group with S2 and two new resonances were observed at 3.75 and 0.92 ppm, corresponding to CH2 and CH3 groups, respectively, of neopentyl group (labeled as d and e of Figure 1d). Molar concentration of the attached S2 in SEBS-S2e was calculated by comparing the resonance integrals of CH3 of PEB block in the polymer chain (labeled as a at 0.83 ppm in Figure 1a) and CH2 of the neopentyl group on the side chain (labeled as e at 3.75 ppm in Figure 1d), and it agreed well with that of the Bpin group in SEBS-Bpin (i.e., 18 mol % of SEBS repeating unit). Deprotection of SEBS-S2e was based on the work of Baek and co-workers: a brief (30 min) thermolysis at 150 °C was reported to cleave neopentyl group and yield a polystyrene functionalized with arylsulfonic acid group.31−33 On the basis of these studies, we placed a thin membrane of SEBS-S2e on a Teflon plate, heated to 155−160 °C under a continuous stream of N2, and monitored the progress of thermolysis with FT-IR spectroscopy (Figure 3). In the case of SEBS-S2e, a minimum heating period of 60 min was necessary to completely remove the neopentyl group as only a partial deprotection was observed when the membrane was heated for 45 min. Figure 3d shows

Scheme 2. Synthesis of Sulfonated and Phosphonated Phenyl Bromides

Figure 1. 1H NMR spectra of (a) SEBS, (b) SEBS-Bpin, (c) SEBS-S1e, (d) SEBS-S2e, and (e) SEBS-Pe.

SEBS-S1e and SEBS-S1. For incorporation of the fluoroalkyl sulfonated group, we adopted the same acid precursor phenyl bromide S1 we employed before22 and removed the 3,5dimethylphenoxy protecting group by treating with a base. The Suzuki coupling reaction using Pd(dppf)Cl2·CH2Cl2 as a catalyst precursor and K3PO4 as a base afforded quantitative conversion of the boronate ester in SEBS to fluoroalkyl sulfonated SEBS-S1e. The 1H NMR of SEBS-S1e showed a new peak at 2.29 ppm corresponding to the terminal methyl group of 3,5-dimethylphenol structure, and the integral area at 1.26 ppm decreased due to the decrease in the Bpin resonance (Figure 1c). Molar concentration of the 3,5-dimethylphenol protected sulfonate group in the polymer was calculated based E

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Figure 2. FT-IR spectra of (a) SEBS-S1e and (b) SEBS-S1 after base hydrolysis.

Figure 3. FT-IR spectra of (a) SEBS-S2e before and after thermolysis for a period of (b) 15, (c) 45, and (d) 60 min [complete deprotection achieved].

Figure 4. FT-IR spectra of (a) SEBS-Pe before hydrolysis and (b) SEBS-P after acid hydrolysis.

that the absorption peak at 1359 cm−1 (SO asymmetric stretching of neopentyl sulfonate ester) became significantly smaller, and a new broad peak appeared at 3433 cm−1 for O−H stretching of water, indicating formation of −SO3H. SEBS-Pe and SEBS-P. Because of the coordinating tendency of phosphorus compounds to palladium catalyst, carbon−

phosphine bond formation generally requires a higher loading of catalyst.34 To achieve complete conversion of Bpin to the phosphonate of P by Suzuki coupling, it was necessary to use 6 mol % of palladium catalyst while the coupling reactions of S1 and S2 required 3 mol % palladium catalyst. After the reaction, new peaks were observed at 4.00 and 3.12 ppm corresponding F

