ORIGINAL PAPER
Comparative study of polyimides containing differentflexible linkages
Irina Bacosca • Elena Hamciuc • Maria Bruma •
Inga A. Ronova
Received: 20 December 2011 / Accepted: 12 May 2012
� Iranian Chemical Society 2012
Abstract Two series of polyimides were synthesized
based on different aromatic dianhydrides containing vari-
ous flexible linkages and two aromatic diamines containing
ether and nitrile groups. The structure of the polymers was
confirmed by FTIR and 1H NMR spectroscopy. The cor-
relation between some physical properties, such as solu-
bility, thermal stability and glass transition temperature,
and conformational rigidity parameters, such as Kuhn
segment, characteristic ratio and rigidity parameter p, was
studied.
Keywords Polyimides � Thermal stability � Glass
transition temperature � Conformational rigidity parameters
Introduction
Due to their unique excellent thermal, mechanical, dielectric
and optical properties along with good chemical resistance
and dimensional stability, polyimides possess many desirable
attributes, so that this class of materials has found applications
in many technologies ranging from microelectronics to high
temperature adhesives and gas separation membranes [1–4].
However, the practical applications of wholly aromatic
polyimides are limited because they are insoluble in organic
solvents and intractable. Therefore, considerable research has
been undertaken in order to identify new ways to avoid these
limitations. The introduction of flexible linkages and/or bulky
units into polymer backbones is a general approach, used
mainly to lower transition temperatures and to improve sol-
ubility. The main concept behind all these approaches is the
reduction of the packing force and the increase of the free
volume of polymers [5–9].
Most of the commercial aromatic polyimides contain
flexible ether or ketone linkages in their repeating units.
Early works in the field demonstrated that dianhydride
monomers having two phthalic anhydride moieties joined
by flexible bonding groups gave more tractable polyimides
[10–12]. But, these changes in polymer chains may lead to
the reduction of thermal stability. Therefore, an adjusted
degree of modification should be applied to optimize the
balance of properties. The introduction of phthalide groups
and ether linkages into the macromolecular chains of an
aromatic polyimide leads to polymers having high thermal
stability, high glass transition temperature, and excellent
mechanical toughness. In addition, the polymers are solu-
ble in different organic solvents, such as N-methylpyrr-
olidone, N,N-dimethylacetamide, N,N-dimethylformamide,
or chloroform, and can be cast into flexible tough films.
Phthalide groups and ether linkages also improve the
optical properties, the polymers exhibiting light color and
high transparency [13–15].
The presence of nitrile substituents may lead to the
increase of thermooxidative resistance and of dielectric
constant comparative to the polymers that do not have this
substituent onto the macromolecular chain. Nitrile groups
possess strong bond dissociation energy, which maintains
the excellent thermal stability of the polyimide [16–19].
Also, the functional nitrile groups may enhance other
physical properties of high performance materials, since it
was shown that some amorphous polymers containing
strong dipoles exhibited piezoelectric response [20].
I. Bacosca (&) � E. Hamciuc � M. Bruma
‘‘Petru Poni’’ Institute of Macromolecular Chemistry,
Aleea Grigore Ghica Voda, 41A, 700487 Iasi, Romania
e-mail: [email protected]
I. A. Ronova
Nesmeyanov Institute of Element-Organic Compounds,
Vavilov Street 28, Moscow 119991, Russia
123
J IRAN CHEM SOC
DOI 10.1007/s13738-012-0107-2
Also, the incorporation of hexafluoroisopropylidene
groups into polymer backbones may enhance certain
properties such as solubility without sacrificing thermal
stability [21–24].
Calculation of conformational parameters, taking into
account the bond lengths, bond angles and rotational bar-
riers, allows to explain the behavior of new materials and
to predict the properties of the polymers synthesized from
new monomers.
We synthesized two series of polyimides containing
different flexible linkages, so that the resulting polymers
have better solubility and high thermooxidative stability.
We studied the correlation between some physical prop-
erties and their conformational parameters. These polymers
are based on aromatic diamines containing one or two
nitrile substituents and two ether linkages, and various
aromatic dianhydrides some of which also contain ether
linkages.
