Effect of conformational parameters on physical properties of polymers containing pendant phenoxyphthalonitrile substituents

Article abstract of DOI:10.1007/s11224-013-0215-3

Heteroaromatic polymers are considered to be high performance organic materials due to their unique and highly attractive properties, including outstanding thermal and mechanical resistance, that arise from their aromatic structure and strong interactions

Full text of DOI:10.1007/s11224-013-0215-3

Struct Chem
DOI 10.1007/s11224-013-0215-3
ORIGINAL RESEARCH
Effect of conformational parameters on physical properties
of polymers containing pendant phenoxyphthalonitrile
substituents
•
Ionela-Daniela Carja Corneliu Hamciuc
•
•
Tachita Vlad-Bubulac Maria Bruma
•
Inga A. Ronova
Received: 22 November 2012 / Accepted: 18 January 2013
Ó Springer Science+Business Media New York 2013
Abstract Heteroaromatic polymers are considered to be
high performance organic materials due to their unique and
highly attractive properties, including outstanding thermal
and mechanical resistance, that arise from their aromatic
structure and strong interactions between macromolecular
chains. Modification or designing new molecular archi-
tectures with tailored physico-chemical characteristics
allows expanding the applications of these materials in
various advanced technologies. Herein, a series of poly-
mers containing bulky phenoxyphthalonitrile pendant units
was synthesized and their physical properties were studied
and correlated with their conformational parameters, as
well as free and van der Waals volumes. For comparison,
the related polymers without lateral moieties were also
investigated to highlight the effect of bulky substituent on
the polymer rigidity. Thus, it is shown that conformational
rigidity determines the packing of macromolecules in solid
state, and, therefore, the free volume, glass transition, and
decomposition temperatures. The values found experi-
mentally for Tg correlate well with those obtained using the
conformational rigidity parameters. The dependence of Tg
of these polymers on Kuhn segment is described by linear
equations, with very good factors of convergence. The
correlations established by Monte Carlo method allow
obtaining the Tg values for related polymers where the
experimental measurement of this parameter is difficult.
Keywords Structure–property relations ꢀ Monte Carlo
simulation ꢀ Free volume ꢀ Kuhn segment ꢀ Glass transition
temperature ꢀ Phthalonitrile-containing polymers
Introduction
Heteroaromatic polymers have found widespread applica-
bility associated with high thermal stability, outstanding
chemical resistance, high glass transition temperature, and
excellent mechanical and electrical properties. However,
their applications are limited due to shortcomings in pro-
cessability and solubility. Therefore, research efforts are
underway to develop soluble macromolecular structures
with an appropriate balance of chemical, physical, and
mechanical properties that allows the use in extreme con-
ditions, for example, at high temperature, while maintain-
ing the structural integrity. The general approaches
employed to enhance the solubility of the polymers are
focused on lowering the cohesive energy and the crystal-
lization tendency by introduction of flexible linkages [1, 2],
bulky side substituents [3], nonsymmetric, alicyclic, or
nonlinear moieties [4, 5]. Copolymerization is also widely
used [6–8]. Among these, the introduction of bulky pendant
units into the rigid polymer backbone has been recognized
as a straightforward and elegant path to increase the solu-
bility and, consequently, the processability and to incor-
porate new chemical functionalities in order to expand the
applications as high performance materials in various fields
on the forefront of scientific research, such as optically
active, luminescent, electrochromic, ionic exchange, flame
resistant, and selective receptors materials.
I.-D. Carja (&) ꢀ C. Hamciuc ꢀ T. Vlad-Bubulac ꢀ M. Bruma
‘‘Petru Poni’’ Institute of Macromolecular Chemistry,
Romanian Academy of Sciences, 700487 Iasi, Romania
e-mail: daniela.carja@icmpp.ro
Nitrile-containing polymers have attracted much interest
over the past decades because of their unique properties
such as remarkable thermal and thermo-oxidative stability,
I. A. Ronova
Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences, Moscow 119991, Russia
123

