Effect of Phosphaphenanthrene Skeleton in Sulfonated Polyimides for

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Effect of Phosphaphenanthrene Skeleton in Sulfonated Polyimides for Proton Exchange Membrane Application Arun Kumar Mandal, Soumendu Bisoi, and Susanta Banerjee ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00128 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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ACS Applied Polymer Materials

Effect of Phosphaphenanthrene Skeleton in Sulfonated Polyimides for Proton Exchange Membrane Application Arun Kumar Mandal, Soumendu Bisoi, Susanta Banerjee*, Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India Keywords: Sulfonated co-polyimides, DOPO, Nuclear magnetic resonance (NMR), Oxidative stability, Proton conductivity. Abstract Sulfonic acid groups containing aromatic polyimides are of great interest as polymer electrolyte membranes owing to their high thermal and mechanical stability, strong resistance to fuel crossover, excellent film forming ability and high proton conductivity. However, these polymers generally experience reduced oxidative permanence. To alleviate the problems associated with the oxidative stability, the present work reports the synthesis and characterization of a new series of sulfonated co-polyimides (co-SPIs) using 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10oxide (DOPO) containing diamine monomer as one of the co-monomers. The synthesized sulfonated co-polyimides were soluble in numerous organic solvents and exhibited reasonable high inherent viscosity that allows to prepare high quality membranes by solution casting route. The structural elucidation and the sulfonic acid content in the polymers was verified from the integral values of the proton NMR signals. FTIR and

31P

NMR was also used for structural 1

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confirmation of the polymers. Transmission electron microscopy (TEM) images revealed well scattered spreading hydrophilic and hydrophobic phases. In general, the membranes from these copolymers showed improved proton conductivity and oxidative stability, and low water uptake. Among all the co-SPI membranes, DPPNH-80 (IECW = 2.58 mequiv g-1) exhibited proton conductivity of 202 mS cm-1 at 80 °C in fully hydrated condition.

1. Introduction The increasing need for renewable energy sources are encouraging researchers to find alternate energy generating device and fuel cells are interesting candidates in this connection. Considering different kinds of fuel cells, polymer electrolyte membrane fuel cells, commonly known as proton exchange membrane fuel cells (PEMFCs) have shown great promise for stationary, transportation and portable application.

1-5

The proton exchange membrane (PEM) is

a central component of PEMFCs that facilitate the transport of proton from the anode to cathode at the same time act as a barrier for fuel and electron. The important characteristics of a PEM required to have high proton conductivity, physicochemical stability and extended durability. The benchmark PEMs, made of per-fluorinated sulfonic acid containing polymers such as Nafion®, Aciplex®, Flemion® are used in existing PEMFCs due to their excellent proton conductive properties.

6

Nevertheless, these membranes have some shortcomings such as

restricted operation temperature (< 80 °C), low glass temperature, difficult synthetic procedure and high oxygen permeability. These shortcomings have triggered the researcher to find for an alternative PEM material with improved property. Accordingly, several classes of aromatic polymers having sulfonic acid were prepared and their PEM properties have been examined.

7-29

Considering different classes of PEM 2


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materials, sulfonated polyimide has gotten significant interest owing to their high thermal and mechanical stability, strong resistance to fuel crossover, excellent film forming ability and high proton conductivity. Naphthalene tetra carboxylic dianhydride (NTDA) based polyimides exhibited superior thermochemical stability than the five-member ring based polyimides due to lower ring strain. 30 However, it is somewhat difficult to process these six-membered polyimides because of their insolubility and infusibility. As an approach to increase the solubility of this class of polymers several research works have been directed towards the introduction of bulky moieties.

31,32

The introduction of bulky moiety disrupts the packing of polymer chains, thus

creating fractional free volume which in turn improves the solubility of the polymer.

32-34

The

aromatic copolymer based PEMs having sulfonic acid in the backbone often have lower conductivity which is largely attributed to their weak acidity and poor hydrophilic-hydrophobic phase separation. It is very challenging to get the phase separation as distinctively as the PFSA membranes owing to its main chain rod like structure, thereby limiting the proton conductivity. To achieve high proton conductivity from these types membranes generally required high ion exchange capacity (IEC), that consequently increases water uptake, swelling ratio and deteriorate the dimensional and mechanical stability of the membranes. Therefore, controlling of the polymer structure is required to acquire a suitable morphological structure and balancing the PEM properties like proton conductivity, fuel crossover andhydrolytic and chemical stability for a practical application. Oxidative stability of the polymer is another crucial factor for PEM application. The literature reports that the presence of fluorinated groups protects the imide linkages and increase their stability . 32-35 In addition, due to the hydrophobic nature of fluorine, the pendant trifluoromethyl groups facilitate formation of the phase separated morphology that subsequently helped in getting high proton conductivity of the membranes.

36

However, the 3



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environmental incongruity of fluorine moiety encourages researchers to find the non-fluorinated polymers.

