Polybenzimidazole block copolymers for fuel cell: Synthesis and ...
Polybenzimidazole block copolymers for fuel cell: Synthesis and studies of block length effects on nanophase separation, mechanical properties and proton conductivity of PEM
Sudhangshu Maity and Tushar Jana*
School of Chemistry University of Hyderabad Hyderabad, India Tel: (91) 40 23134808 Fax: (91) 40 23012460 E-mail: [email protected] [email protected] (* To whom correspondence should be addressed)
Supporting Information
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Materials: Sulfuric acid (98%), orthophosphoric acid (PA) (85%), sodium bicarbonate (NaHCO3) was procured from Merck, India. Methane sulfonic acid (MSA), isophthalic acid (IPA), terephthalic acid (TPA), ammonium hydroxide (NH4OH) and formic acid (FA) were procured from SRL, India. The NMR solvent dimethyl sulfoxide (DMSO-d6), 3,3ʹ4,4ʹ-tetraaminobiphenyl (TAB), polyphosphoric acid (PPA, 115%) were obtained from Sigma-Aldrich. Absolute ethanol was purchased from Finar chemicals, India. All chemicals were used without further purification.
Molecular weight measurement: (a) Viscosity: The viscosity measurements of all the diamine and acid terminated oligomers and m-PBI-b-p-PBI polymers in H2SO4 were carried out at 30°C in a constant temperature water bath using a Cannon Ubbelohde capillary dilution viscometer (model F725). The inherent viscosity (IV) values were calculated from the flow time data. For all the flow time measurements 0.2 g/dL polymer solution in H2SO4 (98%) was used. (b) 1H NMR analysis: All the 1H NMR spectra were recorded using Bruker AV 400 MHz NMR spectrometer at room temperature using DMSO-d6 as NMR solvent to confirm the chemical structures of oligomers and segmented block copolymers. We have calculated number average molecular weights of the oligomers by applying end group analysis of 1H NMR spectra as follows: Calculation of MW using NMR data for one representative oligomeric sample is shown here. The oligomer is diamine terminated oligomer which has IV = 0.32 dL/g. The NMR spectra along with the peak integral and the oligomer structure are also shown here for the clarity.
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Membrane fabrication: All the segmented block copolymers of PBI were dissolved in methane sulfonic acid (MSA) solvent at 2% (w/v) concentration by continuous stirring for 12 h. Then clear solutions were filtered through 0.5 μm PTFE filter paper. The dark brown solution was poured into flat glass petridis to fabricate the membranes and the solution was evaporated in hot air oven at 120°C for 24 h. Homogeneous membranes were obtained, taken out from glass petridis, soaked in boiled deionized water for three times to remove trace amount of MSA. Finally these membranes were kept in vacuum oven at 120°C for two days for complete removal of solvent. The obtained membrane thickness was approximately 30 to 40 μm. These membranes were stored in the desiccator for further studying.
Thermal study: Thermogravimetric and differential thermal analysis (TG-DTA) were carried out on a (Netzsch STA 409PC) TG-DTA instrument from 30-800°C with a scanning rate of 10°C/min in presence of continuous nitrogen flow. Solid polymer samples were used in this study.
Mechanical property study: The temperature dependent dynamical mechanical properties of all meta-PBI-block-para-PBI (m-PBI-bp-PBI) segmented block copolymers films were measured using a dynamic mechanical analyzer (DMA) (TA Instruments, model Q-800). The dimensions of films 25 mm × 5 mm × 0.04 mm (L × W × T) were cut and clamped on the films tension clamp in the pre-calibrated instrument. The samples were annealed at 100°C for 30 minutes and then scanned from 100°C to 450°C at a heating rate 4°C/min. The storage modulus (Eʹ), loss modulus (Eʺ) and tan δ values were measured at a constant linear frequency 10 Hz with a preload force 0.01N.
PA doping level: All the dried segmented block copolymers membranes obtained from MSA solution were immersed in H3PO4 (85%) solution for seven days. After that the membranes were taken out from acid bath and extra acid from membrane sample were wiped by filter paper and stored in air tight packed. The PA doping level of membranes were determined by titrating of a membrane sample with pre standardized 0.1N sodium hydroxide Page 4 of 10
solutions using a Metrohm Titrino Titrator (702). The dry weight of the polymer membrane was obtained by washing the samples with de-ionized water and then drying in vacuum oven at 100°C for 24 hours. The acid doping levels are expressed as mols of phosphoric acid per mole of PBI repeat unit. The acid doping level was calculated using following equation: Acid (PA) doping level
WPA M PBI W dry M PA
Where WPA and Wdry are the weight of acid and dry membrane, respectively and MPBI and MPA are the repeating unit molecular weight of the polymer and phosphoric acid, respectively. For each sample, three similar size pieces of samples were titrated and the reported acid doping level data are the average of these three values.
