proton-conducting poly(phenylene oxide)–poly(vinyl benzyl phosphonic acid) block copolymers via...

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Proton-Conducting Poly(phenylene oxide)–Poly(vinyl benzyl phosphonic acid) Block Copolymers via Atom Transfer Radical Polymerization

Avneesh Kumar, Wojciech Pisula, Dilyana Markova, Markus Klapper,* Klaus Müllen*

Block copolymers containing poly(phenylene oxide) (PPO) and poly(vinyl benzyl phosphonic acid) segments are synthesized via atom transfer radical polymerization (ATRP). Monofunc-tional PPO blocks are converted into ATRP active macroinitiators, which are then used to polymerize a diethyl p -vinylbenzyl phosphonate monomer in order to obtain phosphonated block copolymers bearing pendent phosphonic ester groups. Poly(phenylene oxide- b -vinyl benzyl phosphonic ester) block copolymers are hydrolyzed to corresponding acid derivatives to investigate their proton con-ductivity. The effect of the relative humidity (RH) is investi-gated. The proton conductivity at 50% RH and one bar of vapor pressure approaches 0.01 S cm − 1 .

1. Introduction

For decades, Nafi on has been the material of choice for proton exchange membrane (PEM) in fuel cell applica-tions. In Nafi on, the presence of water is required to trans-port protons through the vehicular mechanism. [ 1 ] The performance of Nafi on decreases dramatically at high temperature due to the evaporation of water molecules located around sulphonic acid groups, thus limiting the practical applications of the proton exchange membrane fuel cells (PEMFCs). [ 2 , 3 ] There has, therefore, been a great need for polymeric ionomers, which can perform at higher operational temperatures ( > 120 ° C) and which do not necessarily require water in order to transport protons. Other advantages of having high temperature proton conductors include rapid oxidation of the fuel and its low reforming costs. To this end, proton exchange materials

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A. Kumar , W. Pisula , D. Markova , M. Klapper , K. Müllen Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, GermanyE-mail: [email protected]; [email protected]

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functionalized with phosphonic acid groups and different nitrogenous heterocycles [ 4–6 ] have been developed and studied for their anhydrous proton conductivity prop-erties in order to utilize them for fuel cell applications. Phosphonated polymeric materials have been used for a variety of applications, for example, tissue engineering, [ 7 ] dental cements, [ 8 ] fl ame-retardants, [ 9 ] and fuel cell mem-brane materials. [ 10 ] It is well known that proton conduc-tion in phosphonated polymeric material takes place via a dynamic hydrogen bond network of phosphonic acid groups. This process, involving proton hopping, is gener-ally called Grotthus mechanism . [ 11 ] However, the presence of a solvent, such as water, that forms hydrogen bonds with polar phosphonic acid groups can lead to enhanced proton conductivity due to the formation of hydronium ions. [ 11 – 13 ]

Polybenzimidazole [ 14 ] doped with phosphoric acid (H 3 PO 4 ) has been reported to show a proton conductivity of about 10 − 3 –10 − 2 S cm − 1 from room temperature to 190 ° C respectively. However, due to its solubility in water, the phosphoric acid gets washed out upon hydration. In order to prevent leaching, therefore, the protogenic groups must be covalently attached to the main copolymer

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MacromolecularChemistry and Physics

chain, either on the backbone or in the side chain. Yu and Benicewicz [ 15 b] have reported a cross-linked poybenzimi-dazole with poly(phosphoric acid) that showed a proton conductivity as high as 0.27 S cm − 1 at 160 ° C. However, the relationship between the morphology of the copoly-mers and the conductivity was not investigated.

Recently, Kreuer [ 15 a] reported that the high local con-centration of phosphonic acid groups is also required in order to achieve high intrinsic proton conductivity in phosphonated polymers. A phosphonated polymer such as poly(vinylbenzyl phosphonic acid) (PVBPA) is known as a polyelectrolyte with a high concentration of ionic groups attached covalently to the repeating unit. However, PVBPA lacks the potential to fulfi ll all of the requirements, such as higher thermal stability and fi lm-forming properties. Therefore, it must be either cross-linked or copolymerized with suitable and chemically different polymer blocks in order to use it as proton exchange material for fuel cell applications.

Poly(phenylene oxide) [ 16 ] (PPO), a high-performance polymer, possesses remarkable fi lm-forming properties. Due to its great thermal, chemical and mechanical sta-bility, PPO can provide stability to the proton exchange material. Usually, PPO can be synthesized either by oxida-tive coupling [ 17 ] or by an aromatic nucleophilic substitu-tion reaction, where a phenol containing a halogen at the para position polymerizes with itself in the presence of a base- [ 18 ] and phase-transfer catalyst [ 18 , 19 ] in an aqueous medium.

Block copolymers with chemically different segments [ 21 ] exhibit phase separation in the nanometer regime ranging from columnar, lamellar, bicontinuous, and gyroid arrangements etc. Polymers with ionic groups such as Nafi on and sulfonated poly(ether ether ketone)s (PEEK)s have the tendency to form continuous nanochannels interconnected with each other via a hydrogen bonded network of ionic groups surrounded by water molecules. Additionally, the water molecules facilitate the dissocia-tion of protons from acidic groups in order to generate “proton carriers,” which leads to higher proton conduc-tivity through interconnected ionic nanoclusters.

