The spirobichroman-based polyimides with different side groups: From structure-property relationships to chain packing and gas transport performance

Article abstract of DOI:10.1039/d0ra10113c

Spirobichroman-based polymers with high gas permeability and selectivity are promising for their applications as membranes in gas separation. In this study, three spirobichroman-based polyimides (PIs; 6FDA-FH, 6FDA-DH, and 6FDA-MH) were synthesised by the

Full text of DOI:10.1039/d0ra10113c

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The spirobichroman-based polyimides with
different side groups: from structure–property
relationships to chain packing and gas transport
performance†
Cite this: RSC Adv., 2021, 11, 5086
a
a
a
a
a
Shuli Wang, Xiaohua Tong, Chunbo Wang, Xiaocui Han, Sizhuo Jin,
a
b
a
Daming Wang, * Jianan Yao* and Chunhai Chen
Spirobichroman-based polymers with high gas permeability and selectivity are promising for their
applications as membranes in gas separation. In this study, three spirobichroman-based polyimides (PIs;
6
FDA-FH, 6FDA-DH, and 6FDA-MH) were synthesised by the polyreaction between diamines containing
0
different
substituents
(benzene
ring,
pyridine
ring,
and
methyl
group)
and
4,4 -
(hexafluoroisopropylidene)-diphthalic anhydride (6FDA). The physical properties, gas transport behaviour,
d-spacing, dihedral angle of molecules, and fractional free volume of the PIs were investigated through
experiments and molecular simulations. The PIs exhibited excellent thermal stability and good solubility
in common organic solvents. The gas permeability of the PIs was investigated; the results highlighted the
critical role of the substituents in the enhancement of the gas separation performance of polymer
membranes. Detailed analysis of the PIs showed that 6FDA-FH exhibits the highest gas permeability. This
can be ascribed to the loose packing of the polymer chain owing to the increased dihedral angle
between the two planes. However, the methyl substituent in 6FDA-MH disrupts the polymer chain
packing rather than changing the dihedral angle between the two planes, thus enhancing the gas
Received 30th November 2020
Accepted 19th January 2021
DOI: 10.1039/d0ra10113c
2 4
permeability of 6FDA-MH. Furthermore, 6FDA-DH exhibited the highest CO /CH selectivity, which is
rsc.li/rsc-advances
attributed to the CO affinity of the polymer containing the pyridine unit.
2
(
separation efficiency), which is illustrated by the empirical
1
. Introduction
5,6
criterion of the Robeson upper bound. This challenge arises
Since the concept of membrane separation was rst proposed because of the inadequate free volume of polymer and random
1
by Graham in 1866, membrane separation technologies have size distribution of micropores. Hence, design of a molecular
been rapidly developed and are widely used in the eld of gas structure with a high free volume distribution and reduced
separation for various purposes such as removal of acidic gas chain mobility can effectively overcome the above-mentioned
7,8
from natural gas, separation and recovery of hydrogen during challenge.
Typically, the approach to tuning molecular
the pyrolysis of petroleum, and enrichment of oxygen and structures focuses on two molecular design strategies: (i)
2,3
nitrogen in the air. Although numerous polymer membranes incorporation of various twisted sites (e.g., ethanoanthracene,
have been studied for their gas separation properties, fewer spirobisindane, spirobiuorene, and triptycene units and
4
than 10 membranes have been used on a commercial scale.
Tr ¨o ger's base) into the polymer skeleton to create intrinsic
One of the serious challenges that limits the wide-scale appli- microporosity and allow for high free volume and appropriate
cation of polymer membranes is the trade-off relationship size distribution and (ii) integration of bulky pendant groups
between their gas permeability (gas throughput) and selectivity into the polymer chain to facilitate the disruption of polymer
9–12
chain packing and increase the free volume of the polymer.
13
For example, Filiz et al. developed a series of ortho-allyloxy
polyimides (PIs) with different ratios of allylation units. Their
results showed that the fractional free volume (FFV) was
improved by increasing the number of allyl ether sidechains.
In addition, a class of polymers of intrinsic microporosity
a
National & Local Joint Engineering Laboratory for Synthesis Technology of High
Performance Polymer, Key Laboratory of High Performance Plastics, Ministry of
Education, College of Chemistry, Jilin University, Changchun 130012, China.