Experimental procedure
Monomers
Aromatic diamine containing one nitrile substituent 1a,
namely 2,6-bis(m-aminophenoxy) benzonitrile, was syn-
thesized by the reaction of 2,6-dichlorobenzonitrile with
m-aminophenol [25]. Aromatic diamine containing two
nitrile substituents 1b, namely 1,3-bis-2-cyano-3-(3-amino-
phenoxy) phenoxybenzene, was synthesized by the reaction of
2-chloro-6-fluoro-benzonitrile with resorcinol, followed by the
reaction of the resulting dihydroxy compound with m-amino-
phenol [26]. M.p. 1a: 136–138 �C, m.p. 1b: 110–120 �C.
Aromatic dianhydrides, such as 2,2-bis[4-(3,4-dicarb-
oxyphenoxy)phenyl] isopropane dianhydride (2a), 2,2-bis
[4-(3,4-dicarboxyphenoxy)phenyl]hexafluoroisopropane
dianhydride (2b), 1,1-bis[4-(3,4-dicarboxyphenoxy) phenyl]
cyclohexane dianhydride (2c) and 3,3-bis[4-(3,4-dicarboxy-
phenoxy)phenyl]phthalide dianhydride (2d), were synthesized
by a multistep reaction starting from 4-nitrophthalonitrile and
a bisphenol compound such as isopropylidene-bisphenol, hex-
afluoroisopropylidene-bisphenol, cyclohexylidene-bisphenol or
phenolphtaleine-bisphenol [27–30]. M.p. 2a = 189–190 �C,
m.p. 2b = 229–231 �C, m.p. 2c = 195–197 �C, m.p. 2d =
191–193 �C.
Biphenyltetracarboxylic dianhydride (2e) and benzo-
phenonetetracarboxylic dianhydride (2f) were provided
from Aldrich. They were purified by recrystallization from
acetic anhydride and washed thoroughly with anhydrous
diethylether and dried in vacuum. M.p. 2e: 230–232 �C,
m.p. 2f: 308–309 �C.
The structures of all these monomers are shown in
Scheme 1.
Synthesis of polymers
Polyimides 3 were synthesized by a two-step polycondensa-
tion reaction of aromatic diamine containing one nitrile group,
namely 2,6-bis-(m-aminophenoxy)benzonitrile (1a), with an
aromatic dianhydride, namely 2,2-bis[4-(3,4-dicarboxyphen-
oxy)phenyl] isopropane dianhydride (2a), 2,2-bis[4-(3,4-di-
carboxyphenoxy)phenyl] hexafluoroisopropane dianhydride
(2b), 1,1-bis[4-(3,4-dicarboxyphenoxy)phenyl]cyclohexane
dianhydride (2c) or 3,3-bis[4-(3,4-dicarboxyphenoxy)phe-
nyl]phthalide dianhydride (2d), respectively. Polyimides 4
1a
CNNH2H2N OO
1b
CNO
CNO NH2OH2N O
2a
O O
C
OC
O
OC
OC
O
O
C
CH3
CH3
2b
O
C
OC
O
OC
OC
O
O
OC
CF3
CF3
O O
C
OC
O
OC
OC
O
O2c
O O
C
OC
O
OC
OC
O
O
C
COO
2d
C
OC
O
OC
OC
O
O 2e
O
C
C
OC
O
OC
OC
O
O2f
Scheme 1 Structure of the monomers
J IRAN CHEM SOC
123
were obtained by a two-step polycondensation reaction of
aromatic diamine containing two nitrile groups, namely 1,3-
bis-2-cyano-3-(3-aminophenoxy) phenoxybenzene (1b) with
an aromatic dianhydride, namely 2,2-bis-[4-(3,4-dicarboxy-
phenoxy)phenyl] isopropane dianhydride (2a), biphenyltetra-
carboxylic dianhydride (2e) or benzophenone tetracarboxylic
dianhydride (2f), using NMP as a solvent.
The reactions were run with equimolar quantities of
monomers, at a concentration of 12–15 % total solids, in
NMP as solvent. The first step of the reaction was carried
out at room temperature and yielded the polyamidic acids.
Small portions of each polyamidic acid solution were
precipitated in water in order to isolate the polymer which
was used afterwards for IR and 1H NMR structural iden-
tification. The rests of polyamidic acid solutions were
converted into the corresponding polyimides by heating at
about 180 �C, under a slow stream of nitrogen to remove
the water formed during the cyclodehydration process.
