Synthesis and characterization of polybenzimidazoles derived from tetraaminodiphenylsulfone for high temperature gas separation membranes

Article abstract of DOI:10.1016/j.polymer.2015.06.021

Abstract A series of polybenzimidazoles containing sulfonyl groups were synthesized in Eaton's reagent for high temperature H₂/CO₂ separation membranes. The key monomer, 3,3′,4,4′-tetraaminodiphenylsulfone, was prepared via a novel and economical synthetic route starting from 4,4′-dichlorodiphenylsulfone. These polybenzimidazoles with sulfonyl moieties had enhanced solubilities in dipolar aprotic solvents relative to the commercial Celazole that is prepared from diaminobenzidine. Thermal gravimetric analysis showed that the materials were stable at elevated temperatures with 5% weight loss values of at least 485 °C in either air or N₂. Glass transition temperatures of three polybenzimidazoles in this series were ascertained by dynamic mechanical analysis to be 438-480 °C. These sulfonyl-containing polybenzimidazoles exhibited excellent gas separation properties for H₂/CO₂. Polymers from tetraaminodiphenylsulfone and either terephthalic or isophthalic acid crossed Robeson's upper bound for H₂/CO_2.

Full text of DOI:10.1016/j.polymer.2015.06.021

Accepted Manuscript
Synthesis and Characterization of Polybenzimidazoles derived from
Tetraaminodiphenylsulfone for High Temperature Gas Separation Membranes
Hailun Borjigin, Kevin A. Stevens, Ran Liu, Joshua D. Moon, Andrew T. Shaver,
Steve Swinnea, Benny D. Freeman, J.S. Riffle, James E. McGrath
PII:
S0032-3861(15)30042-2
10.1016/j.polymer.2015.06.021
JPOL 17919
DOI:
Reference:
To appear in: Polymer
Received Date: 20 April 2015
Revised Date: 11 June 2015
Accepted Date: 12 June 2015
Please cite this article as: Borjigin H, Stevens KA, Liu R, Moon JD, Shaver AT, Swinnea S, Freeman
BD, Riffle JS, McGrath JE, Synthesis and Characterization of Polybenzimidazoles derived from
Tetraaminodiphenylsulfone for High Temperature Gas Separation Membranes, Polymer (2015), doi:
10.1016/j.polymer.2015.06.021.
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ACCEPTED MANUSCRIPT

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Synthesis and Characterization of Polybenzimidazoles derived from
Tetraaminodiphenylsulfone for High Temperature Gas Separation Membranes
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Hailun Borjigin,a Kevin A. Stevens,b Ran Liu,a Joshua D. Moon,b Andrew T. Shaver,a
Steve Swinnea, b Benny D. Freeman,b J. S. Riffle*a and James E. McGratha
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aMacromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061,
USA
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bDepartment of Chemical Engineering, Center for Energy and Environmental
Resources, The University of Texas at Austin, Austin, TX, 78758, USA
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*To whom correspondence should be addressed, judyriffle@aol.com
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Keywords: polybenzimidazole, gas separation, pre-combustion syngas separation
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Abstract
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A series of polybenzimidazoles containing sulfonyl groups were synthesized in
Eaton’s reagent for high temperature H2/CO2 separation membranes. The key
monomer, 3,3’,4,4’-tetraaminodiphenylsulfone, was prepared via a novel and
economical synthetic route starting from 4,4’-dichlorodiphenylsulfone. These
polybenzimidazoles with sulfonyl moieties had enhanced solubilities in dipolar
aprotic solvents relative to the commercial Celazole® that is prepared from
diaminobenzidine. Thermal gravimetric analysis showed that the materials were stable
at elevated temperatures with 5% weight loss values of at least 485 oC in either air or
N2. Glass transition temperatures of three polybenzimidazoles in this series were
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ascertained by dynamic mechanical analysis to be 438-480 C. These sulfonyl-
containing polybenzimidazoles exhibited excellent gas separation properties for
H2/CO2. Polymers from tetraaminodiphenylsulfone and either terephthalic or
isophthalic acid crossed Robeson’s upper bound for H2/CO2.