E-mail: wangdaming@jlu.edu.cn
b
Center for Advanced Low-Dimension Materials, State Key Laboratory for Modication
of Chemical Fibers and Polymer Materials, College of Materials Science and (PIMs) was studied for verifying their applicability as a gas
Engineering, Donghua University, Shanghai 201600, China. E-mail: yjn@dhu.edu.cn
separation membrane. The main chain of these polymers
contains a twisted spiro-centre (two rings that share one carbon
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra10113c
1
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atom) and rigid rings in the form of dioxane units, which limit solubility of CO
2
as a result of the interaction between tertiary
the free rotation of molecules, leading to ultrahigh free volume amines and CO2.
and appropriate distribution of free volume elements as well as Based on the ndings of the above-mentioned studies, we
14
excellent gas permeability and high selectivity. For example, developed new semi-ladder PIM-PIs containing functional
the ladder-type PIMs prepared by the post-treatment of PIM-1 groups consisting of pyridine units in this study. Furthermore,
with thioamide, tetrazole, and carboxyl, which exhibited excel- the structure–property relationships of the new PIM-PIs were
15,16
lent gas permeability and high gas pair selectivity.
investigated by combining experimental data and molecular
Despite the advantageous properties of PIMs as membranes simulations, with the aim of expanding their application as
for gas separations, the commercially application of PIs with membranes for gas separation. Herein, we designed and syn-
high thermal stability and good mechanical properties is thesised three spirobichroman-based PIs (6FDA-FH, 6FDA-DH,
1
7,18
0 0
limited by the poor gas permeability of PIs.
Hence, the and 6FDA-MH) through a series of reactions with 4,4,4 ,4 -tet-
0
0
combination of PIMs with twisted spiro-centres and PIs to ramethyl-2,2 -spirobi[chromane]-7,7 -diol (Spiro-diol; Fig. 1)
obtain PIM-PIs with excellent gas permeability has attracted and 6FDA. We expected that the integration of both the con-
widespread interest. This concept is realised by the introduc- torted spirobichroman-based unit and the functional side
tion of a twisted spiro-centre into a skeleton of PI chains, group into the PIM-PIs will disrupt the packing of polymer
leading to reduced exibility and tight packing of the molecular chains and enhance the overall separation performance of PIs.
chains, thereby increasing the free volume within the polymer Finally, the effects of the substituents on chain packing and gas
19
membrane. Moreover, the PIM-PIs containing two main transport were investigated in detail.
components (diamine and dianhydride) can be used to inde-
pendently adjust the properties of the polymers by changing the
structure of the diamine and/or dianhydride monomer. These
advantages have inspired researchers to focus on PIM-PIs or
2
. Experimental section
2.1. Reagents and methods
PIM-PIs with functional groups to improve the applicability of Resorcinol (99%), mesityl oxide (95%), iron chloride (>99.99%,
these polymers as membranes for gas separation. In particular, FeCl ), 4-chloronitrobenzene (98%), 2-chloro-5-nitropyridine
contorted 6FDA is one of the most suitable anhydride units for (98%), 2-chloro-3-methyl-5-nitropyridine (99%), potassium
manufacturing PIM-PI membranes; thus, it can be used to carbonate (>99%, K CO ), Pd/C (10%), and hydrazine mono-
3
2
3
produce semi-ladder PIs that can serve as membranes that can hydrate (N
H
$H
O, 85%) were purchased from Aladdin and
2
4
2
20,21
0
exhibit excellent gas separation performance.
For example, used as received. 4,4 -(Hexauoroisopropylidene)-diphthalic
22
Li et al. reported three 6FDA-based semi-ladder PIM-PIs that anhydride (6FDA, 99%) was acquired from TCI and subli-
were prepared from spirobichroman-based diamines with mated in vacuum before use. Toluene, N,N-dimethylacetamide
different substituents and 6FDA dianhydride units. The results (DMAc), and N,N-dimethylformamide (DMF) were dried using 4
showed that these PI membranes have signicantly higher gas A˚ molecular sieves.
permeability than commercially available polymers, including
Kapton, Matrimid 5218, polycarbonate, polysulfone, and cellu- ducted on a BRUKER-300 spectrometer with deuterated
lose acetate.
dimethyl sulfone as the solvent. Fourier transform infrared
Nuclear magnetic resonance (NMR) spectroscopy was con-
Despite the high membrane performance of semi-ladder PIs, (FTIR) spectroscopy was performed on a Bruker Vector 22
membranes with higher gas permeability and gas pair selec- spectrometer. Gel permeation chromatography (GPC, PL-GPC
tivity are required for improved gas separation. Hernandez 220) was carried out by using polystyrene as the standard and
23
et al. fabricated a series of blend membranes based on 6FDA–
FpDA PIs and copolyuorenes. Compared with the pure PI
membrane, their blend membranes exhibited higher CO
permeability and CO /CH selectivity (>23% and 15%, respec-
6
2
2
4
tively) when the content of polyuorene was approximately 1%.
In addition, many semi-ladder PIs with functional groups have
been developed to improve the capacity of their interactions
24
with different gases. Tena et al. successfully obtained func-
0
tionalised polymers by the 3,3 -sigmatropic Claisen rearrange-
ment, indicating that the incorporation of specic functional
groups in the polymer backbone can enhance the performance
of the materials. The tertiary amine of the pyridine unit inte-
grated into the polymer was expected to be responsible for the
increase in CO affinity of the polymer, which was anticipated to
2
enhance CO
gases such as CH
2
selectivity and permeability toward other slow
25
26
4
or N
2
.