Conductivity study: We have measured the proton conductivities by using a four probe impedance method by using a Metrohm Autolab (PGSTAT302N) over a frequency range from 1 Hz to 100 kHz. A rectangular size PA doped segmented block copolymer membrane (1.5 cm × 4.0 cm) and four platinum wires were set in a homemade Teflon cell. Two outer electrodes (1.5 cm apart) supply current to the cell, while the two inner electrodes 0.5 cm apart on opposite sides of the membrane measure the potential drop. For the temperature dependence proton conductivity measurement the cell was placed in an oven. The membranes were dried at 100°C by heating and holding at 100°C isothermally for 2 hours. The membrane samples were then cooled in a vacuum oven and taken out just before conductivity measurement to keep the samples dry. The conductivities of the membranes were measured from room temperature to 160°C at 20°C intervals. Before measuring the conductivity, the samples were kept in the oven at each temperature for 30 minutes. Results obtained using the above temperature profile and testing procedure were found reproducible. A two component model with an ohmic resistance in parallel with a capacitor was employed to fit the experimental curve of the membrane resistance across the frequency range (the Nyquist plot). The conductivities of the membrane at different temperatures were calculated from the membrane resistance obtained from the ohmic resistance. Proton conductivity was then calculated from the following equation:
D LBR
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Where, D is the distance between the two current electrodes 0.5 cm apart, L and B are the thickness and width, respectively, and R is the measured resistance value.
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(A)
(B)
Supporting information Figure 1. Double logarithmic plot of I.V. vs Mn of (A) diamine terminated PBI oligomer (m-PBI-Am) and (B) acid terminated PBI oligomer (p-PBI-Ac).
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(B)
Supporting information Figure 2. Thermogravimetric plots of (A) meta oriented diamine terminated oligomers and (B) para oriented acid terminated oligomers in a N2 atmosphere at the heating rate of 10
°C/min. Page 8 of 10
(A)
(B)
(C)
Supporting information Figure 3. Tan δ plots against temperature plots of PBI block copolymers; (A) fixed diamine terminated with varied acid terminated (Amfix-b-Acvary), (B) fixed acid terminated with varied diamine terminated (Amvary-b-Acfix) and (C) equal diamine and acid terminated (Amequal-b-Acequal). These results are obtained from DMA study at the heating rate of 4 °C/min.
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Supporting Information Figure 4. Few representative Nyquist plots of three sets of PBI block copolymers at different temperatures during the second heating scans.
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Sudhangshu Maity and Tushar Jana*
School of Chemistry University of Hyderabad Hyderabad, India Tel: (91) 40 23134808 Fax: (91) 40 23012460 E-mail: [email protected] [email protected] (* To whom correspondence should be addressed)
Supporting Information
Page 1 of 10
Materials: Sulfuric acid (98%), orthophosphoric acid (PA) (85%), sodium bicarbonate (NaHCO3) was procured from Merck, India. Methane sulfonic acid (MSA), isophthalic acid (IPA), terephthalic acid (TPA), ammonium hydroxide (NH4OH) and formic acid (FA) were procured from SRL, India. The NMR solvent dimethyl sulfoxide (DMSO-d6), 3,3ʹ4,4ʹ-tetraaminobiphenyl (TAB), polyphosphoric acid (PPA, 115%) were obtained from Sigma-Aldrich. Absolute ethanol was purchased from Finar chemicals, India. All chemicals were used without further purification.
Molecular weight measurement: (a) Viscosity: The viscosity measurements of all the diamine and acid terminated oligomers and m-PBI-b-p-PBI polymers in H2SO4 were carried out at 30°C in a constant temperature water bath using a Cannon Ubbelohde capillary dilution viscometer (model F725). The inherent viscosity (IV) values were calculated from the flow time data. For all the flow time measurements 0.2 g/dL polymer solution in H2SO4 (98%) was used. (b) 1H NMR analysis: All the 1H NMR spectra were recorded using Bruker AV 400 MHz NMR spectrometer at room temperature using DMSO-d6 as NMR solvent to confirm the chemical structures of oligomers and segmented block copolymers. We have calculated number average molecular weights of the oligomers by applying end group analysis of 1H NMR spectra as follows: Calculation of MW using NMR data for one representative oligomeric sample is shown here. The oligomer is diamine terminated oligomer which has IV = 0.32 dL/g. The NMR spectra along with the peak integral and the oligomer structure are also shown here for the clarity.
Page 2 of 10
( )
( (
) )
(
)
Page 3 of 10
( )
(
)
( )
Membrane fabrication: All the segmented block copolymers of PBI were dissolved in methane sulfonic acid (MSA) solvent at 2% (w/v) concentration by continuous stirring for 12 h. Then clear solutions were filtered through 0.5 μm PTFE filter paper. The dark brown solution was poured into flat glass petridis to fabricate the membranes and the solution was evaporated in hot air oven at 120°C for 24 h. Homogeneous membranes were obtained, taken out from glass petridis, soaked in boiled deionized water for three times to remove trace amount of MSA. Finally these membranes were kept in vacuum oven at 120°C for two days for complete removal of solvent. The obtained membrane thickness was approximately 30 to 40 μm. These membranes were stored in the desiccator for further studying.
