Structural study on rodlike aromatic polyimides derived by solid-state thermal and chemical imidization of poly(amic n-dodecyl ester)

Article abstract of DOI:10.1016/j.molstruc.2004.04.033

Para-linked aromatic poly(amic n-dodecyl ester) (PA-12), in which long n-alkyl side chains are attached on rodlike aromatic polyamide, was prepared by polycondensation of 2,5-bis(1-dodecyloxycarbonyl)terephthaloyl chloride and 1,4-diaminobenzene. In as-cast PA-12 film, formation of layered structure with alternating main-chain-segregated and side-chain-segregated layers is suggested by X-ray diffraction measurement. PA-12 is a precursor of rodlike aromatic polyimide, poly(1,4-phenylene pyromellitimide) (PPPI). Solid-state thermal imidization and base-catalyzed chemical imidization of PA-12 film has been investigated. Although the layered structure of PA-12 was destroyed during the imidization reaction, the obtained PPPI exhibited lower density than densely packed PPPI. This result may indicate that the side chain-segregated domain was transformed to a cavity surrounded by PPPI after the imidization reaction. Chemical imidization in pyridine/toluene mixture and subsequent heating at 200°C was the optimum condition to obtain low density PPPI.

Full text of DOI:10.1016/j.molstruc.2004.04.033

Journal of Molecular Structure 739 (2005) 117–123
www.elsevier.com/locate/molstruc
Structural study on rodlike aromatic polyimides derived by solid-state
thermal and chemical imidization of poly(amic n-dodecyl ester)
*
Katsuhiro Inomata , Yuki Ozeki, Sachi Shimomura, Yuhji Sakamoto, Eiji Nakanishi
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Received 31 March 2004; revised 6 April 2004; accepted 12 April 2004
Available online 24 November 2004
Abstract
Para-linked aromatic poly(amic n-dodecyl ester) (PA-12), in which long n-alkyl side chains are attached on rodlike aromatic polyamide,
was prepared by polycondensation of 2,5-bis(1-dodecyloxycarbonyl)terephthaloyl chloride and 1,4-diaminobenzene. In as-cast PA-12 film,
formation of layered structure with alternating main-chain-segregated and side-chain-segregated layers is suggested by X-ray diffraction
measurement. PA-12 is a precursor of rodlike aromatic polyimide, poly(1,4-phenylene pyromellitimide) (PPPI). Solid-state thermal
imidization and base-catalyzed chemical imidization of PA-12 film has been investigated. Although the layered structure of PA-12 was
destroyed during the imidization reaction, the obtained PPPI exhibited lower density than densely packed PPPI. This result may indicate that
the side chain-segregated domain was transformed to a cavity surrounded by PPPI after the imidization reaction. Chemical imidization in
pyridine/toluene mixture and subsequent heating at 200 8C was the optimum condition to obtain low density PPPI.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Polyamide; Polyimide; Chemical imidization; Layered structure; Density
1. Introduction
(carbon number nO16), ordinary layered mesophase was
observed below the mesophase-isotropic transition tem-
perature. On the other hand, polyesters with shorter alkyl
side chains than nZ12 exhibited hexagonal columnar
phase, in which the alkyl side chains form segregated
cylindrical domain, and the strongly associated main chain
are surrounding the cylindrical domain like honeycomb
structure. These layered and honeycomb structures have
analogy with the microphase separated structure of diblock
copolymer, i.e., lamellar and cylinder structure [10,11]. In
diblock copolymers, the segregation power of the consisting
block chains induces the formation of segregated domains,
and its morphology depends on the volume fraction of each
block. When the volume fraction of the diblock copolymer
is asymmetric, the minor block forms cylindrical domain
and the major block surrounds the cylinder as the matrix
domain. The formation of honeycomb or layered structure
in rodlike polyesters with depending on the alkyl chain
length can also be described by the volume fraction of main
chain and side chain.