In our previous fi ndings, we reported the polymeri-zation of a diethyl p -vinylbenzyl phosphonate (DEVBP) monomer to PEEK block via atom transfer free radical process for obtaining tri-block copolymers with different lengths of phosphonated block. [ 22 ] These tri-block copoly-mers consisted of PEEK and PVBPA, which were thermally stable and showed high proton conductivities under anhydrous and humidifi ed conditions. The morphologies of these tri-block copolymers have also been investigated using atomic force microscopy, scanning electron micro-scopy, and dissipative particle dynamics (DPD) simulation methods. [ 23 ] These morphological studies have shown no clear phase separation between the two different blocks

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of the copolymers. A possible reason for this could be that the phosphonic acid groups of the poly(vinyl benzyl phos-phonic acid) block might form hydrogen bonds with the ketonic functionality of PEEK, thus making both blocks compatible.

In the following approach, a PPO homopolymer was chosen as a stabilizer that can induce phase separation due to its incompatibility with the poly(vinylbenzyl phos-phonic acid). We envisaged that these poly(phenylene oxide- b -vinyl benzyl phosphonic acid) (PPO- b -PVBPA) block copolymers with chemically different segments were likely to have phase-separated domains with proton con-duction pathways. Here, we report the functionalization of the hydroxyl-terminated PPO homopolymer 1 (Scheme 1 ) resulting in ATRP active macroinitiators 2 (Scheme 1 ) in order to polymerize phosphonated monomer 3 (Scheme 1 ). The degree of polymerization was controlled by varying the ratio of monomer to the PPO macroinitiator in order to vary the content of the phosphonated segment. The thermal and proton conducting properties of these poly-mers, along with their nanostructured morphologies, are discussed in order to evaluate the possibilities of obtaining highly stable proton exchange material for fuel cell applications.

2. Experimental Section

2.1. Materials

All chemicals required for the synthesis of block copolymers were purchased from Aldrich and used without further purifi cation. All nitrogen-based ligands such as dTbpy = 4,4 ′ -di-tert-butyl-2,2 ′ -dipyridyl and bpy = 2,2 ′ -bipyridine used in ATRP were purchased from Aldrich. Triethylphosphite, 2,6-dimethyl-4-bromo phenol, and tetrabutylammonium hydrogen sulfate (TBAHS) were pur-chased from Aldrich and used as received. The end capping unit, α -chloro phenyl acetyl chloride and trimethyl silyl bromide (TMSBr), was also purchased from Aldrich. All reaction sol-vents were dried prior to use. Anisole (anhydrous, 99.7%; Sigma Aldrich), triethyl amine (p.a. > 99.5%; Fluka), methanol (HPLC, 99.8%; Fisher Scientifi c), toluene (99.8%; Sigma Aldrich), aqueous HCl (p.a. 37%; Prolabo), and NaOH (98%; Sigma Aldrich) were used as received. DEVBP (3) (Scheme 1 ) with a pendent phospho-nate group for polymerization was synthesized as reported in the literature. [ 22 ]

2.2. Synthesis of Monohydroxy PPO Homopolymers 1 (Scheme 1)

Synthesis of hydroxyl-terminated PPO was carried out according to the procedure described in the literature. [ 19 ] The monomer, bromo-2, 6-dimethylphenol (10.50 g, 50 mmol), was dissolved in an aqueous solution of NaOH (25.01 g, 625.01 mmol in 400 mL water) at 0 ° C (Scheme 1 ). TBAHS (tetrabutylammonium hydrogen sulfate, 1.6 g, 3.10 mmol) and toluene (250 mL) were added and

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Scheme 1 . Synthetic route for the preparation of PPO- b -PVBPA block copolymers via ATRP by applying a mono-functional PPO macroini-tiator for polymerizing diethyl p -vinylbenzyl phosphonate. Reaction conditions: (i) 1. NaOH, H 2 O; 2. TBAHS, toluene, room temperature, 48 h; (ii) α -chloro phenyl acetyl chloride, Et 3 N, toluene, 80 ° C, 30 h; (iii) Diethyl p -vinylbenzyl phosphonate, Cu(I)Cl, dTbpy, anisole, 84 h, 130 ° C; (iv) 1.TMSBr 2.MeOH.

the mixture was stirred vigorously at room temperature for 45 h. The toluene phase was separated and washed with HCl (5%) and water. The polymer was precipitated into methanol (800 mL) and was dried at 80 ° C in a vacuum oven.

Yield: 75%, Mn = 9900 g mol − 1 (GPC) 1 H NMR (250 MHz, CD 2 Cl 2 , ppm): 6.40(broad signal, CH aromatic ),

2.04(s, CH 3 )

2.3. Synthesis of PPO Macroinitiators 2 (Scheme 1)

Poly(phenylene oxide)s with two different molecular weights were functionalized with an end-capping unit in order to obtain macroinitiators for polymerizing DEVBP monomer via ATRP.

The functionalization of PPO block was carried out as follow: The monohydroxy PPO 1 (9900 g mol − 1 ) was dissolved in dried

toluene (25 mL) (Scheme 1 ). Triethylamine (8 eq, 198 mg) was added to this solution at 80 ° C. The end-capping unit, α -chloro phenyl acetyl chloride (8 eq, 198 mg), was added by syringe to the above solution under an atmosphere of N 2 . The reaction mixture was stirred at 80 ° C for 30 h. After this, the PPO macroinitiator

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was precipitated in MeOH (600 mL) and was dried in vacuo. A pure colorless powder of PPO macroinitiator was obtained. Yield: 80%

1 H NMR (250 MHz, CD 2 Cl 2 , ppm): 6.40(broad signal, CH aromatic ), 2.04(s, CH 3 ), 5.66 (s, 1 H), 7.26 (s, 2 H), 7.43, (t, 1 H), 7.62 (d, 2 H). The corresponding molecular weights of various PPO macroinitiator blocks were measured by GPC relative to polystyrene as standard (Entry 4a and 4d, Table 1 ).