Their detailed synthetic procedure was reported previously
[31].
The structures of these polyimides are presented in
Scheme 2.
C
CH3
CH3
n
CNOO
O
C
OC
N
O
O
C
OC
O
N
3a
C
CF3
CF3
n
CNOO
O
C
OC
N
O
O
C
OC
O
N
3b
n
CNOO
O
C
OC
N
O O
O
C
OC
N
3c
n
CNOO
O
C
OC
N
O
C O
C
OC
O
N
OCO
3d
4an
CNO
CNO OO
O
NC
O
CN
O
CO
C
4b
CN
O
CO
CO
NC
O
C On
CNO
CNO OO
4cn
CNO
CNO OO N
O
CO
CO
CH3
CH3
C
O
NC
O
CO
Scheme 2 Structures of
polyimides 3 and 4
J IRAN CHEM SOC
123
Preparation of polymer films
Free-standing thin films of polyimides 3 and 4 were pre-
pared from polymer solutions of 12 % concentration in
NMP by casting onto glass plates, followed by gradually
heating from room temperature up to 220 �C, and an
additional hour at 220 �C. The resulting films showed a
strong adhesion to the glass support and were stripped off
the plates by immersion in water, followed by drying in
oven at 110 �C. These films had the thickness in the range
of 30–80 lm and were used afterwards for various
measurements.
Very thin films having the thickness in the range of
nanometers were prepared from diluted polymer solutions
in NMP with concentration of 2 % by spin-coating onto
silicon wafers at a speed of 5,000 rpm. These films, as
deposited, were gradually heated up to 220 �C in the same
way as described earlier to remove the solvent and they
were used afterwards for atomic force microscopy (AFM)
investigations.
Measurements
Melting points of the monomers and intermediates were
measured on a Melt-Temp II (Laboratory Devices).
Infrared spectra were recorded on a FT-IR Bruker
Vertex 70 analyzer, using KBr pellets or very thin polymer
films.1H-NMR (400 MHz) spectra were obtained on a Bruker
Avance DRX 400 spectrometer. The polymer samples were
dissolved in DMSO-d6 on heating, or in a mixture of sol-
vents (CDCl3:CF3COOD = 9/1, v/v) at room temperature,
and then their spectra were recorded at room temperature.
Model molecules for a polymer fragment were obtained
by molecular mechanics (MM?) by means of the Hyper-
chem program, Version 7.5. The calculations were carried
out with full geometry optimization (bond lengths, bond
angles and dihedral angles) [32].
The quality of the films was investigated by atomic
force microscopy (AFM). The images were taken in air, on
a SPM SOLVER Pro-M instrument.
The glass transition temperature (Tg) was measured on a
Mettler DSC 12E apparatus in nitrogen with a heating rate
of 20 �C/min. The mid-point of the inflection curve
resulting from the second heating cycle was considered as
the Tg of polymers.
Thermogravimetric analysis (TGA) was performed on a
MOM-type Derivatograph made in Budapest, Hungary,
operating in air at a heating rate of 12 �C/min.
Dynamic mechanical analysis was performed on a
Perkin–Elmer Diamond device equipped with a standard
tension attachment. The measurements were run at a fre-
quency of 1 Hz and a heating rate of 3 �C/min from 0 to
300 �C using film samples with dimensions of 10 9
10 9 0.04 mm.
The Kuhn segments were calculated using the Monte
Carlo method as described earlier [33].
Results and discussion
Polyimides studied here were obtained by the classical
two-step polycondensation reaction of two aromatic dia-
mines containing ether and nitrile substituents and various
aromatic dianhydrides, using an aprotic polar solvent NMP.
The structure of the polymers was controlled using
monomers with different degrees of flexibility. All
polymers have flexible ether linkages between the rigid
phenyl rings in the diamine segment; some polymers also
contain ether bridges in the dianhydride segment toge-
ther with other flexible units, such as isopropylidene (3a
and 4c), hexafluoroisopropylidene (3b), cyclohexylidene
(3c) or carbonyl (4b), while other polymers contain more
rigid units such as phthalide (3d) or biphenyl (4a)
(Scheme 2).
