Micro-flowers of poly(p-phenylene pyromelliteimide) crystals

Article abstract of DOI:10.1016/j.polymer.2010.12.022

Morphology control of poly(p-phenylene pyromelliteimide) (PPPI) crystals was examined using reaction-induced crystallization of oligomers during solution polymerization of self-polymerizable N-(4′-aminophenyl)-3-carboxyl-4- alkoxycarbonylphthalimide. Micr

Full text of DOI:10.1016/j.polymer.2010.12.022

Polymer 52 (2011) 837e843
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Micro-flowers of poly(p-phenylene pyromelliteimide) crystals
,
Kanji Wakabayashi a, Tetsuya Uchida b, Shinichi Yamazaki a, Kunio Kimura a
*
a Graduate School of Environmental Science, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
b Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Morphology control of poly(p-phenylene pyromelliteimide) (PPPI) crystals was examined using reaction-
induced crystallization of oligomers during solution polymerization of self-polymerizable N-(40-amino-
phenyl)-3-carboxyl-4-alkoxycarbonylphthalimide. Micro-flowers of the PPPI needle-like crystals were
formed in which the needle-like crystals grew radially from the center part as petals. The molecules
aligned regularly along the long axis of the needle-like crystal. The structure of alkoxy group in the
monomer and the monomer concentration influenced the size of the needle-like crystals, and their
Received 20 September 2010
Received in revised form
28 November 2010
Accepted 12 December 2010
Available online 21 December 2010
average length and width were changeable from 640 nm to 1.69 mm and from 110 nm to 210 nm,
respectively. The average thickness was 20 nm. The obtained micro-flowers possessed high crystallinity
and exhibited excellent thermal stability.
Keywords:
Crystallization
Morphology
Polyimide
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
microspheres of PPPI were obtained so far. The previous results
reveal that the behavior of the phase separation is significantly
Aromatic polyimides have been paid much attention as high
performancepolymers and they have beenwidely used [1e4]. Among
them, poly(p-phenylene pyromelliteimide) (PPPI) is the most rigid
polyimide, and therefore it is expectable to possess the highest
performance [5,6]. Morphology of polymers with molecular orienta-
tion isof great importance to obtain essential properties, and the ideal
morphology must be a one-dimensional structure such as a needle-
like crystal and a fiber, especially for the usage as reinforcements. It is
somewhattroublesometocontrolthemorphologyofPPPIwithhighly
ordered molecular orientation by conventional techniques because of
its intractability. In order to overcome the intractability, PPPI has been
generally prepared by a two-step procedure via the synthesis of the
corresponding soluble poly(amic acid) precursor, and the following
imidization. However, the molecular orientation of poly(amic acid)
precursor is difficult due to the structural irregularity caused by meta
and para catenation of amide linkage, and the rapid crystallization
during imidization prevents controlling the molecular orientation of
PPPI [7,8]. Although various chemical modifications have been also
attempted to improve its processability [7e11], these modifications
usually decrease the essential properties predicted from the rigid
molecular structure.
influenced by the degree of imidization of oligomer leading to the
drastic change in the morphology of PPPI [12]. The structural
irregularity of precursor oligo(amic acid) caused by meta and para
catenation of amide linkage lowers the freezing point of the
precipitated oligomers resulting in the induction of the liquid-
eliquid phase separation to form the microsphere. Further, the
structural irregularity damages the crystallizability of the oligomers
to form the crystals having clear habit. Additionally, the chemical
structure of the oligomer end-groups also affects the morphology of
the crystals [14,15]. In order to prepare the crystals having clear
habit such as needle-like crystals by the crystallization of oligomers,
it is at least necessary to precipitate the fully cyclized oligoimides
having regular structure.
On the basis of these results, self-polymerizable monomers 1a and
1b, which contained an imide linkage, were designed as shown in
Scheme 1 and the morphology control of PPPI was performed using
reaction-induced crystallization during the polymerization of them.
