Enhancing the Gas Permeability of Tr?ger's Base Derived Polyimides of Intrinsic Microporosity

Article abstract of DOI:10.1021/acs.macromol.6b00351

A series of four novel Tr?ger's base (TB) derived polyimides of intrinsic microporosity (PIM-TB-PI) is reported. The TB diamine monomer (4MTBDA) possesses four methyl groups in order to restrict rotation about the C-N imide bonds in the resulting polymers

Full text of DOI:10.1021/acs.macromol.6b00351

Article
pubs.acs.org/Macromolecules
̈
Enhancing the Gas Permeability of Troger’s Base Derived Polyimides
of Intrinsic Microporosity
Michael Lee,† C. Grazia Bezzu,† Mariolino Carta,† Paola Bernardo,‡ Gabriele Clarizia,‡
Johannes C. Jansen,‡ and Neil B. McKeown*,†
†EaStCHEM, School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh, Scotland EH9
3FJ, U.K.
‡Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, Via P. Bucci 17/C, 87036 Rende (CS), Italy
S
* Supporting Information
ABSTRACT: A series of four novel Troger’s base (TB) derived polyimides
̈
of intrinsic microporosity (PIM−TB−PI) is reported. The TB diamine
monomer (4MTBDA) possesses four methyl groups in order to restrict
rotation about the C−N imide bonds in the resulting polymers. The
polymers possess apparent BET (Brunauer, Emmett, and Teller) surface
areas between 584 and 739 m2 g−1, complete solubility in chloroform,
excellent molecular mass, high inherent viscosity and good film-forming
properties. Gas permeability measurements demonstrate enhanced perform-
ance over previously reported polyimide-based Troger’s base (TB) polymers confirming the benefit of the additional methyl
̈
groups within the TB diamine monomer. Notably, a polyimide derived from 4MTBDA and pyromellitic anhydride (PMDA)
demonstrates gas permeability data above the 2008 upper bounds for important gas pairs such as O2/N2, H2/N2, and H2/CH4.
cost of low permeability.9 However, this obstacle has been
INTRODUCTION
■
overcome by application of the concept of intrinsic micro-
porosity to their structural design to give PIM−PIs with high
gas permeabilities.10,11 More recently, a PIM−PI (PIM−EA−
PI) derived from a dianhydride based on the bridged bicyclic
ethanoanthracene demonstrated enhanced gas separation
performance.12 Notable recent work by Pinnau et al. has also
shown similar performance using a novel spirobifluorene-based
dianhydride (SBFDA).13 In addition, exceptional performance
was demonstrated from polyimides (e.g., KAUST-PI-1) derived
from a 9,10-di-iso-propyltriptycene-based dianhydride, with
data that lie well above the 2008 Robeson upper bounds for
several important gas pairs.7a,14
PIM−PI development has focused predominantly on the
synthesis of suitable dianhydride monomers, whereas the
preparation of novel diamines has received less attention.
One attractive component for elaboration is the rigid V-shaped
bridged bicyclic unit with the formal name of 2,8-dimethyl-
6H,12H-5,11-methanodibenzo[b,f ][1,5]diazocine but more
Polymer-based gas separation membranes are of increasing
industrial importance. Presently, membranes fabricated from a
range of different polymers, including polyimides, are utilized in
applications such as O2 or N2 enrichment from air, natural gas
purification and hydrogen recovery.1 A desirable polymer for
such applications requires both high permeability and high
selectivity. However, for each gas pair to be separated (X and
Y) there is a well-established inverse relationship between
permeability (Px) and selectivity (α = Px/Py), which was defined
by Robeson2 and then rationalized by Freeman’s theoretical
analysis.3 Hence, there is a challenge to design polymers with
permeability behavior that surpasses the current Robeson’s
upper bounds for important gas pairs, based on the state of the
art performance of polymers in 2008, and therefore have
potential as membrane materials. Higher gas permeability can
be achieved by increasing the interchain separation and hence,
the polymer’s free volume, whereas selectivity can be improved
by enhancing polymer rigidity.3 In 2004, a new class of