Structural identification
The structure of the polymers was identified using infrared
and 1H NMR spectroscopy. All the polyamidic acids
showed a broad IR absorption band at 3,350–3,450 cm-1
characteristic of NH amide and a narrow strong absorption
peak at 1,660–1,670 cm-1 due to CO group in amide
linkage. In IR spectra of the corresponding polyimides, the
absorption bands from 3,350 to 3,450 cm-1 and 1,660 to
1,670 cm-1 significantly decreased, which means that the
conversion of the intermediate polyamidic acid into final
polyimide structure was quantitatively achieved by poly-
condensation in solution at high temperature. At the same
time, new absorption peaks appeared at 1,780–1,790 cm-1,
1,730–1,740 cm-1 and 720–730 cm-1 which were char-
acteristic to imide ring. In all spectra, the absorption peaks
characteristic to ether, C–H aromatic linkage and nitrile
group appeared at 1,240–1,250 cm-1, 3,070–3,080 cm-1
and 2,230–2,240 cm-1, respectively. The hexafluoroiso-
propylidene unit was evidenced at 1,210 and 1,170 cm-1 in
polyimide 3b and the C–H aliphatic group in polymers 3a,
3c, 3d and 4c appeared at 2,970 cm-1. As an example,
Fig. 1 shows the FTIR spectra of polyimides 3a and 4c
based on 2,2-bis[4-(3,4-dicarboxyphenoxy) phenyl] iso-
propane dianhydride 2a.
The conversion of the polyamidic acids to the corre-
sponding polyimide structures was also confirmed by the
fact that 1H NMR spectra of polyimides showed no residual
resonance in the region 9–11 ppm indicating the absence of
amide NH protons.
J IRAN CHEM SOC
123
Solubility
These polyimides are soluble in organic solvents such as
N-methylpyrrolidinone, N,N-dimethylacetamide, N,N-dim-
ethylformamide and dimethylsulfoxide. The polymers
based on dianhydrides containing flexible units such as
isopropylidene (3a and 4c), hexafluoroisopropylidene (3b),
cyclohexylidene (3c) and carbonyl (4b) units are also sol-
uble in less polar solvents such as tetrahydrofuran, chlo-
roform and methylene chloride. Polyimides 3d and 4a
containing a more rigid segment such as phthalide (3d) or
biphenyl (4a), respectively, are only partially soluble in
methylene chloride and tetrahydrofuran.
The good solubility of the present polymers can be
explained by the presence of flexible bridges such as ether,
isopropylidene, hexafluoroisopropylidene, cyclohexylidene
and carbonyl, which disturb the conjugation along the
polymer chain and do not allow a tight packing of the
macromolecules. Figure 2 shows information on the most
probable conformation for four repeating units of the
polyimides 3c and 4c. The molecular model was obtained
by molecular mechanics by means of the Hyperchem
programme version 7.51. Molecular mechanics calculation
treats atoms as Newtonian particles interacting through a
potential energy function. Potential energy depends on
bond lengths, bond angles, torsion angles and non-bonded
interactions (van der Waals forces, electrostatic interac-
tions and hydrogen bonds). The forces on atoms are
functions of the atomic position. For calculations, we used
the MM? force field. We can notice that the shape of the
polymers is far from the rigid-rod one characteristic for
conventional polyimides.
These polymers have the ability to form thin, flexible
and transparent films by casting technique. The quality of
very thin films prepared from dilute solutions of polymers
at a concentration of 2 % in NMP was investigated by
AFM measurements which showed that the surface of the
films was smooth. As an example, Fig. 3 presents the tri-
dimensional image of polyimide 3d (top) and 4a (bottom)
over an area of 2 9 2 lm2.
Thermal properties
All these polyimides containing flexible ether bridges are
thermally stable, as evaluated by TGA and differential
scanning calorimetry (DSC). The initial decomposition
temperature (Tonset) was above 445 �C for polymers 3
which are based on diamine containing one nitrile group.