Rodlike aromatic polyesters, polyamides, and poly-
imides are expected as high performance materials because
of their excellent mechanical and thermal properties. Some
rodlike polymers, on which flexible side chains are
introduced, have been investigated for the purpose of
good processability and solubility. Because of the difference
in chemical structure of the aromatic main chain and
aliphatic side chain, it has been reported that the main
chains and side chains forms respective segregated domains
[1–7]. As the result of this micro-segregation, these rodlike
polymers form ordered morphology, i.e., layered structure
with alternating main chain- and side chain-segregated
layers.
Recently, Watanabe et al. [8,9] reported a unique
mesophase in rodlike polyesters, poly(p-biphenylene
terephthalate) with flexible alkoxy side chains on the
terephthalate moiety (PBpT-On). For long alkyl side chains
Because the size of the side-chain-segregated domain
depends on the alkyl chain length, the diameter
* Corresponding author. Tel./fax: C81 52 735 5274.
E-mail address: inomata.katsuhiro@nitech.ac.jp (K. Inomata).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.04.033

118
K. Inomata et al. / Journal of Molecular Structure 739 (2005) 117–123
distribution of the cylinder should be sharp with
nanometer order. The segregated alkyl-chain domain
and matrix aromatic main chain domain have different
physical properties, i.e., the former contains flexible non-
polar chains and liquid-like above w50 8C, and the latter
contains rigid aromatic and polar ester group. In this
sense, the surrounded cylindrical domain can be regarded
as a liquid-like path in the rigid matrix, and the diameter
of the path is close to low-molecular-weight substances.
If the alkyl chains can be removed without any influence
on the matrix domain, a unique mesoporous material
[12–14] with nanometer-sized voids or cavities will be
possibly prepared.
with high tensile modulus by drawing the precursor fiber
before imidization. On the other hand, there have been only
limited reports about the chemical imidization of poly(amic
alkyl ester)s [22].
The objective of this report is to perform the
imidization reaction for PA-12 in solid-state. If the
imidization reaction and removal of the alkyl chain
proceed with maintaining the morphology of PA-12, the
alkyl-chain-segregated domains in PA-12 will be trans-
formed to cavities in fully aromatic polyimide, poly(1,4-
phenylene pyromellite imide) (PPPI). For application use,
these ‘molecular cavity’ may be suitable for separating
membranes in order to separate substances by their
molecular size. In this report, we have investigated the
change of morphology formed by PA-12 after the
chemical and thermal imidization reactions to PPPI by
means of infrared spectrum, X-ray scattering, and density
measurements.
Following the above considerations, we have prepared
rigid rodlike polyamide with flexible alkyl side chains. Para-
linked aromatic poly(amic n-dodecyl ester) (PA-12),
polymerized by polycondensation of 2,5-bis(1-dodecyloxy-
carbonyl)terephthaloyl chloride and 1,4-diaminobenzene,
have similar molecular structure with the rodlike polyesters
by Watanabe et al. [8,9], and is expected to form the main-
chain- and side-chain-segregated domains. As is well
known, poly(amic alkyl ester)s have been used as precursor
of polyimides because its chemical stability is better than
that for poly(amic acid)s [15–22]. Thermal imidization of
poly(amic alkyl ester)s with short alkyl chain length have
been investigated in detail [15–21]. In this case, the
cyclization reaction from the amic alkyl ester to imide
ring occurs with removing alkyl alcohol. Kakimoto et al.
[20] reported the structure and mechanical properties
of poly(amic alkyl ester)s with long alkyl chains and
poly(p-phenylene pyromellitimide) derived by thermal
imidization, and succeeded to prepare oriented PI fiber
2. Experimental
2.1. Materials
Outline of polymer synthesis is shown in Scheme 1
[4,18]. Twenty grams of pyromellitic dianhydride and
equimolar 1-decanol was heated at 150 8C for 1 h. Because
the reaction mixture contains para (1) and meta (2) isomers,
400 ml of acetone was added and stirred for 24 h in order to
remove the meta isomer. This extraction process was
repeated for three times. Finally, the precipitate was
thoroughly washed with dichloromethane, and pure para
Scheme 1.