2.4. Synthesis of PPO- b -PDEVBP Block Copolymers 4a–e (Scheme 1) via ATR

All block copolymers were synthesized via ATRP as shown in Scheme 1 . A typical procedure for the synthesis of PPO 81 - b -PVBPA 181 . (4a, Table 1 ) is given below.

In a 10 mL pre-dried Schlenk tube, Cu(I)Cl and dTbpy in a molar ratio of 1:6 were added under N 2 . A solution of PPO macroinitiator 2 (PPO = 9900 g mol − 1 ), and DEVBP (3) in a molar ratio of 1:250 was prepared separately in 1 mL of anisole and was degassed by two cycles of freeze–pump–thaw. This solution was injected into the Schlenk tube. The color of the reaction mixture turned

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Table 1. Results for PPO- b -PDEVBP block copolymers obtained by ATRP.

PPO macroinitiator a) PPO- b -PDEVBP block b ) PPO- b -PVBPA block

Entry [RX]/[M]/[CuCl]/[L]

Mn [g mol − 1 ]

Mw / Mn DEVBP

yield [%] Mn

[g mol − 1 ]Theoretical

Mn [g mol − 1 ]

DP c ) DEVBP

Mn [g mol − 1 ]

4a. 1/250/1/6 9900 1.7 75 68 000 55 000 230 56 000 PPO 81 - b -PVBPA 181

4b. 1/500/1/6 9900 1.7 68 54 000 91 000 170 44 000 PPO 81 - b -PVBPA 134

4c. 1/1000/1/6 9900 1.7 75 130 000 197 000 470 104 000 PPO 81 - b -PVBPA 370

4d. 1/1000/1/6 36000 1.9 58 146 000 168 000 440 124 000 PPO 295 - b -PVBPA 346

4e. 1/2000/1/6 36000 1.9 60 350000 325 000 1200 283 000 PPO 295 - b -PVBPA 795

a) Evaluated by GPC using PS standards; b ) Mn calculated by GPC result of PPO macroinitiator and 1 H NMR of diblock copolymers; c ) DP for DEVBPA calculated by 1 H NMR;Solvent = anisole (50%), DEVBP = Diethyl P- vinylbenzylphosphonate, T ° C = 130.Polymerization time = 84 h.

slightly blue. The polymerization mixture was degassed by four cycles of freeze–pump–thaw. After stirring for 5 min at room temperature, the Schlenk tube was placed in a thermostated oil bath at 130 ° C for 84 h under an atmosphere of N 2 . The color of the reaction mixture turned dark due to the formation of the copper complex. The reaction mixture was cooled down immediately by immersing the Schlenk tube in water. Afterwards, the reaction mixture was diluted with 5 mL of dichloromethane and then poured into 400 mL of methanol in order to precipitate the block copolymer. The precipitate was washed with methanol and dried under vacuum.

Yield: 75% 1 H NMR (250 MHz, CD 2 Cl 2 , ppm): 6.41 (broad signal, CH aromatic

of PPO), 6.99 (broad signal, CH aromatic of PDEVBP), 3.92 (broad signal, OCH 2 ), 3.03 (broad signal, CH 2 P), 2.10 (CH 3 of PPO), 1.16–1.91.

2.5. Hydrolysis of PPO- b -PDEVBP Block Copolymers 4a–e (Scheme 1)

Hydrolysis of the block copolymers 4a–e was realized by silyla-tion followed by methanolysis. An excess of TMSBr was added to a solution of the block copolymer (4a, Scheme 1 ) in 15 mL of dichloromethane, and the reaction mixture was stirred at room temperature for 1 day. All solvents were then removed from the reaction mixture under vacuum. The resulting product was treated with 20 mL of methanol and stirred at room temperature for 1 day in order to carry out ethanolysis. After this, all solvents were evaporated under vacuum, and the hydrolyzed derivative of the block copolymer was obtained. This was further washed with methanol several times and dried under vacuum at 40 ° C for 2 days. The hydrolysis of the block copolymer was proven by 1 H NMR and 31 P NMR spectroscopy.

1 H NMR (250 MHz, DMSO-d 6 , ppm): 7.3–6.2 (broad signal, CH aromatic ), 5.0–5.9 (broad signal, P (OH) 2 ), 3.03 (CH 2 P), 2.35 (CH 3 of PPO), 0.71–1.16 (broad signal, alkyl region).

2.6. Copolymer Characterization

In all ATRP experiments, the monomer DEVBP (3) was poly-merized to the PPO segment by ATRP in order to yield AB type

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of block copolymers. The molecular weights and the PDI indices of the PPO blocks and their corresponding macroinitiators were obtained using gel permeation chromatography (GPC) (Table 1 ). The GPC measurements were performed to polystyrene stand-ards on a Waters 150CALC/GPC device equipped with a Styragel column using THF as eluent at 30 ° C or DMF as eluent at 30 ° C. The molecular weights of the standard were in the range of 600–1600000 g mol − 1 .