2. Experimental
2.1. Materials
We studied the morphology control of PPPI using reaction-
induced phase separation during isothermal polymerization of
pyromellitic dianhydride (PMDA) and p-phenylene diamine (PPDA)
[12,13]. Lozenge-shaped crystals, aggregates of plate crystals and
Pyromellitic dianhydride (PMDA) was purchased from Aldrich
Co. Ltd. and used as received. 4-Nitroaniline purchased from Tokyo
Chemical Incorporation Co. Ltd. was used after recrystallization
from acetone. Tetrahydrofuran (THF), n-hexane, methanol, ethanol
and N-methyl-2-pyrolidone (NMP) were purchased from Nacalai
Tesque Co. Ltd. THF and n-hexane were used after distillation over
* Corresponding author. Tel./fax: þ81 86 251 8902.
E-mail address: polykim@cc.okayama-u.ac.jp (K. Kimura).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.12.022

838
K. Wakabayashi et al. / Polymer 52 (2011) 837e843
O
O
O
O
O
H
N
HO
RO
-n ROH
-n H2O
n
N
NH2
N
RO
O
O
O
n
O
O
R:
1a
(
C2H5
)
1b
(
, (CH2)5CH3
)
N
N
-n ROH
-n ROH
O
O
n
PPPI
O
O
O
O
N
O
O
HO
n
O
NH2
N
H
N
-n H2O
O
O
n
Scheme 1. Preparation of PPPI from self-condensation monomer; monomer 1 (R: eC2H5(1a), e(CH2)5CH3)(1b)).
calcium hydride. Absolute methanol and ethanol were obtained
by distillation over magnesium methoxide and magnesium eth-
oxide, respectively. NMP was used after dehydration over molec-
ular sieves 4A. A mixture of isomers of dibenzyltoluene (DBT) was
purchased from Matsumura Oil Co. Ltd. (Trade name: Barrel Therm
400, MW: 380, b.p.: 382 ꢀC) and purified by distillation under
reduced pressure (170e175 ꢀC/0.2 mmHg).
filtration and dried under vacuum at 40 ꢀC for 12 h. The purity of
PMMA 2 in the crystals was 96% estimated by 1H NMR. Yield: 22 g
(47%), m.p.: 273 ꢀC. IR (KBr): 3100e2500, 1858, 1795, 1778, 1715,
1482, 1425,1254,1144,1107,1006, 890, 731, 683, 1H NMR (300 MHz,
acetone-d6) d(ppm): 8.38 (s, 2H).
2.3.2. 4,5-Dicarboxy N-(40-nitrophenyl)phthalimide (CNPI) (3)
4-Nitroaniline (4.15 g, 0.03 mol) and NMP (50 mL) were placed
into a three-necked flask equipped with a thermometer and
a condenser. The solution was heated to 90 ꢀC under N2 atmosphere
and then PMMA 2 (7.23 g, 0.03 mol) was added. The mixture was
stirred for 30 min. The mixture was graduallycolored to orange with
time. Acetic anhydride (7 mL) and pyridine (0.5 ml) were added into
the mixture at 100 ꢀC and stirred for 3 h. After the filtration of the
mixture, the filtrate was allowed to cool at 50 ꢀC and then poured
into 1 L of the water. The precipitates were collected by filtration
and dried at 50 ꢀC under vacuum for 12 h. Recrystallization from
THF gave pale yellow crystals of CNPI 3 with yield of 52%. m.p.:
233 ꢀC, IR (KBr, cmꢁ1): 3100e2500 (br),1782, 1729, 1718,1618,1594,
1523, 1495, 1346, 1252, 1218, 1142, 1115, 1093, 1011, 928, 895, 845,
2.2. Measurements
Morphology of the products was observed on a HITACHI S-3500N
scanning electron microscope at 20 kV. Samples were dried, sput-
tered with platinum/palladium. Average shape parameters of the
products were determined by taking the average of over 100 obser-
vation values. A selected-area electron diffraction (SAED) was taken
by a JEOL 2000EX (Tokyo, Japan) transmission electron microscope
(TEM) at 200 kV. 1H NMR spectra were recorded on a JEOL AL300 SC-
NMR at 300 MHz. Infrared (IR) spectra were recorded on a JASCO FT/
IR-410 spectrometer. Wide angle X-ray scattering (WAXS) pattern
was measured on a RIGAKU MiniFlex diffractometer with nickel-
filtered CuK
a
radiation (35 kV, 20 mA). Matrix assisted laser
750, 695, 1H NMR (300 MHz, acetone-d6)
d(ppm): 8.46e8.43 (d, 2H,
desorption ionization time-of-flight mass spectrometry (MALDI-TOF
MS) was performed on a Bruker Daltonics AutoFLEX MALDI-TOF MS
system operating with a 337-nm N2 laser. Spectra were obtained in
the linear positive mode with an accelerating potential of 20 kV. Mass
calibration was performed with angiotensin I (MW 1296.69) and
insulin B (MW 3496.96) from a Sequazyme peptide mass standard
kit. The samples were then prepared by the evaporationegrinding
method and measured in 3-aminoquinoline as a matrix doped with
potassium trifluoroacetate salt according to the reported procedure
[13,16]. Thermogravimetric analysis was performed on a Perkin-
Elmer TGA-7 with a scanning rate of 20 ꢀC minꢁ1 in N2.