K. Inomata et al. / Journal of Molecular Structure 739 (2005) 117–123
119
isomer, 2,5-bis(1-dodecyloxycarbonyl)terephthalic acid (1),
3. Results and discussion
was isolated. The purity was deterimed by 1H NMR spectra
(solvent: DMSO-d6): the para isomer has two equivalent
aromatic protons at 8.0 ppm, and meta isomer contains two
inequivalent protons at 7.9 and 8.1 ppm.
3.1. Layered structure of PA-12
WAXD measurement for as-cast PA-12 film was
performed as the film surface is parallel to the incident
X-ray beam, and the obtained WAXD photograph is shown
in Fig. 1. A strong reflection with the spacing dZ1.6 nm
locates on equator line, and a weak reflection at dZ1.2 nm
is observed on meridional line. The spacing for the latter
reflection coincides with the repeating distance of rodlike
PA-12 along the molecular axis. This result suggests that
PA-12 molecule is oriented parallel to the film surface,
which seems reasonable because the rodlike polymers
preferably lie on the film surface. Therefore, the strong
reflection at dZ1.6 nm should correspond to the inter-
molecular distance of PA-12. As reported by Watanabe
et al. [9], the rodlike polyester with n-dodecyl side chains,
PBpT-O12, exhibited hexagonal columnar mesophase and
its d-spacing was observed at 2.34 nm. Extrapolation of the
d-spacing for the layered mesophase of PBpT-O series with
longer n-alkyl side chains estimates the d value as 1.8 nm
for nZ12. From these comparisons, PA-12 is most probably
taking a layered structure in which the main-chain and side-
chain-segregated layers are stacking repeatedly. It should be
noted that there is amorphous halo around dZ0.44 nm but
no reflection from crystallized alkyl chain can be detected.
Because the solvent evaporation was conducted at 100 8C,
alkyl chain was in liquid state and was not crystallized even
at room temperature.
The para isomer of di-n-dodecyl ester (1) (7.0 g) was
added excess thionyl chloride (4 molar equiv.) and a drop of
N,N-dimethylformamide, and heated to reflux for 2 h. After
the reaction, the excess thionyl chloride was removed by
distillation under reduced pressure. Residual thionyl
chloride was further removed by adding benzene to the
remaining solid and continuing the distillation. Finally, 2,5-
bis(1-dodecyloxycarbonyl)terephthaloyl chloride, (3), was
obtained as white solid after drying in vacuum.
1,4-diaminobenzene (0.54 g) was dissolved in hexa-
methylphosphoric triamide (HMPA) (10 ml) in three-
necked round-bottomed flask. Benzene solution (10 ml) of
propylene oxide (0.66 g) and dichloride of di-n-dodecyl
ester (3) (2.997 g) was added to the flask at 0 8C, and stirred
for 24 h at room temperature. After the polycondensation
reaction, and viscous solution was diluted by HMPA/ben-
zene mixture, and poured into excess methanol. The
precipitated solid, PA-12, was washed in refluxing methanol
for 30 min, filtered, and dried in vacuum.
As-cast film of PA-12 was prepared by casting the
HMPA/benzene solution of PA-12 on flat glass plate, and
the solvent was evaporated at 60–100 8C for 7 h under
reduced pressure. Thickness of the obtained film was
1–2 mm.
Thermal imidization was conducted by heating the PA-
12 film at elevated temperature under reduced pressure.
Chemical imidization of the PA-12 film was conducted by
soaking the film in pyridine or pyridine/toluene mixture at
room temperature or 80 8C. After washing the film in
toluene, the imidized film was dried in vacuum at 100 8C.
Further heat treatment was performed at various tempera-
tures under reduced pressure.
3.2. Thermal imidization
Thermal properties and thermal imidization process for
PA-12 was investigated by TGA measurement. As shown in
Fig. 2, the decrease of sample weight starts around 160 8C,
and the degree of the loss is about 55% in the range of
160–550 8C. This value shows good coincidence with the
calculated weight loss of dodecanol during the thermal
imidization and formation of PPPI. The second weight loss
started at w600 8C, corresponds to the degradation of PPPI.