37-39

Several reports are found that phosphine oxide moiety in the polymer structure

helped in achieving PEMs with increased oxidative stability, proton conductivity and adhesive properties of the membrane with the catalyst layer. 19,37,40 Accordingly, our group has prepared numerous sulfonated co-polyimides from phosphine oxide containing diamine monomers, e.g., bis [4-(4'-aminophenoxy) phenyl] phenylphosphine oxide (DATPPO) and bis [4-(4'-aminophenoxy)-3-trifluoromethyl phenyl] phenylphosphine oxide (DFPPO) and their PEM properties have been studied (Figure 1). 27,38 The hydrolytic and oxidative stability and proton conductivity of the membranes were significantly improved when we use the fluorinated monomer DFPPO instead of DATPPO.38

O

O H2N

O

P

O

NH2

H2N

O

P

CF3 O

NH2

F3C

DATPPO O

H2N

DFPPO

P O NH2 CH3

DPPA

Figure 1. Structure of the monomer DATPPO, DFPPO and DPPA. Among different types of phosphorous containing polymers, the phosphinate type, particularly, the polymers with cyclic –(O)P-O-C linkages are of interest due to their excellent thermal and flame retardant properties. several

polymers

with

cyclic

41

With this viewpoint, the researchers have prepared

9,10-dihydro-9-oxa-10-phosphaphenanthrene

10-oxide

(DOPO)moiety and consequently the polymers showed improved solubility, better thermal stability and flame retardency.

42-45

It is attributed that relatively low conformational stress of 4


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DOPO moiety caused the creation of a bulky structure that disrupts polymer chain packing and subsequently improve their solubility. At the same time, it is expected that the above bulky group creates larger free volume cavities where the water molecules can be confined. 1,3,46 It also to be noted that the electron withdrawing effect of –P=O group is more prominent in the DOPO moiety than in the monomer, DATPPO.

27,42

Considering the role of phosphorus containing

groups, it is worthwhile to design and prepare new phosphorus containing polymers and to investigate their properties. In the present work, we have prepared DOPO moiety containing diamine monomer, DPPA (Figure 1) and it has been utilized to prepare several

sulfonated co-polyimides.

Accordingly, the present work deals with the preparation of several new co-SPIs (DPPNH-XX) using DPPA as one of the comonomers, detailed characterization of the copolymers by different spectroscopic methods and investigation of their PEM properties.

2. Experimental 2.1. Materials 9,10-Dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) (> 97%) and 1,4,5,8naphthalenetetracarboxylic dianhydride (NTDA, 98.0%) was procured from TCI (USA). pToluenesulphonic (p-TSA) acid monohydrate was bought from Sigma Aldrich. 4,4'Diaminostilbene-2,2'-disulfonic acid (DSDSA, 95.0%), 4-aminoacetophenone (99%) and Nafion® 117 membrane was obtained from Alfa Aesar (USA). Both DSDSA and NTDA heated at 120 °C for 12 h under vacuum. Nafion® 117 membrane was treated with hot 5 weight % H2O2 aqueous solution for 1 h and washed several times with DI water. Later, the membrane was boiled in 1 M H2SO4 aqueous solution for 1 h and washed several times with deionized water. 5



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All other chemicals were purchased from Spectrochem (India). The synthesis of DPPA and polymers are included in the Supporting Information. 2.2. Preparation of membranes The salt form of the polymers was dissolved in DMSO (~10 % w/v), filtered and transferred on Petri dishes. The Petri dishes were kept at 80 °C for 24 h and subsequently heated at 100 °C, 120 °C, 140 °C, 160 oC for 2 h at each temperature. This allows the formation of good quality films. Later, the Petri dishes were kept at 120 °C for 24 h under vacuum to maximize the removal of any residual solvent from the films. The salt form of the membranes was obtained by immersing the Petri dishes in hot water. Finally, the membranes were transformed to their acid form on treatment with 1.5 M sulfuric acid for 60 h at room temperature. The membranes were thoroughly washed several times with deionized water and dried at 120 °C under vacuum for 24 h. Flexible pale yellow membranes of 68-72 µm (±2 µm) thickness were obtained. 2.3. Measurements Nexus 870 Fourier transform spectrometer was used to obtain the ATR-FTIR spectra of the polymer membranes in a humidity controlled atmosphere at room temperature.

ZnSe

parallelogram crystal with an angle of incidence of 45o was used and 10 scans were given for each sample while recording the spectrum. 1H (600 MHz) and 31P (161.92 MHz) NMR spectra were measured on 600 and 400 MHz NMR instrument (Bruker, Germany) respectively using DMSO-d6 as solvent. The 1H spectra were referred to δ(1H) = 2.56 ppm of the methyl proton peak of DMSO-d6 and the 31P NMR spectra were referenced on external H3PO4 (δ(31P) = 0 ppm). The viscosity of the polymer solution (0.5 g dL-1 in NMP) was measured at 30 °C and termed as inherent viscosity (ɳinh). Thermal stability of the polymers was investigated in air (N2:O2 = 80:20) atmosphere by TGA Q50 (TA Instruments, USA) instrument at a heating rate of 10 6


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K/min. The Densimeter-X22B (Wallace, UK) was used to measure the density of the polymer membranes (isooctane displacement) at 30 °C. The mechanical properties of the acidified polymer films of average thickness 68-72 μm, width 10 mm and length 25 mm were measured at 30 oC using a universal testing machine from TINIUS OLSEN, UK (H5KS) at a speed of 5 mm/min. Three uniform specimens were used and the values reported as an average of the three measurements with a standard deviation below 4% of the mean value.

