Synthesis of silyl-disubstituted poly(p-phenylenevinylene) membranes and their gas permeability

Article abstract of DOI:10.1016/j.polymer.2013.03.007

p-Bis(bromomethyl)benzene with two silyl groups [SiMe₂-n-C ₈H₁₇, SiMe₃ (1a), SiMe₂-n-C ₈H₁₇, SiEt₃ (1b), SiMe₂-n-C ₈H₁₇, SiMe₂

Full text of DOI:10.1016/j.polymer.2013.03.007

Polymer 54 (2013) 2272e2277
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of silyl-disubstituted poly(p-phenylenevinylene) membranes and their
gas permeability
*
Toshikazu Sakaguchi , Miki Sato, Tamotsu Hashimoto
Department of Materials Science and Engineering, Graduate School of Engineering, University of Fukui, Bunkyo, Fukui 910-8507, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
p-Bis(bromomethyl)benzene with two silyl groups [SiMe2-n-C8H17, SiMe3 (1a), SiMe2-n-C8H17, SiEt3 (1b),
SiMe2-n-C8H17, SiMe2-n-C8H17 (1c), SiMe2-n-C8H17, SiMe2-n-C18H37 (1d), SiMe2-n-C18H37, SiMe2-n-C18H37
(1e)] were polymerized by Gilch method to afford silyl-disubstituted poly(p-phenylenevinylene)s (2aee).
The polymers containing SiMe3 groups (2a) showed poor solubility and dissolved in only CHCl3. On the
other hand, the other polymers (2bee) exhibited good solubility in common organic solvents. According
to thermogravimetric analysis (TGA), the silyl-disubstituted poly(p-phenylenevinylene)s showed high
thermal stability (thermal decomposition temperature: Td ꢀ 310 ꢁC). Polymers 2aee had relatively high
molecular weight over 7.6 ꢂ 104, and gave free-standing membranes by solution casting method. Except
2e, the oxygen permeability coefficients (PO2) of membranes of the silyl-disubstituted poly(p-phenyl-
enevinylene)s increased as increasing alkyl length of silyl groups (2a: 5.4, 2b: 6.0, 2c: 15.0, 2d: 20.4
barrers). The membrane of 2e having two dimethyl-n-octadecylsilyl groups in the repeating unit
exhibited the lowest gas permeability among the present polymers, and its PO2 was 1.8 barrers. This is
because polymers 2aed were amorphous while 2e was crystalline. The PO2 values for amorphous poly(p-
phenylenevinylene)s increase in direct proportion to the number of carbon in the side chains.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 15 December 2012
Received in revised form
27 February 2013
Accepted 4 March 2013
Available online 13 March 2013
Keywords:
Poly(p-phenylenevinylene)
Membrane
Gas permeability
1. Introduction
potassium tert-butoxide as a base, namely Gilch polymerization
[23]. It is a simple procedure, and give high molecular weight PPVs.
Gas permeable polymers have been extensively studied because
of their promising application in gas separation membrane [1e5]. A
variety of polymers such as substituted polyacetylenes [6e8], pol-
yimides [9e12], polysulfones [13e15], polycarbonates [16e18] have
been studied as materials for gas separation membrane. The main
chains of these polymers are composed of double bonds, aromatic
rings, etc., and they are rigid and restricted in rotation. When such
polymers possess spherical bulky substituents, their membranes
usually showed large free volume because of the steric repulsion of
bulky substituents. Thus, both stiff main chain and bulky sub-
stituents are important for high gas permeability. However, there
are few polymers which have sufficient performance to apply to
practical use. Therefore, the design and synthesis of novel polymers
are essential from the viewpoint of industrial issue.
PPVs possess rigid main chain composed of benzene rings and
carbonecarbon double bonds, and membranes of PPVs containing
bulky substituents such as silyl groups possibly exhibit high gas
permeability. Silyl groups are effective as a bulky substituent to
improve the gas permeability of polymer membranes because of
the steric repulsion and high local mobility of silyl groups [24,25],
and a number of silyl-substituted polymers indeed show high gas
permeability [26e29]. Among various silyl groups, a trimethylsilyl
group is the best substituent for poly(substituted acetylene)s in
order to obtain high gas permeable membranes. When a silyl group
becomes bulkier than a trimethylsilyl group, the gas permeability of
membrane becomes lower [6]. These findings motivated us to
examine the gas permeability of membranes of silyl-substituted
PPVs, especially trimethylsilyl group-substituted PPV. In our pre-
vious study [30], the gas permeability of PPV having long alkyl silyl
groups were larger than that of PPV having triethylsilyl and
dimethyl-t-butylsilyl groups. This tendency is opposite to that for
poly(substituted acetylene)s. Unfortunately, the membrane of tri-
methylsilyl group-substituted PPV had not been obtained because
the PPV showed poor solubility.