1 H NMR and 13 C NMR spectra were recorded in deuterated solvents on a 250 MHz Bruker DPX spectrometer. 31 P NMR spectra were recorded on a Varian spectrometer 500 MHz. All 1 H NMR chemical shifts are reported in ppm versus residual protons in the deuterated solvents as follows: δ = 7.27; CDCl 3 , δ = 2.50; (CD 3 ) 2 SO. The solvent proton or carbon signals were used as the internal standard for the 1 H NMR and 13 C NMR measurements respectively, whereas triphenylphosphine was applied for the calibration of the 31 P NMR measurements.

The thermal stability of all block copolymers was evaluated by thermogravimetric analysis (TGA) on a PerkinElmer TGA 7 thermo gravimetric analyzer (10 K min − 1 heating rate in nitrogen atmosphere). All TGA results were obtained from 80 to 500 ° C. The temperature dependent proton conductivity in dry conditions as well as in 50% humidity (at 1 bar of vapor pressure) was investigated by using a Schlumberger SI 1260 impedance/gain phase analyzer with a dielectric interface in the frequency range of 10 − 1 –10 6 Hz and a temperature range of 20–160 ° C at increments of 10 ° C. The measurements were carried out in a Novocontrol cryostat (Novocontrol, Hundsangen, Germany), in which the sample was sandwiched between two Pt electrodes under nitrogen atmosphere. Proton conductivity measurements in pure water vapor (p(H 2 O) = 1 atm) were investigated on a Hewlett Packard 4192A LF AC impedance analyzer in a double-walled temperature controlled glass chamber with heated gas inlet and outlet, where the sample was placed in pellet form in a porous glass cylinder and contacted with Pt/C electrodes from E-Tek. All samples were pressed in pellet form and dried in a vacuum oven at 60 ° C for 24 h prior to carrying out the proton conductivity experiments.

A Θ-Θ Siemens D500 Kristallofl ex with a graphite-monochromatized Cu-K α X-ray beam was used for the investigation of the structure of the pure polymer powder. The

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diffraction patters were taken as a function of the scattering vector s ; with s = 2sin Θ / λ where 2 Θ is the scattering angle. The d -spacing was directly derived from the scattering vector.

3. Results and Discussion

3.1. Block Copolymer Synthesis and Characterization

We previously reported the successful synthesis of PVBPA- b -PEEK- b -PVBPA tri-block copolymers via ATRP, where the PEEK homopolymers were used as macroinitiators for the polymerization. [ 22 ]

However, block copolymers consisting of PEEK seg-ment lacked well-defi ned morphology. We therefore made further attempts to copolymerize DEVBP monomer via ATRP to a rather more incompatible homopolymer, namely PPO. Atom transfer radical polymerization can provide well-defi ned polymeric material with narrow molecular distribution. [ 24–26 ] Typically, it is initiated by a one electron-transfer process via the complex of Cu (I) with a nitrogen-based ligand, which generates a propa-gating species and dormant chains. The chemical struc-ture and electronic nature of the ligand not only affect the activity of the metal complex but also the poly-merization process with regard to the solubility of the catalyst.

The effect of the different ligands on the polymeri-zation of DEVBP monomer 3 in ATRP has already been investigated, [ 22 ] and the system with Cu(I)Cl/dTbpy was found effective for polymerizing the phosphonated monomer. This catalytic system, therefore, was further employed for synthesizing block copolymers 4a–e. As

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Figure 1 . 1 H NMR spectrum of the poly(phenylene oxide) macroinitiatoof the end group protons indicating the functionalization of the PPO

ppm (t1)4.05.06.07.08.0

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e

abc

CD2Cl2

e

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f

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the polar group in the monomer may inhibit the cata-lytic activity [ 26–28 ] of the copper complex formed in situ, the monomer with the protected phosphonic group was used for ATRP .

First, PPO homopolymer 1 (Scheme 1 ) with different molecular weights was synthesized via a nucleophilic aromatic substitution (S N AR) reaction involving a phase transfer catalyst in an aqueous medium. The PPO homopolymers were modifi ed with α -chloro phenyl acetyl moiety in the presence of a mild base such as triethylamine at 80 ° C in order to obtain ATRP active PPO macroinitiators with different molecular weights, as shown in Scheme 1 . All macroinitiators were fully characterized by GPC and 1 H NMR. In the 1 H NMR spec-trum of PPO macroinitiator (Figure 1 ), a chemical shift was observed at 5.66 ppm, corresponding to the proton of –CHCl group that suggests the formation of an ATRP active site. Second, block copolymers 4a–e with different molecular weights were synthesized by employing ATRP involving Cu(I)Cl, dTbpy ligand and PPO macroinitiator 2 (Scheme 1 ) in the ratio of 1:6:1 with different molar concentrations of monomer 3 in order to achieve a varied length of PDEVBP block (Table 1 ). Anisole was chosen as the solvent for the polymerization process because of the insolubility of PPO macroinitiators 2 in DEVBP monomer (3). The polymerization of monomer 3 initiated by PPO macroinitiator/Cu(I)Cl/dTbpy was found to be slow at 110 ° C, which may be due to the strong C–Cl bond in PPO macroinitiator 2 since the strong bond does not allow rapid formation of the radical at this temperature. Additionally, the polar nature of the monomer may fur-ther inhibit the metal ligand complex formation thereby affecting the polymerization. Usually, the complex Cu(I)Cl/