Struct Chem
1
published procedure [19]. H NMR (400 MHz, DMSO-d6,
high glass transition temperature, easy processing, excel-
lent mechanical properties, adhesion to many substrates,
strong interaction with the applied electric field and supe-
rior fire resistance [9–13]. Considering the outstanding
characteristics of the nitrile-containing polymers, the cur-
rent research is twofold directed: (a) the designing of new
piezoelectric materials for use at high temperature [14, 15]
and (b) the development of a new class of highly thermo-
setting polymers with high thermo-oxidative stability and
high performance structural properties at elevated tem-
peratures by thermal cross-linking polymerization [16–18].
Herein, the thermal properties of different type of
polymers containing 4-phenoxyphthalonitrile units have
been discussed in connection with conformational param-
eters of the macromolecular chains. The corresponding
polymers derived from 4,40-diaminodiphenylmethane were
also investigated to highlight the effect of bulky side sub-
stituent on the polymer rigidity. The aim of this work was
to establish new correlations by Monte Carlo method that
could permit to obtain the values of glass transition tem-
perature for related polymers where the experimental
determination of Tg values is difficult.
3
4
d, ppm): 8.08 (d, J = 8.8 Hz, 1H), 7.79 (d, J = 2.4 Hz,
1H), 7.35 (dd, 3J = 8.8 Hz, 4J = 2.4 Hz, 1H), 7.17 (d,
3J = 8.4 Hz, 2H), 7.09 (d, 3J = 8.4 Hz, 2H), 6.76 (d,
3
3J = 8.4 Hz, 4H), 6.49 (d, J = 8.4 Hz, 4H), 5.22 (s, CH,
1H), 4.93 (s, NH2, 4H). DHTM was obtained by the
reaction of 4-hydroxybenzaldehyde and aniline in the
presence of an acid catalyst (aniline hydrochloride), as
previously reported [20].
Terephthaloyl chloride, 3a, was synthesized by refluxing
the corresponding dicarboxylic acid in thionyl chloride. 1H
NMR (400 MHz, CDCl3, d, ppm): 8.25.
2,2-Bis[N-(4-chloroformylphenyl)phthalimidyl]hexaflu-
oropropane, 3b, was obtained by treating with thionyl
chloride the corresponding dicarboxylic acid resulting from
the condensation reaction of 4,40-(hexafluoroisopropylid-
ene)diphthalic anhydride with 4-aminobenzoic acid, in
glacial acetic acid as solvent and dehydrating reagent, at
1
reflux [21]. H NMR (400 MHz, DMSO-d6, d, ppm): 8.22
3
3
4
(d, J = 8.4 Hz, 2H), 8.11 (dd, J = 6.8 Hz, J = 2 Hz,
4H), 7.99 (d, 3J = 8 Hz, 2H), 7.77 (s, 2H), 7.61 (dd,
H2N
CH
O
NH2
Experimental
Materials
CN
CN
4,40-Diaminodiphenylmethane, 4-nitrophthalonitrile, tere-
phthalic acid, 4-aminobenzoic acid, 4,40-(hexafluoro-
isopropylidene)diphthalic anhydride, thionyl chloride,
N,N-dimethylformamide (DMF), and 1-methyl-2-pyrrolid-
inone (NMP) (HPLC grade) were purchased from Aldrich
and used as received. 4,40-Oxydiphthalic anhydride and
benzophenone-3,30,4,40-tetracarboxylic dianhydride were
provided by Aldrich and purified by recrystallization from
acetic anhydride, followed by thoroughly washing with
anhydrous diethyl ether. 4-Hydroxybenzaldehyde, aniline,
and aniline hydrochloride were purchased from Merck and
used as received. Potassium hydroxide and hydrochloric
acid were provided by Lach-Ner. Potassium carbonate,
toluene, glacial acetic acid, acetic anhydride, and ethanol
were purchased from Chemical Company Iasi. Diethyl
ether from Chemical Company Iasi was dried over sodium
wires and freshly distilled.
1
H2N
CH2
2
NH2
Cl
C
O
C
O
Cl
3a
O
O
N
CF3
C
N
C
O
Cl
Cl
C
O
CF3
O
O
O
3b
O
O
O
O
O
O
O
4a
Synthesis of the monomers
O
4,40-Diamino-400-(3,4-dicyanophenoxy)triphenylmethane, 1,
was prepared by nucleophilic substitution reaction of
4-nitrophthalonitrile and 4,40-diamino-400-hydroxy-triphe-
nylmethane (DHTM), in DMF as solvent and in the pres-
ence of anhydrous potassium carbonate, according to a
C
O
O
O
O
O
4b
Scheme 1 Structures of the monomers
123