Wang et al. prepared PIM-PIs by
functionalisation with a tertiary amine, and the resultant poly-
mer membrane offered high gas permeability and improved gas
Fig.
1
Energy-minimised conformation of spirobichroman-based
2 4 2 2
pair selectivity for CO /CH and CO /N
, owing to the enhanced diphenol.
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DMF as the eluent to determine the molecular weight of the universal force eld. Once the balanced state of the polymer was
29
polymer. Differential scanning calorimetry (DSC, TA Q100) was achieved, the simulation was continued.
performed to evaluate the glass transition temperature (T ) of Furthermore, the dihedral angle of the two adjacent planes
g
ꢀ
ꢁ1
the polymer at a heating rate of 10 C$min , and the T value was calculated using Materials Studio 2017. The dihedral angles
g
was calculated aer the second heating cycle. Thermogravi- were incrementally varied from ꢁ180ꢀ to +180 using the
ꢀ
metric analysis (TGA, TA 2050) was carried out at a heating rate Conformers module. Energy minimisations of all conformers
ꢀ
ꢁ1
of 10 C min under nitrogen and air atmospheres, respec- were conducted on the Forcite module by the COMPASS force
tively. Wide-angle X-ray diffraction (WAXD) analysis was con- eld. Then, the torsional degree of freedom was estimated when
30
ducted on a Bruker D-8 Advance diffractometer with Cu Ka the thermal energy was close to 3 RT.
radiation. The d-spacing of the polymer was determined using
˚
Bragg's law (d ¼ l/2 sin q, l ¼ 1.54 A).
2
.2. Membrane preparation
The density (r) of the polymer was measured using sophis-
ticated electronic balance equipment by the buoyancy method
in deionised water at 25 C. The FFV of the polymer was
The membrane (60–70 mm) was prepared by the solution casting
method. Specically, the polymer was dissolved in chloroform
ꢀ
(
w/w ¼ 5–8%) and ltered using a 4.5 mm lter to remove any
27
calculated using eqn (1)–(3):
insoluble material. The ltrate was carefully poured into a clean
glass Petri dish and placed in a sealed device; then, the solvent
V ¼ 1.3Vw
(1)
(2)
0
ꢀ
was evaporated at 25 C for 3 days. Finally, the membrane was
M
V ¼
r
ꢀ
heated in vacuum at 120 C for 12 h to remove the residual
solvent.
V ꢁ V
0
V
f
¼
(3)
2.3. Monomer synthesis
V
0
0
0
2
.3.1 Synthesis of 4,4,4 ,4 -tetramethyl-2,2 -spirobi[chro-
Here, V
w
derived from Bondi's group contribution theory, is the
0
ꢁ1
mane]-7,7 -diol (Spiro-diol). Spiro-diol was obtained according
specic van der Waals volume, M (g mol ) is the molar mass
from the repeat unit of the polymer, V (cm mol ) is the molar
volume of the polymer, and V is the FFV of the polymer.
31
3
ꢁ1
to a previously reported method, as shown in Scheme 1.
Details of the synthesis process are provided in the experi-
mental section of the ESI.†
f
2 2 2 4
The permeability of pure gases (e.g., O , N , CO , and CH )
2
.3.2 Synthesis of spirobichroman-based dinitro mono-
was measured by the constant-volume method at 2 barrer and
ꢀ
mers. The dinitro monomers, Spiro-FN, Spiro-DN, and Spiro-
MN were synthesised by a nucleophilic substitution reaction
using the appropriate reagents; the synthetic route is shown in
Scheme 1. The specic synthesis process of 4,4,4 ,4 -tetra-
methyl-7,7 -bis(4-nitrophenoxy)-2,2 -spirobi[chromane] (Spiro-
FN) is used as an example and is presented in the experi-
3
5 C. Gas permeability (P) and selectivity (aA/B) are calculated
28
according to eqn (4) and (5), respectively.
2
73 Vl dpðtÞ
0
0
P ¼ DS ¼
(4)
7
6 ATP
0
dt
0
0
P
A
D
A
S
A
D
A=B
S
A=B
a
A=B
¼
¼
ꢂ
¼ a
ꢂ a
(5) mental section of the ESI.†
.3.3 Synthesis of spirobichroman-based diamine mono-
Here, V (cm ) is the downstream chamber volume, A (cm ) and l mers. The diamine monomers (Spiro-FH, Spiro-DH, and Spiro-
cm) are the effective area and thickness of the membrane, MH) were obtained by a reduction reaction, using Pd/C and
respectively; T (K) is the trial temperature of the experiment; P0 N H $H O as the catalyst and reductant, respectively. The
P
B
D
B
S
B
2
3
2
(
2
4
2
(
cmHg) is the upstream pressure of the instrument; and dp/dt is synthetic process of the diamine monomer was explained using
0
0
0
0
0
the rate at which the pressure changes during the experiment D 4,4 -((4,4,4 ,4 -tetramethyl-2,2 -spirobi[chromane]-7,7 -diyl)
and S are the diffusion and solubility coefficients, respectively.
a
A/B is the ideal selectivity coefficient of the gas, which is derived
D
D
from the diffusion selectivity (aA/B) and solubility selectivity (aA/
).