materials termed polymers of intrinsic microporosity (PIMs)
was introduced.4 PIMs possess high free volume due to their
contorted and rigid structures and provide not only excellent
gas permeability but also moderate selectivities. Recently
developed PIMs, in particular polymers fabricated from highly
rigid bridged bicyclic units, such as triptycene, have
demonstrated a substantial improvement in gas transport
performance.5−7
commonly known as Troger’s base (TB). In recent years TB
̈
has been incorporated within PIMs to provide enhanced
performance.5,6,15 Wang et al. incorporated the TB structure
into the polyimides (e.g., TBAD2−6FDA; Figure 1) using TB-
based diamine monomers.16 It was found that by using a TB
component with a single methyl group ortho to the imide
linking group produced polyimides (e.g., TBDA2−6FDA;16b
Figure 1) with significant microporosity and high gas
Polyimides (PIs) are an important class of polymers for
membrane-based gas separations.8 Known for their good
thermal and chemical stabilities, and excellent mechanical
properties, PIs often demonstrate high gas selectivity but at the
Received: February 18, 2016
Revised: May 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.6b00351
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N-(2,6-Dimethylphenyl)-4-methylbenzenesulfonamide (M1). 2,6-
Dimethylaniline (12.00 g, 99.0 mmol) and p-toluenesulfonyl chloride
(20.77 g, 108.9 mmol) were dissolved/suspended in pyridine (75 mL)
and stirred for 16 h. The reaction mixture was poured while stirring
into 2 M hydrochloric acid solution (250 mL), filtered, and washed
thoroughly with water. The resulting solid was triturated in hot n-
hexane to yield N-(2,6-dimethylphenyl)-4-methylbenzenesulfonamide
(15.83 g, 58%) as a white powder; mp 131−133 °C (lit. 130 °C).18
νmax (CHCl3/cm−1): 3269, 2925, 1597, 1474, 1379, 1326, 1158, 900,
673. 1H NMR (500 MHz, CDCl3): δ 7.64 (d, 2H, J = 8.1 Hz, TsAr H),
7.27 (d, 2H, J = 8.1 Hz, TsAr H), 7.11 (t, 1H, Ar H), 7.03 (d, 2H, Ar
H), 6.55 (br s, 1H, NH), 2.45 (s, 3H, TsCH3), 2.08 (s, 6H, CH3). 13
C
NMR (126 MHz, CDCl3): δ 143.4, 137.6, 132.4, 129.4, 128.6, 127.6,
127.0, 21.4, 18.6; LRMS (EI, m/z): calculated, 275.1; found, 275.1
[M+].
Figure 1. Structures of polyimides TBDA2−6FDA,16b PI−TB-1,17 and
4MTBDA−6FDA.
N-(2,6-Dimethyl-4-nitrophenyl)-4-methylbenzenesulfonamide
(M2). N-(2,6-Dimethylphenyl)-4-methylbenzenesulfonamide (3.00 g,
10.9 mmol) was suspended in a 50:50 mixture of acetic acid and water
(120 mL) and a catalytic amount of sodium nitrite was added.
Concentrated nitric acid (16 mL) was carefully added and the reaction
was heated at 140 °C for 3h. The reaction mixture was cooled to room
temperature and quenched over ice. The resulting off white solid was
filtered and washed repeatedly with water until the washings were
neutral. After drying, N-(2,6-dimethyl-4-nitrophenyl)-4-methylbenze-
nesulfonamide was isolated (2.61 g, 75%) as a white solid; mp 163−
166 °C (lit. 165−167 °C).19 νmax (CHCl3/cm−1): 3268, 1512, 1334,
permeability. In an alternative approach to give similar
polyimides (e.g., PI−TB-1; Figure 1), Guiver et al.17 used a
polymerization reaction involving the formation of the TB unit
as the polymerization reaction.
It was anticipated that additional reduction of conformational
freedom about the imide linkages, by the introduction of a
second methyl substituent adjacent to the two amino groups on
the TB monomer, would further enhance gas separation
performance. Here we report the efficient synthesis of the TB
monomer (4MTBDA) with the desired methyl substitution and
the gas permeability data for a series of PIM−PIs prepared from
this novel monomer.
1
1158, 675. H NMR (500 MHz, CDCl3): δ 7.88 (s, 2H, Ar H), 7.60
(d, 2H, J = 8.2 Hz, TsAr H), 7.28 (d, 2H, J = 8.2 Hz, TsAr H), 6.47 (br
s, 1H, NH), 2.44 (s, 3H, TsCH3), 2.15 (s, 6H, CH3);. 13C NMR (126
MHz, CDCl3): δ 146.3, 144.6, 139.4, 138.8, 137.3, 130.1, 127.2, 123.7,
21.8, 19.3. LRMS (EI, m/z): calculated, 320.1; found, 320.0 [M+].