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Introduction
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Polymeric membranes for gas separation have become an important
technology for various industrial refinery processes. In contrast to traditional
separation technologies such as cryogenic distillation, pressure swing adsorption and
chemical absorption, membrane separations offer several advantages, including lower
energy consumption, lower capital investment and ease of operation. [1] Due to a
significant growth in interest over the last ~30 years, numerous polymers have been
developed as membranes for a variety of gas separations. [2] An inherent trade-off
relationship between permeability and gas selectivity based on empirical observations
of available gas transport data has been reported by Robeson, [3,4] and the theory
behind this phenomenon was described by Freeman. [5] Most of the available gas
transport data on polymeric membranes from research laboratories have been
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measured in the temperature range of 25-35 C. However, for many industrial
applications, the ideal operating temperature may vary significantly from ambient
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conditions. For example, a high operation temperature (150-300 C) is required to
improve the thermal efficiency for H2 separation from pre-combustion syngas in the
Integrated Gasification Combustion Cycle (IGCC) system for electricity production.
[6,7] These harsh conditions eliminate most polymer membranes from consideration
due to thermal instabilities that lead to degradation and loss of mechanical properties.
[8]
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Polybenzimidazoles (PBIs), initially developed by Marvel, are well known for
their outstanding thermal stability, often exhibiting glass transition temperatures
greater than 400oC as well as flame retardance and chemical stability. [9,10] Due to
these characteristics, they are promising candidates for gas separation membranes that
can be used at high temperatures. Membranes prepared from a commercial
polybenzimidazole, Celazole®, have been shown to have attractive gas transport
properties. [11,12] Celazole (sometimes referred to as m-PBI in the literature) [7,12]
is prepared from 3,3’-diaminobenzidine and isophthalic acid. However,
polybenzimidazoles based on the 3,3’-diaminobenzidine monomer have very limited
solubilities in common solvents due to their rigid rod structures and intermolecular
hydrogen bonding. [13] For instance, m-PBI is only partially soluble in
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dimethylacetamide and insoluble in other common solvents, and PBIs based on 3,3’-
diaminobenzidine and terephthalic acid are insoluble in common organic solvents.
[10] Structural modification of polymer backbones to include flexible linkages usually
increase the solubility of PBIs. [14] However, a reduction in rigidity causes a decrease
in the glass transition temperature, thus compromising the high temperature properties
of these glassy polymers. [15]
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This work describes the synthesis and characterization of the sulfonyl-
containing tetraamine monomer, 3,3’,4,4’-tetraaminodiphenylsulfone (TADPS), and
of a series of PBIs containing this monomer. This tetraamine monomer was prepared
via a novel synthetic route from the economical reagent 4,4’-dichlorodiphenylsulfone.
Due to the sulfonyl linkage between the two diaminophenyl groups,
tetraaminodiphenylsulfone has a bent structure compared to diaminobenzidine with a
linear structure. We hypothesized that the kinked structure introduced by the sulfonyl
linkages in the PBI backbones would reduce the chain packing efficiency and
therefore enhance the gas transport properties of the PBIs. Initial investigations of
pure gas transport properties are also discussed herein.
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Experimental
Materials
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4,4’-Dichlorodiphenylsulfone (DCDPS) was kindly provided by Solvay and
recrystallized from toluene before use. Isopropanol, acetic acid, ammonium hydroxide
solution (29%), nitric acid (69.3%) and sulfuric acid were purchased from Spectrum
Chemicals and used as received. Hydrazine hydrate, palladium on carbon, sodium
bicarbonate and 4,4’-oxybis(benzoic acid) were purchased from Sigma-Aldrich and
used as received. Dimethylsulfoxide (DMSO), dimethylacetamide (DMAc) and N-
methyl-2-pyrrolidone (NMP) were purchased from Fisher and used as received.
Eaton’s Reagent was purchased from Alfa Aesar. Celite was purchased from EMD
Chemicals. Isophthalic acid was provided by Amoco and recrystallized from
methanol before use. Terephthalic acid was provided by Eastman and recrystallized
from methanol before use.
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Synthesis of the 3,3’,4,4’-tetraaminodiphenylsulfone monomer (TADPS)
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Synthesis of 3,3’-dinitro-4,4’-dichlorodiphenylsulfone
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Excess 4,4’-dichlorodiphenylsulfone (100.5 mmol, 28.75 g) and 290 mL of
96% H2SO4 were added to a 500-mL three-necked flask equipped with a condenser,
mechanical stirrer and addition funnel. The reaction mixture was stirred at room
temperature until the 4,4’-dichlorodiphenylsulfone completely dissolved. Nitric acid
(69.3%) (201.0 mmol, 18.28 g) was added dropwise via the addition funnel. Upon
completion of addition, the solution was stirred for 6 h at room temperature. The final
heterogeneous solution containing a pale yellow precipitant was poured into 2 L of
deionized water, and NaHCO3 was added until the solution reached a pH of 7. Then
the crude product was filtered and dried in vacuo at 100 °C. The product was
recrystallized from acetic acid to obtain a 92% yield. 1H NMR (d6-DMSO): δ8.70 (d,
2H), δ 8.32 (d, 2H), δ 8.29 (dd, 2H), δ 8.05 (d, 2H).
