Highly Proton Conducting Polyelectrolyte Membranes with Unusual

Subscriber access provided by READING UNIV

Article

Highly Proton-conductive Polyelectrolyte Membranes with Unusual Water Swelling Behavior Based on Triptycenecontaining Poly(arylene ether sulfone) Multiblock Copolymers Joseph Aboki, Benxin Jing, Shuangjiang Luo, Yingxi Zhu, Liang Zhu, and Ruilan Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13542 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Highly Proton-conductive Polyelectrolyte Membranes with Unusual Water Swelling Behavior Based on Triptycene-containing Poly(arylene ether sulfone) Multiblock Copolymers Joseph Aboki,† Benxin Jing,‡ Shuangjiang Luo,† Yingxi Zhu,‡ Liang Zhu,§ Ruilan Guo,†* †

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, United States ‡

Department of Chemical Engineering and Material Science, Wayne State University, Detroit, MI 48202, United States §

Department of Materials Science and Engineering, The Pennsylvania State University, University Park 16802, United States * Corresponding author: [email protected], +1-574-631-3453 (tel), +1-574-631-0317 (fax) KEYWORDS: polyelectrolyte membranes, triptycene-containing poly(arylene ether sulfone), multiblock copolymers, supramolecular interaction, water swelling ABSTRACT: Multiblock poly(arylene ether sulfone) copolymers are attractive for polyelectrolyte membrane fuel cell applications due to their reportedly improved proton conductivity under partially hydrated conditions and better mechanical/thermal stability compared to Nafion®. However, the long hydrophilic sequences required to achieve high conductivity usually lead to excessive water uptake and swelling which degrade membrane dimensional stability. Herein we report a fundamentally new approach to address this grand challenge by introducing shape-persistent triptycene units into the hydrophobic sequences of multiblock copolymers, which induce strong supramolecular chain threading and interlocking interactions that effectively suppress the water swelling. Consequently, unlike previously reported multiblock copolymer systems, the water swelling of the triptycene-containing multiblock copolymers did not increase proportionally with the water uptake. This combination of high water uptake and low swelling behavior of these copolymers resulted in excellent proton conductivity and membrane dimensional stability under fully hydrated conditions. In particular, the triptycene-containing multiblock copolymer film with the longest hydrophilic block length (i.e., BPSH100-TRP0-15K-15K) had a water uptake of 105%, an excellent proton conductivity

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of 0.150 S/cm and a volume swelling ratio of just 29% (more than 42% reduction compared to Nafion® 212).

1 ACS Paragon Plus Environment

Page 2 of 36

Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) are a promising class of alternative energy devices with potential applications in transportation, portable electronics and stationary power devices that possess high efficiencies and are also environmentally friendly.1,2 Perfluorinated sulfonic acid (PFSA) membranes such as Nafion® are the most widely used polyelectrolyte membranes for PEMFC applications.3 Although PFSA membranes possess high proton conductivity at high relative humidity (RH) as well as excellent chemical stability, their relatively high cost, high methanol permeability in direct methanol fuel cells (DMFCs),4 and insufficient thermal stability have largely hindered the full scale commercialization of PEMFCs.5,6 Attempts focused on decreasing the swelling of Nafion® membranes via low energy plasma treatment have shown that although the modification decreases the membrane hydrophobicity, the reduced water uptake results in a significant decrease in the proton conductivity of Nafion®.7 Thus, much effort has been focused on the development of alternative and economically viable polyelectrolyte membranes. The disulfonated random copolymers of poly(arylene ether sulfone) and poly(arylene ether benzonitrile) were initially studied as alternatives to Nafion® because of their low cost of production, excellent stability, and low fuel permeability compared to Nafion®.8,9 However, despite their attractive PEM properties, their proton conductivity under low RH levels was still inferior to that of Nafion®. Moreover, the excessive swelling of the polysulfone random copolymers with high degrees of sulfonation also resulted in loss of performance under extended DMFC operations.10,11 However, it has been shown that the proton conductivity under low relative humidity of sulfonated aromatic PEMs can be enhanced by employing synthetic strategies that yield copolymer architectures which feature well-defined hydrophilic channels via nanophase separation.12 Guiver et al. reported a series of fully aromatic comb-shaped copoly(arylene ether sulfone) copolymers13 and ABA triblock copolymers with highly sulfonated blocks14 which possessed low in-plane swelling ratios and high proton conductivities at low and high relative humidity levels compared to Nafion® 112. Over the last two decades, disulfonated multiblock copolymers, in particular those based on the poly(arylene ether sulfone)s, have emerged as very promising alternative PEMs for addressing the drawbacks associated with Nafion®.15–18 These multiblock copolymers, prepared via coupling hydrophilic and hydrophobic telechelic oligomers, exhibited improved thermal, 2 ACS Paragon Plus Environment