N-(4-Amino-2,6-dimethylphenyl)-4-methylbenzenesulfonamide
(M3). Under a nitrogen atmosphere, N-(2,6-dimethyl-4-nitrophenyl)-
4-methylbenzenesulfonamide (7.10 g, 22.2 mmol) was dissolved in dry
tetrahydrofuran (150 mL). Raney nickel (∼40 mg) and hydrazine
monohydrate (5.55 g, 110.8 mmol, 5 equiv) was added, and the
mixture was refluxed for 24 h. The colorless mixture was cooled in ice
and filtered under nitrogen. The organic phase was extracted with
diethyl ether and the solvent was removed under vacuum at 25 °C to
afford the desired product N-(4-amino-2,6-dimethylphenyl)-4-methyl-
benzenesulfonamide as colorless crystals (5.62 g, 87%); mp 201−204
°C. νmax (CHCl3/cm−1): 3378, 3269, 2924, 1598, 1323, 1158. 1H
NMR (500 MHz, (CD3)2CO): δ 7.64 (br s, 1H, NH), 7.59 (d, 2H, J =
8.0 Hz, TsAr H), 7.34 (d, 2H, J = 8.0 Hz, TsAr H), 6.26 (s, 2H, Ar H),
4.49 (br s, 2H, NH2), 2.42 (s, 3H, TsCH3), 1.88 (s, 6H, CH3).13C
NMR (126 MHz, (CD3)2CO): δ 148.5, 143.9, 140.7, 139.7, 130.4,
128.1, 123.7, 115.1, 21.6, 19.2. HRMS (EI+, m/z): calculated,
290.1089; found, 290.1088 [M+].
EXPERIMENTAL SECTION
■
General Methods and Equipment. The synthesis of monomer
SBFDA is given in the Supporting Information. Commercially
available reagents were used without further purification. Anhydrous
dichloromethane was obtained by distillation over calcium hydride
under nitrogen atmosphere. Anhydrous N,N-dimethylformamide was
bought from Aldrich. All reactions using air/moisture sensitive
reagents were performed in oven-dried or flame-dried apparatus,
under a nitrogen atmosphere. TLC analysis refers to analytical thin
layer chromatography, using aluminum-backed plates coated with
Merck Kieselgel 60 GF254. Product spots were viewed either by the
quenching of UV fluorescence, or by staining with a solution of cerium
sulfate in aqueous H2SO4. Flash chromatography was performed on
silica gel 60A (35−70 μm) chromatography grade (Fisher Scientific).
Microwaved irradiation synthesis was performed with a CEM Discover
microwave reactor. Melting points were recorded using a Gallenkamp
1
Melting Point Apparatus and are uncorrected. H NMR spectra were
recorded in the solvent stated using an Avance Bruker DPX 400 (400
MHz) or DPX 500 (500 MHz) instruments, with 13C NMR spectra
recorded at 100 or 125 MHz, respectively. Low-resolution mass
spectrometric data were determined using a Fisons VG Platform II
quadrupole instrument using electron impact ionization (EI) unless
otherwise stated. High-resolution mass spectrometric data were
obtained in electron impact ionization (EI) mode unless otherwise
reported, on a Waters Q-TOF micromass spectrometer. Low-
temperature (77 K) N2 adsorption/desorption measurements of
methanol treated powders were made using a Coulter SA3100. CO2
sorption measurements (273 K) were made using a Quadrasorb Evo.
Samples were degassed for 800 min at 120 °C under high vacuum
prior to analysis. The TGA was performed using the device Thermal
Analysis SDT Q600 at a heating rate of 10 °C/min from room
temperature to 1000 °C. Viscosity measurement were carried out with
an Ubbelhode capillary (Nr 532 00/0, Schott Germany) on a
SCHOTT Instruments GmbH: AVS 370, maintaining 25 °C, using
CHCl3 as solvent with 0.0013 g cm−3 concentration. Gel Permeation
Chromatography was carried out using a Viscotek GPC Max1000
system which includes a refractive index detector and two 2 columns
(KF-805L Shodex). The analysis used dilute solution of polymer in
chloroform (1 mL min−1).
