Gas permeability of hexaphenylbenzene based polymers of intrinsic microporosity

Article abstract of DOI:10.1021/ma501925j

The synthesis and characterization of a series of novel hexaphenylbenzene (HPB) based polymers of intrinsic microporosity (PIM-HPBs) containing methyl, bromine, and nitrile substituents are reported. The successful formation of thin films from these polymers allowed the evaluation of the influence of the substituents on intrinsic microporosity and gas permeability. Analysis by the time-lag method also yielded information about gas diffusion coefficients and, indirectly, the gas solubility. The gas permeability varies as a function of the polarity of the substituents and shows a significant increase after treatment of the samples with methanol, especially for films cast from THF as the solvent. This enhancement, which is mostly due to an increase in the diffusion coefficient, is only partially lost upon aging of the membranes for 5 months. Measurements at different feed pressures confirm the typical dual mode sorption behavior, with increasing diffusivity and decreasing permeability and solubility as a function of the feed pressure.

Full text of DOI:10.1021/ma501925j

Article
pubs.acs.org/Macromolecules
Gas Permeability of Hexaphenylbenzene Based Polymers of Intrinsic
Microporosity
Mariolino Carta,† Paola Bernardo,‡ Gabriele Clarizia,‡ Johannes C. Jansen,‡ and Neil B. McKeown*,†
†School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, Edinburgh, U.K.
‡Institute on Membrane Technology, ITM-CNR, Via P. Bucci 17/C, 87036 Rende (CS), Italy
S
* Supporting Information
ABSTRACT: The synthesis and characterization of a series of novel hexaphenylbenzene
(HPB) based polymers of intrinsic microporosity (PIM-HPBs) containing methyl, bromine, and
nitrile substituents are reported. The successful formation of thin films from these polymers
allowed the evaluation of the influence of the substituents on intrinsic microporosity and gas
permeability. Analysis by the time-lag method also yielded information about gas diffusion
coefficients and, indirectly, the gas solubility. The gas permeability varies as a function of the
polarity of the substituents and shows a significant increase after treatment of the samples with
methanol, especially for films cast from THF as the solvent. This enhancement, which is mostly
due to an increase in the diffusion coefficient, is only partially lost upon aging of the membranes
for 5 months. Measurements at different feed pressures confirm the typical dual mode sorption behavior, with increasing
diffusivity and decreasing permeability and solubility as a function of the feed pressure.
greater detail and demonstrate the effects of introducing simple
substituents (Me, Br, and CN) onto the HPB unit on the gas
permeability.
INTRODUCTION
■
Microporous organic polymers1 (i.e., with pore diameters <2 nm)
are of growing interest due to their potential applications in gas
storage,2 sensors,3 heterogeneous catalysis,4 CO2 capture,5 and
as membranes for gas separation.6 For the latter application it is
necessary that the microporous polymer is also solution
processable to facilitate membrane fabrication.7 A decade ago,
a strong impetus to the field of polymer membranes was
provided by the invention of the polymers of intrinsic
microporosity (e.g., PIM-1),8 a class of processable polymers
that combines high gas permeability, due to their high free
volume, with moderate selectivity for one gas over another.9
Many PIMs have now been reported10 with recent studies
focused on the introduction of highly rigid units into the
polymer backbone.11 However, very few studies describe the
effect of a systematic variation of polymer substituents in order
to establish structure−property relationships for gas transport
properties,12 and for PIMs only modification of the nitrile
functionality in PIM-1 has been probed systematically.13
RESULTS AND DISCUSSION
■
Synthesis. In our previous work, it was established that the
attainment of high molecular mass PIM-HPB was related to
the relative positions of the catechol moieties in the central
core of the HPB.21 When the monomer containing two ortho
catechol units was polymerized (Figure 1a), only small cyclic
oligomers were obtained. In contrast, a mixture of predom-
inantly p-bis(catechol) monomer (Figure 1b), containing a
small amount (∼10%) of the meta regioisomer (Figure 1c),
provided a polymer with considerably higher molecular mass,
which proved suitable for the formation of a thin film for the
measurement of gas permeation. For the present study we
designed a more appropriate synthetic pathway to provide
exclusively p-bis(catechol) HPB monomers in order to over-
come the inhomogeneity provided by the mixture of para and
meta catechol substitution of the HPB monomers.