chemical, and mechanical stabilities as well as improved conductivity under low RH conditions compared to the random copolymer membranes, and Nafion®.15–18 The improved properties of the multiblock copolymers were largely attributed to their ability to form nanophase separated morphologies with interconnected hydrophilic domains17 that facilitated proton transport even under low humidity conditions.19,20 Similarly, several multiblock copolymers with different hydrophobic blocks such as poly(arylene ether nitrile), poly(arylene ether ketone) were reported to exhibit much improved DMFC performance than Nafion® 212.21–24 In all these studies, it has been demonstrated that long hydrophilic sequences (typically > 10,000 g/mol) are needed in the multiblock copolymers in order to generate continuous proton conducting nano-channels that facilitate proton transport, especially at low RH levels.17,20,25 However, increasing the proton conductivity by increasing the hydrophilic sequence length usually comes at the expense of high water uptake and excessive swelling, which severely deteriorates the mechanical properties and dimensional stability of the membranes.17,25–27 Different approaches have been explored to address the challenge of excessive swelling. Kreuer et al. utilized ionic crosslinking of acidic multiblock copolymers with polybenzimidazole and reported a 61% decrease in membrane water uptake.28 Lee et al. synthesized crosslinked sulfonated poly(arylene ether) which possessed low water uptakes and low in-plane swelling ratios.29 Although crosslinking generally improves dimensional stability and reduces methanol permeability of membranes, such gains are usually accompanied with reduced proton conductivity and sometimes lead to an overall reduction of electrochemical single cell performances.30 Guo et al. showed that the swelling ratio could be greatly reduced by annealing multiblock copolymer membranes at temperatures higher than the Tg of the hydrophobic blocks.25,26 However, the thermal annealing approach is limited by the thermal stability of the hydrophilic sulfonated sequences. As such, fundamentally new ways are needed to effectively suppress water swelling of highly proton conductive polyelectrolyte membranes. Recently, iptycene-based polymers have attracted a lot of attention in the membrane field because of the unique molecular configurations of iptycenes that induce strong supramolecular reinforcement via a chain threading and interlocking mechanism.31–37 For example, Swager et al. reported a triptycene-based polyester with a nearly 3-fold increase in strength and Young’s modulus, and a greater than 20-fold increase in strain-to-failure compared to a non-triptycene polyester.32 Our group also demonstrated in a series of gas separation membranes based on 3 ACS Paragon Plus Environment