DOI: 10.1021/acs.macromol.7b01848 Macromolecules XXXX, XXX, XXX−XXX

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phosphonic acid in the polymer and incomplete neutralization by hydroxide solution in titration. Another possibility is due to a partial loss of free phosphonic acids by the formation of anhydrides linkage in the polymer film during the drying process of SEBS-P.36,37 Water Uptake. Water uptake is an important parameter which influences mechanical properties and proton conductivity in ionic polymer membranes. In sulfonated polymers, IEC values of membranes generally dominate water uptake of membranes. In contrast, high degrees of hydrogen bonding, bulkiness, and lower acidity of phosphonic acid groups can result in lower water uptake in phosphonated polymers.38 As shown in Table 1, the SEBS PEM functionalized with phosphonic acid has a significantly lower water adsorption than that with sulfonic acid: SEBS-P showed only 4% water uptake despite its high IEC (3.56 mequiv/g) while SEBS-S2 with IEC of 1.94 mequiv/g showed 32% water uptake. SEBS-S1 showed a lower water uptake than SEBS-S2 since it has hydrophobic fluorine atoms near sulfonic acid groups and low IEC value (1.55 mequiv/g). Thermal Stability. The thermal stability of acid-functionalized polymer samples was studied with TGA under a nitrogen atmosphere at a heating rate of 10 °C/min (Figure 5). While thermal decomposition of pristine SEBS did not occur until 400 °C, the TGA curve of SEBS-S2 was characterized by three distinct regions of mass loss at lower temperature: (a) loss of water molecules bound to sulfonic acid of the membrane (50− 200 °C), (b) elimination of sulfonic acid (around 260 °C), and (c) thermal decomposition of polymer backbone (around 400 °C). A comparison of thermal stability behavior of the phosphonated polymer and pristine SEBS is also shown in Figure 5. The initial minor weight loss (5−10%) at 170−300 °C was attributed to the formation of anhydrides by creation of P−OP anhydride linkages. These anhydrides may be formed intermolecularly or intramolecularly. Jiang et al. studied thermal degradation of poly(vinylphosphonic acid) and observed formation of these anhydrides using a combination of TGA and FT-IR spectroscopy.36 The second weight loss was observed around 440 °C, which was attributed to the degradation of the polymer backbone. Proton Conductivity. Proton conductivity of PEMs is one of the most important properties affecting fuel cell performance because membrane conductivity can directly influence power density. Proton conductivity is heavily dependent upon the hydration level of conducting membrane when the Vehicular mechanism dominantly contributes to proton conduction. The SEBS-S2 sample maintained a consistent proton conductivity

to OCH2 and benzylic CH2 (labeled as g and h in Figure 1e), respectively. Molar concentration of the attached aryl phosphonate in SEBS-Pe was calculated by comparing the resonance integrals of CH3 of PEB unit of polymer main chain at 0.83 ppm (labeled as a) and OCH2 at 4.0 ppm (labeled as g) or benzylic CH2 at 3.1 ppm (labeled as h) of the phosphonate side chain. Similar to the Suzuki coupling reactions with sulfonated phenyl bromides, full conversion (18 mol % phosphonate) was confirmed. To remove an ethyl group from the phosphonate ester of SEBS-Pe, acidic hydrolysis using 12 M HCl under reflux condition was carried out in membrane form. The conversion of hydrolysis reaction was confirmed by FT-IR spectroscopy. In the FT-IR spectrum of SEBS-Pe, the strong absorption band at 1249 cm−1 was assigned to PO stretching (Figure 4a). After acidic hydrolysis, the PO stretching band became broad and shifted to lower frequencies. This is caused by the hydrogen bonding between −OH and PO of phosphonic acid.35 Also, strong intense bands at 1050 and 1026 cm−1 that were assigned to O−C stretching of phosphonate ester disappeared in the FTIR spectrum of SEBS-P. A broad band from O−H stretching of phosphonic acid appeared between 2800 and 2200 cm−1. After the hydrolysis, the SEBS-P membrane was observed to have lighter color compared to SEBS-Pe. Membrane Properties and Morphology. Ion Exchange Capacity (IEC). Successful conversion of sulfonate and phosphonate esters to the corresponding acids was confirmed quantitatively by comparing the experimentally measured IECs (IECExp) with theoretical IECs (IECTheo) calculated from 1H NMR data (i.e., 18 mol % functionalization). As listed in Table 1, the titrated IECExp values of SEBS-S1 and SEBS-S2 matched Table 1. Membrane Properties of Acid-Functionalized SEBS PEMs and Nafion IEC (mequiv/g) ionomer

water uptake (60 °C)

exptla

calcdb

σc (mS/cm)

SEBS-S1 SEBS-S2 SEBS-P Nafion 212

22 32 4 26

1.55 1.94 3.56 0.92

1.58 1.94 3.78 0.90

43 73 0.4 56

a Measured from the titration method. bEstimated values from 1H NMR spectra of protected sulfonated/phosphonated SEBS. cProton conductivity measured at 60 °C and 100% relative humidity.

well with those obtained from the NMR spectra. However, the IECExp of SEBS-P was slightly lower than the IECTheo, which could be caused by a lower acidity of second proton of