Hexaphenylbenzene (HPB) has been known for many
decades,14 but only when its structure was properly deter-
mined15 did its potential start to be fully exploited. To date,
substituted HPB have been widely used for different
applications, including crystal engineering,16 crystalline photo-
conductors,17 preparation of synthetic graphene fragments,18
photoinduced electron transfer,19 and lately as a structural unit
for the formation of microporous materials.20,21 In a previous
communication the synthesis and gas permeability of a micro-
porous polymer derived from HPB were reported.21 It was
shown that the HPB unit could frustrate the packing of poly-
mer chains in the solid state. This feature does not allow the
material to pack efficiently, thereby inducing microporosity.
Here we discuss the gas transport properties of PIM-HPB in
The synthesis of HPB derivatives is well established and
relies on a reaction between diphenyl acetone and benzil to give
a cyclopentadienone that can then undergo a Diels−Alder
reaction with a diphenylacetylene. As illustrated in Figure 2,
by using 1,3-di(3′,4′-tetramethoxyphenyl)acetone as precursor,
it is possible to ensure that the resulting catechol units are
placed in a para orientation on the resulting HPB monomers.
By the appropriate choice of the benzil (Figure 2b) and phenyl
Received: September 18, 2014
Revised: November 10, 2014
© XXXX American Chemical Society
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Figure 1. Three regioisomers of bis(3′,4′-dihydroxyphenyl)tetraphenylbenzene. In addition, there could be conformers due to restricted rotation
about the aryl−aryl C−C bonds.
Table 1. Summary of the Physical Properties of the PIM-
HPB Polymers
BET surface total pore volume
area
(cm3 g−1) at
Mw × 10−3
polymer
(m2 g−1
)
P/P0 = 0.98
(g mol−1
)
PDI
PIM-HPB
537
560
410
440
0.65
0.53
0.45
0.37
422
83
2.55
3.40
1.74
1.85
PIM-CH3-HPB
PIM-Br-HPB
PIM-CN-HPB
281
65
Figure 2. Precursors required for hexaphenylbenzene synthesis: (a)
substituted 1,3-(3,4-dimethoxy)diphenyl acetone, (b) substituted
benzil, and (c) substituted diphenylacetylene.
standards (Table 1). The solubility and high molecular mass of
the HPB polymers facilitated the preparation of robust thin films
for gas separation measurement by simple casting of a 2.5% w/v
solution, at atmospheric pressure and at room temperature, of
the polymer into a Teflon dish and allowing a very slow evap-
oration of the solvent over a period of a few days.
acetylene precursors (Figure 2c), it is possible to introduce sub-
stituents R in the final monomer.
Scheme 1 outlines the HPB monomer synthesis by means of
the self-condensation of commercial (3,4-dimethoxyphenyl)-
acetic acid gave bis(3,4-dimethoxyphenyl)acetone, 1, which is
the key precursor that was used for the preparation of all the
novel HPB monomers reported in this paper. The literature
precedent for the synthesis of bis(4-methoxyphenyl)acetone
using a modification of the Dakin−West reaction was
followed.22,23 The aldol condensation of 1 with appropriate
benzil precursors gave the substituted cyclopentadienones
2a−c. Thermal Diels−Alder reactions of 2a−c with the
appropriate diphenyl acetylenes 3a−c gave the substituted
hexaphenylbenzenes 4a−c. With the exception of 1,2-di-p-
tolylacetylene, 3b, which was synthesized via the procedure of
Park et al.,24 commercially available benzil and diphenylacety-
lene precursors were used. The nitrile containing HPB 4d was
synthesized from 4c via the Rosemund von Braun reaction with
CuCN.16b Efficient demethylation with boron tribromide
afforded the catechol containing HPB monomers 5a−d,
which were polymerized by reaction with commercial 2,3,5,6-
tetrafluoroterephthalonitrile, via aromatic nucleophilic substitu-
tion, affording PIM-HPB, PIM-CH3-HPB, PIM-Br-HPB, and
PIM-CN-HPB, respectively.