Page 4 of 36

Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


segmented pentiptycene-poly(ethylene oxide) (PEO) copolymers that the strong supramolecular interactions suppressed PEO crystallization and strengthened the membranes with high PEO content, leading to superior CO2 separation performance.33 Zhang et al. introduced iptycene structures into random copolymers based on post-sulfonated poly(arylene ether sulfone)s,34,35 which showed reduced membrane swelling, decent proton conductivity and excellent thermaloxidative stability when compared with Nafion® 117. In this research, we have developed, for the first time, sulfonated poly(arylene ether sulfone) multiblock copolymers containing alternating triptycene-based hydrophobic blocks and biphenol-based hydrophilic blocks prepared from a directly disulfonated monomer. It should be noted that all previously reported triptycene-based poly(arylene ether sulfone)s have been based on statistical random copolymers in which the ionic groups were randomly distributed in the polymer backbone. Additionally, all previous systems have utilized a post-sulfonation approach to introduce sulfonic acid groups into the polymer main chain. By utilizing sulfonated monomers directly in the synthesis of our multiblock copolymers, we have successfully circumvented the harsh post-sulfonation conditions reported in previous studies. The copolymers herein were synthesized via coupling a telechelic hydrophilic oligomer and a telechelic hydrophobic oligomer with systematically varied molecular weights. This synthetic procedure allowed us to explore the effects of block length and the triptycene units on the fundamental PEM properties of the copolymer membranes. Water uptake, water swelling ratio, thermal, and oxidative stabilities of the multiblock copolymers were directly measured as a function of block length to determine the effectiveness of supramolecular interactions induced by triptycene units. The morphologies of the multiblock copolymers were studied to determine the effect of block length on hydrophilic/hydrophobic

phase

separation.

The

proton

conductivity

and

methanol

uptake/swelling were also investigated systematically as a function of block length and temperature, which were compared to Nafion® 212 to evaluate the potential of the copolymer membranes for PEMFC applications. 2. EXPERIMENTAL SECTION 2.1. Materials. 4,4’-dichlorodiphenylsulfone (DCDPS), decafluorobiphenyl (DFBP) and anhydrous potassium carbonate (K2CO3) were purchased from Alfa Aesar and dried in vacuum for 24

h

at

110

°C

before use.

4,4’-biphenol

(BP)


and

3,3’-disulfonated-4,4’-


dichlorodiphenylsulfone (SDCDPS) were purchased from Akron Polymer Systems and vacuum dried at 120 °C for 24 h prior to use. Anhydrous N-Methyl-2-pyrrolidinone (NMP) and hydrogen peroxide (30%) were purchased from EMD Millipore and used without further purification. Anhydrous N,N-dimethylacetamide (DMAc), cyclohexane, toluene, xylenes, glacial acetic acid, and hydrobromic acid (48%) were purchased from Sigma-Aldrich and used as received. Chloroform, 2-propanol (IPA) and methanol were purchased from Sigma-Aldrich. Triptycene1,4-hydroquinone was synthesized from anthracene and p-benzoquinone according to our recently published procedure.36 Nafion® 212 membrane was purchased from Ion Pow. Inc., USA. The membranes were subsequently treated by boiling for 1 h in deionized (DI) water, boiling for 1 h in 3% H2O2 solution, washing for 1 h in boiling DI water, boiling for 1 h in 1 M sulfuric acid solution, and finally rinsing in DI water at 25 °C. 2.2. Synthesis of Phenoxide-terminated, Disulfonated Hydrophilic Oligomers (BPS100). BP-based fully disulfonated poly(arylene ether sulfone) hydrophilic oligomers (BPS100) with molecular weights ranging from 5,000 to 15,000 g/mol were synthesized via condensation polymerization following reported procedure.19 Molecular weight of oligomers and phenoxide end-group functionality were controlled by offsetting the stoichiometric molar feeding ratio of BP and SDCDPS. 2.3. Synthesis of Unsulfonated Hydrophobic Oligomers End-capped with DFBP (TRP0). A series of triptycene-based poly(arylene ether sulfone) hydrophobic oligomers (TRP0) with molecular weights ranging from 5,000 to 15,000 g/mol were prepared via a two-step process: condensation polymerization producing phenoxide-terminated oligomers followed by an endcapping reaction with DFBP to introduce fluoride end groups for ensuing coupling reactions with hydrophilic oligomers (Scheme 1). During polymerization, molecular weight of oligomers and phenoxide terminal functionality were controlled by offsetting the stoichiometric molar ratio of monomers. A sample synthesis of a 5,000 g/mol TRP0 oligomer (TRP0-5K) with phenoxide end group is provided as follows: A 150 ml three-necked round bottom flask was charged with 7.1910 g (25.1 mmol) of triptycene-1,4-hydroquinone, 6.5826 g (22.7 mmol) of DCDPS and 4.1651 g (79.8 mmol) of anhydrous K2CO3 and equipped with a condenser, an overhead stirrer, a Dean-Stark trap and a nitrogen inlet. Anhydrous DMAc (60 mL) and toluene (30 mL) were then