The polymers 4 based on diamine containing two nitrile
groups have the initial decomposition temperature above
435 �C (Table 1). These polymers contain four ether
bridges in the repeating unit: in polymers 3, the ether
bridges are located equally in both monomer segments,
while in polymers 4, all ether groups are located in the
diamine segment, except polymer 4c. It seems that the
presence of all ether groups in the same monomer segment
Fig. 1 FTIR spectra of
polyimides 3a and 4c, both
containing isopropylidene
groups
J IRAN CHEM SOC
123
leads to a slightly lower thermal stability of the corre-
sponding polyimides 4. The temperature of 10 % weight
loss for both series of polymers was in the range of
460–525 �C. The degradation process exhibited two max-
ima of decomposition for all polyimides. The temperature
corresponding to the first maximum of decomposition was
in the range of 490–555 �C and it was due to the cleavage
of the flexible ether, carbonyl, isopropylidene, hexaflu-
oroisopropylidene or cyclohexylidene groups, while the
second step of decomposition was in the domain of
610–645 �C and it was attributed to the degradation of the
whole polymer chain (Table 1).
The glass transition temperature (Tg), determined by the
movement of large segments from the polymers chain as a
result of the heating process, was determined by DSC
analyses. The Tg values of these polyimides are situated in
the range of 169–232 �C, and they correlate well with their
structure (Table 1). Among polymers 3, the highest value
of Tg (200 �C) corresponds to the more rigid polyimide 3d
containing phthalide unit in the dianhydride segment;
among polymers 4, the highest value of Tg (232 �C) cor-
responds to polyimide 4a containing the rigid biphenyl unit
in the dianhydride segment.
As expected, the lowest value of Tg (187 �C) in series 3
corresponds to the more flexible polyimide 3a containing
isopropylidene bridge in the dianhydride segment; in series
4, the lowest value of Tg (169 �C) corresponds to
Fig. 2 Molecular models of
segments containing four
repeating units of polyimides
3c and 4c
Fig. 3 AFM tridimensional images of polymers 3d (top) and 4a(bottom)
Table 1 Thermal properties of polyimides 3 and 4
Polymer Tonseta (oC) T10
b (oC) Tmax1c (oC) Tmax2
d (oC) Tge (oC)
3a 490 525 520 645 187
3b 445 505 555 600 200
3c 465 485 495 645 191
3d 480 525 520 630 232
4a 435 465 490 640 200
4b 445 460 510 610 187
4c 445 495 510 630 169
a Initial decomposition temperature (temperature of 5 % weight loss)b Temperature of 10 % weight lossc Temperature of the first maximum speed of decompositiond Temperature of the second maximum speed of decompositione Glass transition temperature, measured by DSC
J IRAN CHEM SOC
123
polyimide 4c based on the same dianhydride containing
isopropylidene groups. It can be noticed that the presence
of isopropylidene groups in the repeating unit leads to a
slight decrease of glass transition temperature.
There is a large interval between the glass transition
temperature and the decomposition temperature of all these
polymers which can be advantaging for their processing by
thermoforming technique.
Conformational parameters
A correlation between the physical properties of the poly-
mers and the conformational rigidity of their chain shows
that the contribution of the conformational rigidity to their
properties is significant. The conformational rigidity of a
polymer can be estimated using different parameters, such
as Kuhn segment Afr, characteristic ratio C?, and rigidity
parameter p.
Kuhn segment Afr can be calculated using the following
Eq. (1):
Afr ¼ limn!1
hR2inl0
ð1Þ
where hR2i/nl0 is the ratio of the average square end-to-end
distance of a chain to its contour length (L = nl0 is a
parameter independent of the chain conformation), n is the
number of repeating units, and l0 is the contour length of a
repeating unit. In the case of polyheteroarylenes in which
the repeating unit contains virtual bonds with different
lengths and different angles between them, the length of
the zig-zag line connecting the mid-points of the virtual
bonds is taken as the contour length. The Kuhn segment
length was calculated by Monte Carlo method. We used
Volkenstein [34] rotational isomeric state approximation
by consideration of only discrete values of rotation angles
and the Flory approximation [35] by the assumption that
rotations around virtual bonds are independent. The term
‘‘virtual bond’’ is used to indicate a rigid section of a chain
approximated by a straight line about which rotation is
possible. In a particular case, it can be an ordinary valence
bond; more generally, it can contain rings, as well.