Nitrogen adsorption measurements at 77 K of precipitated
powders of the PIM-HPB polymers (Figure 3) allowed apparent
BET surface area and pore volume to be calculated, which
confirmed the intrinsic microporosity generated by the HPB
unit (Table 1). The methyl substituents of PIM-CH3-HPB did
not significantly influence the nitrogen adsorption isotherm, as
compared to that of PIM-HPB, whereas the N2 uptake of PIM-
Br-HPB and PIM-CN-HPB indicated reduced intrinsic micro-
porosity. This could be explained by the presence of stronger
intermolecular dipolar interactions increasing polymer cohesion
and resulting in a more efficient packing.
Polymer Properties. All HPB based polymers proved
readily soluble in at least one common organic solvent, which
facilitated their purification by reprecipitation in acetone to
remove oligomeric product and their characterization by solu-
1
tion H NMR and gel permeation chromatography (GPC).
Both PIM-HPB and PIM-CH3-HPB proved fully soluble in
chloroform, whereas PIM-Br-HPB was soluble in both
chloroform and THF and PIM-CN-HPB only in THF. As anti-
cipated, the para substitution of the two catechol moieties within
the HPB monomers ensured an efficient polymerization with
high average molecular mass (Mw) of the reprecipitated polymers
in the range 65−400 × 103 g mol−1 as compared to polystyrene
⧫
Figure 3. Nitrogen adsorption isotherms at 77 K for PIM-HPB (red
,
◊
adsorption; , desorption); PIM-CH3-HPB (blue ∗, adsorption);
□
●
PIM-CN-HPB (purple , adsorption) and PIM-Br-HPB (green
,
adsorption). Except for PIM-HPB, the desorption curves are removed
for clarity but are reported in the Supporting Information.
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a
Scheme 1. Synthesis of the Substituted Hexaphenylbenzenes
a
Reagents and conditions: (i) DCC, DMAP, DCM, RT; (ii) KOH, EtOH, reflux; (iii) Ph2O, 270 °C, μW; (iv) CuCN, DMF, reflux; (v) BBr3, DCM,
RT; (vi) H2O; (vii) DMF, K2CO3, 65 °C.
Figure 4. TGA data PIM-H-HPB (top, black) and PIM-CN-HPB (bottom, red) as cast membranes showing the far greater retention of THF
(∼10 mass %) as compared to chloroform (2%).
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Table 2. Gas Permeability Px, Diffusivity Dx, Solubility Coefficient Sx, and Ideal Selectivity α with Respect to N2 Measured at
25 °C and 1 bar Feed Pressure on the PIM-HPB Polymers in Different States
sample
transport parameters
Px [barrer]
O2
CO2
CH4
H2
He
N2
68
PIM-HPB as cast
221
3.28
97.6
1.70
534
2.81
195
2.05
340
3.18
130
1.97
154
3.36
78.7
1.47
408
3.33
125
2.45
256
4.28
106
1.81
30.4
4.82
19.0
1.20
301
3.26
102
2.21
203
3.77
82.2
1.85
37.3
4.76
26.7
1.05
386
3.15
134
2.16
205
3.90
85.6
1.79
1640
24.3
54.8
22.4
3800
20.0
110
124
1.83
14.8
6.28
361
1.90
34.3
7.89
192
1.79
19.3
7.46
82
671
320
α (Px/PN2)
9.94
1560
0.323
1413
7.44
2490
0.43
1000
9.35
2070
0.36
501
4.74
2160
0.111
592
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
35.7
1.42
190
]
PIM-HPB after MeOH
α (Px/PN2)
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
3.11
3630
0.12
437
78.6
1.81
107
]
]
]
25.8
2390
22.3
72.7
29.7
990
PIM-HPB aged 145 days
PIM-CH3-HPB as cast
α (Px/PN2)
4.08
3630
0.09
249
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
47.5
1.69
46
α (Px/PN2)
21.5
43.9
16.9
2620
21.4
65.3
30.1
1630
27.3
48.9
25.1
189
1.77
11.5
5.30
230
1.88
20.5
8.41
132
2.20
15.1
6.57
10.1
1.60
1.6
10.9
1390