2,8-Di(4-methylbenzenesulfonamido)-1,3,7,9-tetramethyl-
6H,12H-5,11-methanodibenzo[b,f ][1,5]diazocine (M4). N-(4-
Amino-2,6-dimethylphenyl)-4-methylbenzenesulfonamide (5.60 g,
19.3 mmol) was dissolved in trifluoroacetic acid (50 mL) and cooled
in an ice bath. Dimethoxymethane (3.8 mL, 42.4 mmol, 2.2 equiv) was
added dropwise, and the reaction was stirred for 16 h. The mixture was
poured slowly into an aqueous ammonia solution (200 mL) and
stirred thoroughly. The precipitate was extracted with chloroform, and
the solvent was removed under vacuum. After reprecipitation from
chloroform with n-hexane, the desired product of 2,8-di(4-methyl-
benzenesulfonamido)-1,3,7,9-tetramethyl-6H,12H-5,11-
methanodibenzo[b,f ][1,5]diazocine was obtained (5.81 g, 98%) as a
white solid; mp 184−188 °C. νmax (CHCl3/cm−1): 3267, 2923, 1598,
1471, 1321, 1158, 1092. 1H NMR (500 MHz, CDCl3): δ 7.54 (d, 4H J
= 8.2 Hz, TsAr H), 7.21 (d, 4H, J = 8.0 Hz, TsAr H), 6.76 (s, 2H, Ar
H), 6.21 (s, 2H, NH), 4.41 (d, 2H, J = 16.7 Hz, CH2), 4.15 (s, 2H,
CH2), 3.99 (d, 2H J = 16.7 Hz, CH2), 2.41 (s, 6H, TsCH3), 1.88 (s,
6H, CH3), 1.82 (s, 6H, CH3). 13C NMR (126 MHz, CDCl3): δ 147.4,
143.5, 137.7, 136.3, 136.0, 129.5, 128.2, 127.0, 124.6, 124.5, 65.3, 57.4,
21.5, 18.4, 14.0. HRMS (ES−, m/z): calculated, 616.2178; found,
615.2108 [M − H+]−.
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2,8-Diamino-1,3,7,9-tetramethyl-6H,12H-5,11-methanodibenzo-
[b,f ][1,5]diazocine (4MTBDA). 2,8-Di(4-methylbenzenesulfonami-
do)-1,3,7,9-tetramethyl-6H,12H-5,11-methanodibenzo[b,f ][1,5]-
diazocine (3.0 g, 9.4 mmol) was dissolved in concentrated sulfuric acid
(15 mL) and heated at 50 °C for 3 h. The reaction mixture was poured
slowly into ice water, and ammonia solution was added dropwise until
a value of pH 8 was observed. The precipitate was extracted with
dichloromethane, and the solvent was removed under reduced
pressure. The resulting solid was purified via reprecipitation from
dichloromethane with n-hexane to yield 2,8-diamino-1,3,7,9-tetrameth-
yl-6H,12H-5,11-methanodibenzo[b,f ][1,5]diazocine (4MTBDA, 1.13
g, 75%) as cream colored crystals; mp >235 °C (dec.). νmax (CHCl3/
4MTBDA−SBIDA. General procedure A was followed using 2,8-
diamino-1,3,7,9-tetramethyl-6H,12H-5,11-methanodibenzo[b,f ][1,5]-
diazocine (0.5925 g, 1.92 mmol), bisanhydride of 3,3,3′,3′-tetramethyl-
6,6′,7,7′-tetracarboxy-1,1′-spirobisindane (0.8000 g, 1.92 mmol),
ethanol (15 mL), triethylamine (0.97 g, 1.34 mL, 9.61 mmol), and
NMP:toluene (5 mL) to afford PIM−SBIDA−TB-PI (3) (0.95 g,
72%) as an off-white powder. νmax (cm−1): 2958, 2865, 1776, 1717,
1348, 733. 1H NMR (601 MHz, CDCl3): δ 7.79 (br s, 2H, Ar H), 7.35
(br s, 2H, Ar H), 7.00 (br s, 2H, Ar H), 5.01−3.63 (br m, 6H, CH2),
2.75−2.21 (br m, 4H, CH2), 2.24−1.68 (br m, 12H, CH3), 1.73−1.00
(br m, 12H, CH3). 13C NMR (151 MHz, CDCl3): δ 168.1, 167.7,
160.1, 157.1, 149.7, 132.4, 126.0, 125.5, 59.2, 58.4, 57.7, 44.8, 32.0,
1
cm−1): 3430, 3356, 2935, 2880, 2844, 1625, 1476, 1421. H NMR
30.2, 18.8, 18.6, 13.9, 13.7. GPC (chloroform): Mn = 31 700, Mw
=
70 700; BET surface area = 733 m2/g; total pore volume = 0.71 cm3/g
at (P/Po = 0.98). TGA analysis: Initial weight loss due to thermal
degradation commences at ∼488 °C.