B
The FFVs of the polymer were calculated using Materials
Studio 2017. First, an amorphous cell containing 15 repeat units
was established as the initial model, and the density of the
ꢁ
3
polymer cell was set to 0.01 g cm . Then, the cell of the polymer
was compressed by the normal pressure and temperature (NPT)
ensemble until the density approached the measured value. The
simulation step size and time for the NPT ensemble were 1 fs
and 100 ps, respectively. The structure of the polymer was then
balanced by the normal volume and temperature ensemble, and
the equilibration time and step size were 100 ps and 1 fs,
respectively. The dynamics simulation was performed using the
Scheme 1 Synthetic routes to Spiro-FH, Spiro-DH, and Spiro-MH.
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1
bis(oxy))dianiline (Spiro-FH) as an example, as shown in the The chemical structure of the PIs was conrmed by FTIR and H
experimental section of the ESI.†
NMR spectroscopy. As shown in Fig. S4 (ESI†), the representa-
tive characteristic peaks of the imide group were observed at
ꢁ
1
2
.4. Synthesis of spirobichroman-based PIs
1780–1785 cm
1
(C]O symmetric stretching) and 1728–
ꢁ
1
738 cm (C]O asymmetric stretching). The peak ascribed to
N–H bending was observed at 1375–1388 cm . The H NMR
spectrum of the PIs is shown in Fig. S5 (ESI†). All chemical
The PIs (6FDA-FH, 6FDA-DH, and 6FAD-MH) were synthesised
by chemical imidisation, as shown in Scheme 2. The prepara-
tion of 6FDA-FH serves as an example to illustrate the detailed
synthesis process and is presented in the experimental section
of the ESI.†
ꢁ
1
1
1
shis of the protons on the H NMR spectrum of the polymer
repeat unit were satisfactorily assigned, and the integral values
corresponded to the chemical structure of the polymer. The
intrinsic physical behaviour of the PIs was investigated in detail
by determining the molecular weights of the three polymers
3
3
. Results and discussion
.1. Synthesis and characterisation of spirobichroman-
(
7
Table 1). The polymers exhibited high molecular weights (M
n
¼
4
.7–17.2 ꢂ 10 ) and narrow molecular weight distribution (PDI
based monomers and PIs
¼
1.14–1.79), which is benecial for the processing technology
The diamine monomers were synthesised through a multistep
reaction process, as shown in Scheme 1. From the FTIR and
NMR spectra, the chemical structure of the three monomers
was conrmed. As depicted in Fig. S1 (ESI†), the –NO
tion peaks at 1509–1523 cm
of the membrane. The molecular weights of the polymers
decreased in the order 6FDA-FH > 6FDA-DH > 6FDA-MH; this
decrease can be primarily attributed to the reactivity of the
2
absorp-
3
2
diamine moiety. As shown in Fig. 2, the net charges of the N
atom of the amine unit decrease in the following order: Spiro-
FH > Spiro-MH > Spiro-DH. Therefore, Spiro-FH has the highest
nucleophilicity among the three diamines and can be used to
ꢁ
1
ꢁ1
and 1342–1355 cm
dis-
appeared, and the N–H stretching band of amine appeared at
ꢁ
1
1
13
3
426–3473 cm . The H NMR and C NMR spectra of the
diamine monomers are shown in Fig. S2 and S3, respectively
ESI†). All the resonance signals were appropriately assigned to
22
obtain the longest PI molecular chain. Although the nucleo-
philicity of Spiro-DH is lower than that of 6FDA-MH, the high
molecular weight of 6FDA-DH, which is higher than that of
(
the chemical structure of these molecules. These results
conrmed that the diamine monomers were successfully syn-
thesised. Theoretically, the chemical shis of the –CH groups
3
at the spiro-centres on the diamine monomers should be the
same because they are in the same chemical environment.
However, the bulky –CH groups to the sides of the spiro-centre
limit the rotation of the molecules, resulting in the formation of
chiro isomers. In addition, the chemical shis of the amine
protons in Spiro-FH, Spiro-DH, and Spiro-MH are 4.97, 5.09,
and 5.02 ppm, respectively. Compared with the amine proton at
6
FDA-MH, could be attributed to the low polarity of the amine
of Spiro-MH when –CH3 is present at the meta-position of
amine. The relatively low polarity effect weakens the reactivity of
the diamine moiety to the dianhydride monomer and results in
the formation of the polymer with a relatively low molecular
3
21
weight.