0.27
1220
9.99
2080
0.44
856
5.41
2840
0.07
578
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
29.0
1.19
122
PIM-CH3-HPB after MeOH
PIM-CH3-HPB aged 147 days
PIM-Br-HPB as cast
α (Px/PN2)
4.72
4090
0.11
413
Dx [10−12 m2/s]
46.2
1.99
60
Sx [cm3 (STP) cm−3 bar−1
]
Px [barrer]
α (Px/PN2)
14.3
2183
0.29
154
6.90
4800
0.06
91.4
14.5
1660
0.04
392
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
34.8
1.29
6.30
]
]
α (Px/PN2)
30.0
7.9
24.5
580
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
5.5
0.86
92
17.8
2130
23.0
52.5
30.4
1430
26.5
41.4
25.8
228
4.80
177
1.92
15.3
8.66
102
1.90
10.7
7.19
12.8
1.63
2.3
0.20
930
PIM-Br-HPB after MeOH
PIM-Br-HPB aged 146 days
PIM-CN-HPB as cast
α (Px/PN2)
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
10.1
1740
0.40
694
4.24
2970
0.1
36.2
1.92
54
]
312
α (Px/PN2)
12.9
1700
0.31
181
5.81
2900
0.08
113
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
26.2
1.54
7.8
]
]
α (Px/PN2)
29.2
8.4
23.1
719
14.4
1790
0.05
541
Dx [10−12 m2/s]
Sx [cm3 (STP) cm−3 bar−1
Px [barrer]
8.0
20.5
2390
19.50
55.6
32.3
1300
24.7
32.5
29.9
4.23
212
1.73
20.1
7.90
93
0.19
1200
9.82
2480
0.36
734
0.73
123
PIM-CN-HPB after MeOH
PIM-CN-HPB aged 147 days
α (Px/PN2)
4.42
4040
0.10
361
Dx [10−12 m2/s]
48.7
1.89
52
Sx [cm3 (STP) cm−3 bar−1
]
Px [barrer]
α (Px/PN2)
1.78
10.9
6.44
14.0
1820
0.30
6.88
3480
0.08
Dx [10−12 m2/s]
27.1
1.45
Sx [cm3 (STP) cm−3 bar−1
]
Gas Permeability Measurements. The results of the gas
permeation measurements are reported in Table 2. For each
polymer the order of gas permeability is CO2 > H2 > He > O2 >
N2 > CH4, reproducing the behavior observed for most PIMs
reported to date.9a,10 Moreover the permeation data show
reduced permeation rates in the substituted HPB polymers.
Generally, permeability and diffusion coefficients follow the trend
PIM-HPB > PIM-CH3-HPB > PIM-Br-HPB > PIM-CN-HPB. It
is known that polymer gas transport properties are influenced by
the type of solvent used for film preparation, both by changing
the final microstructure of the polymer via interactions with its
backbone during the membrane formation process and by the
amount of residual solvent remaining after fabrication.9a,25
Permeability, diffusion, and solubility coefficients of all gases in
PIM-CN-HPB and PIM-Br-HPB films, both processed from
THF, are significantly lower than the values measured for films
cast from CHCl3 solutions. Indeed, TGA analysis (Figure 4,
single TGA plots are shown in the Supporting Information)
confirmed that THF retention within the films is greater (ca. 10%
for PIM-CN-HPB) than chloroform (ca. 2% for PIM-HPB).
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Figure 5. Robeson plots for the CO2/CH4, CO2/N2, and O2/N2 separations. Red = PIM-HPB, green = PIM-CH3-HPB, gray = PIM-Br-HPB,
and purple = PIM-CN-HPB. Circles: as-cast membranes. Triangles: MeOH-treated membranes. Thick line: 2008 upper bound. Thin line:
1991 upper bound.
between selectivity and permeability was quantified by Robeson
in his famous plots for different gas pairs of interest,26
identifying also the so-called upper boundthe existence of
which was supported by theoretical considerations.27 The
permeability and selectivity of the PIM-HPBs polymers are
plotted as Robeson diagrams in Figure 5 for some relevant gas
pairs (CO2/N2, CO2/CH4, and O2/N2; for other gas pairs see
the Supporting Information).