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Synthesis of 3,3’-dinitro-4,4’-diaminodiphenylsulfone
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3,3’-Dinitro-4,4’-dichlorodiphenylsulfone (125.1 mmol, 21.80 g), NH4OH
(312.7 mmol, 43.6 mL) and 300 mL of DMSO were added into a 500-mL pressure
reactor equipped with heating coils and an overhead stirrer. The reactor was
pressurized to 60 psi with nitrogen then heated to 140 °C. After 16 h, the reaction
mixture was cooled and precipitated in deionized water. The yellow precipitant was
filtered, washed with copious amounts of water and dried in vacuo at 80 ℃. A 90%
yield of product was obtained. 1H NMR (d6-DMSO): δ8.42 (d, 2H), δ 8.04 (bs, 4H), δ
7.75 (dd, 2H), δ 7.10 (d, 2H).
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Synthesis of 3,3’,4,4’-tetraaminodiphenylsulfone.
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3,3’-Dinitro-4,4’-diaminodiphenylsulfone (30.9 mmol, 10.46 g), 1.05 g Pd/C
and 700 mL of isopropanol were added to a 1000-mL three-necked flask equipped
with a condenser, mechanical stirrer and addition funnel. The reaction mixture was
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heated in a thermocouple-regulated oil bath set at 100 C and stirred. Hydrazine
hydrate (10.2 mL, 216.4 mmol) was added dropwise through the addition funnel.
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After complete addition, the solution was stirred and refluxed for 12 h in the 100 C
oil bath. The reaction mixture was hot-filtered through Celite. The product started to
crystallize from the filtrate upon cooling. The light grey lustrous crystals were filtered
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and washed with water. The monomer product was dried in vacuo at 100 °C overnight
to afford a 62% yield. Melting point: 176 oC.
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Synthesis of tetraaminodiphenylsulfone-isophthalic acid polybenzimidazole
(TADPS-IPA)
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The TADPS-IPA polymer was synthesized by direct polycondensation of the
tetraamine and dicarboxylic acid in Eaton’s reagent (phosphorus pentoxide/
methanesulfonic acid, PPMA) which served as both a condensing agent and solvent.
This polymerization procedure was modified from previous literature. [9] TADPS
(11.6 mmol, 3.2200 g), IPA (11.6 mmol, 1.9220 g) and Eaton’s reagent (22 mL) were
added to a 100-mL three-necked flask equipped with a mechanical stirrer, nitrogen
inlet, and a condenser. A stirred, thermocouple-regulated oil bath was used to heat
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the reaction to 145 C. After refluxing for 24 h, the hot viscous solution was poured
into 1 L of a stirring saturated NaHCO3 solution to precipitate a highly fibrous solid.
The solid was filtered again on an aspirator and then boiled in 500 mL of deionized
water for 2 h (repeated 4 times with new DI water each time) to remove any residual
salts. The solid polymer was finally dried at 150oC in vacuo for 24 h. Yield was
95%.
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Synthesis of tetraaminodiphenylsulfone-terephthalic acid polybenzimidazole
(TADPS-TPA)
and
tetraaminodiphenylsulfone-oxybis(benzoic
acid)
polybenzimidazole (TADPS-OBA)
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The TADPS-TPA and TADPS-OBA polymers were synthesized in the exact
manner as the TADPS-IPA polymer, except TPA (11.6 mmol, 3.2200 g) or OBA
(11.6 mmol, 2.9854 g) were used instead of IPA. Yields were 96 and 95%
respectively.