22
3
.2. Thermal properties of PIs
The thermal behaviour of the PIs was investigated by DSC and
TGA; the T results are shown in Fig. 3 and Table 1. The results
conrm that all PIs have an amorphous structure. The T
the polymers ranges from 270 to 278 C in the order 6FDA-DH >
FDA-FH > 6FDA-MH. According to Flory's theory, the factors
inuencing the T of polymers are the rigidity of the polymer
molecular chains and the interactions. In this regard, 6FDA-
MH has a bulky substituent in the form of the –CH group,
which increases the exibility of the polymer chain. Hence, the
of 6FDA-MH is the lowest among the three polymers, whereas
FDA-DH had the highest T , which is attributed to the stron-
gest interactions between the molecular chains in all syn-
4
.97 ppm in Spiro-FH, the amine protons in Spiro-DH and
Spiro-MH are shied downeld. This is because the pyridine
unit in the Spiro-DH and Spiro-MH contains lone-pair electrons,
resulting in a strong electron-withdrawing effect.
Spirobichroman-based PIs (6FDA-FH, 6FDA-DH, and 6FDA-
MH) were successfully synthesised by reacting 6FDA with the
corresponding diamines via chemical imidisation (Scheme 2).
g
33
g
of
ꢀ
6
g
34
3
T
g
6
g
35
thesised polymers.
The thermostability of the polymers under N
2
and air
atmospheres is shown in Fig. 4 and S6 (ESI†), respectively, and
the specic results are summarised in Table 1. The thermal
decomposition temperature of the polymers at 5% weight loss
ꢀ
ꢀ
2
was 438–469 C and 435–466 C in N and air, respectively. The
residual weight of the polymers was observed to be 41–47% at
ꢀ
8
00 C. These results demonstrate that the polymers exhibit
good thermal stability. Nevertheless, the thermal decomposi-
tion temperature at 5% weight loss is the lowest for 6FDA-MH,
which is attributed to the presence of an aliphatic structure,
Scheme 2 Synthetic routes to 6FDA-FH, 6FDA-DH, and 6FDA-MH.
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Table 1 Thermal properties and molecular weights of the polyimides
a
g
ꢀ
b
b
c
ꢀ
ꢀ
T
( C)
T5% ( C)
T10% ( C)
Molecular weights
d
Polymer
DSC
N
2
Air
N
2
Air
R
w
%
M
n
M
w
w n
PDI (M /M )
4
4
4
4
4
4
6
6
6
FDA-FH
FDA-DH
FDA-MH
273
278
270
469
446
438
466
440
435
481
460
451
482
461
456
41
47
46
17.2 ꢂ 10
8.1 ꢂ 10
7.7 ꢂ 10
30.8 ꢂ 10
11.7 ꢂ 10
8.9 ꢂ 10
1.79
1.44
1.14
a
b
c
g
T was determined by DSC. The thermal decomposition temperatures at 5% and 10% weight loss were determined by TGA. Molecular weights
d
ꢀ
2
were obtained using GPC. Residual rates at 800 C were recorded by TGA under N atmosphere.
Fig. 2 Net charges of the nitrogen atom in the amine units calculated
by Materials Studio 2017. (a), (b), and (c) represent Spiro-FH, Spiro-DH,
and Spiro-MH, respectively.
2
Fig. 4 TGA curves of the three PIs under N .
relationship between the spirobichroman-based polymers and
solubility was studied; the results are summarised in Table 2.
The three PIs exhibited excellent solubility in common organic
solvents, including chloroform and tetrahydrofuran that have
low boiling points. Following are the reasons for the high
solubility of the polymer: (i) the spirobichroman-based struc-
3 2
ture and the presence of large bulky –C(CF ) – groups increased
the noncoplanarity of the molecule and reduced the density of
the polymer packing; (ii) an electron-decient heterocyclic
pyridine unit was incorporated into the polymer, enhancing the
dipole–dipole interaction between the polymer and solvent,
thus increasing polymer solubility; and (iii) the ether bond is
considered to increase the movement of chain segments and
37–39
enhance the solubility of the polymer in organic solvents.
Fig. 3 DSC curves of the three PIs.
Furthermore, the morphology of the synthesised PIs was
investigated through WAXD analysis. The results are shown in
Fig. 5, and the corresponding values measured for the chain
spacing (d-spacing) are listed in Table 3. All polymers had
36
namely, the –CH3 group. In addition, owing to the higher
aromaticity of the phenyl ring compared with that of the pyridyl
ring, at 5% weight loss, the thermal stability of 6FDA-FH is
higher than that of 6FDA-DH.
a
broad diffraction peak, conrming the amorphous
morphology of the polymers in correspondence with the DSC
40
measurements. The d-spacing of the polymers decreased in
the order 6FDA-FH > 6FDA-MH > 6FDA-DH, because of the
presence of different substituents of the polymers. The dihedral
angles between the substituent and the plane of the adjacent
3.3. Solubility and morphology of PIs
benzene ring were determined; these angles are denoted as a
1
,
Owing to the strong interaction between the intermolecular
chain and the rigid structure of the imine ring, aromatic PIs are
typically poorly soluble in common organic solvents. Thus,
optimisation of polymer performance is necessitated. And the
a , and a for 6FDA-FH, 6FDA-DH, and 6FDA-MH, respectively.