The removal of residual solvent from the membrane by
alcohol treatment causes an increase in permeability for all
gases and a slight decrease in selectivity, without any
Figure 6. Diffusion selectivity vs diffusivity for the CO2/N2, and
O2/N2 separations. Red = PIM-HPB, green = PIM-CH3-HPB, gray =
modification of the gas permeation order. The PIM-Br-HPB
and PIM-CN-HPB containing more polar substituents are
more affected by this treatment. However, the relative
differences in permeability between the PIM-HPBs become
less marked due to the removal of tightly held residual solvent,
particularly THF. In the Robeson diagrams, the alcohol-treated
samples follow the usual trade-off between permeability and
PIM-Br-HPB, and purple = PIM-CN-HPB. Circles: as-cast mem-
branes. Triangles: MeOH-treated membranes.
Selectivity of the cast films follows the trade-off between
permeability and selectivity, which is typical for polymeric
materials applied in membrane gas separation; i.e., the more the
polymer is permeable, the less it is selective. This trade-off
Figure 7. CO2 permeation data in the PIM-HPB membranes in different states: as cast, immediately after MeOH treatment, and aged for 5 months.
(a) CO2 permeability. (b) CO2/N2 ideal selectivity. (c) CO2 diffusivity. (d) CO2 solubility.
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Figure 8. CO2 permeation in the PIM-HPB series at different feed pressures: (a) permeability, (b) diffusivity, (c) solubility, and (d) concentration.
Lines in (a−c) are provided as a guide to the eye; lines in (d) represent a least-squares fit of the data with eq 1 and are indicated for qualitative
evaluation only (note: 1 mg CO2 = 0.51 cm3 (STP)).
selectivity. For the CO2/N2 and CO2/CH4 gas pairs the
performance of each of the PIM-HPB polymers is close to the
2008 upper bounds.26a The data points for the O2/N2
separation lie close to the 1991 upper bound. Overall, the gas
permeability performance of the PIM-HPBs is similar to that
reported for many other PIMs, especially those based on
spirobisindan as site of contorsion.10
The differences in gas permeability among the PIM-HPBs
can be better understood by analyzing the contribution of
diffusivity and solubility coefficients, according to the solution-
diffusion transport model. For molecules with comparable sizes
(e.g., CO2 and N2) the absolute value of the diffusion
coefficient depends on the polymer structure, but their
selectivity is ∼1 and is independent of the molecular structure
(Figure 6a). For gas pairs with different sizes such as O2/N2
(Figure 6b), H2/N2, and H2/CH4, the diffusion selectivity
follows a Robeson-like trade-off.
polymers such as polyimides (15−30 m2 g−1),28 but lower
than the value typically reported for other PIMs (e.g., PIM-1 =
700−800 m2 g−1).10 The apparent BET surface areas and total
pore volumes accessible during nitrogen adsorption are known
to correlate roughly with permeability,10 and this correlation is
also observed for PIM-HPBs. Hence, the lower values for gas
permeabilities and diffusivities, coupled with an increase in gas
selectivities, observed for the PIM-HPBs with more polar
substituents (Br and CN) appear to be related to less micro-
porosity, presumably caused by enhanced polymer cohesion
interactions. Similarly, a reduced porosity was observed in
modified PIM-1, by replacing its nitrile groups with other more
polar or bulkier functionalities which can induce a rigidification
of the polymeric matrix and a finer ultramicroporosity of
the porous texture, by an extensive network of intermolecular
H-bonding.29,30
The permeability data for PIM-HPB measured after the
methanol treatment are much higher than the values reported
previously for a film subjected to vacuum and heating21a
procedure that is known to cause accelerated physical aging.
Table 1 reports the apparent BET surface area of the PIM-
HPB polymers, which lie in the range 410−500 m2 g−1. These
values are larger than a typical value for common glassy
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Five months after the MeOH treatment, the PIM-HPB films all
show a notable decrease in permeability due to physical aging
(Figure 7). However, the permeability remains significantly
above the original value of the as cast films, especially for PIM-
CN-HPB and PIM-Br-HPB, once again demonstrating that the
relatively low permeability of the as-cast films is due to solvent
retention. Aging is somewhat more evident for the slower gases
(N2 and CH4), thus leading to a slight increase in selectivity
(Figure 7a). Because of aging in PIM-CN-HPB, the values of
P and D for CO2 decrease, but the CO2 solubility remains
significantly higher than in the other HPBs. For other gases like
methane and oxygen, values for S are relatively lower for PIM-
CN-HPB. Therefore, this enhanced solubility may be due to
specific interactions of CO2 with the additional nitrile groups in
the polymer.