27

The microstructure of

the membranes was analysed using a TEM instrument, FEI -TECNAI G2 20S TWIN. The acid form (H+) of the samples was converted in their silver ion (Ag+) by putting the membranes in in 0.5 M AgNO3 aqueous solution for 24 h. The membranes were washed with deionized water, dried at room temperature for 12 h and sectioned Leica Ultra cut UCT EM FCS, Austria) and positioned on copper grids. The surface morphology of the membranes was investigated by atomic force microscopy instrument (AFM 5500, Agilent Technology) in tapping mode using 10 μm × 10 μm specimens in ambient conditions. The small-angle X-ray scattering (SAXS) study of the silver ion (Ag+) stained copolymer membranes were performed using XEUSS, SAXS instrument in transmission mode. The Cu Kα radiation source was used to generate X-rays with λ = 1.54 A°. The scattering vectors (q) were calculated using the Eqation (1). q = 4πsin θ/λ----------------------(1) where θ represent scattering angles. The inter-domain spacings (d) were calculated from the scattering maxima using Equation (2). d = 2π/q-----------------------------(2) The weight based ion exchange capacity (IECw) (equiv. g-1 or mmol g-1) of the polymer membranes (EW = 1000/IECw) was calculated from the Eqation (3). IECw = (1000/MWrepeat unit) ×DStheo×2 --------------------------(3) 7



Page 8 of 41

where DStheo corresponds the mole fraction sulfonic acid containing monomer (DSDSA) and defined as degree of sulfonation (DS) of the copolymers. The ion exchange capacities (IECw) of the membranes were determined following the procedure reported earlier. 27,38 IECW values were also calculated from the peak integrals of the 1H NMR spectra. Water uptake (WU) and consequently dimensional swelling of the membranes were measured at 30 oC and 80 oC following the literature method.

27,40

The volume based ion exchange capacity of the dry

membranes [IECV(dry)] were calculated by multiplying the density of the films with the IECW values and the values were used to calculate the volume based ion exchange capacity of the wet membraes [IECV (wet)] using the Equation (4). IECv (wet) = (IECv (dry) / (1 + 0.01 WU))----------------------(4) Oxidative stability of the membrane specimens of area 10 mm×10 mm was studied by dipping them into freshly prepared Fenton’s reagent (2 ppm FeSO4 in 3% H2O2) at 80 °C. The membranes were heated at 80°C for 24 h in deionized water and the retention of the mass of the membranes are termed as Hydrolytic stability.

27,38

AC impedance spectroscopic technique

(Gamry Reference 3000 potentiostat/galvanostat/ZAR instrument) was used to measure in-plane proton conductivity of the polymer membranes in deionized water in the frequency and temperature ranges from 100 Hz – 1 MHz

and 30 to 90 oC respectively. The hydrated

membranes of dimensions of 2 × 1 cm2 were clamped between two platinum electrodes in a 4probe conductivity cell (Electrochem Inc., FC-BT-115) and placed in a temperature controlled water bath. The resistance values (R) of the membranes were determined from the Nyquist type plot and used to calculate the proton conductivity (σ) of the membranes using Equation (5). σ = L/ (A×R)-------------------------------------(5) where A and L are the area and the length of the membranes respectively. 8


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3. Results and discussions 3.1. Synthesis and characterization of the polymers A series of new co-SPIs were synthesized by single pot high temperature solution polycondensation reaction of an equimolar amount of NTDA and two different diamines namelyDSDSA and DPPA in m-cresol (Scheme 1). The compound DSDSA was first dissolved in m-cresol at 80 0C by addition of excess trimethylamine to liberate the amine group from Zwitterionic structure of DSDSA followed by other two co-monomers DPPA and NTDA were added in the reaction flask. The reaction was proceeded by the two-step reaction.

