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: ibacosca@icmpp.ro

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.

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