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Structural Characterization
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1H NMR analysis was performed on a Varian Inova spectrometer operating at
400 MHz. All spectra were obtained from 15% (w/v) 1-mL solutions in DMSO-d6.
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Size exclusion chromatography (SEC) was conducted on the TADPS-IPA,
TADPS-TPA and TADPS-OBA polybenzimidazoles to measure molecular weight
distributions. The solvent was DMAc that was distilled from CaH2 and that contained
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dry LiCl (0.1 M). The column set consisted of 3 Agilent PLgel 10-µm Mixed B-LS
columns 300x7.5 mm (polystyrene/divinylbenzene) connected in series with a guard
column having the same stationary phase. The column set was maintained at 50 °C.
An isocratic pump (Agilent 1260 infinity, Agilent Technologies) with an online
degasser (Agilent 1260), autosampler and column oven was used for mobile phase
delivery and sample injection. A system of multiple detectors connected in series was
used for the analyses. A multi-angle laser light scattering (MALLS) detector (DAWN-
HELEOS II, Wyatt Technology Corp.), operating at a wavelength of 658 nm, a
viscometer detector (Viscostar, Wyatt Technology Corp.), and a refractive index
detector operating at a wavelength of 658 nm (Optilab T-rEX, Wyatt Technology
Corp.) provided online results. The system was corrected for interdetector delay and
band broadening. Data acquisition and analysis were conducted using Astra 6
software from Wyatt Technology Corp. Validation of the system was performed by
monitoring the molar mass of a known molecular weight polystyrene sample by light
scattering. The accepted variance of the 21,000 g/mole polystyrene standard was
defined as 2 standard deviations (11.5% for Mn and 9% for Mw) derived from a set of
34 runs.
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Membrane preparation
For each polymer, 0.5 g of polymer was weighed into a scintillation vial, 10
mL of DMAc was added, and the mixture was stirred until a homogeneous solution
was obtained. The solution was syringe-filtered through a 0.45 ꢀm PTFE filter into a
new vial. Each vial was sonicated for 30 min to remove dissolved gases. A 10x10
cm2 glass plate was cleaned with acetone and dried before use. The solution was cast
on the glass plate on a leveled casting surface in the vacuum oven and allowed to dry
under full vacuum at room temperature overnight. The temperature was then
increased to 60°C under full vacuum for 4 h. The temperature was increased to 100
°C for another 1 h under full vacuum. The film was removed from the glass plate
with the aid of water and treated in boiling water for 4 h to remove remaining solvent.
The following day the film was dried in the oven at 140 °C under full vacuum.
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X-ray diffraction
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Powder X-ray diffraction (PXRD) was performed using a Scintag X-1 theta-
theta diffractometer, with a Cu X-ray source and a Si(Li) solid state detector tuned to
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Cu Kα radiation of 1.54 Å wavelength to characterize the amorphous nature of the
PBIs in this study.
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Solubility
Solubilities of the PBIs were determined by stirring 0.5 grams of polymer
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powder in 10 mL of solvent for 24 h at room temperature or 100 C. The solvents
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were NMP, DMAc, DMSO and THF.
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Water uptake
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The membrane water uptake was determined by the weight difference between
dry and wet membranes. Membranes (~0.2 grams) that had been vacuum-dried at 120
oC for 24 h were weighed (Wdry) and then immersed in deionized water at room
temperature for 24 h. The wet membrane was blotted dry and immediately weighed
again (Wwet). The water uptake of the membranes was calculated according to
Equation 1. The water uptake measurements of the membranes were carried out in
triplicate independently with different pieces of membranes to check the
reproducibility of the results.
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ꢂ
ꢆꢂ
ꢇꢈꢉ
ꢃꢄꢅ
ꢀ ꢁ
(1) Water Uptake % =
17
× 100
ꢂ
ꢃꢄꢅ
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Thermal analysis
The TADPS-based PBIs were characterized by thermogravimetric analysis
(TGA) and dynamic mechanical analysis (DMA). TGA scans were conducted using a
TA Instruments Q5000 thermogravimetric analyzer under nitrogen and air
atmospheres. A heating rate of 10oC min-1 was employed from 25 to 700 oC. Dynamic
mechanical analysis was performed using a TA Instruments Q800 configured in
tensile geometry. Storage modulus (E’) and tan δ were measured in a temperature
sweep mode (1 Hz, 2 °C min-1) at temperatures ranging from 150 to 550 °C under a
N2 atmosphere.