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dTbpy does not induce polymerization of DEVBP monomer at low temperature even after 24 h. [ 22 ] Therefore, all polymerization experiments (Table 1 ) were carried out at 130 ° C for 84 h, resulting in block copolymers with high molecular weights. However, the degree of polymeriza-tion for DEVBP did not correspond to that of the theoret-ical one. The lower solubility of the propagating polymer chains in anisole can be related to the above deviation. The polymerization experiment (4a, Table 1 ) with the PPO macroinitiator 2 (9900 g mol − 1 ) and with the 1/6/250 molar ratio of Cu(I)Cl/dTbpy/DEVBP monomer, respec-tively, led to the copolymer with 68 000 g mol − 1 , which was slightly higher than the theoretically calculated molecular weight ( Mn ). However, in another poly-merization experiment (4b, Table 1 ) where a similar PPO macroinitiator (9900 g mol − 1 ) was employed with a higher molar ratio of DEVBP, the degree of polymeriza-tion was found to be lower with low conversion value. This suggests that the molecular weight of the resulting copolymer remained unaffected even with increasing molar concentration of the monomer. In a separate exper-iment (4c, Table 1 ), where the molar ratio of the monomer 3 to PPO macroinitiator 2 was four times higher and the polymerization was carried out in similar conditions, led to the copolymer 4c with a high molecular weight and a high conversion. However, the degree of polymerization for the monomer was still low even with the increased molar concentration of the monomer. From this observa-tion, it may be inferred that the formation of the catalytic species is hindered by the polar monomer in possible complex with metal (Cu) through phosphoryl oxygen thereby lowering the reactivity of the catalyst. Further polymerization experiments were conducted in similar ATRP conditions using the higher molecular weight PPO macroinitiator 2 (36 000 g mol − 1 ) while keeping the molar ratio of Cu(I)Cl and the ligand constant, as employed before with the relative smaller PPO macroinitiator (9900 g mol − 1 ). The block length of the PPO and the con-centration of the monomer can also infl uence the degree of polymerization signifi cantly due to the solubility effect and the polar nature of the monomer. [ 28 ] The poly-merization procedure involving the PPO macroinitiator 2 (Scheme 1 ) with 36000 g mol − 1 and a similar catalytic system gave a conversion in the range of 58% – 60% (4d–e, Table 1 ). The low conversion with the PPO macroinitiator of 36 000 g mol − 1 as compared with the PPO macroini-tiator 2 (Scheme 1 ) of 9900 g mol − 1 could be due the low solubility of the propagating unit in the reaction solution. A deviation in the degree of polymerization from the one theoretically calculated was also observed for the poly-merization experiments involving the PPO macroinitiator with 36000 g mol − 1 molecular weight ( 4d − e, Table 1 ). It is obvious from the above observations that the molar ratio of the monomer to the macroinitiator greatly affects the

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molecular weight of the desired polymer synthesized via ATRP under similar conditions. Other factors such as the solubility of the macroinitiators, molar concentration, and polarity of the monomer played a crucial role during the polymerization.

Polymers with ionic segments have the tendency to aggregate in the solution via noncovalent interac-tions leading to a self-assembled species. [ 27 ] Similarly, the phosphonated block copolymers synthesized via a Cu (I)-mediated ATRP-formed aggregations in organic solvent, as evidenced by the dynamic light scattering data (not shown). Because of this, it was not possible to determine the molecular weights of the block copoly-mers (5a-e, Scheme 1 ) by GPC. The relative molecular weights of the copolymers, therefore, were determined by combining the GPC data of the PPO macroinitiator 2 with the 1 H NMR data of the block copolymers 4a-e. The relative signal intensities of the methyl groups of the PPO block (2.0 ppm) and the methylene protons of the benzyl position in PDEVBP block (2.95 ppm) were used to evaluate the molecular weight of the block copolymers 4a-e.

Figure 1 and 2 shows the typical 1 H NMR spectra of PPO macroinitiator 2, PPO- b -PDEVBP 4e and PPO- b -PVBPA block copolymers 5e. The corresponding chemical shifts for –CH 2 P and –OCH 2 were observed at 2.95 ppm and 3.86 ppm, respectively (Figure 2 a), suggesting the presence of PDEVBP segment in copolymer. The appear-ance of a broad signal in the region of 1.10–1.9 ppm indicates the alkyl regions of the PDEVBP main chain. Furthermore, a distinct signal at 2.0 ppm corresponds to the methyl protons of the side group of the PPO block. All chemical shifts for the aromatic protons of the PPO and the PDEVBP segments were observed at 6.1–7.0 ppm with a broad signal indicating overlapping of the aro-matic protons. The appearance of the corresponding signals suggested the formation of copolymer with PPO and PDEVBP block. In addition, the appearance of a signal at 26.5 ppm in the 31 P NMR spectra of the block copolymers (ester form) indicated the presence of P–O–CH 2 –CH 3 .