30

Firstly, the

formation of polyamic acid followed by solution imidization at high temperature using benzoic acid as catalyst. The sulfonic acid containing monomer (DSDSA) was used to incorporate the sulfonation acid groups in the copolymers. The mole fraction of the sulfonated monomer was adjusted by changing the molar ratio of the DPPA to DSDSA in DPPNH-XX copolymers (where, XX = mole percentage of sulfonated amine monomer, DSDSA or the degree of sulfonation) and termed as the theoretical degree of sulfonation (DS) of the copolymers. The viscosity of the reaction medium was increased gradually during the progress of the reaction. At the completion of the reaction the viscosity of the medium increased greatly. The polymer solution was diluted with certain quantity m-cresol and precipitated in excess of isopropanol that resulted. Afterward the salt form of the precipitate was isolated, washed with isopropanol and deionized water and dried under vacuum at 100 oC. The salt form of the polymers was fairly soluble [~10% (w/v)] at 30°C in different organic solvents such as NMP, CH3CN, DMAc and DMSO and were insoluble in THF, DCM, methanol and water despite heating. This was attributed that the presence of bulky DOPO moiety in the polymer backbone that creates internal fractional free volume by disrupting the regularity of the polymer chain stacking, which in turn 9



improve polymer solubility.

34,46

Page 10 of 41

The inherent viscosity and the DS values of the co-SPIs are

reported in Table 1. The salt form of the membranes was prepared through solution casting technique and were converted into their acid form by acidification (treatment of 1.5 M H2SO4 aqueous solutions).

Table 1. Inherent viscosity and degree of sulfonation of the DPPNH-XX copolymers DSDSA (mole %)

Polymer

Degree of ηinh

Sulfonation (DS)

(dLg-1) Theo.a

NMRb

DPPNH-60

60

1.01

0.60

0.58

DPPNH-70

70

1.16

0.70

0.69

DPPNH-80

80

1.17

0.80

0.79

DPPNH-90

90

1.22

0.90

0.88

a Calculated b Calculated

from the monomer feed composition. from 1H NMR signal integral values.

10


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Scheme 1. Synthesis DPPA monomer and DPPNH-XX copolymers. NH2

NH2 pTSA

O

CH3

O

130 °C, 24 h

P O

O

H2N

NH2 CH3

excess

H

P O

DOPO

DPPA

SO3H SO m H2N NH2 m DSDSA

N(CH2CH3)3/m-cresol

-

H N 3

+

H2N

80 oC/4 h

NH2

HO3S

+N

-

O3S

H

O

H2N

m = 0.6, DPPNH-60 0.7, DPPNH-70 0.8, DPPNH-80 0.9, DPPNH-90

P O CH3

(1-m) DPPA

O

O

N

N

O

O

Benzoic acid 180 oC, 16 h 200 oC, 3 h

O

O

O

O

O NTDA

N+ H SO3 O

Salt form

O NH2

O O

+N

O3S

m

N

N

O

O

P O CH3

(1-m)

H 1.5 M H2SO4, 60 h

O

O

N

N

O

O

SO3H O

O O

H3OS

m

N

N

O

O

P O CH3

(1-m)

Sulfonated polyimides (DPPNH-XX), acid form

11



Page 12 of 41

The structure of the co-SPIs were established by ATR-FTIR and NMR spectroscopic techniques. The FTIR spectra of the polymer membranes indicating the complete imidization. No absorption bands were found at 1770 and 1743 cm-1 corresponds to the carbonyl stretching of 6membered anhydride and peak around 1780 cm-1 typical of the amide stretching band of poly(amic acid). The absorption bands at 1711 cm-1 (-C=O asymmetric stretching), 1668 cm-1 (C=O symmetric stretching) and 1346 cm-1 (-C-N asymmetric stretching) confirm the creation of the naphthalene imide of the DPPNH-XX copolymers. 27,38 The bands at 1248 cm-1 and 1076 cm1

confirm the presence of P=O and SO2 is stretching bands in the polymers (Figure 2). 38

Figure 2. ATR-FTIR spectra of DPPNH-XX membranes.

The incorporation of the of the sulfonic acid containing monomer (DSDSA) in the DPPNH-XX copolymers was checked by 1H NMR spectroscopy. The experimental DS values of the co-SPIs were determined from signal integral values of the 1H NMR spectra (Figure 3a).

30

12


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The NMR spectra were of acid form of the polymers recorded to circumvent signal overlay with the -NH proton of triethylamine. 47 The representative 1H NMR spectrum of DPPNH-70 is shown in Figure 3a. Absence of any peak in the region of ~10-12 ppm (corresponding to amide protons) in Figure 3a is indicating complete imidization. 46 The signal broadening of the spectra was observed and was attributed to the restricted chain mobility, typical

of sulfonated

polyimides, that arises due to the formation of hydrogen bonds with solvent. 38 However, the DS values of the copolymers determined from the 1H-NMR spectra of the respective copolymers are in concurrence with theoretical DS values of the copolymers, indicating successful incorporation of the sulfonated monomer (Table 1). The phosphorus atoms in 31P NMR spectrum of DPPNH70 (Figure 3b) appeared at around 37.1 ppm, similar to DPPA (37.5 ppm) (Figure S1a in Supporting Information) indicating the DOPO moiety was non-reactive under polymerization conditions.