The conformational energy maps for several aromatic
polyesters, and polycarbonates were calculated and the
minimum energy structures were found, in which the
rotation angles about virtual bonds passing through
aromatic rings were 0� and 180�, both values being
equally probable. This means that these virtual bonds
behave as statistically free rotating bonds. With these
assumptions, one can write coordinates of any vector in the
reference frame associated with the first vector as Eq. (2):
Vj ¼ T1. . .Tj�2Tj�1lj ð2Þ
where T is the Flory matrix:
Tj ¼cos hj sin hj 0
sin hj cos /j � cos hj cos /j sin /j
sin hj sin /j � cos hj sin /j � cos /j
24
35
and (p-hj) are the angles between virtual bonds. On con-
structing the polymer chain, the angles hj recur periodically
with the period depending on the number of virtual bonds,
N, in the repeating structural unit. The index j runs from 1
to nN. The values of the rotation angles, uj, were chosen in
one of the following two ways depending on the nature of
the bond: (a) they were determined by Monte Carlo pro-
cedure on the assumption of a uniform distribution within
the interval (0, 2p); (b) u was constant, i.e., any rotation
about a bond was forbidden. The ensemble average
hR2/nNi was obtained by generating on a computer a set of
independent chain sequences (in each sequence n runs from
1 to 2,500) and averaging over the set of hR2/nNi values
relating to the same n. For each of the polymers, the value
of n = n’ beyond which the average hR2/nNi as a function
of n tended to converge was found. Usually n’ was found to
be of the order of 1,000 [33].
Knowing the value of Kuhn segment, one can calculate
the characteristic ratio C? which shows the number of
repeating units in Kuhn segment [Eq. (3)]:
C1 ¼Afr
l0
ð3Þ
and the rigidity parameter, p [Eq. (4)]:
p ¼ Afr
lok ð4Þ
where k is the number of aromatic rings in a repeating unit.
The rigidity parameter p takes into account both factors:
aromatic character of polyheteroarylenes and their rigidity.
Table 2 presents the calculated values of conformational
parameters of polyimides 3 and 4. The dependence of glass
transition temperature Tg on the characteristic ratio C?, is
shown in Figs. 4 and 5. It should be mentioned that the
Table 2 Conformational parameters of polymers 3 and 4
Polymer loa (A) Afr
b (A) C?c pd
3a 36.73 15.27 0.416 2.910
3b 36.71 15.24 0.476 2.906
3c 36.68 15.08 0.432 2.878
3d 36.62 15.07 0.535 3.292
4a 35.33 17.45 0.494 3.457
4b 36.62 15.45 0.422 2.947
4c 46.45 15.37 0.331 2.978
a Contour length over repeating structural unitb Kuhn segmentc Characteristic ratiod Conformational rigidity parameter
J IRAN CHEM SOC
123
correlation between glass transition temperature and con-
formational parameters could be used to calculate the value
of glass transition temperature when its experimental
measurement is difficult.
Using the least-squares method, the dependence of glass
transition temperature of polyimides 3 and 4 on charac-
teristic ratio can be described by two linear equations: the
first equation for polymers containing one nitrile group in
their repeating unit is: Tg = 21.75 ? 399.67 C?, with a
good factor of convergence (R = 96.86 %); the second
equation for polymers containing two nitrile groups in the
repeating unit is: Tg = 106.14 ? 190.51 C?, with a very
good factor of convergence (R = 99.97 %).
The good solubility of the polyimides is due to the
flexibility of the macromolecular chains, which is in
agreement with relative low values of Kuhn segment being
in the domain of 15.07–17.45 A (Table 2). The glass
transition temperature of a polymer increases when the
macromolecular chains have a rigid structure and conse-
quently a higher value of rigidity parameter p. Thus, for
polyimides 3, the highest value of p (3.292) corresponds to
the polymer 3d containing the rigid phthalide unit which
shows the highest value of Tg (232 �C); for polyimides 4,
the highest value of rigidity parameter p (3.457) corre-
sponds also to the most rigid structure 4a containing
biphenyl unit which shows the highest value of Tg
(200 �C).
It can be concluded that the glass transition temperatures
of polyimides based on the same diamine are correlated
quite well with the conformational rigidity of these
polymers.
Molecular relaxations
Dynamic mechanical analysis (DMA) of free standing
films of these polyimides was performed to get more
information about the imidization reaction and of the
phenomena which take place during the heating process. It
is known that most of the polyimides films exhibit relax-
ation processes (c, b, a) when are subjected to dynamo-
mechanical measurements.