Gas transport
Pure gas permeabilities of H2, He, O2 and CO2 (UHP grade, Airgas, Radnor,
Pennsylvania, USA) through the TADPS-based PBIs were measured via a constant-
volume, variable-pressure method. [16] The upstream pressure was measured by a
Honeywell Super TJE transducer (Honeywell Sensotec, Columbus, Ohio, USA) with
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a 1500 psig range. The downstream pressure was maintained under vacuum and
measured by an MKS Baratron 626B (MKS Instruments, Andover, Massachusetts,
USA). Coupons of each film were masked to a metal disk with a pre-machined hole
using Master Bond EP46HT-2 epoxy (Master Bond Inc., Hackensack, New Jersey,
USA), and the exposed film area was measured. Prepared samples were stored in a
desiccator prior to placement in the pressure cell to reduce exposure to moisture.
Mounted membrane samples were placed in a 47-mm high-pressure filter holder
(Millipore, Billerica, Massachusetts, USA) and degassed at 35 °C overnight. The
downstream pressure rise was measured over a range of upstream pressures, and the
calculated permeabilities are reported herein at 10 atm and 35 °C.
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Results and Discussion
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Synthesis and characterization of the 3,3’,4,4’-tetraaminodiphenylsulfone
monomer (TADPS)
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A synthesis procedure for the TADPS monomer has been previously reported
as a four-step method starting from 4,4’-diaminodiphenylsulfone (DDS). [17] We
developed a three-step synthetic route to make polymer-grade TADPS monomer
starting with dichlorodiphenylsulfone (DCDPS), which is a widely used monomer for
polysulfone synthesis (Figure 1).
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Figure 1. Two possible routes for TADPS synthesis
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TADPS is the key monomer in these PBIs. The SO2 linkage was introduced to
provide a bend in the structure to increase solubility of the PBIs (Figure 2). In the
previously-reported synthetic route, protection and deprotection of the amino groups
from DDS were required before and after nitration. [17] As a result, synthesis of
TADPS via this method consisted of more synthetic steps, which contributed to
inefficiency and increased cost. In this study, nitration of DCDPS was conducted first
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to activate the sites with the chlorine substituents toward amination by nucleophilic
substitution with ammonium hydroxide. Finally TADPS was achieved by reducing
the nitro groups to amines. This method also produced an overall yield of 57% which
is higher than the reported yield (34%) of TADPS derived from DDS. [17]
180°
106°
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Figure 2. 3,3’-Diaminobenzidine (left) vs TADPS (right) molecules
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The H NMR spectrum of TADPS is shown in Figure 3. All of the peaks
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integrate appropriately to confirm the molecular structure. The H NMR spectrum
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was also free of “extra” peaks that would correspond to organic side products or
contaminants. The melting point of the recrystallized product was in good agreement
with the reported value. [17]
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Figure 3. 1H-NMR spectrum of the TADPS monomer
Synthesis and characterization of polybenzimidazoles derived from TADPS
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The TADPS-based PBIs in this study were synthesized by direct
polycondensation using Eaton's reagent as both a solvent and condensing agent
(Figure 4). [9] PBI synthesis in acid solution was originally carried out in
poly(phosphoric acid) (PPA), which is a beneficial method for preparing acid-doped
PBI fuel cell membranes. [18] However, using Eaton’s reagent as a reaction solvent
has some obvious advantages for general PBI synthesis. First, Eaton’s reagent is much
less viscous than PPA and thus it is easier to handle. Secondly, the method conducted
in Eaton’s reagent can be accomplished at a lower temperature (135-145 °C) than that
required in PPA (180-200 °C). Finally, Eaton’s reagent is easier to remove than PPA
once the polymerization is completed. [19]
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Figure 4. Synthesis of polybenzimidazole using Eaton’s reagent as a solvent
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After polymer isolation and drying, the TADPS-based PBIs were
characterized by NMR spectroscopy. The 1H NMR spectra shown in Figure 5 confirm
the expected polymer structures and show that the solvent was effectively removed.
The integral values were consistent with the expected chemical structures and no
extraneous peaks were observed in the spectra.
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The TADPS-based PBIs were sufficiently high molecular weight to form
transparent and ductile films. SEC of the PBIs quantitatively substantiated the
molecular weights (Figure 6 and Table 1). The chromatograms of the polymers
showed a monomodal Gaussian distribution with reasonable polydispersities. It is
noted that the polydispersities in Table 1 are somewhat lower than the value of two
that is expected for polymers prepared by polycondensation. The reported molecular
weights were measured by static light scattering in the SEC which directly measures
weight average molecular weight. Thus, the Mw values may be more accurate than the
Mn values that were calculated from the software.