13



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13

1

2

3 H3OS

12

O

P O

10

SO3H 4

11 O

5

O

N

N

O

O

9

6

14 15

8

7

CH3 16

Figure 3. (a) 1H NMR and (b) 31P NMR spectra of DPPNH-70 (in DMSO-d6, 600 MHz). [peak at 2.56 ppm and at 3.1 ppm corresponds to H2O and DMSO] [Region I (S1) represent 2 sulfonated protons and 2 non-sulfonated protons, region II (S2) represent 3 non-sulfonated protons. DS = {(S1/2) – (S2/3)} / [{(S1/2) - (S2/ 3)} + (S2/ 3)] = 0.69] 30

14


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3.2. Thermal and mechanical properties The TGA plot of the DPPNH-XX copolymers are shown in Figure 4. The polymers were dried under vacuum at 120 oC for 1 h before performing TGA. The polymers were thermally stable and showed a typical two-step degradation response as anticipated from sulfonated polyimides. The thermal stability of the copolymers in terms 10% weight loss are given in Table 2. The values indicating reasonable high thermal stability (> 300 oC) of the copolymers that are required for their application as PEM.1,2 The polymers showed some amount residues (~2-3 wt %) even at 800 oC is attributed to the presence of phosphorus in the polymers. 27

In DSC, no thermal transition corresponds to glass transition (Tg) or melting temperature were

observed up to 350 °C, a typical characteristic of sulfonated polymers and was accredited to the strong ionic interactions among the sulfonic acid groups, –P=O functionality of the DPPA and – C=O of NTDA unit in the polymer backbone 27,38,48

Table 2. Physical properties of the DPPNH-XX co-polymers. Td, 10a

TS (MPa)

YM (GPa)

EB (%)

Polymer

(oC)

Dry

Wet

Dry

Wet

Dry

Wet

DPPNH-60

321

69

59

1.44

1.07

15

17

DPPNH-70

315

60

51

1.32

1.28

13

15

DPPNH-80

301

52

42

1.22

0.95

10

12

DPPNH-90

295

45

37

1.02

0.82

7

10

Nafion® 117

-

38

22

0.26

0.16

288

301

a 10

% weight loss temperature.

TS = Tensile strength; YM = Young’s modulus; EB = Elongation at break. 15



Page 16 of 41

Figure 4. TGA thermogram of the DPPNH-XX copolymers.

The mechanical property of the membranes was measured at room temperature (30 °C) both in dry and wet condition. The tensile plot of the dry and wet membranes is represented in Figures 5a and 5b respectively. The tensile strength

of the DPPNH-XX membranes both in

dry (45-69 MPa) and wet (37-59 MPa) states were considerably higher than that of Nafion® 117 (Table 2). The elongation at break of the membranes were considerably lower (less than 17 %) in comparison to Nafion® 117 (~300 %), typical of rigid co-SPIs.

27,38

But, the values both in

their dry and hydrated states were similar that justifies their suitability as a better candidate for PEM applications. 49

16


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Figure 5. Tensile plots of the DPPNH-XX (a) dry and (b) wet state.

3.3. Ion exchange capacity, water uptake, hydration number and swelling ratio The PEM properties such as the water uptake, proton conductivity, etc. were greatly influenced by the ion exchange capacity of the membranes. The IECw depends on the feed composition and the IECW values were determined experimentally from the peak integration of 1H

NMR spectra (allows to calculate the DS values) and titrimetric measurements. The values 17



Page 18 of 41

are presented in Table 3. The experimental IECw values were in concurrence with the theoretical values which attributed to the quantitative incorporation of DSDSA in the polymer backbone. 30 The water uptake values of the PEM are critical as it significantly influences the mechanical integrity and dimensional stability of the membranes. Excessive water uptake values worsen the mechanical and dimensional constancy of the membrane.

50

At the same time, the

water holding capacity of the membrane was an important criterion for PEM. The absorbed water molecules dissociate the acid functionalities and assist the transportation of protons by creating interconnected networks and ultimately improved proton conductivity. 51,52 Hence, it is of great challenge in designing of new PEM materials that can address this trade-off relationship. In the current study, the non-sulfonated monomer (DPPA) contributes a vital part in regulating the swelling ratio of the membrane as there is an internal fractional free volume generating from the bulky DOPO moiety. 46 The WUvalues of the DPPNH-XX membranes were lower related to the previously reported sulfonated polyimide membranes represented in Table 3. 27,38 This was attributed to the greater dipole-dipole interaction of polar –P=O functionality present in these co-SPIs. 14 In plane proton conductivity of the PEM materials can be correlated to the length and is independent of the mass of the membranes. Thus, to understand this effect more clearly both IECw and IECv of the membranes were measured. Weight-based parameters have important limitations compared to the volume-based parameters when fuel cell performance of PEM is correlated as the density of the membranes are not always same in their dry and wet states. 53 Besides, the transportation of the protons in PEM happened in length scales that better characterized by volume rather than mass.