Poly(p-phenylenevinylene)s [PPV] bearing various substituents
have been intensively researched because of their optical and
electronic properties [19e22]. Substituted PPVs are generally syn-
thesized by the polymerization of bis(halomethyl)benzene using
To reveal the tendency of gas permeability and the effect of
trimethylsilyl groups for the membranes of PPV, in the present
* Corresponding author. Tel.: þ81 776 27 9779; fax: þ81 776 27 8904.
E-mail address: sakaguchi@matse.u-fukui.ac.jp (T. Sakaguchi).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.03.007

T. Sakaguchi et al. / Polymer 54 (2013) 2272e2277
2273
study, bis(bromomethyl)benzenes having two silyl groups were
synthesized and then polymerized by Gilch polymerization. The
PPVs obtained by this route have at least one long alkyl silyl group
in a repeating unit. So, they would exhibit good solubility and afford
the free-standing membranes by solution casting. We investigated
the gas permeability of membranes of silyl-disubstituted PPVs and
discussed the effect of silyl groups on the gas permeability for PPV
membranes (Scheme 1).
[32,33]. The details of the procedures and analytical data of
monomers are stated below.
2.3. 2-Dimethyl-n-octylsilyl-5-trimethylsilyl-p-bis(bromomethyl)
benzene (1a)
A 500 mL three-necked flask was equipped with a dropping
funnel, a three-way stopcock, and a magnetic stirring bar. After
flushing the flask with nitrogen, 2,5-dibromo-p-xylene (20 g,
75.9 mmol) and ether (300 mL) were added and cooled at 0 ꢁC. At
2. Experimental
the same temperature,
a hexane solution of n-butyllithium
(47.4 mL, 1.6 M, 75.9 mmol) was added dropwise, and the mixture
was stirred for 1 h. Then, a solution of chlorotrimethylsilane (9.9 g,
91.1 mmol) in ether (50 mL) was added dropwise at 0 ꢁC, and
stirring was continued further for 12 h at room temperature. The
reaction mixture was washed with water, and dried over anhydrous
sodium sulfate. Ether was evaporated, and the crude product was
purified by silica gel column chromatography (eluent: hexane) to
afford 2-bromo-5-trimethylsilyl-p-xylene (yield 83%).
Silylation of 2-bromo-5-trimethylsilyl-p-xylene was performed
by the same method as for 2-bromo-5-trimethylsilyl-p-xylene us-
ing chlorodimethyl-n-octylsilane instead of chlorotrimethylsilane.
The obtained crude product was purified by silica gel column
chromatography (eluent: hexane) to afford 2-dimethyl-n-octylsilyl-
5-trimethylsilyl-p-xylene (yield 82%).
2.1. Measurements
The molecular weight distributions (MWDs) of polymers were
measured by gel permeation chromatography (GPC) in chloroform
(at a 1.0 mL/min flow rate) at 40 ꢁC on a Shimadzu LC-10AD chro-
matograph equipped with three polystyrene gel columns (Shodex
K-807L, K-805L, K-804L) and a Shimadzu RID-6A refractive index
detector. The number-average molecular weight (Mn) and poly-
dispersity ratio [weight-average molecular weight/number-
average molecular weight (Mw/Mn)] were calculated from chro-
matograms based on a polystyrene calibration. 1H (500 MHz) and
13C (125 MHz) NMR spectra were recorded on Jeol LA-500 instru-
ment in CDCl3 at room temperature. Thermogravimetric analyses
(TGA) were conducted with Rigaku TG-DTA 8078G1 at a 10 ꢁC/min
heating rate. Thermal decomposition temperature (Td) was defined
as the temperature of 5% weight loss of the sample. Differential
scanning calorimetry (DSC) was performed with Rigaku Thermo
Plus DSC 8230L. The temperature range was ꢃ50 to þ300 ꢁC and
the heating and cooling rates were 5 ꢁC/min. During the mea-
surement, the sample was purged by nitrogen gas. Gas permeability
coefficients were measured with a Rikaseiki K-315-N gas perme-
ability apparatus at 25 ꢁC under 1 atm upstream pressure.