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Figure 5. 1H-NMR spectra of TADPS-based polybenzimidazoles
Table 1. Molecular weights of TADPS-based polybenzimidazoles by SEC
Mn (kDa)
Mw (kDa)
PDI
1.5
1.6
1.8
dn/dc (mL/g)
0.31
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TADPS-TPA
TADPS-IPA
TADPS-OBA
0.28
0.30
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Figure 6. Light scattering SEC chromatograms of TADPS-based polybenzimidazoles
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X-ray diffraction
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XRD was used to probe whether any regions of crystallinity existed in these
sulfonyl-containing PBI membranes. As shown in Figure 7, the broad amorphous
halos observed for all three of the polymers confirmed that they were completely
amorphous. This can be desirable for gas separation membranes since crystalline
domains reduce both gas diffusivity and solubility resulting in a reduction in
permeability. [20]
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Figure 7. XRD of the TADPS-based polybenzimidazoles
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Solubility and water uptake
The low solubility of PBIs often limits their processibility by solvent-casting
to form thin membranes. The solubilities in various solvents and water uptake of these
PBIs are listed in Table 2. With introduction of sulfonyl linkages into the PBI chains,
their solubilities were significantly enhanced compared to poly-[2,2’-(m-phenylene)-
5,5’-bisbenzimidazole], the commercial polybenzimidazole (Celazole , m-PBI). All
three of the TADPS-based PBIs were completely soluble in common dipolar aprotic
solvents at a 5.0 wt% polymer concentration, which is in the range of concentrations
for solution processing to form thin membranes. As expected, all of these polymers
were still insoluble in common organic solvents such as THF and methanol.
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PBIs, in general, have high water uptake due to the hydrophilicity of the
imidazole ring. [21] All of these sulfonyl-containing PBIs exhibited high
hydrophilicity, with TADPS-TPA showing a water uptake of 25 wt%.
Table 2. Solubility in common solvents at 25 °C and water uptake of
polybenzimidazoles
NMP
++
+
DMAc
++
DMSO
++
THF Water Uptake
-
-
-
-
18%
25%
12%
15%
TADPS-IPA
TADPS-TPA
TADPS-OBA
m-PBI*
++
++
++
+
++
++
+
++
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Soluble at room temperature (++), partially soluble at room temperature and fully soluble at
refluxing temperature (+), and insoluble (-).
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*The data for m-PBI (Celazole ) is from previous literature. [22,21]
Thermal gravimetric analysis
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PBIs are renowned for their excellent thermal stability. [10,11] Introduction of
the SO2 linkage into the polymer chain resulted in polymers that did not show any
weight loss before 400°C in either air or N2 (Figure 8a-b). The 5% weight loss
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temperatures are listed in Table 3. It was observed that all of the TADPS-based PBIs
exhibited lower decomposition temperatures than m-PBI, and this was likely due to
the introduction of the relatively less stable sulfonyl groups. Overall, the high thermal
stabilities of all three TADPS-based PBIs make them potential candidates for high
temperature membrane separations.
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Table 3. Thermal properties of polybenzimidazoles
5% weight loss in
N2
5% weight loss in
Air
Tg
428 °C
480 °C
447 °C
417 °C
485 °C
499 °C
503 °C
576 °C
510 °C
525 °C
532 °C
-
TADPS-OBA
TADPS-TPA
TADPS-IPA
m-PBI *
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* The reported data for m-PBI (Celazole ) is from previous literature [23]
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b
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Figure 8. Thermal gravimetric analysis of sulfone-containing polybenzimidazoles in
(a) N2 and (b) air
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Dynamic mechanical analysis
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DMA results for the series of TADPS-based PBIs under N2 are shown in
Figure 9a-b. These samples were treated with boiling water for 4 hours and then dried
in vacuo at 180 °C for at least 24 hours before DMA testing. Two motional relaxation
processes were observed with increasing temperature (designated β and α), consistent
with results reported for other PBIs such as m-PBI. [23] Here, the β processes
correspond to localized sub-glass transition motions of limited range, while the α
process corresponds to the glass-rubber relaxation.