Figure 5. TGA data of SEBS, SEBS-S2, and SEBS-P. G

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Macromolecules value at 60 °C and 100% RH, suggesting that this membrane is stable under this measurement condition (Figure 6). Although

lower water uptake lead to significantly lower proton conductivity in spite of the higher IEC value than the corresponding sulfonated polymers. Morphology. Phase separation between hydrophilic and hydrophobic domains is known to affect proton conductivity as interconnected hydrophilic ionic domains can form efficient ion conducting channels.39 Size and spatial arrangements of microdomains of SEBS and its sulfonated and phosphonated polymers were studied using small-angle X-ray scattering (SAXS). Morphology of such ionic block copolymers can range from spherical domains in a cubic lattice to cylindrical in hexagonal packing (HEX) and lamellar structures (LAM) depending on the overall volume fraction of hydrophilic domains.40 In addition, the increase of interaction parameter (χ) upon acid functionalization will affect the morphology structure. To enhance the X-ray scattering contrast, the acid groups of SEBS ionomers were ion-exchanged to Na+ form, and their SAXS profiles were compared with that of pristine SEBS (Figure 7a). The pristine SEBS sample with 30 wt % PS displayed three broad scattering peaks at q*, √3q*, and √4q*, suggesting the HEX morphology with PS cylinders. With the attachment of acid groups to all repeating units of PS blocks of SEBS, the calculated overall chemical compositions of the acidfunctionalized PS domains are 61, 52, and 53 wt % for SBES-S1, SEBS-S2, and SEBS-P, respectively. The significant shift in increasing the acid-functionalized hydrophilic domain compositions should favor the formation of LAM morphology for the acid-functionalized SEBS triblock copolymers. The experimental SAXS profiles of SEBS-S1, -S2, and -P with scattering peaks at q*, 2q* and 3q* suggest the formation of LAM morphology for the proton-conducting acid-functionalized block copolymers (Figure 7). To emulate the conditions of morphology development at high and low RH conditions, SAXS experiments were performed in ambient air on “hydrated” membranes (Figure 7a) and in vacuum on “dried” membranes (Figure 7b), respectively. The q* values are related with lamellar interdomain distance between hydrophilic and hydrophobic domains, and they can be calculated from the Bragg’s law [d(SAXS) = 2π/q*]. Table 2 summarizes the d(SAXS) values of hydrated and dried ionic polymer samples, which emulate high and low RH conditions, respectively. The SEBS-P sample did not show a significant difference in interdomain spacing of

Figure 6. Proton conductivity of SEBS-S1, SEBS-S2, SEBS-P, and Nafion as a function of relative humidity at 60 °C.

the SEBS-S2 membrane exhibited greater proton conductivity than Nafion 212 above 90% RH, its conductivity values dropped gradually at lower RH. SEBS-S1 showed higher proton conductivity than SEBS-S2 at below 50% RH despite having a smaller IEC and a lower water uptake. We attributed this greater proton conductivity at lower RH to the higher acidity of fluoroalkylsulfonic acid group in SEBS-S1 and better selfaggregation of fluoroalkyl side chains (see the Morphology section). Compared with sulfonic acid PEMs, 1 or 2 orders of lower proton conductivity was generally observed for phosphonic acid-functionalized polymer PEMs. While sulfonated aromatic polymers are typically prepared by direct electrophilic sulfonation (via SO3+ electrophile) of aromatic rings of the polymers, phosphonation requires preparation of intermediate polymer (either brominated or lithiated) that can form C−P bond by subsequent reactions. Because they employ independent synthetic routes, it is not easy to synthesize ionic aromatic polymers with the same molar concentration of sulfonic acid and phosphonic acid.15,19 Since all three SEBS PEMs in this report were prepared from the same borylated precursor polymer, they should have the same concentration and distribution of ionic groups along the polymer chain. In the case of SEBS-P, a combination of the lower acidity of phosphoric acid (pKa = 2) than that of sulfonic acid (pKa = −3) and the

Figure 7. SAXS profiles of SEBS and SEBS ionomer samples with different acid functional groups (SEBS-S1, SEBS-S2, and SEBS-P). SAXS experiments were performed (a) in ambient air on “hydrated” samples prepared by drying the membranes overnight at room temperature and soaking in water for an hour and (b) in vacuum on “dried” samples by drying the membranes at room temperature for a month. H

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anticipated from noticeable 3q* peaks in SAXS, the TEM images also suggest the well-developed lamellar-like morphology for SEBS-S2 and SEBS-P. In contrast, the TEM image of SEBS-S1 indicates weakly ordered lamellar morphology without long-range ordering. Furthermore, as shown in Figure 8b, the TEM image of the acid-functionalized PS-based hydrophilic domains in SEBS-S1 suggests the development of dark/gray/dark sandwich-like stained-layer within the PS-based domains. Upon careful control of RuO4 staining time, we were able to differentiate the staining characteristics within the PS-based hydrophilic domains of SEBS-S1 (Figure S1). As the cohesive aggregation of polymer chains promoted by the fluorinated tether of SEBSS1 provides a kinetic barrier for the RuO4 diffusion in staining PS-containing chains, it could have resulted in less darkly (i.e., gray) stained domains in the middle of the hydrophilic lamellae. Thus, we believe the difference in thickness of ionic lamellar layers is caused by more efficient self-aggregation of −OCF2CF2SO3H side chains of SEBS-S1 than that of SEBSS2. As known from Teflon, the fluorinated tethered moiety of SEBS-S1 will be more cohesive and likely to introduce a greater tendency to aggregate each other. Thus, in addition to the stronger acidity, the more densely aggregated morphology of fluoroalkylsufonate side chains of SEBS-S1 contributes to enhance proton conductivity at low RH conditions and lower swelling at high RH conditions.