2-Dimethyl-n-octylsilyl-5-trimethylsilyl-p-xylene
(8.8
g,
25.2 mmol), NBS (9.0 g, 50.5 mmol), BPO (68 mg, 0.28 mmol), and
CCl4 (200 mL) were charged into a 500 mL flask equipped with
a reflux condenser. The mixture was stirred at 80 ꢁC under N2 for
1 h. The solution was washed with water three times, and then
dried over anhydrous sodium sulfate. After filtration and evapo-
ration, the residue was purified by silica gel column chromatog-
raphy (eluent: hexane) to afford 2-dimethyl-n-octylsilyl-5-
trimethylsilyl -p-bis(bromomethyl)benzene (yield 25%). 1H NMR
(CDCl3, ppm): 7.52 (s, 1H, Ar), 7.50 (s, 1H, Ar), 4.60 (s, 2H, CH2Br),
4.59 (s, 2H, CH2Br), 1.35e1.20 (m, 12H, SiCH2-(CH2)6-CH3), 0.87 (t,
J ¼ 7.1 Hz, 5H, SiCH2-(CH2)6-CH3), 0.40 (s, 9H, Si(CH3)3), 0.38 (s, 6H,
Si(CH3)2). 13C NMR (CDCl3, ppm): 142.2, 142.0, 140.8, 140.3, 137.7,
137.4, 34.2, 34.1, 33.5, 31.9, 29.2, 23.9, 22.7, 16.4, 14.1, 0.3, -1.5. Anal.
Calcd for C21H38Br2Si2: C, 49.80; H, 7.56; Br, 31.55; Si, 11.09. Found:
C, 49.48; H, 7.16.
2.2. Materials
2,5-Dibromo-p-xylene, n-butyllithium in hexane, N-bromo-
succinimide (NBS), benzoyl peroxide (BPO), potassium t-butoxide,
and common organic solvents were commercially obtained from
Wako Pure Chemicals Ind., Ltd. and used without further purifica-
tion. Various chlorosilanes were purchased from Tokyo Chemical
Industry Co., Ltd. THF as polymerization solvent was purified by
distillation twice. Monomers 1c [31] and 1e [32] were synthesized
according to the literatures. The other monomers were synthesized
from 2,5-dibromo-p-xylene by silylation followed by radical
bromination as shown in Scheme 2 according to the literatures
2.4. 2-Dimethyl-n-octylsilyl-5-triethylsilyl-p-bis(bromomethyl)
benzene (1b)
This monomer was prepared by the same method as for 1a
(Overall yield 12%). 1H NMR (CDCl3, ppm): 7.50 (s, 1H, Ar), 7.48 (s,
1H, Ar), 4.58 (s, 2H, CH2Br), 4.57 (s, 2H, CH2Br), 1.34e1.18 (m, 12H,
SiCH2-(CH2)6-CH3), 1.00e0.85 (m, 20H, SiCH2-(CH2)6-CH3,
Si(CH2CH3)3), 0.38 (s, 6H, Si(CH3)2). 13C NMR (CDCl3, ppm): 142.4,
141.9, 139.9, 138.4, 137.8, 137.7, 34.4, 34.3, 33.5, 31.9, 29.2, 23.9, 22.7,
16.4, 14.1, 7.6, 4.2, ꢃ1.6. Anal. Calcd for C24H44Br2Si2: C, 52.55; H,
8.08; Br, 29.13; Si, 10.24. Found: C, 52.30; H, 8.20.
2.5. 2-Dimethyl-n-octylsilyl-5-dimethyl-n-octadecylsilyl-p-
bis(bromomethyl)benzene (1d)
This monomer was prepared by the same method as for 1a
(Overall yield 9%). 1H NMR (CDCl3, ppm): 7.50 (s, 2H, Ar), 4.58 (s, 4H,
Scheme 1. Synthesis of silyl-disubstituted poly(p-phenylenevinylene)s.
Scheme 2. Monomer synthesis scheme.

2274
T. Sakaguchi et al. / Polymer 54 (2013) 2272e2277
Table 1
CH2Br), 1.34e1.21 (m, 44H, SiCH2e(CH2)6eCH3, SiCH2e(CH2)16e
CH3), 0.90e0.83 (m, 10H, SiCH2e(CH2)6eCH3, SiCH2e(CH2)16e
CH3), 0.38 (s, 12H, Si(CH3)2, Si(CH3)2). 13C NMR (CDCl3, ppm):
142.1,140.1,137.7, 34.3, 33.52, 33.51, 31.92, 31.91, 29.70, 29.68, 29.66,
29.59, 29.4, 29.3, 29.24, 29.22, 23.9, 22.69, 22.66, 16.4, 14.1, ꢃ1.5.