14

IECv values of the co-SPI membranes were increased with an increase in the IECw

value, but the IECV (wet) value of the DPPNH-80 and DPPNH-90 are similar, 2.79 and 2.78 18


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mequiv g-1 respectively. The nearly same IECV (wet) indicating that the difference in mass normalized IEC (IECW for DPPNH-80 and DPPNH-90 are 2.58 and 2.95 mequiv g-1, respectively) is offset by the higher water uptake (presented in Table 3). WUvol% values of the co-SPI membranes are presented against IECv (dry and wet) in Figures 6a and 6b. IECv dry and wet exhibited a alike trend both at 30 and 80 oC, indicating no unexpected swelling and better water managements of the membranes. The hydration number (the water molecules per sulfonic acid group, λ) of the membranes rises progressively from 7.7 to 10.5 with increase in IECW values from 1.88 to 2.95 mequiv g-1. However, the values are inferior in contrast to Nafion® (λ~15) but sufficient to facilitate the proton transfer. The inflexibility of aromatic structure in comparison to flexible construction of Nafion® accredited this. Moreover, strong ionic interaction between sulfonic acid groups confines free volume for water absorption nearby the ionic –SO3H groups after definite limit.

19



Page 20 of 41

Figure 6. Water uptake (vol%) dependence of (a) IECV (dry) and (b) IECV (wet) values of DPPNH-XX co-polymers.

Further, to understand better, the swelling behavior of the membranes and the water uptake values are plotted against time (Figure 7). As the time increases, the water uptake increases, but after certain time (here 72 hrs) there is no further weight gain of the membranes and an equilibrium sorption is attained. 54

20


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Figure 7. Swelling behavior of DPPNH-XX membranes.

The dimensional change of these co-SPI membranes is provided in Table 4. The dimensional change of the membranes increases with an increase in the DS values and attributed to the more ionic nature of the membranes. The swelling ratio of the DPPNH-XX membranes were lower compared to the previously reported semi-fluorinated polymers despite they have similar water uptake values.

38,46

The lower swelling of DPPNH-XX copolymers are accredited

to the bulky DOPO moiety that inhibits the polymer chains packing and thereby expected to create larger free volume cavities that can be responsible for more water to occupy without much affecting the dimensional swelling.

The membranes exhibited larger swelling in

towards

thickness than in length and that is better for construction of superior characteristic membrane electrode assembly (Figure S2 in Supporting Information). 26

21



Page 22 of 41

Table 3. Ion exchange capacity and other PEM properties of DPPNH-XX copolymers Polymer

IECw

IECv

(mequiv/g)

(mequiv/cm3) Dry

WU (wt %)

λ [H2O/SO3]

30 °C

80 °C

30 °C

80 °C

Reference

Wet

Theo

Titr

NMR

DPPNH-60

1.88

1.84

1.82

2.96

2.35

26

31

7.7

9.0

This study

DPPNH-70

2.23

2.21

2.19

3.54

2.68

32

36

8.0

9.2

This study

DPPNH-80

2.58

2.56

2.55

3.93

2.79

41

46

8.8

9.9

This study

DPPNH-90

2.95

2.90

2.87

4.34

2.78

56

64

10.5

12.1

This study

DFPNH-90

2.86

2.62

2.53

3.23

2.00

61

70

11.9

13.6

[27]

TPPO-60

1.84

1.83

1.81

2.35

1.79

31

41

9.4

12.3

[38]

sPAE 1.00

1.75

-

-

3.55

2.55

27

32

-

-

[9]

BAPS-70

1.82

-

-

-

-

65

-

20

-

[30]

O

O

N

N

O

SO3H O

O

N

N

O O

P

O

O HO3S

0.6

O

O

0.4

TPPO-60 O

O

N

N

O

O

SO3H O

O O

HO3S

0.9

N

N

O

O

O

P F3C

CF3 O 0.1

DFPNH-90

3.4. Oxidative and hydrolytic stability The oxidative stability (accelerated test) was measured with the freshly prepared Fenton’s reagent at 80 °C. The oxidative and hydrolytic stability of DPPNH-XX membranes are presented 22


Page 23 of 41 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


in Table 4. As usual like other sulfonated polymers, both hydrolytic and oxidative stability of the of DPPNH-XX membranes declines to rise in DS. 27,55 But, the copolymer membranes under preset investigation displayed improved oxidative stability compared to reported sulfonated polyimides with comparable IEC values.

14,27,37,38

several previously The high oxidative

stability of the membranes attributed to more electron withdrawing effect of of DPPA (cyclic – P=O functionality) compared to previously reported polymers that utilize DATPPO (linear – P=O functionality) as one of the co-monomer. 27 As a result, the analogous polymers with similar IEC values, as for example DPPNH-90 showed higher oxidative stability compared to DFPN-90. 38

Similarly, DPPNH-60 has higher peroxide stability in comparison to TPPO-60.

27

Phosphine

oxide group also has the capability to chelate with Ferrous ion that and helped to stop the secession of the polymer backbone. 56 A plausible mechanism of oxidative attack of OOH radical is shown in the Scheme 2.