(500 MHz, CDCl3): δ 6.80 (s, 2H, Ar H), 4.51 (d, 2H, J = 16.4 Hz,
CH2), 4.22 (s, 2H, CH2), 4.11 (d, 2H, J = 16.5 Hz, CH2), 3.37 (s, 4H,
NH2), 2.14 (s, 6H, CH3), 1.89 (s, 6H, CH3). 13C NMR (126 MHz,
CDCl3): δ 139.2, 124.3, 124.2, 121.7, 118.2, 66.2, 57.9, 17.8, 11.8.
HRMS (EI+, m/z): calculated, 308.2001; found, 308.2005 [M+].
General Procedure A. Polyimide Synthesis (via Ester-Acid
Precursor). Under a nitrogen atmosphere, the bis-anhydride was
dissolved in ethanol in a two-necked flask equipped with Dean−Stark
apparatus and reflux condenser. Triethylamine was injected and the
mixture was refluxed for 1 h. The side arm was opened to remove the
solvent under a stream of nitrogen to give a highly viscous solution.
The trap was filled with toluene before a solution of the diamine in
NMP:toluene (4:1 mixture) was added and the reaction was heated at
80 °C for 1 h. The reaction mixture was gradually heated to 200 °C
and maintained at this temperature until the desired viscosity was
achieved. The mixture was cooled to room temperature and diluted
with chloroform. The mixture was poured into ethanol to precipitate a
solid. The solid was collected by filtration, washed with ethanol until
the washings were clear. The resulting powder was dissolved in
chloroform and methanol was added dropwise until the solution
became turbid. The solution was stirred for a further 30 min to
precipitate a gel. The reprecipitation from chloroform was repeated
twice. The polymer was dissolved in chloroform and added dropwise
to n-hexane with vigorous stirring and the precipitated fine powder was
filtered. The powder was refluxed in methanol for 24 h, filtered and
then dried in a vacuum oven at 120 °C for 9 h to afford the desired
polymer.
4MTBDA−SBFDA. General procedure A was followed using 2,8-
diamino-1,3,7,9-tetramethyl-6H,12H-5,11-methanodibenzo[b,f ][1,5]-
diazocine (0.7500 g, 2.43 mmol), spirobifluorene-based dianhydride
(1.1099 g, 2.43 mmol), ethanol (15 mL), triethylamine (1.23 g, 1.70
mL, 12.16 mmol), and NMP:toluene (5 mL) to afford PIM−SBFDA−
TB-PI (5) (1.43 g, 81%) as a light brown powder. νmax (cm−1): 2953,
1776, 1719, 1374, 750. 1H NMR (500 MHz, CDCl3): δ 8.40 (br s, 2H,
Ar H), 8.00 (br s, 2H, Ar H), 7.53 (br s, 2H, Ar H), 7.36−7.16 (br m,
4H, Ar H), 7.12−6.70 (br m, 4H, Ar H), 4.99−3.66 (br m, 6H, CH2),
2.08 (br s, 6H, CH3), 1.88 (br s, 6H, CH3). 13C NMR (126 MHz,
CDCl3): δ 168.2, 167.8, 152.7, 149.7, 135.9, 134.8, 130.7, 125.9, 125.3,
117.1, 57.4, 44.6, 35.2, 19.2, 18.5, 13.6. GPC (chloroform): Mn
=
25 800, Mw = 57 600; BET surface area = 739 m2/g; total pore volume
= 0.54 cm3/g at (P/Po = 0.98). TGA analysis: Initial weight loss due to
thermal degradation commences at ∼486 °C.
Film Preparation and Gas Permeation Measurements. Films
were prepared by slow evaporation (in ambient conditions, typically
over 96 h) of a 2.5−3% (w/w) solution of the polymer in chloroform.
The reported data refer to membranes after a soaking in methanol.