CONCLUSIONS
■
A series of novel hexaphenylbenzene based polymers of
intrinsic microporosity (PIM-HPBs) was prepared with
sufficiently high molecular mass to enable the fabrication of
cast films suitable for gas permeation studies. The chosen
synthetic pathway for the preparation of the bis(catechol)
monomers facilitates the introduction of methyl, bromine, and
nitrile substituents to allow quantification of their effect on
microporosity and gas permeabilities. The introduction of polar
groups modulated intrinsic microporosity and decreased
permeability albeit with increased selectivity for some
important gas pairs. This approach could be useful for tuning
the performance of membranes for gas separation, particularly
those involving CO2.
It is likely that the aged membranes correspond more closely
to the samples subjected to nitrogen adsorption analysis be-
cause the pretreatment (i.e., thermal degassing under reduced
pressure) carried out before analysis causes accelerated aging.31
Indeed, the results obtained after 5 months of aging on the
unsubstituted PIM-HPB are much closer to the data reported
by Short et al. for the same polymer in which the heat
treatment after MeOH soaking induced accelerated aging.21
For example, the CO2 permeability in the aged HPB polymer is
2400 barrer, while Short et al. reported a value of 1750 barrer.
Methanol treatment is known to swell the PIMs (e.g., 10%
increase in film thickness) and thus helps to remove tightly
bound solvent but also introduces additional free volume which
is lost with aging.
ASSOCIATED CONTENT
■
S
* Supporting Information
Full experimental details for the synthesis of precursors and
polymers; nitrogen adsorption isotherms and gel permeat-
ion chromatograhy; measurements of the gas permeabilities.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail neil.mckeown@ed.ac.uk (N.B.M.).
Notes
The authors declare no competing financial interest.
The interaction of CO2 with the different PIM-HPB poly-
mers was further investigated by means of CO2 permeability
measurements at variable feed pressures (Figure 8a). For each
polymer CO2 permeability decreases with the feed pressure,
particularly for the unsubstituted PIM-HPB, while the effect is
smallest for PIM-CH3-HPB. The decrease of the permeability as
a function of the CO2 feed pressure is the result of the decrease
in gas solubility (Figure 8b,c), consistent with the dual mode
sorption model. This analysis confirms that the CO2 sorption is
highest in PIM-CN-HPB and decreases in the order PIM-Br-
HPM > PIM-CH3-HPB > PIM-HPB (Figure 8c). The relatively
narrow pressure interval considered in the measurements did not
allow an accurate quantitative evaluation of the model
parameters. Although the indirect calculation of the solubility
from the relation P = D × S should be done with care in the case
of pressure-dependent transport parameters,32 qualitatively the
sorption data fit well with the dual mode model:
ACKNOWLEDGMENTS
■
The work leading to these results has received funding from the
European Community’s Seventh Framework Programme
(FP7/2007-2013) under grant agreement no. NMP3-SL-
2009-228631, project DoubleNanoMem, and from the Italian
National Program, Programma Operativo Nazionale Ricerca e
Competitivita 2007-2013, project PON01_01840 “Micro-
̀
PERLA”.
REFERENCES
■
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CHbp
1 + bp
c = kDp +
(1)
where c is sorbate concentration, p is sorbate pressure, kD is the
Henry’s law constant, CH is the Langmuir (monolayer) sorption
capacity constant, and b is the Langmuir affinity constant.
For PIM-CH3-HPB the CO2 concentration c has an almost
linear pressure dependence, indicating that Langmuir sorption
is relatively small compared to the Henry sorption. This may be
due to the methyl groups reducing the interchain interactions
so that the polymer is more prone to dilation especially
when compared with PIM-Br-HPB and PIM-CN-HPB
containing more polar substituents. Because of a very high
affinity for CO2, a significant portion of the Langmuir sorption
sites is already saturated below 1.3 bar, as is commonly ob-
served in PIMs.11a,13b,32
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