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Storage moduli (Figure 9b) reveal that all three of these sulfonyl-containing
PBIs maintain a stable plateau up to at least 230 °C. Up to this temperature, TADPS-
TPA, TADPS-IPA and TADPS-OBA maintain a storage modulus of 3.5, 2.6 and 2.1
GPa, respectively. Above that temperature, the storage moduli of these PBIs started to
decrease slowly as the temperature was increased up to 430, 397 and 371 °C
respectively. The initial reductions in storage moduli corresponded to the beta
relaxations of these polymers. [23] Afterwards, a large drop in storage modulus was
observed for all three of the polymers which corresponds to the glass transition. The
storage modulus curves indicated that the TADPS-based PBIs are high-temperature
amorphous polymers that maintain their structural stiffness up to 430, 397 and 371 °C
respectively. For all three of these materials, at temperatures beyond 496, 481 and 456
°C respectively (very close to the 5% weight loss values in N2 from TGA), a
stiffening occurs that is likely associated with degradation by in-situ crosslinking.
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In comparison with m-PBI, the three TADPS-based PBIs had higher glass
transition temperatures, which is likely due to the enhanced rigidity imparted by the
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double bond feature of the C-S linkage. [24] Similar glass transition temperature
enhancements in polymers have been observed with comparisons between
poly(arylene ether) and poly(arylene ether sulfone). [8] The TADPS-OBA PBI had a
lower glass transition temperature than the other two polymers. This can be attributed
to the flexible ether linkages in the TADPS-OBA PBI that reduces the rigidity of the
polymer chain.
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Figure 9. (a) Dynamic mechanical analysis (tan δ vs temperature) of TADPS-based
polybenzimidazoles under N2, and (b) dynamic mechanical analysis (storage modulus
vs temperature) under N2
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Gas transport properties
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Gas permeation properties of four gases (H2, He, O2 and CO2) through the
three sulfonyl-containing PBIs were measured at 35 °C by single gas permeation
experiments. Tables 4-5 show the permeabilities and ideal selectivities of these
materials. The permeability coefficients for TADPS-OBA were higher than those of
TADPS-IPA and TADPS-TPA and increased from TADPS-IPA to TADPS-TPA to
TADPS-OBA. For example, permeability of CO2 was 0.11 Barrer for TADPS-IPA,
0.28 Barrer for TADPS-IPA, and 0.56 Barrer for TADPS-OBA. The ether linkages in
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TADPS-OBA introduce additional kinks in the polymer backbone that disrupt chain
packing, leading to higher gas permeability coefficients relative to those for the
TADPS-TPA and TADPS-IPA polymers. This same phenomenon has also been
observed for aromatic polyimides. [25,26] Permeability measurements of slower
gases (N2, CH4) were attempted, but the estimated permeabilities were below the
detection limit of the instrument. Further work with thinner films will be necessary to
measure accurate permeabilities of these species at 35 °C and will be pursued as part
of a future study.
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Table 4. Permeabilities for TADPS-based polybenzimidazoles tested at 35 °C and
10 atm
Pure Gas Permeabilities (Barrer)
Samples
H2
He
O2
CO2
Thickness (ꢀm)
5.7 ± 0.2
5.5 ± 0.3
3.6 ± 0.3
6.7 ± 0.2 0.17 ± 0.01 0.56 ± 0.02
6.7 ± 0.4 0.09 ± 0.01 0.28 ± 0.02
5.1 ± 0.4 0.05 ± 0.04 0.11 ± 0.01
TADPS-OBA
TADPS-TPA
TADPS-IPA
19.4 ± 0.7
19.5 ± 1.1
21.5 ± 1.7
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The TADPS-IPA and TADPS-TPA PBIs are meta- and para-linked isomers.
Generally, para-linked linear aromatic polymers pack less efficiently and have more
segmental mobility than meta-oriented aromatic polymers. [27] Thus, higher
fractional free volumes and higher permeabilities for para-oriented aromatic polymers
are often observed. [28,29] TADPS-based PBIs follow this meta/para isomer effect.