J ¼ 9.2 Hz), 8.29 (s, 2H), 7.91e7.88 (d, 2H, J ¼ 9.2 Hz). Anal. Calcd for
C16H8O8N2 (356.24): C, 53.94; H, 2.26; N, 7.86. Found: C, 55.29; H,
3.54; N, 8.98.
2.3.3. CNPI anhydride (4)
CNPI 3 (3.5 g) and acetic anhydride (100 mL) were heated at 140 ꢀC
for 1 h in N2 atmosphere. The reaction mixture was condensed by the
distillation of aceticanhydride, andwhitesolidswere precipitated after
cooling at 25 ꢀC. The solids were collected by filtration and then
recrystallized from acetic anhydride. CNPI anhydride4was obtained as
white crystals with yield of 85%. m.p.: 272 ꢀC. IR (KBr, cmꢁ1): 3109,
1866, 1778, 1726, 1596, 1525, 1516, 1459, 1375, 1348, 1299, 1206, 1120,
2.3. Monomer synthesis
903, 839, 809, 746, 720,1HNMR(300 MHz,acetone-d6)
d(ppm): 8.60 (s,
2H) 8.49e8.44 (m, 2H), 7.94e7.89 (m, 2H). Anal. Calcd for C16H6O7N2
2.3.1. Pyromellitic monoanhydride (PMMA) (2)
(338.22): C, 56.78; H, 1.79; N, 8.28. Found: C, 60.81; H, 2.30; N, 6.28.
PMDA (43.6 g, 0.20 mol) and THF (400 mL) were placed into
a three-necked round bottom flask equipped with a thermometer,
a dropping funnel and a drying tube. THF solution (30 mL) con-
tained water (5 mL, 0.28 mol) was added dropwise into the mixture
at 20 ꢀC over 48 h. After the addition, the solution was stirred for
another 24 h. The solution was dried with sodium sulfate. 250 mL
of THF was evaporated and absolute n-hexane was slowly added
into the solution at 20 ꢀC until precipitation occurred. The precip-
itates were removed by filtration and then filtrate was stood under
ꢁ20 ꢀC for 12 h. White needle crystals were collected by suction
2.3.4. 4-Carboxy 5-ethoxycarbonyl N-(40-nitrophenyl)phthalimide
(CENPI) (5a)
CNPI anhydride 4 (3.0 g) and ethanol (100 mL) were refluxed for
6 h. Ethanol was evaporated to obtain yellow solids. Recrystalliza-
tion from ethanol gave white needle crystal of CENPI 5a with yield
of 81%. m.p.: 217 ꢀC. 1H NMR (300 MHz, acetone-d6)
d(ppm):
8.47e8.42 (m, 2H) 8.33 (d, 1H, J ¼ 0.5 Hz), 8.22 (d, 1H, J ¼ 0.5 Hz),
7.92e7.87 (m, 2H), 4.44e4.37 (q, 2H, J ¼ 7.2 Hz), 1.39e1.35 (t, 3H,
J ¼ 7.2 Hz). IR (KBr, cmꢁ1): 3117, 3059, 2998, 3100e2500 (br), 1787,

K. Wakabayashi et al. / Polymer 52 (2011) 837e843
839
O
O
O
O
O
O
HO
HO
(i)
O
O
O
O
O
2
O
O
O
O
O
O
HO
HO
HO
(ii)
(iii)
O
N
NO2
O
N
H2N
NO2
NO2
+
HO
O
O
O
O
O
O
3
4
O
O
O
O
RO
RO
HO
(iv)
(v)
N
NH2
N
NO2
-C2H5, -(CH2)5CH3
HO
O
O
O
O
1
5
Scheme 2. Synthesis of monomer 1 (R: eC2H5(1a), e(CH2)5CH3(1b)) (i) H2O, THF; (ii) (1) NMP, 100 ꢀC, (2) Acetic anhydride/pyridine; (iii) Acetic anhydride, reflux; (iv) Ethanol or
1-hexanol, reflux; (v) H2, ethanol or methanol. 25 ꢀC.