Anal. Calcd for C38H72Br2Si2: C, 61.27; H, 9.74; Br, 21.45; Si, 7.54.
Found: C, 60.96; H, 9.65.
Polymerization of monomer 1a in THF.
[t-BuOK]0 Temp. Time [h] Yielda [%] Mn
Mw/Mn
b
b
Run [M]0
[mM] [mM]
[ꢁC]
1
2
3
4
5
20
20
20
40
80
60
60
0
25
0
2
15
2
49
41
49
63
82
12,600 8.48
17,300 6.53
32,300 7.22
76,600 4.11
insoluble
120
240
480
0
0
2
2
2.6. Polymerization
a
Acetone-insoluble product.
Measured by GPC.
b
Polymerization was carried out in a round-bottom flask equip-
ped with a dropping funnel and a three-way stopcock under dry
nitrogen. A detailed procedure of polymerization is as follows:
Monomer 1a (1.01 g) and dry THF (30 mL) were charged in the flask.
To this solution was slowly added a solution of potassium tert-
butoxide (1.35 g) in dry THF (20 mL) at 0 ꢁC. After 2 h, the reaction
mixture was then poured into the mixture of methanol and acetone
(1:1). The precipitate was dissolved in chloroform and reprecipi-
tated in the mixture of methanol and acetone. Its yield was deter-
mined by gravimetry.
resultant polymers (runs 1 and 2), but increase of concentration of
base improved the Mn of the resultant polymers (run 3). When the
polymerization was operated at higher concentrations of monomer
and base, the yield and Mn of the polymer became 63% and 76,600,
respectively (run 4). However, the polymerization at much higher
concentrations of monomer and base afforded an insoluble poly-
mer (run 5). Therefore, the polymerizations of the other monomers
were examined under the same conditions as those of run 4 in
Table 1. The silyl-disubstituted PPVs (3b-e) were obtained in rela-
tively high yields, whose Mn’s were in the range of 98,700e260,000
(Table 2).
2.7. Membrane preparation
The 1H NMR spectra of polymers were measured in CDCl3
(Fig. 1). In all the spectra, two broad peaks at around 7.90 and
7.45 ppm were observed, which are assigned to the protons on the
vinylene segments and benzene rings, respectively. The peaks
assigned to the protons on alkyl silyl groups were observed at
around 1.40, 0.85, and 0.45 ppm. In the spectra of 2a and 2b, a very
weak peak around 3.1 ppm is observed, which originates from the
resonance of bisbenzyl moieties. This is a type of structural defect in
the PPV backbone. It is reported that such defect is formed by the
head-to-head linkage of p-xylylene intermediates during poly-
merization [35]. The amount of bisbenzyl defects is calculated to be
approximately 8.5% based on the peak intensity ratio of bisbenzyl
moiety to vinylene moiety. Polymers 2cee should have the struc-
tural defect in the backbone, but the peak could not be observed
probably because the peak intensity is too low toword the peak
intensity of alkyl groups.
Membranes of 2bee were prepared by casting toluene solution
of polymers (concn. 1.0e2.0 wt%) onto a glass plate. The plate was
covered with a glass vessel to slow down solvent evaporation (ca. 3
days). After drying, the membrane was peeled off, and it was
further dried at 25 ꢁC under reduced pressure for 24 h. However,
membrane of 2a was prepared by casting CHCl3 solution of the
polymer because 2a did not dissolved in toluene completely. The
thickness of membranes used for measurement is in the range of
50e80 mm.
2.8. Fractional free volume (FFV) of polymer membranes
The densities of membranes were determined by hydrostatic
weighing using a Mettler Toledo balance and a density determi-
nation kit. In this method, a liquid with known density (r0) is
needed, and the membrane density (r) is given by the following
equation:
3.2. Solubility and thermal stability of silyl-disubstituted PPV
r
¼
r0 ꢂ MA=ðMA ꢃ MLÞ
The solubility of silyl-dusbstituted substituted PPVs is summa-
rized in Table 3. Trimethylsilyl-substituted PPV (2a) showed poor
solubility. Polymer 2a was soluble only in CHCl3, and partly soluble
in hexane, toluene, CH2Cl2, and THF. Specifically, most part of 2a
was completely soluble in toluene and THF, but a small amount of
insoluble part was observed in those solvents. On the other hands,
PPVs having two long alkyl silyl groups in a repeating unit (2bee)
exhibited relatively good solubility, and they totally dissolved in
toluene, CHCl3, CH2Cl2, and THF.