At very low temperatures, a c process called sub-
ambient secondary relaxation can be evidenced in rigid
polymers. It is associated with phenyl ring motions and is
influenced by moisture absorption content, aging history
and morphology [36]. At higher temperatures, a b transi-
tion can appear and it is associated with the non-coopera-
tive motions of the phenylene groups from the diamine or
the dianhydride units, around flexible linkages such as –O–,
–CH2–, etc. [37]. The primary relaxation a observed at
higher temperatures is attributed to the glass transition
temperature of the polymer. It is mainly correlated with the
rigidity of the analyzed structure.
Figure 6 shows the dependencies of mechanical loss
tangent (tan d) of polyimides 3a and 4b on temperature.
The peaks in tan d plots indicate the physical transitions
which take place in the macromolecules.
As it can be seen on tan d curve, at low temperature of
about -90 �C for polymer 3a and at about -75 �C for
polymer 4b, a secondary c relaxation appears. At higher
values of temperature, of about 30 �C, the b relaxation
process can be noticed in the curve of polymer 3a. The
broadness of this relaxation is due to the high values of
relaxations times which characterize a heterogenic struc-
ture. For polyimide 4b, it cannot be detected a sharp peak
characteristic for this secondary relaxation, just a constant
rise in the shape of the curve. At very high temperature, of
about 200 �C for polyimide 3a and at approximately
240 �C for polyimide 4b, the primary relaxation a is evi-
denced. In this region, the macromolecular chains begin to
Fig. 4 Dependence of glass transition temperature (Tg) on charac-
teristic ratio (C?) for polyimides 3
Fig. 5 Dependence of glass transition temperature (Tg) on charac-
teristic ratio (C?) for polyimides 4
J IRAN CHEM SOC
123
coordinate large scale motions. One classical description is
that the amorphous regions have begun to melt. The raise
of the temperature induces motions of the whole polymer
chain and the slippage of the chains past each other can
take place, and as result the experiment cannot be con-
ducted over these temperatures.
Conclusions
The properties of two series of polyimides based on aro-
matic diamines containing flexible ether bridges and nitrile
groups and various aromatic dianhydrides were compared.
The solubility of the polymers increases with the rise of
number of flexible units, these polyimides being soluble in
aprotic organic solvents and in less polar solvents such as
tetrahydrofuran and chloroform. The present polymers
show high thermal stability with initial decomposition
temperature being in the range of 435–490 �C, depending
on the number of flexible ether bridges. The thermogravi-
metric curves evidence two steps of decomposition of these
polyimides: the first is attributed to the cleavage of the
flexible moieties, and the second step is due to the break-
down of the macromolecular chain. Glass transition tem-
perature of these polymers is in the range of 169–232 �C
being in a good agreement with their structure: the highest
value of Tg corresponds to the most rigid structure, while
the lowest value of Tg corresponds to a more flexible chain.
Conformational parameters were calculated by Monte
Carlo method and they were used to characterize the
dependence of glass transition temperature on the rigidity
of polymers. Molecular relaxations of the polymer chains
were evidenced using dynamic mechanical analyses which
showed the secondary relaxations c and b and the primary
transition a.
Acknowledgments The research leading to these results has
received funding from the European Union’s Seventh Framework
Program (FP7/2007–2013) under grant agreement no. 264115,
STREAM.