According to the TGA results, thermal imidization
reaction of PA-12 was performed at temperatures of 150 to
2.2. Measurements
Degree of imidization was investigated by Fourier
transform infrared (FT-IR) spectra recorded with AVATAR
320-FT-IR (Nicolet). Wide-angle X-ray diffraction
(WAXD) photographs were recorded with Ni-filtered
Cu Ka radiation and a flat film as detector. The reflection
spacing, d, was evaluated by using Bragg’s equation,
2d sinqZl, where 2q and l are scattering angle and
wavelength of the X-ray beam (0.15418 nm), respectively.
The value of 2q was calibrated against the 111 reflection of
silicone powder. Thermogravimetry analysis (TGA) was
done using TG30 (Seiko Instruments) under nitrogen
atmosphere with heating rate of 10 8C/min. Densities of
the samples were determined by floatation method using
potassium bromide aqueous solution.
Fig. 1. WAXD photograph for as-cast film of PA-12. The normal of the film
is placed in the horizontal direction.

120
K. Inomata et al. / Journal of Molecular Structure 739 (2005) 117–123
Table 1
Thermal imidization of PA-12
Temperature (8C)
aimide
aalkyl
r (g cmK3
)
150
160
170
180
190
200
250
300
350
0.28
0.32
0.42
0.42
0.39
0.76
0.92
0.80
0.85
0.85
0.77
0.68
0.72
0.71
0.42
0.13
0.03
0.02
1.24
1.29
1.40
1.46
subsequently heated sample showed highest degree of
imidization and was defined as aimideZ1.0. aimide and
aalkyl are listed in Table 1, and plotted against the
temperature for thermal imidization in Fig. 4. The value of
aimide and aalkyl shows sharp increase and decrease,
respectively, in the range of 190–200 8C. Fig. 5 shows the
relation between aimide and aalkyl (C). The linear relation-
ship along the straight line of aalkylZ1Kaimide suggests that
the removal of 1-dodecanol occurs immediately after the
formation of the imide ring.
Fig. 2. TGA curve for PA-12 measured at the heating rate of 10 8C/min.
375 8C under reduced pressure for 12 h. Progress of the
imidization reaction was monitored by FT-IR spectrum. In
Fig. 3(a), the IR spectrum of the as-cast PA-12 is indicated.
We can recognize characteristic amide absorptions at
approximately 3400 cmK1 for the N–H stretch, 1655 cmK1
for the amide carbonyl stretch (amide I), and absorption for
C–H stretch in alkyl chain around 2800–3000 cmK1. The
sample after the thermal imidization at 350 8C is shown in
Fig. 3(b). As the result of imidization and elimination of n-
dodecanol, amide and alkyl absorptions disappeared and the
characteristic imide absorptions begin to be observed at
1778 cmK1 for imide carbonyl stretch (imide I). Degree of
progress of the imidization reaction from PA-12 to PPPI,
aimide, was evaluated by the peak height of imide I band at
1778 cmK1, and the fraction of the remaining alkyl chain,
aalkyl, was evaluated by the absorption at 2922 cmK1, with
using the aromatic ring absorption at 1515 cmK1 as internal
reference. As will be shown below, chemically imidized and
WAXD patters for the thermally imidized samples are
shown in Fig. 6. The strong reflection at dZ1.6 nm,
assigned to the repeating distance for layered structure, is
still observed for the sample imidized at 150 8C as
indicated by arrow in Fig. 6(a). However, this reflection
becomes broad after imidized at 200 8C, and cannot be
detected after the imidization above 250 8C as shown in
Fig. 6(b). This means that the layered structure for
PA-12 was not maintained by the thermal imidization
reaction above 250 8C. Instead, sharp reflections at
dZ0.60 nm (indicated by the arrow in Fig. 6(b)) and
0.30 nm started to be appeared, these correspond to 002
and 004 reflections for the crystallized PPPI [23] . From
these results, we can conclude that the layered structure
in PA-12 cannot be maintained and densely packing of
Fig. 3. FT-IR spectra for (a) PA-12, (b) thermally imidized sample at
350 8C, (c) chemically imidized sample in pyridine/toluene mixture at
80 8C for 96 h, and (d) the sample of (c) was heated at 200 8C.