This treatment was carried out to cancel the history of the sample and
to remove traces of the solvent used for preparation. Each membrane
sample was put in methanol and left there overnight. The successive
day, it was removed from the alcohol and put between two porous
glass disks and then left to dry under room conditions overnight. The
permeation tests were carried out the subsequent day. Single gas
permeation measurements were carried out in a fixed volume/pressure
increase apparatus at 25 °C using the time-lag method and at a feed
pressure of 1 bar, as described elsewhere.20 The gas diffusivity and
solubility data for each polymer was indirectly obtained assuming the
solution-diffusion model for the gas permeation (Table 2). Samples
were aged under ambient conditions.
4MTBDA−6FDA. General procedure A was followed using 2,8-
diamino-1,3,7,9-tetramethyl-6H,12H-5,11-methanodibenzo[b,f ][1,5]-
diazocine (0.5900 g, 1.91 mmol), 4,4′-(hexafluoroisopropylidene)-
diphthalic anhydride (0.8498 g, 1.91 mmol), ethanol (15 mL),
triethylamine (0.97 g, 1.33 mL, 9.56 mmol), and NMP:toluene (5 mL)
to afford PIM−6FDA−TB-PI (1) (1.17 g, 85%) as an off-white
1
powder. νmax (cm−1): 2955, 2927, 1785, 1727, 1343, 725. H NMR
(601 MHz, CDCl3): δ 8.26−7.73 (br m, 6H, Ar H), 7.03 (br s, 2H, Ar
H), 4.77−3.99 (br m, 6H, CH2), 2.13 (br s, 6H, CH3), 1.94 (br s, 6H,
CH3). 13C NMR (151 MHz, CDCl3): δ 167.0, 166.7, 150.0, 139.6,
136.4, 135.9, 134.8, 133.3, 132.9, 125.6, 124.7, 65.7, 57.7, 18.8, 13.9;
GPC (chloroform): Mn = 18 800, Mw = 54 000; BET surface area =
584 m2/g; total pore volume = 0.72 cm3/g at (P/Po = 0.98). TGA
analysis: Initial weight loss due to thermal degradation commences at
∼457 °C.
4MTBDA−PMDA. General procedure A was followed using 2,8-
diamino-1,3,7,9-tetramethyl-6H,12H-5,11-methanodibenzo[b,f ][1,5]-
diazocine (0.7000 g, 2.27 mmol), pyromellitic dianhydride (0.4951 g,
2.27 mmol), ethanol (15 mL), triethylamine (1.15 g, 1.58 mL, 11.35
mmol), and NMP:toluene (5 mL) to afford PIM−PMDA−TB-PI (2)
(0.855 g, 77%) as a light yellow powder. νmax (cm−1): 1778, 1721,
1373, 733. 1H NMR (500 MHz, CDCl3): δ 8.48 (br s, 2H, Ar H), 7.03
(br s, 2H, Ar H), 4.57 (br s, 2H, CH2), 4.22 (br s, 4H, CH2), 2.10 (br
s, 6H, CH3), 1.91 (br s, 6H, CH3). 13C NMR (126 MHz, CDCl3): δ
165.4, 165.1, 149.6, 137.3, 135.3, 134.2, 125.1, 124.9, 119.6, 57.3, 31.9,
29.7, 29.4, 22.7, 18.2, 14.1, 13.3. GPC (chloroform): Mn = 26 500, Mw
= 43 500; BET surface area = 651 m2/g; total pore volume = 0.57
cm3/g at (P/Po = 0.98). TGA analysis: Initial weight loss due to
thermal degradation commences at ∼461 °C.
RESULTS AND DISCUSSION
■
Synthesis. The synthetic route used to prepare monomer
4MTBDA, starting from commercially available 2,6-dimethyla-
niline, is presented in Scheme 1. Overall the route proved
efficient with the key TB-forming reaction to produce the tosyl-
protected monomer being achieved in near-quantitative yield.
Because of its greater stability, it proved convenient to store the
monomer in its protected state and perform the removal of the
tosyl groups immediately prior to polymerization.
A series of TB-based PIM−PIs (Figure 2) were prepared via
the cycloimidization reaction between 4MTBDA and four
dianhydride monomers: 4,4′-(hexafluoroisopropylidene) diph-
thalic anhydride (6FDA), pyromellitic dianhydride (PMDA),
spirobisindane-based dianhydride (SBIDA)11 and spirobifluor-
ene-based dianhydride (SBFDA).13
Physical Characterization. All four PIs proved readily
soluble in chloroform, which enabled characterization by NMR
spectroscopy and gel permeation chromatography (GPC), the
latter of which indicated molecular weights (Mw) for the four
C
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a
Nitrogen adsorption isotherms for the TB-based polyimides
were obtained at 77 K and each showed significant uptake at
low relative pressures (p/p0 < 0.05) indicative of intrinsic
microporosity (Supporting Information). Apparent BET sur-
face areas calculated from these data were in the range 584−
739 m2 g−1.