23



Page 24 of 41

Scheme 2. Proposed mechanism of oxidative attack of perhydroxyl radical with the phosphaphenanthrene skeleton. Fe (II) + H2O2

Fe (III) + OH

Fe (III) + H2O2 + OH

-

+ OH

Fe (II) + OOH + O2 H

H

H OOH

OOH

OOH

O

O

O

P

P

P O

O

O

(IA)

(I)

O

O

O

P

P

P

O H OOH

O

O H (II)

OOH

H OOH (IIA)

OOH O

OOH O

OOH O

P

P

P

O

O

O

(III)

(IV)

The hydroxyl and per hydroxyl radical generated by the Fenton’s reagent attacks the aromatic backbone present in the membrane. The phosphaphenanthrene skeleton present in the polymer backbone resists the radical attack by stabilizing the radical. The –I & -R effect of –OP=O groups of the phosphaphenanthrene skeleton stabilize the radical and scavenge the hydroxyl and per hydroxyl radical. In the above scheme, structure I & II are stabilized by the –I effect of 24


Page 25 of 41 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


the –O-P=O group. These radicals are also stabilized through resonance by the involvement of the phenyl ring (such as in IA & IIA). The structure III & IV are highly stable as these resonance structures are identical.

57

The DPPNH-XX membranes

also showed improved hydrolytic

stability in contrast to several structurally similar sulfonated copolymers studied earlier.

27,38

This could be owing to the captivity of water molecules in the free volume cavities created by the bulky DOPO moiety. However, the oxidative and hydrolytic stabilities of these copolymers were inferior compared to Nafion® but the membranes displayed substantial enhancement of these properties than earlier reported NTDA based polyimide membranes. 26,27,38 Table 4. Different PEM properties of the DPPNH-XX copolymers

Polymers

IECw (mequivg1)

Oxidative stability (h) Hydrolytic stability c (%) τ1a τ2b

Swelling Ratio (%) Reference 30 °C Length

80 °C

Thickness Length Thickness

DPPNH-60

1.88

6

>24

>99

4.0

8.0

6.0

13.0

This study

DPPNH-70

2.23

5

>17

~98

5.0

11.0

7.0

17.0

This study

DPPNH-80

2.58

4

>15

~97

7.0

14.0

9.0

20.0

This study

DPPNH-90

2.95

2

>7

~95

8.0

17.0

10.0

22.0

This study

p-DTN- 70

2.1

-

9

~98

2.0

12.0

-

-

[26]

DFPNH-90

2.86

1

>2

94

10.0

25.0

14.0

30.0

[27]

TPPO-60

1.84

3.1

20

95

5.0

5.5

10.0

13.8

[38]

DAN-70

1.85

-

>21

-

-

4.0

-

14.0

[37]

SPAES-36

1.86

-

>2.5

7.6

9.1

16.8

22.9

[14]

a Start

breaking; b Dissolved; c %Weight retention. 25



Page 26 of 41

3.5. Copolymer microstructure The membrane morphology is very important

to understand the proton transport

behavior in PEM. Accordingly, AFM was used to study the surface morphology of the DPPNHXX membranes and the images are shown in Figure 8. The images displayed both bright and dark regions. The average roughness of the membranes decrease with increasing the DS values corresponding to 8.51 nm, 5.82 nm and 5.28 nm respectively for DPPNH-70, DPPNH-80 and DPPNH-90. Thus, it can be attributed that the dark regions resembles to hydrophilic sulfonic acid groups and the bright regions are corresponding to the hydrophobic polymer matrix.

38

Also, the interconnectivity of the soft structures (ionic domains) develop more protuberant with increasing the DS values of the membranes. 38

26


Page 27 of 41 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


Figure 8. AFM images of DPPNH-XX membranes.

The small-angle X-ray scattering (SAXS) was also performed to further

study the

microstructure of DPPNH-XX membranes for better understanding the membrane morphology and the SAXS profiles are shown in Figure 9. The SAXS profile indicated that the ionic scattering maxima shifted to a larger q value with the increase in DS of the copolymers which is in accordance to the size of the ionic clusters obtained from the TEM micrographs. With the increase in DS values, the size of the ionic cluster increases and their distance reduces which in 27



turn shifted the scattering maxima towards the larger q value.

Page 28 of 41

58

The broad SAXS profile of the

polymer membranes is attributed to the long range order of ionic domains present in the DPPNH-XX membranes.

Figure 9. SAXS profiles of DPPNH-XX membranes. To further justification of the morphology of the DPPNH-XX membranes, the TEM images of the silver ion-stained ultra-microtome polymer samples were captured. Figure 10 shows the cross section TEM micrographs of DPPNH-XX membranes that showed outstanding nano-phase separated morphology. Similar to AFM, The images displayed both dark and bright regions where the black spherical region resembles to the hydrophilic soft structure and the 28


Page 29 of 41 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


bright regions are indicating the hydrophobic hard structure. The size and the quantity of the hydrophilic domains per unit area were increased with the increase of the sulfonic acid content in the polymers. The nano-phase separated morphology and interconnected hydrophilic ionic channels are responsible for the higher proton conductivity of the copolymers. 19,26,27

Figure 10. TEM images of DPPNH-XX membranes.