Fig. 4. Degree of imidization (aimide, C) and fraction of remained alkyl
chain (aalkyl, &) plotted against temperature for thermal imidization.

K. Inomata et al. / Journal of Molecular Structure 739 (2005) 117–123
121
Fig. 7. Density for thermally imidized (C), chemically imidized (&), and
chemically imidized and subsequently heated samples (:, heated at
150 8C (a) and 200 8C (b)). The continuous line indicates the calculated
density.
Fig. 5. aimide vs. aalkyl for thermally imidized (C), chemically imidized
(&), and chemically imidized and subsequently heated samples (:).
the PPPI chains occurs during the thermal imidization
above 250 8C.
3.3. Chemical imidization
Results of density measurements for the thermally
imidized samples are shown in Table 1. The density, r,
for PPPI via poly(amic acid) has been reported in the range
of 1.50–1.56 in Refs. [23–26]. As shown in Table 1, r values
for PPPI prepared by high-temperature thermal imidization
from PA-12 fall close to this range, but PPPI imidized at
lower temperature exhibit smaller r value. In Fig. 7, r
values are plotted against aimide (C). Continuous line in the
figure shows the calculated density with assuming the
additive rule of PA-12 (1.15 g cmK3) and PPPI via
poly(amic acid) (1.53 g cmK3). With the increase of
aimide, the density tends to increase to that for PPPI.
However, some experimental values locate under the
continuous line, means that the density cannot be rep-
resented by simple additive rule of PA-12 and PPPI and
smaller than the expected. This result may suggest that,
although the ordered layered structure cannot be detected
from the WAXD pattern, the alkyl-chain-segregated domain
still exists as a cavity after the imidization and removal of
1-dodecanol, which make the experimental density of PPPI
Solid-state chemical imidization was performed by
soaking the PA-12 film in basic solvent, pyridine [22].
The imidization conditions are listed in Table 2, and the IR
spectrum for the imidized sample no. 5 is shown in Fig. 3(c).
As like that for the thermally imidized sample, decrease in
amide absorption and increase in imide absorption can be
recognized. aimide and aalkyl were also evaluated and listed
in Table 2. Comparison of no. 1 and 3 suggest that the
dilution of the base by toluene (volume ration of pyridine/
tolueneZ1:5) depress the imidization reaction. Decrease of
the soaking time and temperature also decrease the value of
aimide. Plot of aimide vs. aalkyl for chemical imidization is
shown in Fig. 5 by filled square. These plots also lie on the
straight line, means that the elimination and removal of
1-dodecanol occurs as the progress of the imidization
reaction. From these results, we can point out that the
chemical imidization is a suitable method to control the
degree of imidization.
WAXD patterns for these imidized samples of no. 2
and no. 5, shown in Fig. 8(a) and (b), respectively,
indicate that the layered structure in PA-12 is respect-
ively destroyed and maintained after the chemical
imidization. From the comparison of the WAXD patters
lower than the predicted one from aimide
.
Table 2
Chemical imidization of PA-12 at various conditions
No. Solvent
Tem-
Time (h) aimide aalkyl r (g cmK3
)
perature
1
2
3
Pyridine
Pyridine
Pyridine/
toluene
80 8C
80 8C
80 8C
24
96
24
0.59
0.67
0.29
0.44
0.24
0.85
1.33
4
5
Pyridine/
toluene
r. t.
96
96
0.23
0.48
0.74
0.65
Pyridine/
toluene
80 8C
1.19
Fig. 6. WAXD photographs for thermally imidized samples at (a) 150 8C
and (b) 250 8C.