Scheme 1. Synthesis of Troger’s Base Diamine (4MTBDA)
̈
The polymers also display excellent thermal stability with
decomposition temperatures up to around 490 °C measured by
thermogravimetric analysis (TGA). A summary of the physical
properties of these PIM−PIs is shown in Table 1. Nitrogen
adsorption isotherms and gel permeation chromatography
(GPC) traces are presented in Figure S2. An enhancement of
microporosity is indicated by an apparent BET surface area of
584 m2 g−1 for 4MTBDA-6FDA as compared to 544 m2 g−1 for
PI−TB-117 and 325 m2 g−1 for TBDA2−6FDA16b (Figure 1),
demonstrating the effect due to the two methyl groups adjacent
to the C−N bond of the imide linking group restricting
rotation.
a
Key: (i) p-toluenesulfonyl chloride, pyridine; (ii) KNO3, NaNO2,
CH3COOH/H2O, 140 °C; (iii) NH2NH2·H2O, Raney-Ni, THF; (iv)
DMM, TFA; (v) H2SO4, 50 °C.
CO2 adsorption at 273 K was also performed on the
polymers, both to test the gas uptake (Figure 3a) and to
perform evaluation of micropore size distribution (PSD). All
polyimides demonstrate significant uptake of CO2 (2−3 mmol
g−1), in the order 4MTBDA−SBIDA ∼ 4MTBDA−PMDA >
4MTBDA−SBFDA > 4MTBDA−6FDA, showing the presence
́
of ultramicropores (<0.7 nm). Horvath−Kawazoe analysis
(Figure 3b) indicates that 4MTBDA−PMDA has a particularly
narrow and well-defined micropore structure, which is desirable
for gas separation (see below).
Gas Permeability. Films suitable for gas permeation
measurements were successfully prepared for all polyimides.
It is of interest to assess the relative gas permeability
performance of 4MTBPA−6FDA versus that of the isomeric
TB monomer-based PIs PI−TB-117 and similar TBDA2−
6FDA16b (Figure 1). In addition, very few diamines have been
used as monomers for the synthesis of PIM−PIs or high free
volume PIs with the commercially available 3,3′-dimethylnaph-
thidine (DMN), 2,3,5,6-tetramethyl-1,4-phenyldiamine
(4MPDA) and 2,3,6-trimethyl-1,3-diamine being employed
most often (Figure 2). Therefore, it is of value to compare the
performance of the four novel PIs derived from diamine
4MTBDA related to previously reported PIs based on the same
bisanhydride monomers but made using these three commer-
cial diamines (e.g., DMN−SBFDA, 4MPDA−6FDA, 3MPDA−
PMDA, 4MPDA−SBIDA (PIM−PI-9), and DMN−SBIDA
(PIM−PI-10). Direct comparisons of polymer gas perme-
abilities are made difficult due to the differences in film history
prior to measurement. For example, those films freshly treated
with methanol demonstrate higher permeabilities but lower
selectivities than films made from the same polymer that have
been annealed in vacuum to remove casting solvent. Therefore,
to assess the relative potential performance of polymers in the
separation of a given gas pair, it is best to compare the distance
of their data points relative to the relevant 2008 Robeson upper
bound (Figure 4). For example, to evaluate the molecular
Figure 2. Chemical structures of TB-based polyimides and three
aromatic diamines commonly used for PIM−PI synthesis.
polyimides in the range of (43−71) × 103 g mol−1. As a result,
robust films for each PI were cast from chloroform, which
allowed gas permeability measurements to be carried out.
Table 1. Physical Characterization of PIM−PI-TBs
BET surface
total pore volume
CO2 (cm3/g)
at 273 K/1 bar
Mw × 10−3
inherent viscosity
TGA decomposition
temperature (°C)
PIM−PI
area (m2 g−1
)
(cm3 g−1) at P/P0 = 0.98
(g mol−1
)
PDI
(cm3 g−1
)
4MTBDA−6FDA
4MTBDA−PMDA
4MTBDA−SBIDA
4MTBDA−SBFDA
584
650
733
739
0.72
0.57
0.71
0.54
49
61
65
59
59
43
71
58
2.25
1.79
2.25
2.23
87
53
86
64
457
461
488
486
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Figure 3. (a) CO2 adsorption isotherms for 4MTBDA-based polyimides measured at 273 K. (b) Estimated pore size distribution obtained from CO2
isotherms using the Horvath−Kawazoe (HK) method assuming carbon slit-pore geometry.