29



Page 30 of 41

3.6. Proton conductivity AC impedance spectroscopy was employed to measure the in plane proton conductivity of the DPPNH-XX copolymer membranes in the temperature range 30-90 °C in water. The membranes showed temperature dependence proton conductivity that increases with the increase of DS of the polymers and were in the range of 40 to 135 mS cm-1 and 87 to 235 mS cm-1 respectively at 30 and 80 °C (Table 5). These values were significantly higher than many other SPIs with comparable IECw values.

26,27

The phosphine oxide moiety in DPPNH-70 (proton

conductivity ~ 104 mS cm-1 at 80 oC; IECw ~ 2.23 mequiv g-1) helped in achieving significantly higher proton conductivity when compared with p-DTN-70 (proton conductivity ~ 45 mS cm-1 at 80 oC; IECw ~ 2.10 mequiv g-1), a sulfonoated polyimidie without phosphine oxide moiety. This is accredited to the existence of phosphine oxide in DPPNH-70 that form hydrogen bonds with the sulfonic acid groups and assist proton transport. 37 The activation energy of the proton conduction in the membranes was determined from the temperature dependence behavior proton conductivity of the membranes. For this, the proton conductivity of the membranes was plotted against 1/T (Figure S3 in Supporting Information) and the activation energies (Ea) were calculated from the slope of the plots the Arrhenius equation. The Ea values of the DPPNH-XX membranes are provided in Table 5 (10.8 to 13.3 kJ mol-1) and the values are as good as Nafion® 117, indicating the practicality of these polymer in PEM application.

30


Page 31 of 41 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


Table 5. Proton conductivity of the DPPNH-XX membranes Polymers

 (mS cm-1)

IECw (mequiv

Ea

30 °C

80 °C

90 oC

kJmol-1

Reference

g-1) DPPNH-60

1.88

40

87

92

13.3

This study

DPPNH-70

2.23

48

104

118

14.1

This study

DPPNH-80

2.58

97

202

222

12.5

This study

DPPNH-90

2.95

135

235

252

10.8

This study

DFPNH-90

2.86

111

237

261

13.2

[27]

TPPO-60

1.84

46

99

107

12.3

[38]

DAN-70

1.85

32

75

-

-

[37]

p-DTN- 70

2.1

24

45

53

-

[26]

sPAE 1.00

1.75

68.2

135.4

-

-

[9]

BAPS-70

1.82

90

-

-

-

[30]

Nafion® 117

0.90

90

165

-

13.6

This study

4. Conclusion A series of new co-polyimides containing a cyclic phosphinate (DOPO) in the polymer backbone with controlled degree sulfonation has been successfully prepared. The polymers displayed decent solubility in many common organic solvents accredited to the existence of the bulky DOPO moiety that create a larger free volume cavity are by disordering the chain regularity . The chemical structures of the repeat unit of the polymers were confirmed by FTIR 31



and

NMR techniques.

Page 32 of 41

The 1H NMR spectroscopy was utilized to know the polymer

compositions and was used to calculate the degree of sulfonation and IECw values of the copolymers. The polymers showed high inherent viscosity demonstrating the formation of high molar mass products during the course of polymerization. The high mechanical integrity of the polymer membranes both in dry and wet states verbatim the same. It is attributed that the bulky DOPO moiety creates the fractional free volume in which water molecules are confined. As a result, the polymers showed a lower swelling ratio than the polymers without bulky moiety, but sufficient water uptake for better proton conduction compared to several previously studied sulfonated polyimides with similar IECw values. The high oxidative stability of these nonfluorinated co-polymers were attributed to the existence of cyclic phosphinate groups in the DPPA co-monomer and a plausible mechanism is provided. The polymer showed high proton conductivity (40-135 mS/cm at 30 °C, 87-235 mS/cm at 80 oC. At large, the DPPNH-XX copolymers displayed an improved set of PEM properties. The proton conductivity of one of the co-polymers (DPPNH-80) were as high as 202 mS/cm at 80 oC (Nafion® 117 ~ 165 ms/cm) with a reasonably good set of other PEM properties. AFM, TEM and SAX analysis indicated the formation of phase separated morphology through connected ionic channels that became prominent with the increase in DS value of the co-polymers and attributed to their high proton conductivity.

32


Page 33 of 41 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


ASSOCIATED CONTENT Supporting Information Author information *Corresponding

author: S. Banerjee (E-mail: [email protected])

Notes The authors declare no competing financial interests. Acknowledgements AKM acknowledges the CSIR, New Delhi for the research assistantship.

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Effect of Phosphaphenanthrene Skeleton in Sulfonated Polyimides for

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