2
3
As shown in Fig. 6 and Table 3, a and a are smaller than a .
2
3
1
This is because the substituted pyridine unit has higher polarity
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Table 2 Solubility of the three PIs in common organic solvents
a
Solubility
Polymer
DMAc
DMF
NMP
DMSO
THF
CHCl
3
1,4-Dioxane
Ethanol
6
6
6
FDA-FH
FDA-DH
FDA-MH
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
ꢁ
ꢁ
ꢁ
a
1
0 mg polymer was dissolved in 1 mL organic solvent at room temperature for 24 h. + completely dissolved; ꢁ insoluble.
Fig. 5 WAXD patterns of the three PIs.
Fig. 6 Dihedral angles a
1
(a), a
2
(b), and a
3
(c) of 6FDA-FH, 6FDA-DH,
than the benzene ring in 6FDA-DH and 6FDA-MH, thus and 6FDA-MH, respectively.
affording a higher degree of coplanarity between the planes of
the substituent and the adjacent benzene ring. Hence, the d-
spacing of 6FDA-DH and 6FDA-MH was smaller than that of
FDA-FH. The dihedral angle between the plane of the pyridine
ring and that of the adjacent benzene ring of 6FDA-DH and
FDA-MH did not differ signicantly, which conrmed that the
CH substituent had a negligible effect on the dihedral angle of
the two planes. However, the bulky –CH substituent could
the polymers, derived from the polymer density and molecular
dynamics simulation, are listed in Table 3. The highest FFV
value of 6FDA-FH is attributable to the large dihedral angle
between the plane of the benzene ring and that of the adjacent
plane in 6FDA-FH, which resulted in more loosely packed
6
6
–
3
3
molecular chains. Furthermore, 6FDA-MH, with the –CH
substituents, has higher FFV than 6FDA-DH. This is because the
CH group introduces additional steric hindrance and disrupts
3
disrupt the packing of the polymer chains, resulting in a larger
d-spacing of 6FDA-MH than that of 6FDA-DH.
–
3
the packing of the polymer chain. In this regard, molecular
dynamics simulation is considered an effective method to gain
deep insight into the microporous structure and free volume
3
.4. Fractional free volume (FFV) of the PIs
The free volume and free volume distribution of glassy polymers
are known to affect gas permeability and selectivity. The FFVs of
32
distribution of glassy polymers. As illustrated in Fig. 7, the
Table 3 Fraction free volume (FFV), chain spacing (d-spacing), and dihedral angle (a) of the three PIs
FFV
M
(g mol
a
ꢁ3
ꢁ1
b
w
3
ꢁ1
ꢀ
˚
d (A)
Polymer
r (g cm
)
)
V
(cm mol
)
Found
Simulated
a ( )
6
6
6
FDA-FH
FDA-DH
FDA-MH
1.231
1.350
1.264
931
933
961
504.42
487.62
507.56
0.133
0.083
0.132
0.107
0.101
0.106
5.89
5.57
5.79
65
53
54
a
b
Density of the polymer. Specic van der Waals volume.
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Fig. 7 Three-dimensional view of the free volume distribution of the polymers calculated using Material Studio 2017. The images in (a), (b), and
c) represents 6FDA-FH, 6FDA-DH, and 6FDA-MH, respectively.
(
grey and blue areas represent the ektexine of free space and order in which the gases permeate through the polymer
cross-section of cut free space, respectively. The existence of membranes is consistent with that of the molecular kinetic
46
three-dimensional continuous cavities proved the loose packing diameters and followed the molecular sieve effect. In partic-
4
1,42
structure of the polymers.
Although the tendency of the FFVs ular, N allows for a more obvious molecular sieve effect than
2
32
is consistent with the experimental values and molecular CO because of its relatively large molecular kinetic diameter.
2
dynamics simulation, the differences in the values obtained by Hence, the permeability of N
the two methods are attributed to the following: (i) the multiple be lower than that of CO . 6FDA-FH exhibited the highest gas
.3 molecular chain packing was reasonable in Bondi's group permeability among the three PI membranes, owing to its
2
for all membranes is observed to
2
1
contribution method but not accepted in the actual molecular relatively large FFV and d-spacing, which improve gas
chain packing of glassy polymers; (ii) the semi-empirical passageways. Compared with the 6FDA-DH membrane, the
formulas were used to optimise the force eld in the process 6FDA-MH membrane has higher gas permeability; this is
43–45
of simulation, resulting in an inherent error.
because the –CH3 substituent disrupts chain packing and
increases the FFV of the polymer. This result indicates that the
–
3
CH substituent could contribute to improve the gas perme-
3
.5. Pure-gas transport for PI membranes
ability of the polymer membrane. In addition, although the
6FDA-DH and 6FDA-MH membranes contain the tertiary amine
The permeability of pure gases (N , CH , O , and CO ) and ideal
2
4
2
2
selectivity of the three spirobichroman-based PI membranes are
listed in Table 4. The three PI membranes produced high gas
from pyridine units, the CO
2
permeability of these two
membranes was lower than that of the 6FDA-FH membrane.