Page 6 of 36

Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


added and the reaction was heated to 155 °C with stirring under N2 purge. The reaction refluxed at 155 °C for 4 h to azeotropically dehydrate the system. After 4 h, toluene was removed from the reaction by draining the Dean-Stark trap and the temperature was slowly increased to 185 °C. The reaction mixture was maintained at 185 °C for 48 h. Afterwards, the resulting viscous solution was cooled to room temperature, filtered to remove salts, and coagulated in IPA. The precipitated oligomer was dried in vacuo at 120 °C for 24 h.

Cl HO

OH

O S O

Cl

DCDPS

K2CO3 DMAc/toluene

triptycene-1,4-hydroquinone

~155 °C reflux 4h 185 °C 48h

+ -

KO

O S O

O

O-K+

O n

unsulfonated hydrophobic oligomer F

FF

K2CO3

F F

F F

FF

F

(300% excess)

F

F F

DMAc/cyclohexane 130 °C reflux 4h 125 °C 15h

F

F

FF

F

O O

F F

F F

O

O

S O

F

O n F

F

FF

F

DFBP-ended TRP0 oligomer

Scheme 1. Synthesis of telechelic unsulfonated hydrophobic oligomers (TRP0) with terminal phenoxide groups and their end-capping with DFBP To facilitate coupling reactions with telechelic hydrophilic oligomers, the resulting phenoxide-terminated TRP0 oligomers were end-capped with DFBP via a SNAr reaction. An example end-capping reaction of a 5,000 g/mol TRP0 oligomer is: A 125 ml three-necked round bottom flask was charged with 2.5000 g (0.5 mmol) of TRP0 oligomer, 0.2764 g (2.0 mmol) of K2CO3, 50 mL anhydrous DMAc and 15mL cyclohexane, and equipped with a mechanical stirrer, a condenser, a Dean-Stark trap, and a nitrogen inlet. The solution was heated to reflux at 130 °C for 4 h to azeotropically remove water from the system. Afterwards, the cyclohexane was removed from the reaction. The temperature was then decreased to 125 °C and 2.3388 g (7.0 mmol) of DFBP was added. The reaction was carried out at 125 °C for 15 h. The resulting



solution was cooled to 25 °C, filtered to remove salts and coagulated in methanol. The precipitated oligomer was collected and dried in vacuo for 24 h at 110 °C. 2.4. Synthesis of Multiblock Copolymers (BPS100–TRP0-x-y). Salt form BPS100-TRP0-xy multiblock copolymers with varying hydrophilic-hydrophobic block lengths were synthesized via coupling phenoxide-terminated BPS100 oligomers and DFBP end-capped TRP0 oligomers. Herein, x and y refer to the molecular weights of the hydrophilic and hydrophobic blocks respectively (Scheme 2). A sample coupling reaction that produced a BPS100-TRP0-7K-7K multiblock copolymer is as follows: 3.5000 g (0.5 mmol) of BPS100-7K (Mn = 7,000 g/mol), 0.8293 g (6.0 mmol) of anhydrous K2CO3, 70 mL of anhydrous NMP, and 21 mL of cyclohexane were charged into a three-necked round bottom flask equipped with a mechanical stirrer, a nitrogen inlet, a Dean-Stark trap and a condenser. The mixture was refluxed for 4 h at 130 °C to dehydrate the system. After refluxing, the cyclohexane was removed from the reaction system and the temperature was decreased to 110 °C. Then, 3.5000 g (0.5 mmol) of DFBP end-capped TRP0-7K (Mn = 7,000 g/mol) was added to the mixture and the reaction was allowed to proceed at 110 °C for 72 h. The final dark-brown viscous solution was filtered and coagulated in IPA. The precipitated copolymer was washed in methanol/water (1:3, v:v) for 15 h and then in chloroform for another 15 h to remove any unreacted oligomers. The copolymer was then collected and dried in vacuo for 24 h at 120 °C.