References
1. P.M. Hergenrother, High Perform. Polym. 15, 3 (2003)
2. D.J. Liaw, W.H. Chen, C.C. Huang, in Polyimides and OtherHigh Temperature Polymers, ed. by K.L. Mittal (Utrech, Boston,
2003), pp. 47–70
3. A.L. Rusanov, D.Y. Likhacev, L.G. Komarova, K.A. Shevelev,
M.D. Dutov, I.A. Vatsatze, Polym. Sci., Ser. B. Polym. Chem. 48,
109 (2006)
4. T.J. Dingemans, E. Mendes, J.J. Hinkley, E.S. Weiser, T.L. St
Clair, Macromolecules 41, 2474 (2008)
5. D.J. Liaw, C.Y. Hsu, P.N. Hsu, S.L. Lin, J. Polym. Sci., Part A:
Polym. Chem. 40, 2066 (2002)
6. H. Behniafar, A. Banihashemi, Eur. Polym. J. 40, 1409 (2004)
7. A. Banihashemi, A. Abdolmaleki, Eur. Polym. J. 40, 1629 (2004)
8. Y. Chikashige, Y. Chikyu, K. Miyatake, M. Watanabe, Macro-
molecules 38, 7121 (2005)
9. C. Hamciuc, E. Hamciuc, A.M. Ipate, L. Okrasa, Polymer 49, 681
(2008)
10. Z. Hu, M. Wang, S. Li, X. Liu, J. Wu, Polymer 46, 5278 (2005)
11. E. Krause, G.M. Yang, G.M. Sessler, Polym. Int. 46, 59 (1998)
12. J. de Abajo, J.G. de la Campa, in ‘‘Advances in Polymer Science’’
(Springer, Berlin, 1999), pp. 23–59
13. C.P. Yang, S.H. Hsiao, H.W. Yang, Macromol. Chem. Phys. 201,
409 (2000)
14. D.S. Reddy, C.F. Shu, F.I. Wu, J. Polym. Sci., Part A: Polym.
Chem. 40, 262 (2002)
15. N.H. You, Y. Suzuki, D. Yorifuji, S. Ando, M. Ueda, Macro-
molecules 41, 6361 (2008)
16. M. Bruma, F. Mercer, B. Schulz, R. Dietel, J. Fitch, P. Cassidy,
High Perform. Polym. 6, 183 (1994)
17. B. Lin, X. Xu, Polym. Bull. 59, 243 (2007)
18. E. Hamciuc, C. Hamciuc, M. Cazazu, M. Ignat, G. Zarnescu, Eur.
Polym. J. 45, 182 (2009)
19. L. Li, R. Kituchi, M.A. Kakimoto, M. Jikei, A. Takahashi, High
Perform. Polym. 17, 135 (2005)
20. C. Park, Z. Ounaies, K.E. Wise, J.S. Harrison, Polymer 45, 5417
(2004)
21. M. Bruma, B. Schulz, F.W. Mercer, J. Macromol. Sci. Part A 32,
259 (1995)
22. R.H. Vora, S.H. Gosh, T.S. Chung, Polym. Eng. Sci. 40, 1318
(2000)
23. C. Hamciuc, E. Hamciuc, M. Bruma, Polymer 46, 5851 (2005)
24. I. Bacosca, E. Hamciuc, M. Bruma, M. Szesztay, Rev. Roum.
Chim. 54, 1023 (2009)
25. D.R. Hearth, J.G. Wirth, US Patent 3763211, 1973
26. B. Gonzalo, J.L. Vilas, T. Breczewski, M.A. Perez-Jubindo, M.R.
de la Fuente, M. Rodriguez, L.M. Leon, J. Polym. Sci., Part A:
Polym. Chem. 47, 722 (2009)
27. D.R. Heath, J.G. Wirth, US Patent 3956320, 1976
28. W.T. Schwartz Jr, High Perform. Polym. 2, 189 (1990)
29. C.P. Yang, C.W. Yu, J. Polym. Sci., Part A: Polym. Chem. 39,
788 (2001)
30. C.P. Yang, Y.Y. Yu, J. Polym. Sci., Part A: Polym. Chem. 44,
3140 (2006)
31. I. Bacosca, E. Hamciuc, M. Cristea, G. Lisa, M. Bruma, J. Appl.
Polym. Sci. 124, 1956 (2012)
32. Hypercube Inc. (Ontario), Hyperchem, 2002, Version 7.5
33. I.A. Ronova, Structur. Chem. 21, 541 (2010)
-150 -100 -50 0 50 100 150 200 2500.01
0.1
1
10ta
n δ
Temperature (°C)
3a 4b
Fig. 6 Tan d curves for polymers 3a and 4b
J IRAN CHEM SOC
123
34. M.V. Volkenstein, Configuration statistics of polymer chains,
(Ed. Akad. Nauk SSSR 1959)
35. J.Flory, Statistical mechanics of chain molecules (Ed. Inter-
science, New York, 1969)
36. A. Jonas, R. Legras, Macromolecules 26, 813 (1993)
37. J.P. Habas, J. Peyrelasse, M.F. Grenier-Loustalot, High Perform.
Polym. 8, 515 (1996)
J IRAN CHEM SOC
123
Top Related