122
K. Inomata et al. / Journal of Molecular Structure 739 (2005) 117–123
Fig. 8. WAXD photographs for chemically imidized samples of (a) no. 2, (b) no. 5, and (c) no. 5-2 in Tables 2 and 3. The arrow in (b) indicate the reflection at
dZ1.6 nm, assigned to the layered structure.
and the value of aimide, maintenance of the layered
structure seems to be achieved only when aimide is less
than 0.5.
the layered structure, the density of PPPI should be smaller
than that for PA-12. All the imidized samples in this report
had larger r values than PA-12, so coagulation and packing
of PPPI chains should be occurring to some extent.
However, their densities are certainly smaller than that for
the densely packed PPPI, suggesting the existence of cavity
originated by the removal of alkyl chain. The degree of
positional order of the layered structure become lower, and
the cavity should be located randomly in PPPI. This low-
density structure can be maintained until 200 8C because
PPPI is rigid rodlike polymer with high glass transition
temperature.
The density after the chemical imidization is also listed
in Table 2. Although the imidization for no. 5 proceeded
to 48%, its density remained at almost same value for
PA-12. We can point out that the imidized PA-12 of no. 5
contains a lot of cavity with maintaining the layered
structure.
The sample of no. 5 in Table 2 was subsequently heated
under vacuum for 12 h in order for progress of the
imidization reaction. Temperatures for the thermal treat-
ment are listed in Table 3, and the resulting aimide, aalkyl
,
and r values are also indicated. The thermal treatment at
150 8C did not influence these values as shown in no. 5-1
and arrow (a) in Fig. 7. On the other hand, heating
above 200 8C accelerate the imidization reaction as
shown in FT-IR spectrum in Fig. 3(d), which is consistent
with the thermal imidization results in Table 1. It should be
noted that the sample heated at 200 8C, no. 5-2 in Table 3
and arrow (b) in Fig. 7, exhibits small density value,
1.21 g cmK3, instead of the high degree of imidization
(aimideZ0.83). This value is lower than that for thermally
imidized sample at 200 8C.
The WAXD patter for this sample revealed that the
reflection at dZ1.6 nm, assigned to the layered structure, is
disappeared (Fig. 8(c)). From these results, we may assume
the solid-state structure of PPPI prepared from PA-12 as
follows. Chemical imidization and elimination of 1-do-
decanol by using pyridine as base progress in solid-state. If
the alkyl chains are successfully removed with maintaining
4. Conclusion
Structure change during solid-state thermal and chemical
imidization of para-linked aromatic poly(amic n-dodecyl
ester)s (PA-12) have been investigated. PA-12 formed the
layered structure with main-chain-segregated and side-
chain-segregated layers, and the repeating distance was
1.6 nm. Thermal imidization and base-catalyzed chemical
imidization of PA-12 progressed with the formation of
imide ring and elimination of 1-dodecanol in solid-state.
The layered structure of PA-12 was destroyed during the
imidization reaction when the degree of reaction was larger
than 0.5. However, the obtained PPPI exhibited lower
density than the predicted by simple additive rule of PA-12
and PPPI. This result may indicate that the side-chain-
segregated domain was maintained as a cavity, as the result
of the removal of side chains, surrounded by PPPI after the
imidization reaction.
Table 3
Effect of the thermal treatment after the chemical imidization of PA-12a
No.
Temperature (8C)
aimide
aalkyl
r (g cmK3
)
Acknowledgements
5-1
5-2
5-3
5-4
150
200
250
300
0.48
0.83
1.00
0.88
0.62
0.22
0.18
0.07
1.16
1.21
The authors gratefully acknowledge the financial support
from a Grant-in-Aid for Scientific Research on Priority
Areas (B) “Novel Smart Membranes Containing Controlled
Molecular Cavity” from the Ministry of Education, Culture,
Sports, Science and Technology (Japan).
aAll the samples was chemically imidized in pyridine/toluene at 80 8C for
96 h before the thermal treatment. Thermal treatment was performed at the
indicated temperature for 12 h.

K. Inomata et al. / Journal of Molecular Structure 739 (2005) 117–123
123
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