Table 2. Summary of Lamellar Interdomain Distance (d) of Acid-Functionalized SEBS PEM Ionomers from SAXS and TEM Experiments d(SAXS)a (nm)

d(TEM)b (nm)

ionomers

on hydrated samples

on dried samples

on dried samples

SEBS-S1 SEBS-S2 SEBS-P

37 48 40

36 36 42

35 36 41

a d(SAXS) = 2π/q*. bd(TEM), measured by statistical analysis of TEM images as shown in Figures S2 and S3.

lamellar morphology after drying possibly due to the lower acidity and lower water uptake of the phosphonated polymer. However, the SEBS-S2 sample underwent significant reduction of d(SAXS) upon drying, while the SEBS-S1 sample exhibited nearly unchanged interdomain spacing of lamellar after drying. This structural changes of the lamellar-forming ionomers are consistent with the impacts of RH on the proton conductivity (Figure 6): the increase of lamellar interdomain spacing for SEBS-S2 upon hydration correlates well with its greater proton conductivity than SEBS-S1 at high RH conditions. TEM images also support the development of lamellar structure for all three samples of SEBS-S1, SEBS-S2, and SEBSP (Figure 8). Dried samples were used for the TEM morphological study, and the statistical histogram analysis of lamellar interdomain spacing, d(TEM), was performed by analyzing the gray scale image profile analysis of the TEM images (see Figures S2 and S3 in the Supporting Information). As shown in Table 2, the average values of d(TEM) are in good agreement with d(SAXS) of dried ionomer samples. As

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CONCLUSIONS In summary, a series of acid-functionalized SEBS PEMs were successfully prepared by attaching fluoroalkylsulfonic acid, arylsulfonic acid, and arylphosphonic acid groups along the

Figure 8. TEM images of dried (a, b) SEBS-S1 with different magnifications, (c) SEBS-S2 and (d) SEBS-P. RuO4 vapors were used for staining the acid-functionalized PS-based hydrophilic domains (dark), while the PEB domains remain unstained (bright) in the TEM images. I