[27,28] Furthermore, whereas the permeabilities increase from TADPS-IPA to
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TADPS-TPA to TADPS-OBA, the selectivities decrease as expected. For TADPS-
OBA, the H2/CO2 selectivity was 10.1, which increased to 19.5 for TADPS-TPA and
32.2 for TADPS-IPA. A high H2/CO2 selectivity coupled with a H2 permeability only
moderately lower than those of TADPS-TPA and TADPS-OBA causes TADPS-IPA
to cross both the prior H2/CO2 upper bound (initially reported in 1994) [4] and the
present upper bound (revised in 2008). [30]
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Table 5. Ideal selectivities for TADPS-based polybenzimidazoles tested at 35 °C
and 10 atm
Ideal Gas Selectivity
Samples
He/H2
H2/O2
He/O2
H2/CO2
CO2/O2
1.2
1.2
1.4
33
65
67
38
79
94
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3.2
3.3
2.1
TADPS-OBA
TADPS-TPA
TADPS-IPA
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As discussed previously, H2/CO2 separation was the focus for these PBIs. In
Figure 10, the TADPS-based PBIs are plotted with the H2/CO2 upper bound to
compare with m-PBI and other PBI derivatives reported by the Benicewicz group at
near-ambient temperature (30-43 °C). [31] By introduction of fluorinated bulky
linkages from the diacid monomers into the PBI backbone, the PBI derivatives had
much higher H2 permeabilities than m-PBI. [31] However, the H2/CO2 selectivities of
these PBI derivatives were substantially lower than the m-PBI. As a result, the PBI
derivatives still fell below the prior upper bound. Adding sulfonyl rather than
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fluorinated linkages in the tetraamine monomer did not significantly improve H2
permeabilities compared with m-PBI. The TADPS-OBA polymer also shows a lower
H2/CO2 selectivity, but the selectivities of TADPS-TPA and TADPS-IPA were
enhanced.
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The upper bounds reported by Robeson were based on experimental results of
gas transport properties of existing polymers measured at ambient temperature (25-35
°C). Freeman later provided the fundamental theory of the upper bound which agreed
well with empirical observations. [5] Both Freeman et al. [5] and Robeson [3,32]
showed that the slope of the upper bound is related to the ratio of the penetrant
diameters and is unlikely to change as the state of the art develops. [5] As mentioned
by Robeson, [30] limited data are available at the low-permeability limit of the
H2/CO2 upper bound. A more complete structure-property study of PBIs could
contribute to this region of the upper bound plot.
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Figure 10. H2/CO2 upper bound plot comparison of TADPS-based
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polybenzimidazoles with other PBIs. The gas transport data is reported by Li et al.
[31] for m-PBI measured at 43 °C and other PBI derivatives were measured at
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different temperatures in the range of 30-41 °C.
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It would be desirable to operate H2/CO2 separations at elevated temperatures
for H2 production from pre-combustion syngas. [6,12] TADPS-based PBIs are
potential candidates for high temperature gas separation in part due to their thermal
stabilities. Permeability obeys an Arrhenius-van't Hoff relation with temperature. [20]
For H2/CO2 separation, Li et al. demonstrated that PBIs and their derivatives move
toward the upper right and cross the upper bound as temperature increases. [31]
TADPS-based PBIs show a similar behavior with temperature. The H2 permeability
and H2/CO2 selectivity of TADPS-IPA at 190 °C are shown in Figure 10. As
temperature increases, the permeabilities of both H2 and CO2 increase while
selectivity stays relatively constant. A full study and analysis of temperature-
dependent gas separation properties of TADPS-based PBIs will be the focus of a
subsequent publication.
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Conclusions
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3,3’,4,4’-Tetraaminodiphenylsulfone was synthesized via a novel route with
fewer overall steps and improved yield relative to previously-reported methods,
starting from an economical commercial monomer (dichlorodiphenylsulfone). A
series of high molecular weight PBIs based on the 3,3’,4,4’-
tetraaminodiphenylsulfone monomer were synthesized by solution polymerization in
Eaton's Reagent and their properties were compared to the commercial m-PBI
(Celazole®) and other PBI derivatives for H2/CO2 gas separation. The TADPS-based
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PBIs had increased glass transition temperatures and better organo-solubilities
compared to m-PBI. Better solubility in organic solvents may likely be a major
advantage since this may aid in solvent-casting thin, defect-free membranes. These
TADPS-based PBIs were fabricated into films by solution casting for gas transport
measurements at 35 °C. These TADPS-based PBIs exhibited good gas separation
properties for H2/CO2 separation. TADPS-IPA and TADPS-TPA demonstrated a
significant enhancement on the upper bound graph compared with m-PBI, and
TADPS-IPA crossed both the prior and present upper bounds. Based on their
attractive H2/CO2 transport properties, TADPS-based PBIs are promising candidates
for further study.
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Acknowledgements
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The authors gratefully acknowledge the support of the National Science
Foundation under grant numbers DMR-1126564 and AIR-1237857.
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