1730, 1697, 1594, 1520, 1494, 1448, 1385, 1343, 1289, 1266, 1221,
1138, 1104, 849, 720. Anal. Calcd for C18H12O8N2 (384.30): C, 56.22;
H, 4.16; N, 7.29. Found: C, 56.14; H, 2.91; N, 7.16.
mixture was vigorously stirred under H2 atmosphere at 25 ꢀC for
6 h. Pd/C was removed by filtration with celite and the filtrate was
evaporated at 25 ꢀC. Recrystallization from the mixed solvent of
methanol and water (v/v 5/1) gave pale yellow needle crystals of
monomer 1a with the yield of 39%. m.p.: 227e255 ꢀC. 1H NMR
2.3.5. 4-Carboxy 5-hexyloxycarbonyl N-(40-nitrophenyl)
phthalimide (CHNPI) (5b)
(300 MHz, DMSO-d6) d(ppm): 8.12 (s, 1H), 8.09 (s, 1H), 7.04e7.01 (d,
CNPI anhydride 4 (4.5 g) and 1-hexanol (50 mL) were heated at
120 ꢀC for 3 h. The mixture was allowed to cool at 25 ꢀC. White leaf-
like crystal was precipitated. Recrystallization from 1-hexanol gave
white leaf-like crystals of CHNPI 5b with yield of 3.6 g (62%). m.p.:
2H, J ¼ 8.4 Hz), 6.65e6.62 (d, 2H, J ¼ 8.6 Hz), 4.36e4.29 (q, 2H,
J ¼ 7.0 Hz), 1.33e1.28 (t, 3H, J ¼ 7.2 Hz). IR (KBr): 3476, 3377, 2978,
2871, 2612, 2104, 1780, 1719, 1620, 1601, 1519, 1469, 1427, 1375,
1344, 1261, 1220, 1177, 1131, 738. Anal. Calcd for C18H14O6N2
(354.31): C, 61.00; H, 3.98; N, 7.91. Found: C, 60.10; H, 4.40; N, 6.31.
176 ꢀC. 1H NMR (300 MHz, DMSO-d6)
d(ppm): 13.99 (s, 1H),
8.44e8.41(d,2H,J ¼ 9.0 Hz), 8.23(s,1H), 8.19(s,1H), 7.81e7.79(d, 2H,
J ¼ 9.2 Hz), 4.30e4.26 (t, 2H, J ¼ 6.6 Hz),1.74e1.65 (m, 2H),1.39e1.29
(m, 6H), 0.89e0.84 (t, 3H, J ¼ 6.8 Hz). IR (KBr, cmꢁ1): 3400e2400 (br),
2922, 2856,1784,1736,1692,1592,1523,1496,1404,1341,1285,1223,
1126,1114,1054, 858, 751, 720.Anal. CalcdforC22H20O8N2 (440.40):C,
60.00; H, 4.58; N, 6.36. Found: C, 59.09; H, 3.78; N, 7.46.