́
Figure 4. Robeson plots for different gas pairs showing the data for methanol treated PIM-TB-PIs. Color code: red = polymers prepared with 6FDA;
blue = SBIDA; light brown = PMDA; purple = SBFDA. Points with no fill but same color code refer to aged data (>330 days). Numbers correspond
to previously reported polymers whose data are shown in Table 2.
sieving properties of a polymer (i.e., diffusivity selectivity), its
position relative to the upper bound for O2/N2 is particularly
informative due to the similar solubility of these gases in
polymers and the small difference in size and mass. For
4MTBDA-6FDA, its much higher values of gas permeability
relative to other 6FDA-containing polymers (i.e., 4MPDA−
6FDA, TBAD2−6FDA-PI, and PI−TB-1) are likely due to
being obtained from a freshly methanol treated film, never-
theless, the position of its data point just below the 1991 upper
bound in the O2/N2 Robeson plot is consistent with those from
the other 6FDA based polymers. The relatively low
permeability of 4MTBDA−6FDA relative to the other
polyimides in this study is consistent with its lower BET
surface area and uptake of CO2 (Table 1 and Figure 3).
All O2/N2 data for the SBIDA-based PIs (i.e., 4MTBDA−
SBIDA, 4MPDA−SBIDA (PIM−PI-9), and DMN−SBIDA
(PIM−PI-10)) were obtained from methanol treated films. The
data points from each of these polymers lie between the 1991
and 2008 upper bounds but 4MTBDA−SBIDA lies closest to
the 2008 upper bound, thus indicating a slightly enhanced
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performance for 4MTBDA as a component diamine as
compared to 4MPDA or DMN. Similarly, the O2/N2 data
point for 4MTBDA−SBFDA lies closer to the upper bound
relative to that from the recently reported methanol treated film
of DMN−SBFDA, which also suggests that 4MTBDA provides
enhanced performance over DMN. Similar trends are observed
for the H2 containing gas pairs (i.e., H2/N2 and H2/CH4, Figure
4) both of which are important gas commercial gas separation
application.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b00351.
■
S
Full experimental details for the synthesis of precursors
and polymers (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail neil.mckeown@ed.ac.uk (N.B.M.).
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
Pyromellitic dianhydride (PMDA), despite its low cost, is
rarely used as a monomer for the preparation of PIs of interest
for polymer membranes. This is presumably due to the
insolubility of the PIs arising from the highly linear macro-
molecular structures obtained when combined with diamines
commonly used to make high free volume PIs (e.g., DMN and
4MPDA).21 However, Tanaka et al.22 reported the modest gas
permeabilities of 3MPDA−PMDA from a thermally annealed
film (Figure 4 and Table 2, entry 1). In contrast, the data from
a methanol treated film of 4MTBDA−PMDA showed high
permeability, particularly for the smallest hydrogen molecules
and is consistent with the narrow pore size distribution
centered at 0.5 nm obtained from CO2 adsorption data (Figure
3b). The high permeability and good selectivity of 4MTBDA-
PMDA places its data points over the 2008 upper bound for
O2/N2, H2/N2, and H2/CH4, joining a select few PIs to achieve
this feat (Figure 4 and Table 2, entry 9 and 11).7a,14,12 This
result illustrates the value of 4MTBDA as a component diamine
for PIs with potential for gas separation but also confirms the
recent suggestion23 that PMDA is also a useful precursor if its
diamine comonomer possesses a suitably nonlinear structure to
induce solubility. PMDA provides rigidity, which is the key
factor for enhancing selectivity in gas separations.3 The
combination of the rigid bridged bicyclic monomer 4MTBDA
and PMDA gives a PI of excellent potential as a membrane
material.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the EPSRC Industrial CASE award Scheme and
Defense Science and Technology Laboratory (Dstl), U.K., for
funding (M.L.) and Dr. Corinne Stone and Martin Smith for
useful discussions.
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■
A series of four polyimides based on a novel Troger’s base
̈
diamine monomer (4MTBDA) has been prepared. All TB-
based polyimides exhibited good solubility, excellent thermal
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