All PPO- b -PDEVBP block copolymers 4a–e (ester deriva-tives) were transformed into their phosphonic acid deriv-atives 5a–e by treating them with TMSBr and methanol successively at room temperature for several hours. [ 29 ] An alternative procedure for the hydrolysis, that is, refl ux in aqueous HCl, was avoided because of the reactivity of the end-capping unit of the PPO with strong acid, which could lead to the cleavage of the ester linkage, resulting in a separate individual homopolymer. A typical 1 H NMR spectrum of PPO- b -PVBPA 5e (Figure 2 ) was used to con-fi rm the hydrolysis of PDEVBP blocks (ester derivative). The absence of the signals arising from the protons of the diethyl ester groups (–OCH 2, 3.86 ppm) in the 1 H

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Figure 2 . 1 H NMR and 31 P NMR spectrum of a) block copolymer 4e (ester form) and b) block copolymer 5e (acid form).

ppm (t1)0.01.02.03.04.05.06.07.08.0

aromatic protons

CD2Cl2

-CH2P

P-OCH2

CH3 from PPO

alkyl region

a)

ppm (t1)5.010.015.020.025.030.035.040.0

31PNMR

P-OCH2

ppm (t1)0.01.02.03.04.05.06.07.08.0

aromatic protons

CD2Cl2

-CH2P

P-OCH2

CH3 from PPO

alkyl region

ppm (t1)0.01.02.03.04.05.06.07.08.0

aromatic protonsaromatic protons

CD2Cl2

-CH2P

P-OCH2

CH3 from PPO

alkyl region

a)

ppm (t1)5.010.015.020.025.030.035.040.0

31PNMR

ppm (t1)5.010.015.020.025.030.035.040.0

31PNMR

P-OCH2

ppm (t1)5.010.015.020.025.030.035.0

31P NMR

ppm (t1)0.01.32.53.85.06.37.58.8

DMSO

b)

aromatic region

P(OH)2

P(OH)2

ppm (t1)5.010.015.020.025.030.035.0

31P NMR

ppm (t1)0.01.32.53.85.06.37.58.8

DMSO

b)

ppm (t1)5.010.015.020.025.030.035.0

31P NMR

ppm (t1)0.01.32.53.85.06.37.58.8

DMSO

ppm (t1)5.010.015.020.025.030.035.0

31P NMR

ppm (t1)5.010.015.020.025.030.035.0

31P NMR

ppm (t1)0.01.32.53.85.06.37.58.8

DMSO

b)

aromatic region

P(OH)2

P(OH)2

Figure 3 . TGA curves of block copolymers PPO- b -PDEVBP 4d–e (ester) carried out at 10 ° C min − 1 under nitrogen.

0 100 200 300 400 500 600 700 800 90030

40

50

60

70

80

90

100

110

Wei

ght (

%)

Temperature (°C)

PPO295-b-PDEVBP346 PPO295-b-PDEVBP795

NMR spectrum of the PPO- b -PVBPA 5e (Figure 2 b) further indicates the complete hydrolysis of the ester deriva-tive block copolymers. Additionally, the appearance of a broad band between δ = 5.8 ppm and 6.4 ppm can be attributed to the hydrogen-bonded hydroxyl groups of the phosphonic acid although the shift is slightly overlapped with aromatic region. Also, all peaks were slightly shifted for the hydrolyzed block copolymer 5e as compared with the unhydrolyzed one. The hydrolysis of the PPO- b -PVBPA 5a–e was further realized by the 31 P NMR spectroscopy as shown in Figure 2 b. In the typical 31 P NMR spectrum of the PPO- b -PVBPA 5e, the resonance peak for the phosphonic acid appeared as a broad band at 22.5 ppm.

3.2. Thermal Analysis

Since the higher thermal stability of a proton exchange material is one of the most desirable properties for its practical application, the thermal stability of PPO- b -PDEVBP (ester form) and PPO- b -PVBPA (acid form) block copolymers was investigated by TGA under N 2 atmosphere at 10 ° C min − 1 . The degradation of ester derivatives of the block copolymers occurred in three distinct steps, followed by the evaporation of the residual solvent as shown in Figure 3 . For the PPO- b -PDEVBP 4d–e, the fi rst weight loss was observed at 250 ° C, which corresponds to the loss of diethyl ester groups of –P(O)(C 2 H5O) 2 via formation of ethylene and phosphonic acid groups. [ 30 ] The degradation of C–P bonds also takes place along with this at similar temperatures. The second weight loss appeared above 400 ° C and is attributed to the PPO and aromatic region of the PDEVBP block (Figure 3 ). Phosphonic acid- containing

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polymers are well known for their anhydride forma-tion [ 31 , 32 ] via a P–O–P bond generated by intermolecular self-condensation reactions at higher temperatures (Scheme 2 ). Water molecules are generated during anhy-dride formation, thereby reducing the proton conduc-tivity. Consequently, a gradual weight loss was observed at 110 ° C due to self-condensation reactions between the phosphonic acid groups (Figure 4 ), which was correlated further with the decreased proton conductivity in dry state. Furthermore, it is suggested that the water absorbed by the acid derivatives of the block copolymers also evapo-rates at 110 ° C, and thus the TGA traces for water produced via anhydride formation and absorbed water overlap.

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Scheme 2 . Self-condensation reaction between phosphonic acid groups resulting in the formation of P–O–P bonds at higher tem-perature ( > 110 ° C).

Figure 5 . Anhydrous proton conductivity of the copolymers with varied fraction of PVBPA block.