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(6) Chen, X.; Chen, K.; Chen, P.; Higa, M.; Okamoto, K.-I.; Hirano, T. Effects of tetracarboxylic dianhydrides on the properties of sulfonated polyimides. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 905−915. (7) Roziere, J.; Jones, D. J. Non-Fluorinated Polymer Materials for Proton Exchange Membrane Fuel Cells. Annu. Rev. Mater. Res. 2003, 33, 503−555. (8) Rikukawa, M.; Sanui, K. Proton-Conducting Polymer Electrolyte Membranes based on Hydrocarbon Polymers. Prog. Polym. Sci. 2000, 25, 1463−1502. (9) Won, J.; Park, H. H.; Kim, Y. J.; Choi, S. W.; Ha, H. Y.; Oh, I.-H.; Kim, H. S.; Kang, Y. S.; Ihn, K. J. Fixation of Nanosized Proton Transport Channels in Membranes. Macromolecules 2003, 36, 3228− 3234. (10) Elamathi, S.; Nithyakalyani, G.; Sangeetha, D.; Ravichandran, S. Preparation and Evaluation of Ionomeric Membranes based on Sulfonated-poly (styrene_isobutylene_styrene) Membranes for Proton Exchange Membrane Fuel Cells (PEMFC). Ionics 2008, 14, 377− 385. (11) Kim, S. Y.; Park, M. J.; Balsara, N. P.; Jackson, A. Confinement Effects on Watery Domains in Hydrated Block Copolymer Electrolyte Membranes. Macromolecules 2010, 43, 8128−8135. (12) DeLuca, N. W.; Elabd, Y. A. Polymer Electrolyte Membranes for the Direct Methanol Fuel Cell: A Review. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2201−2225. (13) Elabd, Y. A.; Hickner, M. A. Block Copolymers for Fuel Cells. Macromolecules 2011, 44, 1−11. (14) Peckham, T. J.; Holdcroft, S. Structure-Morphology-Property Relationships of Non-Perfluorinated Proton-Conducting Membranes. Adv. Mater. 2010, 22, 4667−4690. (15) Parcero, E.; Herrera, R.; Nunes, S. P. Phosphonated and Sulfonated Polyhphenylsulfone Membranes for Fuel Cell Application. J. Membr. Sci. 2006, 285, 206−213. (16) Abu-Thabit, N. Y.; Ali, S. A.; Javaid Zaidi, S. M. New Highly Phosphonated Polysulfone Membranes for PEM Fuel Cells. J. Membr. Sci. 2010, 360, 26−33. (17) Subianto, S.; Choudhury, N. R.; Dutta, N. K. PalladiumCatalyzed Phosphonation of SEBS Block Copolymer. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5431−5441. (18) Jakoby, K.; Peinemann, K. V.; Nunes, S. P. Palladium-Catalyzed Phosphonation of Polyphenylsulfone. Macromol. Chem. Phys. 2003, 204, 61−67. (19) Jung, H. Y.; Kim, S. Y.; Kim, O.; Park, M. J. Effect of the Protogenic Group on the Phase Behavior and Ion Transport Properties of Acid-Bearing Block Copolymers. Macromolecules 2015, 48, 6142−6152. (20) Lafitte, B.; Jannasch, P. Phosphonation of polysulfones via lithiation and reaction with chlorophosphonic acid esters. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 273−286. (21) Chang, Y.; Brunello, G. F.; Fuller, J.; Disabb-Miller, M. L.; Hawley, M. E.; Kim, Y. S.; Hickner, M. A.; Jang, S. S.; Bae, C. Polymer Electrolyte Membranes based on Poly(arylene ether sulfone) with Pendant Perfluorosulfonic acid. Polym. Chem. 2013, 4, 272−281. (22) Chang, Y.; Brunello, G. F.; Fuller, J.; Hawley, M.; Kim, Y. S.; Disabb-Miller, M.; Hickner, M. A.; Jang, S. S.; Bae, C. Aromatic Ionomers with Highly Acidic Sulfonate Groups: Acidity, Hydration, and Proton Conductivity. Macromolecules 2011, 44, 8458−8469. (23) Jo, T. S.; Kim, S. H.; Shin, J.; Bae, C. Highly Efficient Incorporation of Functional Groups into Aromatic Main-Chain Polymer Using Iridium-Catalyzed C−H Activation and Suzuki− Miyaura Reaction. J. Am. Chem. Soc. 2009, 131, 1656−1657. (24) Shin, J.; Jensen, S. M.; Ju, J.; Lee, S.; Xue, Z.; Noh, S. K.; Bae, C. Controlled Functionalization of Crystalline Polystyrenes via Activation of Aromatic C−H Bonds. Macromolecules 2007, 40, 8600−8608. (25) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.; Miyaura, N.; Hartwig, J. F. Mechanism of the Mild Functionalization of Arenes by Diboron Reagents Catalyzed by Iridium Complexes. Intermediacy and Chemistry of Bipyridine-Ligated Iridium Trisboryl Complexes. J. Am. Chem. Soc. 2005, 127, 14263−14278.

polystyrene block by borylation of aromatic C−H bonds and Suzuki coupling reactions. The water uptake of SEBS ionomers was dependent on the structure of acid side chains and IEC values. Although all block copolymers maintained distinctive hydrophilic and hydrophobic phase separation, their morphology structures were changed from the pristine SEBS, and the interdomain distances between hydrophilic and hydrophobic domains were dependent on the chemical structure of acid functional groups. Furthermore, the morphological difference among the acid-functionalized SEBS PEMS suggests that less humidity dependence of proton conductivity of fluoroalkyl sulfonate polymer compared to other acid-functionalized polymers can be ascribed to not only the higher acidity strength of superacidic side chains but also more dense aggregation of the fluoalkyl tether chains. This relationship study of ionic group structure and membrane properties demonstrates the importance of acid side chain structures in the development of PEM materials.

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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.7b01848. TEM images of SEBS-S1, SEBS-S2, SEBS-P, TEM image analysis, and their corresponding lamellar interdomain distances (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(C.Y.R.) E-mail [email protected]; Ph +1-518-276-2060; Fax +1518-276-4887. *(C.B.) E-mail [email protected]; Ph +1-518-276-3783; Fax +1518-276-4887. ORCID

Chulsung Bae: 0000-0002-9026-3319 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank NSF (DMR Polymers 1506245) and Rensselaer Polytechnic Institute for their generous support. REFERENCES

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K

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