where MA is membrane weight in air and ML is membrane weight in
the auxiliary liquid. An aqueous sodium nitrate was used as the
auxiliary liquid. FFV is calculated by the following equation:
À
Áꢀ
À
Áꢀ
FFV ¼ vsp ꢃ v0 vspz vsp ꢃ 1:3vw vsp
where vsp is the polymer specific volume, and v0 is the occupied
volume of the polymer. The occupied volume is typically estimated
as 1.3 times the van der Waals volume (vw), which is calculated
using the group contribution method [34].
The thermal stability of silyl-disubstituted PPVs in N2 was
evaluated by TGA (Fig. 2). Except for polymer 2c, thermal
3. Results and discussion
Table 2
Polymerization of monomers 1bee.a
3.1. Synthesis of silyl-disubstituted PPV
c
c
Run
Monomer
yieldb [%]
Mn
Mw/Mn
1d
2
3
1b
1c
1d
1e
49
40
77
81
125,000
98,700
260,000
226,000
4.59
3.97
2.91
2.23
The monomers can be easily polymerized by the Gilch poly-
merization using potassium tert-butoxide as a base in dry THF. In
the present study, relatively high molecular weight polymers are
desired in order to prepare free-standing membranes by solution
casting. Then, the polymerizations of monomer 1a were examined
under various conditions (Table 1). Increase of polymerization
temperature hardly affected the yield and molecular weight of the
4
a
In THF at 0 ꢁC for 2 h; [M]0 ¼ 40 mM, [t-BuOK] ¼ 240 mM.
Acetone-insoluble product.
Measured by GPC.
[M]0 ¼ 80 mM, [t-BuOK] ¼ 480 mM.
b
c
d

T. Sakaguchi et al. / Polymer 54 (2013) 2272e2277
2275
Fig. 1. 1H NMR spectra of 2aee in CHCl3.
decomposition temperatures (5% weight loss) of the PPVs were
over 310 ꢁC, indicating good thermal stability. The weight loss
patterns are similar for all the silyl-disubstituted PPVs. The weight
losses became from around 230 ꢁC, and the weights greatly
decreased to around 500 ꢁC, and then they were slightly decreased.
On the basis of the percentage of weight loss consideration, it was
found that the PPVs lose silyl side groups in the first, and then
decomposition of the main chains occurred. The decomposition
temperature of 2c was 410 ꢁC, which is higher than those of the
other silyl-disubstituted PPVs. The weight residue was approxi-
mately 40% at 500 ꢁC, and the value does not correspond to the
Table 3
Solubility of polymers.a
Polymer Hexane Toluene CHCl3 CH2Cl2 THF Acetone DMF DMSO
2a
2b
2c
2d
2e
ꢄ
ꢄ
ꢄ
þ
þ
ꢄ
þ
þ
þ
þ
þ
þ
þ
þ
þ
ꢄ
þ
þ
þ
þ
ꢄ
þ
þ
þ
þ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
a
Symbols; þ: soluble, ꢄ: partly soluble, ꢃ: insoluble.
Fig. 2. TGA thermograms of polymers 2aee (in N2, heating rate 10 ꢁC/min).

2276
T. Sakaguchi et al. / Polymer 54 (2013) 2272e2277
Table 4
Gas permeability coefficients (P)a and densities of membranes at 25 ꢁC.
Membrane PO2 PN2 PCO2 PO2/PN2 PCO2/PN2 density [g/cm3] FFV
2a
2b
2c
2d
2e
5.4 1.5
6.0 1.6
22
23
67
3.7
3.8
3.0
2.9
15
15
14
14
15
1.02
1.00
0.950
0.929
0.976
0.095
0.095
0.14
0.15
0.10
15
20
4.9
7.1
100
9.2 3.0
1.8 0.60
a
In the unit of barrer (1 barrer ¼ 1 ꢂ 10ꢃ10 cm3(STP) cm cmꢃ2 sꢃ1 cmHgꢃ1).
percentage of weight loss for silyl side groups. However, a
convincing explanation could not be provided for the distinct result
of 2c.
3.3. Gas permeability of membranes of silylated PPVs
The free-standing membranes of 2aee could be prepared by
solution casting. The gas permeability coefficients, separation fac-
tors, and fractional free volume (FFV) are shown in Table 4. The
oxygen permeability coefficients (PO2) of membranes of 2a and 2b
were 5.4 and 6.0 barrers, respectively. The other gas permeability
and separation factors of 2a were almost the same as those of 2b.