The reasons for the low permeability were as follows: the pres-
ence of tertiary amine groups from the pyridine unit in 6FDA-
DH and 6FDA-MH leads to strong CO2 adsorption, thereby
pair selectivity of aCO /CH > 37 and aCO /N > 24. The gas
2
4
2
2
permeability of the membranes decreased in the order P (CO
2
) >
P (O ) > P (N ) > P (CH ), and the results demonstrated that the
2
2
4
resulting in a high CO
2
solubility coefficient (S) and improved
26
CO
2
permeability. However, the introduction of polarity unit
Table 4 Pure-gas permeability (P) and ideal selectivity (a) of the three
PI membranes
pyridine ring into the polymer enhanced the coplanarity
between the plane of the pyridine ring and that of the adjacent
benzene ring and led to denser chain packing. Therefore, the
FFVs of the 6FDA-DH and 6FDA-MH membranes were lower
than that of the 6FDA-FH membranes and reduced the CO2
permeability performance. The results indicated that FFV of
a
Permeability (barrer)
Selectivity (aA/B
)
Polymer
N
2
CH
4
O
2
CO
2
CO /N CO /CH O /N
2
2
2
4 2 2
6
6
6
FDA-FH
FDA-DH
FDA-MH
0.66 0.44 4.06 17.11 25.9
0.29 0.19 1.80 7.99 27.6
0.54 0.36 3.41 13.34 24.7
38.9
42.1
37.1
50.2
41.3
27.6
25.8
25.1
—
—
—
—
30.2
6.2
6.2
6.3
6.8
5.2
3.92
4.60
4.38
—
—
—
—
5.0
polymer exerts a greater effect on CO
strong CO –polymer interaction.
2
permeability than the
2
XS1 (ref. 48)
XS4 (ref. 48)
0.14 0.09 0.98
0.23 0.12 1.19
4.32 29.9
5.05 22.1
Table 4 also lists the membrane gas transport properties of
different types of PIs, which contain different diamine mono-
mers and 6FDA dianhydride. Compared with XS1 and XS4,
which have the binaphthyl structure, the FFVs of the three
spirobichroman-based PIs were enhanced because of the pres-
ence of their twisted spirobichroman-based structure disrupts
the chain packing, thereby improving gas permeability. It is
2
2
6
FDA-MSBC2
1.42 0.77 5.55 21.2
3.69 2.55 17.0 66.0
1.87 1.28 8.19 32.1
—
—
—
—
—
—
—
—
2
6
FDA-FSBC
2
2
6
FDA-SBC
4
7
6
FDA-OHB
1.23 0.81 6.4
1.44 0.84 7.1
2.5 1.29 11.8
9.7 5.2 36
35
36
55
160
190
4
7
6
6
FDA-6HpDA4
7
FDA-6FpDA
4
7
6
FDA-OFB
2
noteworthy that the CO permeability of 6FDA-FH was 3.96 and
46
TB-PI
10
6.3 50
3.39 times higher than those of XS1 and XS4, respectively. These
a
barrer ¼ 10ꢁ10 cm (STP) cm cm
3
ꢁ2 ꢁ1
ꢁ1
1
s
cmHg .
results indicate that the spirobichroman-based PI membranes
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48
hold potential as promising high-performance gas separation and XS4, the performance of the three spirobichroman-based
membrane materials. The gas permeability of the three PI PI membranes is overall higher in terms of the permeability/
membranes in the present work is lower than that of 6FDA- selectivity trade-off for CO /CH and O /N gas pairs (see
2
4
2
2
22
MSBC, 6FDA-FSBC, and 6FDA-SBC. This low permeability Fig. 8). The signicant increase in FFV induced by the incor-
can mainly be ascribed to the presence of the bulky methyl or poration of spirobichroman-based units in PI markedly
aromatic uoro-substituent in 6FDA-MSBC, 6FDA-FSBC, and enhances the gas separation performance of the membranes
6FDA-SBC, which disrupts the chain packing and increases the compared with that of the PI membrane that contains
FFV of the polymers. PI membranes based on 6FDA that contain a binaphthyl structure (XS1 and XS4). However, the compre-
rigid V-shape bridged bicyclic linking groups (e.g., Tr ¨o ger's hensive separation performance of the three spirobichroman-
46,47
base)
exhibit considerably higher gas permeability than the based PI membrane for the CO /CH and O /N gas pairs is
2 4 2 2
PI membrane with the spirobichroman-based structure. This lower than that of the reported polymers, which contain bulky
phenomenon could be attributed to the fact that Tr ¨o ger's base substituting groups (6FDA-MSBC, 6FDA-FSBC and 6FDA-SBC)
22,46
unit signicantly disrupts chain packing, resulting in higher or possess rigidity arising from Tr ¨o ger's base (TB-PI).