2.3.7. N-(40-Aminophenyl)-3-carboxyl-4-
hexyloxycarbonylphthalimide (1b)
Monomer 1b was prepared from CENPI 5b and methanol in the
similar manner of monomer 1a. Recrystallization from mixed solvent
of methanol and water (v/v 5/1) below 50 ꢀC afforded needle crystals
of monomer 1b with the yield of 44%. m.p.: 174 ꢀC, 1H NMR
2.3.6. N-(40-Aminophenyl)-3-carboxyl-4-
(300 MHz, DMSO-d6) d(ppm): 8.12 (s, 1H), 8.07 (s, 1H), 7.03e7.01 (d,
ethoxycarbonylphthalimide (1a)
2H, J ¼ 8.6 Hz), 6.65e6.62 (d, 2H, J ¼ 8.6 Hz), 4.29e4.24 (t, 2H,
J ¼ 6.4 Hz), 1.73e1.64 (m, 2H, J ¼ 6.4 Hz), 1.39e1.28 (m, 6H),
0.88e0.84 (t, 3H, J ¼ 6.4 Hz). IR (KBr): 3483, 3388, 3051, 3002, 2932,
2605, 1780, 1727, 1719, 1621, 1602, 1580, 1518, 1469, 1428, 1372, 1264,
CENPI 5a (1.5 g) and absolute ethanol (300 mL) were placed into
a three-necked round bottom flask equipped with a thermometer
and gas inlet and outlet tubes. 10% Pd/C (0.15 g) was added and the
Fig. 1. (a) A SEM and (b) a TEM image with a selected-area electron diffraction pattern of the PPPI crystals prepared from monomer 1a at a concentration of 0.25% (run no. 2).

840
K. Wakabayashi et al. / Polymer 52 (2011) 837e843
1221, 1132, 1023, 1024, 784. Anal. Calcd for C22H22O6N2 (410.65): C,
64.82; H, 5.40; N, 6.82. Found: C, 64.28; H, 5.27; N, 6.73.
110
2.4. Polymer preparation
200
211
DBT (20 mL) was placed into a cylindrical flask equipped with gas
inlet and outlet tubes, and heated up to 330 ꢀC in N2 atmosphere.
Monomer 1a (0.061 g, 0.17 mmol) was added into the solution at
330 ꢀC under stirring. The stirring was stopped when monomer 1a
was entirely dissolved. The polymerization was carried out at 330 ꢀC
for 6 h with no stirring. Polymerization concentration, defined as
(calculated polymer weight/solvent volume) X 100 in this study, was
0.25%. The solution became turbid immediately and the pale yellow
crystals were formed with time. After 6 h, the crystals were collected
by filtration at 330 ꢀC to avoid the precipitation of oligomers during
cooling. The collected crystals were washed with n-hexane and
acetone several times, and then dried at 25 ꢀC for 12 h. Oligomers
dissolving in the solution at 330 ꢀC were collected by pouring the
filtrate into n-hexane. The precipitated oligomers were collected by
filtration, washed with n-hexane and fried at 40 ꢀC under vacuum for
12 h. Polymerizations of monomer 1b were also carried out in
a similar manner.
210
212
310
204
004
002
006
001
5
10
20
30
2θ (degree)
40
50
Fig. 3. WAXS intensity profile of PPPI crystals prepared from monomer 1a at
a concentration of 0.25% (run no. 2).
Fig. 2 is the spectrum of the needle-like crystals prepared at
aconcentration of 0.25%(run no. 2). Characteristic bands of C]O and
CeN stretching of imide linkage clearly appeared at 1780,1720 cmꢁ1
and 1380 cmꢁ1, respectively. The bands of an amide linkage and
a carboxylic acid were not observed at all. Furthermore, the bands of
end-groups such as amino, an ethyl ester, a carboxyl and an anhy-
dride group had disappeared. These results show that high molec-
ular weight PPPI was prepared as a form of the needle-like crystal. In
a WAXS intensity profile of the needle-like crystal, refraction peaks
were verysharp and diffuse halo caused byan amorphous part could
hardly be detected as shown in Fig. 3.