0 40 80 120 160

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Co

nd

uc

tiv

ity

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m)

Temperature ( C)

PPO81-b-PVBPA 134

PPO295-b-PVBPA795PPO81-b-PVBPA181

°
However, this event was not observed in the TGA traces (Figure 3 ) of the block copolymers (ester derivatives). In addition, the corresponding TGA traces showed that the degradation of the ester and acid derivatives of the PPO block copolymers was very similar.
3.3. Proton Conductivity

For fuel cells operating at higher temperatures ( > 100 ° C), the proton exchange material must be able to trans-port protons under an anhydrous state. [ 33 ] The proton conducting properties of all the PPO- b -PVBPA block copoly mers synthesized via ATRP were investigated by impedance spectroscopy under anhydrous condi-tions and with defi ned relative humidity. The anhy-drous proton conductivity was measured in the range of 20–160 ° C under nitrogen atmosphere. The correla-tion between the composition of AB block copolymers and the proton conductivity was investigated. For this reason, block copolymers with different concentrations of the PVBPA polyelectrolyte were analyzed by imped-ance spectroscopy. Figure 5 and 6 shows the proton con-ductivity of the PPO 81 - b -PVBPA 181 and PPO 81 - b -PVBPA 134

Figure 4 . TGA curves of block copolymers PPO- b -PEVBPA (acid) carried out 10 ° C min − 1 under nitrogen.

0 100 200 300 400 500 600 700 800 90030

40

50

60

70

80

90

100

Wei

ght (

%)

Temperature (°C)

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block copolymers in an anhydrous state and in 50% relative humidity (RH). The proton conductivity of the PPO 81 - b -PVBPA 134 block copolymer increases with the temperature and approaches 10 − 6 S cm − 1 above 80 ° C. The linear behavior of the proton-conducting curve in the higher temperature range indicated temperature dependent proton conductivity. However, the impact of the anhydride formation on the proton conductivity was also observed. For the PPO 81 - b -PVBPA 181 and PPO 81 - b -PVBPA 134 block copolymers, proton conductivity begins to decrease at above 120 ° C due to the formation of P–O–P linkages. A higher concentration of the phosphonic acid groups can also increase the anhydrous proton conduc-tivity at higher temperature. [ 27 ] In Figure 5 , the proton conductivity of the PPO 295 - b -PVBPA 795 block copolymer up to 10 − 5 S cm − 1 at 160 ° C was related to the higher concentration of the phosphonic acid groups as com-pared with the PPO 81 - b -PVBPA 181 and PPO 81 - b -PVBPA 134 . In literature, various polymers such as the phospho-nated poly(arylene ether)s [ 34 ] have been reported to have

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Figure 6 . Proton conductivity of various block copolymers under 50% of relative humidity (RH) and 1 bar of water vapor pressure.

110 120 130 140 150 160 170

10-5

10-4

10-3

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PPO295-b-PVBPA795

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uctiv

ity (S

/cm

)

Temperature (°C)

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Figure 7 . Wide-angle X-ray scattering diffractograms of PPO- b -PVBPA block copolymers with different lengths of PPO and PVBPA unit.

1 2 3

PPO81-b-PVBPA134

PPO295-b-PVBPA795

Inte

nsity

/ a.

u.

s / nm-1

PPO81-b-PVBPA181

shown a proton conductivity of up to 10 − 6 S cm − 1 in the anhydrous state.

Furthermore, the proton conducting experiments were carried out at 50% RH and 1 bar (10 5 Pa) of water vapor pressure. For the copolymer PPO 81 - b -PVBPA 134 , proton conductivity at 50% RH increased by two orders of magni-tude at 120 ° C and 1 bar (10 5 Pa) of water vapor pressure. The increment in proton conductivity could be related to the order of PVBPA domain in PPO 81 - b -PVBPA 134 compared with the PPO 81 - b -PVBPA 181, as evidenced by the wide angle X-ray scattering (WAXS) experiments (Figure 7 ). However, at above 110 ° C, the proton conductivity fur-ther decreases due to the loss of water from the system as it becomes critical to maintain the relative humidity constant at 1 bar of vapor pressure. Also, at this temper-ature, it is diffi cult to study the proton conductivity due to the dynamic between absorbed and free water mol-ecules. Phosphonic acid-containing polymers are known for their lower water absorbing capacity compared with their sulfonated counterparts. [ 27 , 32 , 33 , 35 ] The copolymer with a higher fraction of PPO block (4e, Table 1 ) showed a slight deviation in the proton conductivity under 50% RH, as shown in Figure 6 . The relatively longer hydrophobic PPO segment in PPO 295 - b -PVBPA 795 led to a lower proton conductivity that could be due to the low water absorb-ance of the long PPO block. As shown in Figure 6 , block copolymers with a relative small length of hydrophobic PPO segment showed a higher proton conductivity at 50% relative humidity (50% RH, 1 bar of water vapor pressure). Proton conductivity of the PPO 81 - b -PVBPA 134 block copol-ymer (Figure 6 ) at 110 ° C (50% RH) increases drastically and approaches 0.01 S cm − 1 . Under hydration, the number of water molecules per phosphonic acid group also plays a very crucial role in enhancing the proton conductivity via protonic channels formed by the hydration of phosphonic acid groups. [ 32 ] Therefore, it is necessary to investigate the

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solid state morphologies of PPO- b -PVBPA block copoly-mers in order to correlate the proton-conducting proper-ties with the macromolecular structures.