Scheme 2. Synthesis of sulfonated multiblock copolymers (BPS100–TRP0) via a coupling reaction


Page 8 of 36

Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5. Film Casting and Acidification. Membranes of salt-form copolymers were prepared via a solution casting method. The polymers were dissolved in DMAc to give a 7% w/v solution which was then filtered through 0.45 µm Teflon syringe filters and cast onto clean leveled glass substrates. The casting solutions were placed under an infrared lamp at ~55 °C for 24 h to form thin films. Afterwards, the films were removed from the glass plates by immersing in DI water for a few minutes. The films were then dried in a vacuum oven at 110 °C for 24 h to completely remove residual solvent. The membranes were converted from salt form to acid form by boiling for 2 h in 0.5 M sulfuric acid solution, followed by boiling for 2 h in DI water. After acidification, the films were stored in DI water for further characterizations. The acidified films are referred to as BPSH100-TRP0-x-y in the following discussion to reflect they are in acid form. 2.6. Polymer and Membrane Characterization. 1H and

19

F nuclear magnetic resonance

(NMR) analyses were done on a Bruker 400 MHz or 500 MHz spectrometer using deuterated dimethyl sulfoxide (DMSO-d6) as solvent and tetramethylsilane (TMS) as internal reference to confirm the chemical structures of the oligomers and multiblock copolymers. Additionally, the 1

H NMR spectra of telechelic oligomers were analyzed and their number average molecular

weights (Mn) were determined through end group analysis. The weight based ion exchange capacity (IECw) of multiblock copolymers usually depends on the stoichiometric molar feed ratios (m and n) and the block lengths (x and y) of the hydrophilic and hydrophobic oligomers, respectively.38 ∗∗∗

IECw meq g -1 = ∗∗

(∗

∗!" )

(1)

where $%&'() (606.5 g/mol) and $%&'*+ (502.5 g/mol) refer to the molecular weights of the repeating units of the hydrophilic and hydrophobic oligomers, respectively. In this work, by setting the stoichiometric molar feed ratios to 1 (i.e., m = n), copolymers with fixed IECw were synthesized by coupling hydrophilic and hydrophobic oligomers with equal block lengths (i.e. x = y). Since the IECw is very sensitive to the block length, the experimentally obtained block lengths (i.e., x and y values), determined from 1H NMR end-group analysis, were used in Eq. 1 in place of the theoretically calculated values. The measured IECw values of the multiblock copolymers were calculated based on integrations of protons relating to the sulfonated and 8 ACS Paragon Plus Environment


Page 10 of 36

unsulfonated groups in the 1H NMR spectra of the multiblock copolymers, and compared with calculated target values based on the designed copolymer structures. Intrinsic viscosities (IV) of the multiblock copolymers in salt form were measured in 0.05 M LiBr-NMP at 25 °C using a Cannon Ubbelohde viscometer. The water uptake of acid-form films was evaluated by measuring the difference in weight between dry and wet membranes. The membranes were dried under vacuum at 110 °C for 24 h and weighed (%,- ). The dry membranes were then soaked in DI water at different temperatures for 24 h. The wet membranes were removed from water, wiped dry using a Kimwipe® and quickly weighed again (%./0 ). Similarly, the methanol uptake was determined by measuring the difference in weight between dry and wet membranes at different temperatures following reported procedures.38 In particular, the dry membranes ( %,- ) were immersed in aqueous methanol solution (64 wt %) at 25 °C or 50 °C for 24 h and the wet weights were recorded. The water uptake and methanol uptake of the membranes were determined as follows. MeOH uptake, or, water uptake (

Highly Proton Conducting Polyelectrolyte Membranes with Unusual

Recommend Documents