The needle-like crystals possessed quite high crystallinity. All
peaks could be assigned by the orthorhombic unit cell of PPPI
previously reported [5,17]. To examine the molecular alignment in
the crystal, a selected-area electron diffraction was taken as shown
in Fig. 1. A diffraction pattern was not a true fiber pattern of
cylindrical symmetry and it was like a single crystal pattern. These
diffractions could be also assigned according to the orthorhombic
unit cell of PPPI and the c axis was identical with the direction of
the long axis of the needle-like crystal. This result reveals that the
PPPI molecules align regularly along the long axis of the crystal.
This needle-like morphology with the molecular orientation is
desirable to obtain the essential properties as aforesaid.
3. Results and discussion
Three polymerization routes are possibly thought as also depicted
in Scheme 1, that is, via a direct amidation with elimination of water,
via an amidation with elimination of alcohol, and via a formation of
anhydride. The monomers were synthesized from the pyromellitic
monoanhydride and p-nitroaniline as shown in Scheme 2. Poly-
merizations were carried out at 330 ꢀC in a mixture of dibenzylto-
luene isomers at a concentration of 0.15e0.5%. The solution was
stirred until the monomers were entirely dissolved and then the
polymerization was continued without stirring in N2 atmosphere.
The solution became turbid in the initial stage of the polymerization,
and then the crystals grew with time. After 6 h, the crystals formed in
the solution were collected by filtration. Table 1 presents results of
the polymerizations.
Yields of the crystals were 67e82% and they increased with the
concentration. Fine flowers comprised of needle-like crystals were
formed under these conditions. The flower-like crystals obtained
from monomer 1a at a concentration of 0.25% were representa-
tively shown in Fig. 1.
The needle-like crystals grew radially from the center part as
petals. The average length, width and thickness of the needle-like
crystals were 710, 110 and 20 nm, respectively. The width and the
thickness did not change depending on the concentration, but the
length slightly increased with the concentration. Chemical structure
of the needle-like crystals was measured on FT-IR spectrometry.
In order to clarify the structure of the precipitated oligomers, the
oligomers recovered from the solution at the initial stage of the
polymerization were analyzed by MALDI-TOF mass spectrometry.
The spectrum and the peak assignments were shown in Fig. 4 and
Table 2. A monomer and oligomers up to tetramers mainly composed
(a
(b
400
600
800
1000
1200
1400
1600
1800
2000
Mass (m/z)
4000
3000
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4. MALDI-TOF mass spectra of oligomers recovered from solution after 10 min of
polymerization of monomer 1a at a concentration of 0.25%. (a) Dithranol and (b)
3-aminoquinoline were used as matrices. Peak code “a-K” means one potassium
adduct of an oligomer assigned by peak code ‘a’ shown in Table 2.
Fig. 2. IR absorption spectrum of PPPI crystals prepared from monomer 1a at
a concentration of 0.25% (run no. 2).

K. Wakabayashi et al. / Polymer 52 (2011) 837e843
841
Table 1
spiral growth mechanism caused by the screw dislocation had been
proposed as one of the possible mechanisms of the whiskers. Based
on the robust similarity of the morphological feature with the
molecular alignment, it is likely thought that the PPPI needle-like
crystals may be formed according to the mechanism similar to the
whisker. As aforesaid, the needle-like crystals grew radially from the
center part as petals. The nuclei for needle-like crystals might
aggregate, bringing about the flower-like morphology. The forma-
tion mechanism of the PPPI needle-like crystal has not been clarified
so far, and further study is needed.