The FFVs of these two membranes were also the same, which
suggests that long alkyl silyl groups mainly control the formation of
membranes and the difference between trimethylsilyl and trie-
thylsilyl dose not affect the gas permeability. Dimethyl-n-octylsilyl
group-disubstituted PPV (2c) exhibited higher gas permeability
than 2a and 2b, and dimethyl-n-octylsilyl and dimethyl-n-octade-
cylsilyl groups-substituted PPV (2d) showed even higher gas
permeability than 2c. For example, the PO2 of 2c and 2d were 15
and 20 barrers, respectively, which are more than twice as large as
those of 2a and 2b. The separation factors of 2c and 2d were smaller
than those of 2a and 2b, which agrees with the general tendency
that highly gas permeable polymers show low gas separation
ability. The FFV values of 2c and 2d were also much larger than
those of 2a and 2b. These results indicated that a long alkyl chain is
important for the gas permeability in substituted PPV. The high gas
Fig. 4. Relationship between the PO2 and the number of carbon in polymer side chains
for poly(p-phenylenevinylene)s (C) and poly(p-phenyleneethynylene)s (,).
permeability of PPV having long alkyl chains would be due to that
flexible alkyl chains assist gas diffusion and long chains prevent
pe
p
stacking of conjugated main chains. On the contrary, dimethyl-n-
octadecylsilyl group-disubstituted PPV (2e) showed the lowest gas
permeability among the disubstituted PPVs in the present study.
The low gas permeability of membrane of 2e is due to that 2e is a
crystalline polymer. The membrane of 2e was translucent, and the
membranes of 2aed were transparent. The DSC thermograms on
first heating scan of membranes of 2aee are shown in Fig. 3. In the
thermograms of 2aed, there is no peak in the present temperature
range, while polymer 2e showed melting of crystallites around
28 ꢁC, indicating that membrane of 2e contains crystalline parts.
The relation between the gas permeability and the length of side
chains in the membranes of PPVs is the same as that in poly(p-
phenyleneethynylene)s (PPE) having long alkyl groups we reported
[36]; i.e., the gas permeability increases as the alkyl side chains
become longer in PPV and PPE. In poly(diphenylacetylene)s, how-
ever, the effect of substituents is opposite to PPV and PPE [37].
When poly(diphenylacetylene), one of the highest gas permeable
polymers [6], has spherical bulky substituents such as trimethylsilyl
groups as a side group, the polymer forms a sparse membrane
(FFV ¼ 0.26 [38]) because the stacking between polymer chains is
prevented by steric repulsion of bulky substituents [39]. On the
other hands, the FFV value drastically decreases when poly(-
diphenylacetylene) possesses long alkyl groups. This would be due
to that the flexible long alkyl chains occupy the molecular-scale
voids indigenous to poly(diphenylacetylene) membrane. In
contrast to poly(diphenylacetylene), PPV and PPE afford rather
dense membranes and they have little molecular-scale voids.
Therefore, the role of long alkyl chains on PPV and PPE is different
from that on poly(diphenylacetylene).
In PPV and PPE, the PO2 versus the number of carbon in the side
chains is plotted in Fig. 4. Interestingly, the PO2 values increase in
direct proportion to the number of carbon regardless of polymer
backbone, suggesting that the percentage of flexible alkyl side
chain mainly affects the gas permeability. It is concluded that the
long alkyl chains would play a role of gas diffusion area as well as
generate free volume on PPV and PPE.
4. Conclusions
This paper revealed the gas permeability of membranes of silyl-
disubstituted poly(p-phenylenevinylene)s. Poly(p-phenyleneviny
lene)s with long alkyl silyl groups such as dimethyl-n-octylsilyl and
Fig. 3. DSC thermograms of membranes 2aee (in N2, heating rate 5 ꢁC/min, first
heating scan).

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dimethyl-n-octadecyl groups had relatively high molecular weights
and good solubility, and afford their tough free-standing membranes
by solution casting. The PPVs having two long alkyl silyl groups in a
repeating unit showed higher gas permeability than PPVs having a
long alkyl silyl group in a repeating unit. However, the PPV having two
dimethyl-n-octadecylsilyl groups in a repeating unit exhibited the
lowest gas permeability because of its crystallinity. It is found that the
long alkyl chains would play roles of gas diffusion matrix as well as
generate free volume in PPV and PPE.
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