The
FFV than that of spirobichroman-based PIs. However, the results further highlight that the introduction of bulky side
selectivity of the gas pairs containing the rigid Tr ¨o ger's base groups or rigid chain structures in the polymer promotes loose
unit PI (TB-PI) is obviously lower than that of spirobichroman- polymer chain packing and improves the comprehensive sepa-
based PIs (6FDA-FH, 6FDA-DH, and 6FDA-FSBC). This reveals ration performance of gas pairs.
that the introduction of the spirobichroman-based moieties
To understand the contribution of gas diffusivity (D) and
into the 6FDA-FH, 6FDA-DH, and 6FDA-FSBC backbones solubility (S) to CO and CH transport behaviour in polymer
2
4
enhances microporosity and results in higher gas pair membranes, the diffusivity and solubility of CO and CH were
2
4
37
34
selectivity.
calculated by the time-lag method. As displayed in Table 5, the
To estimate the gas separation performance of the diffusivity of gas in different polymer membranes decreases in
spirobichroman-based PI membranes, the trade-off relation- the order 6FDA-FH > 6FDA-MH > 6FDA-DH, which is consistent
ship between gas permeability and selectivity for CO
2 4
/CH and with the trends for FFV and d-spacing of the polymers (Table 3).
O
2
2
/N was investigated using the Robeson upper bounds as
references, as shown in Fig. 8. Polymer membranes with high
gas pair selectivity are known to typically couple with low gas
permeability. Obviously, this general trade-off relationship is
Table 5 Pure-gas (CO
2
and CH
4
) diffusivity (D), solubility (S), diffusion
selectivity (a
D
), and solubility selectivity (a
S
) of the three PI membranes
b
ꢁ2
3
applicable to the present work. The CO
DH is lower than that of 6FAD-FH, and this was accompanied by
an increase in CO /CH selectivity owing to the sharp decline in
CH permeability. Although the comprehensive separation
performance of the membranes was lower than the Robeson
2
permeability of 6FDA-
S (10 cm
(STP) cm
a
ꢁ9
2
ꢁ3
D (10 cm
ꢁ1
ꢁ1
s
)
cmHg
)
a
D
a
S
2
4
4
Polymer
CO
2
CH
4
CO
2
CH
4
CO
2
/CH
4
2 4
CO /CH
upper bound, 6FDA-FH exhibited the most promising gas 6FDA-FH
18.17
8.96
11.5
3.77
3.43
3.76
9.42
8.92
11.6
1.17
0.55
0.96
4.82
2.61
3.06
8.05
16.22
12.08
6
6
FDA-DH
FDA-MH
separation performance among the three PI membranes. The
excellent performance is attributed to the greatly enhanced gas
permeability of 6FDA-FH relative to that of 6FDA-DH and 6FDA-
MH, and the 6FDA-FH membrane maintains high gas pair
selectivity. Compared with the reported PI membranes of XS1
a
2
D was calculated by the time-lag method using D ¼ l /6q (l and q are the
b
membrane thickness and the lag time, respectively). S ¼ P/D (P is the
permeability of gas).
2 4 2 2
Fig. 8 Robeson's upper bounds relevant to the three PIs (6FDA-FH, 6FDA-DH, and 6FDA-MH) for CO /CH and O /N . The data points are those
48
48
22
22
22
46
of XS1, XS4, M (6FDA-MSBC), F (6FDA-FSBC), S (6FDA-SBC), and TB-PI.
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Compared with the CO
2
solubility and diffusivity of 6FDA-FH 6FDA-FH with benzene. This was attributed to the presence of
and 6FDA-DH, the CO diffusivity of 6FDA-FH is signicantly the polarisable pyridine unit, which enhanced the CO affinity
2
2
higher than that of the 6FDA-DH. This implies that the of 6FDA-DH and improved the CO /CH solubility selectivity.
2
4
improvement of CO permeability from 6FDA-DH to 6FDA-FH is Overall, a systematic study of the relationships among the
2
mainly attributable to the increase in FFV, which is consistent different substituents and the gas separation performance of
with the result for the d-spacing of the polymer. Notably, the the polymers membranes was conducted, demonstrating the
CO
MH because of the enhancement of both gas diffusivity and separation performance of polymer membranes.
solubility. In addition, the solubility values of CO and CH for
FDA-FH are approximately 1.06 and 2.13 times higher than
2 4
and CH permeabilities increase from 6FDA-DH to 6FDA- critical role of the substituents for the enhancement of the gas
2
4
6
Conflicts of interest
that of 6FDA-DH, respectively. This is because 6FDA-DH
contains a tertiary amine in the pyridine unit, which causes
strongly CO affinity and increased CO solubility.
2 2
There are no conicts to declare.
According to the solution-diffusion mechanism for gas
separation, the gas selectivity of glassy polymers is governed by
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