Hexyl ester monomer 1b was polymerized instead of ethyl ester
monomer 1a to investigate the influence of oligomer end-groups on
the morphology. Results were also summarized in Table 1. The
polymerization of monomer 1b also afforded the PPPI needle-like
crystals at a concentration of 0.15e2.0% with the yield of 86e98% as
shown in Fig. 5. The sizes of the crystals prepared at a concentration
of 0.1% were averagely 660 nm in length,120 nm in width and 20 nm
in thickness. Higher concentration gave longer the needle-like
crystals. The sizes of the crystals prepared at a concentration of 2.0%
Preparation of PPPI crystals.a
c
Run no. Polymerization
conditions
Yield (%) Average size of needle-like T10 (ꢀC)
crystals
Monomer Conc. (%)
Lengthb (nm) Width (nm)
1
2
3
4
5
6
7
1a
1b
0.15
0.25
0.50
0.1
66.7
73.0
81.6
86.0
87.6
94.9
97.5
640
710
720
660
810
110
110
110
120
130
150
210
674
610
598
653
677
687
656
0.25
1.0
2.0
1230
1690
a
Polymerizations were carried out in a mixture of dibenzyltoluene isomers at
330 ꢀC for 6 h.
b
Length of needle-like crystals from center part.
Temperatures of 10 wt% loss were measured on a TGA with a heating rate of
c
20 ꢀC minꢁ1
.
of imide linkages were detected. The oligoimides larger than pen-
tamers might be precipitated to form the needle-like crystal and the
polymerization proceeds in the crystals to increase molecular
weight. In this self-polymerization, both the structural regularity of
oligoimide end-groups and the stoichiometric balance between two
reactive end-groups are strictly compensated comparing with the
polymerization of PMDA and PPDA. The drawbacks for the prepa-
ration of the crystals caused by the structural irregularity of oligo
(amic acid) were completely conquered. The needle-like crystals, so-
called whiskers, of aromatic polymers had been prepared by the
crystallization during the isothermal polymerization [18,19]. The
were averagely 1.69
mm in length and 210 nm inwidth. The thickness
was 20 nm. From the MALDI-TOF mass spectroscopy, the needle-like
crystals from monomer 1b were formed by the crystallization of fully
cyclized oligoimides. Based on this fact, the cyclized oligoimides are
precipitated to form the crystals, and hence, the morphology of the
crystals is not so susceptible to the chemical structure of monomers.
The width of the needle-like crystals prepared from monomer 1b is
larger than that from monomer 1a at the same concentration. The
size of the crystals is generally determined by both the size of
nucleus, which relates to the degree of super-saturation, and the
Table 2
Structural assignments of peaks detected in MALDI-TOF mass spectra.
Peak codea
Mass (m/z)
Structure
n
Obs.
Calc.
a
428.78
430.52
b
c
634.75
926.87
636.58
926.81
1
2
d
490.82
490.53
e
f
761.06
1069.1
761.68
1069.9
1
2
g
1289.9
1069.9
2
a
Peak codes were shown in Fig. 4.

842
K. Wakabayashi et al. / Polymer 52 (2011) 837e843
Fig. 5. SEM images of micro-flowers of needle-like PPPI crystals prepared from monomer 1b at concentrations of (a) 0.1%, (b) 0.25%, (c) 1.0% and (d) 2.0%.
growth rate. It is well known that if the degree of super-saturation is
low when the nucleation occurs, the nuclei having larger size are
usually formed [19e21]. The larger size of the crystals prepared from
monomer 1b may be attributed to the lower degree of super-satu-
ration of the oligomers prepared from monomer 1b because of
longer alkoxy end-group. The length of the needle-like crystals
prepared from monomer 1b is also larger. This is due to the higher
yield of the crystals. The thicknesses of the crystals are almost 20 nm,
being not influenced by the structure of monomer and the poly-
merization concentration. This seems to be attributed to the direc-
tion of the crystal growth, and further study is needed to clarify.
Thermal stability of the PPPI needle-like crystals prepared from
monomers 1a and 1b was estimated on thermogravimetric anal-
ysis. Temperatures of 10 wt% loss in N2 atmosphere were in the
range of 598e687 ꢀC as presented in Table 1 and these crystals
exhibited excellent thermal stability.
crystals was controllable by the structure of a monomer and the
monomer concentration. The micro-flowers possessed high crystal-
linity and exhibited excellent thermal stability.
Acknowledgment
This work was supported inpart bya Grant-in-Aid (No. 21350127)
for Scientific Research from the Ministry of Education, Science, and
Culture, Japan.
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