3.4. Solid State Organization of PPO- b -PVBPA Block Copolymers

Macromolecules consisting of rigid polymer rods reveal dif-ferent types of packing modes depending upon their side chain concentration. The organization can include cylin-drical, lamellar, or interdigitated packing of the macro-molecular chains. PPO is an amorphous and rigid polymer. However, it can be crystallized by organic solvent treat-ment. [ 35 ] Ionic homopolymers, such as sulfonated polysty-rene, are known to form chlatrate in the presence of guest solvent molecules in nanocavities organized in an ordered array. [ 36 ] Obviously, the proton conductivity increases tre-mendously in the case of an ordered morphology [ 32 ] where each domain of the block copolymer phase separates in nanometer regime allowing phosphonic acid groups to form “ continuous channels ” . It is imperative, therefore, to study the microstructure of these block copolymers in order to investigate the phase separation phenomenon related to the proton-conduction pathway. WAXS techniques are widely used to investigate the crystallinity in ionic poly-mers. [ 38–40 ] The morphology of the PPO block copolymers, therefore, was investigated by wide-angle X-ray to cor-relate the proton conductivity with the morphology. Figure 7 shows the wide-angle X-ray powder diffracto-grams recorded at room temperature for the block copoly-mers with different relative lengths of the PPO and PVBPA blocks. Independently from the copolymer composi-tion, a major peak appears corresponding to a d -spacing of 1.75 nm for PPO 81 - b -PVBPA 181 , 1.96 nm for PPO 81 - b -PVBPA 134 , and 1.61 nm for PPO 295 - b -PVBPA 795 . These were attributed to the chain-to-chain distance between neigh-boring PPO chains. The identical value for the distance can be explained by the same chemical nature of the rigid PPO block, which induces self-organization and macroscopic order. The range in the diffractogram between the scat-tering vectors 0.91–3.5 nm differs signifi cantly between the copolymers with relatively short PPO blocks (PPO 81 - b -PVBPA 181 and PPO 81 - b -PVBPA 134 ) in comparison to the PPO 295 - b -PVBPA 795 for which only a broad amorphous halo appeared. In contrast, the block copolymers with short PPO revealed a large number of high intensity peaks. In the same conditions, the PVBPA block of PPO 295 - b -PVBPA 795 remains amorphous, which can be related to the extended block length. These distinct peaks are associated with the semicrystalline morphology of the phosphonated polysty-rene block observed also for the chlatrate form of sulfonated polystyrene. [ 37 ] In these chlatrates, the low molecular weight guest molecules, such as water, can organize in the formation of arrays. The poly(vinyl phosphonic acid) block

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has the tendency to absorb the water from the atmosphere due to its hygroscopic nature. The wide-angle X-ray data shown in Figure 7 for the PPO 81 - b -PVBPA 181 and PPO 81 - b -PVBPA 134 indicate order in the PVBPA block induced by water molecules since these low molecular weight mole-cules can intercalate between phosphonic acid groups and induce crystalline domain formation. Furthermore, the WAXS experiment investigations (Figure 7 ) of the PPO 81 - b -PVBPA 181 and PPO 81 - b -PVBPA 134 show that the PPO and PVBPA domains have a rather high order compared with PPO 295 - b -PVBPA 795 . Additionally, the presence of water molecules around phosphonic acid groups helps PPO domains to reorganize, as evidenced by WAXS studies. The formation of hydrogen bonds in the PVBPA block might further affect the reorganization of the PPO block. The WAXS analysis, therefore, has revealed order in phospho-nated fraction, as indicated by sharp peaks corresponding to the crystalline region of the PVBPA segment. This sug-gests that the proton conductivity increased under the rel-ative humidity due to the formation of ordered domains in the protogenic block.

4. Conclusion

We have demonstrated the successful synthesis of phos-phonated block copolymers via copper-mediated ATRP. PPOs with different molecular weights were converted into their corresponding macroinitiators. The size of the phosphonated PPO- b -PDEVBP copolymers was controlled by using different concentrations of monomer versus macroinitiator. The ester derivatives of the block copoly-mers were converted readily into their corresponding acid derivatives under mild conditions. Hydrolyzed copolymers showed high thermal stability with less than 10% weight loss up to 200 ° C, making them favorable for operating at the high temperatures required for fuel cell applications. The proton conductivity of the PPO- b -PVBPA in dry state and under well defi ned humidity was investigated by impedance spectroscopy. The proton conductivity of the block copolymers under anhydrous state as well as under 50% relative humidity indicated that these block copoly-mers have great potential for use in proton exchange materials for fuel cell applications. As expected, the solid state morphology of PPO- b -PVBPA showed order in both segments of the copolymer with crystallinity in the PVBPA fraction that was dependent on the molecular weight. The PPO 81 - b -PVBPA 181 and PPO 81 - b -PVBPA 134 copolymers showed the ordered crystalline domain that imparted into the formation of the chlatrates in the presence of water, thereby increasing proton conductivity. Future work will focus on developing new materials with improved mor-phology in order to achieve high proton conductivity in the anhydrous state.

Macromol. Chem. Phys.© 2012 WILEY-VCH Verlag Gm

Acknowledgements : The author is grateful to the Bundesministerium für Bildung und Forschung (BMBF), Germany for fi nancial support, and to Dr. Anke Kaltbeitzel and Christoph Sieber for carrying out proton conductivity measurements.

Received: August 23, 2011 ; Revised: Published online: February 6, 2012; DOI: 10.1002/macp.201100429

Keywords: ATRP; anhydrous proton conduction; block copolymers; morphologies; phosphonic acid; PEMFC

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