Thermal study: Thermogravimetric and differential thermal analysis (TG-DTA) were carried out on a (Netzsch STA 409PC) TG-DTA instrument from 30-800°C with a scanning rate of 10°C/min in presence of continuous nitrogen flow. Solid polymer samples were used in this study.
Mechanical property study: The temperature dependent dynamical mechanical properties of all meta-PBI-block-para-PBI (m-PBI-bp-PBI) segmented block copolymers films were measured using a dynamic mechanical analyzer (DMA) (TA Instruments, model Q-800). The dimensions of films 25 mm × 5 mm × 0.04 mm (L × W × T) were cut and clamped on the films tension clamp in the pre-calibrated instrument. The samples were annealed at 100°C for 30 minutes and then scanned from 100°C to 450°C at a heating rate 4°C/min. The storage modulus (Eʹ), loss modulus (Eʺ) and tan δ values were measured at a constant linear frequency 10 Hz with a preload force 0.01N.
PA doping level: All the dried segmented block copolymers membranes obtained from MSA solution were immersed in H3PO4 (85%) solution for seven days. After that the membranes were taken out from acid bath and extra acid from membrane sample were wiped by filter paper and stored in air tight packed. The PA doping level of membranes were determined by titrating of a membrane sample with pre standardized 0.1N sodium hydroxide Page 4 of 10
solutions using a Metrohm Titrino Titrator (702). The dry weight of the polymer membrane was obtained by washing the samples with de-ionized water and then drying in vacuum oven at 100°C for 24 hours. The acid doping levels are expressed as mols of phosphoric acid per mole of PBI repeat unit. The acid doping level was calculated using following equation: Acid (PA) doping level
WPA M PBI W dry M PA
Where WPA and Wdry are the weight of acid and dry membrane, respectively and MPBI and MPA are the repeating unit molecular weight of the polymer and phosphoric acid, respectively. For each sample, three similar size pieces of samples were titrated and the reported acid doping level data are the average of these three values.
Conductivity study: We have measured the proton conductivities by using a four probe impedance method by using a Metrohm Autolab (PGSTAT302N) over a frequency range from 1 Hz to 100 kHz. A rectangular size PA doped segmented block copolymer membrane (1.5 cm × 4.0 cm) and four platinum wires were set in a homemade Teflon cell. Two outer electrodes (1.5 cm apart) supply current to the cell, while the two inner electrodes 0.5 cm apart on opposite sides of the membrane measure the potential drop. For the temperature dependence proton conductivity measurement the cell was placed in an oven. The membranes were dried at 100°C by heating and holding at 100°C isothermally for 2 hours. The membrane samples were then cooled in a vacuum oven and taken out just before conductivity measurement to keep the samples dry. The conductivities of the membranes were measured from room temperature to 160°C at 20°C intervals. Before measuring the conductivity, the samples were kept in the oven at each temperature for 30 minutes. Results obtained using the above temperature profile and testing procedure were found reproducible. A two component model with an ohmic resistance in parallel with a capacitor was employed to fit the experimental curve of the membrane resistance across the frequency range (the Nyquist plot). The conductivities of the membrane at different temperatures were calculated from the membrane resistance obtained from the ohmic resistance. Proton conductivity was then calculated from the following equation:
D LBR
Page 5 of 10
Where, D is the distance between the two current electrodes 0.5 cm apart, L and B are the thickness and width, respectively, and R is the measured resistance value.
Page 6 of 10
(A)
(B)
Supporting information Figure 1. Double logarithmic plot of I.V. vs Mn of (A) diamine terminated PBI oligomer (m-PBI-Am) and (B) acid terminated PBI oligomer (p-PBI-Ac).
Page 7 of 10
(A)
(B)
Supporting information Figure 2. Thermogravimetric plots of (A) meta oriented diamine terminated oligomers and (B) para oriented acid terminated oligomers in a N2 atmosphere at the heating rate of 10
°C/min. Page 8 of 10
(A)
(B)
(C)
Supporting information Figure 3. Tan δ plots against temperature plots of PBI block copolymers; (A) fixed diamine terminated with varied acid terminated (Amfix-b-Acvary), (B) fixed acid terminated with varied diamine terminated (Amvary-b-Acfix) and (C) equal diamine and acid terminated (Amequal-b-Acequal). These results are obtained from DMA study at the heating rate of 4 °C/min.
Page 9 of 10
Supporting Information Figure 4. Few representative Nyquist plots of three sets of PBI block copolymers at different temperatures during the second heating scans.
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