Struct Chem
4
3J = 6.8 Hz, J = 2 Hz, 4H). The structures of all these
monomers are shown in Scheme 1.
Measurements
The infrared spectra of the polymers were recorded on
FTIR Bruker Vertex 70 Spectrophotometer in transmission
mode, using KBr pellets.
Synthesis of the polymers
1H NMR (400 MHz) spectra were performed at room
temperature on a Bruker Avance DRX 400 spectrometer,
using DMSO-d6 or CDCl3 as solvent and tetramethylsilane
as internal standard.
Aromatic polyamides Ia and I’a and poly(amid imide)s Ib
and I’b have been prepared by solution polycondensation
reaction at low temperature of equimolar amounts of two
aromatic diamines containing diphenylmethane units with
terephthaloyl chloride, 3a, or with the fluorinated diacid
chloride containing preformed imide rings, 3b, in NMP, in
the presence of pyridine as acid acceptor, at a concentration
of 10–12 % total solids, according to a published procedure
[19, 22]. Owing to the low solubility of polyamide I’a in
NMP, 3.5 % CaCl2 at a concentration of 7 % total solids
was added to the solution.
Model molecules for a polymer fragment were obtained
by molecular mechanics (MM?) by means of the Hyper-
Chem Programme, Version 7.5. The same software was
used to visualize the structures obtained after energy
minimization. The computations were carried out with full
geometry optimization (bond lengths, bond angles, and
dihedral angles) [24].
Polyimides II and II0 have been synthesized by two-step
polycondensation reaction using the same aromatic dia-
mines and 4,40-oxydiphthalic anhydride, 4a, or benzophe-
none-3,30,4,40-tetracarboxylic dianhydride, 4b, according to
a published procedure [23]. The first step consisted in
solution polycondensation reaction at room temperature of a
mixture of 1:1 molar ratio diamine to dianhydride, in NMP,
at a concentration of 10–12 % total solids. The second step
consisted in thermal imidization of the resulted poly(amidic
acid) solution by heating at reflux temperature, under a slow
stream of nitrogen to remove the water of imidization.
1H NMR spectra of the synthesized polymers: Polyam-
The thermal stability of the polymers was investigated
by thermogravimetric analysis (TGA) using a MOM deri-
vatograph, made in Budapest, Hungary, operating at a
heating rate of 10 °C min-1, in air, from room temperature
to 700 °C. The temperature of 5 % weight loss on the TG
curve was considered to be the beginning of decomposition
or the initial decomposition temperature (Td).
The glass transition temperature (Tg) of the polymers was
determined using a differential scanning calorimeter (DSC)
(Mettler-Toledo, Greifensee, Switzerland). Approximately
3–8 mg of each polymer were encapsulated in aluminum
pans having pierced lids to allow escape of volatiles, and
run in nitrogen with a heat-cool-heat profile from room
temperature to 300 °C at 10 °C min-1. The mid-point
temperature of the change in slope of the DSC signal of the
second heating cycle was used to determine the glass
transition temperature values of the polymers.
1
ide Ia: H NMR (400 MHz, DMSO-d6, d, ppm): 10.40
(s, CONH, 2H), 8.09 (m, 5H), 7.82 (s, 1H), 7.78 (d, 4H),
7.41 (d, 1H), 7.28 (d, 2H), 7.19 (d, 6H), 5.66 (s, 1H).
1
Poly(amide imide) Ib: H NMR (400 MHz, DMSO-d6, d,
ppm): 10.35 (s, CONH, 2H), 8.22 (d, 2H), 8.09 (s, 1H),
8.07 (d, 4H), 8.00 (d, 1H), 7.82 (s, 1H), 7.77 (d, 6H), 7.62
(d, 4H), 7.40 (d, 1H), 7.27 (d, 2H), 7.17 (d, 6H), 5.65 (s,
Dynamic mechanical analysis experiments (DMA) were
performed in tension mode on a PerkinElmer Diamons
device. The apparatus was heated in nitrogen atmosphere
1
1H). Polyimide IIa: H NMR (400 MHz, DMSO-d6, d,
from 0 °C up to 350 °C, at a heating rate of 2 °C min-1
.
ppm): 8.16 (d, 3H), 7.84 (s, 1H), 7.65 (m, 5H), 7.44 (d,
4H), 7.37 (d, 6H), 7.20 (d, 2H), 5.86 (s, 1H). Polyimide
IIb: 1H NMR (400 MHz, DMSO-d6, d, ppm): 8.24 (s, 2H),
8.15 (d, 4H), 8.07 (d, 1H), 7.83 (s, 1H), 7.46 (m, 5H), 7.39
(d, 6H), 7.20 (d, 2H), 5.87 (s, 1H). Poly(amide imide) I’b:
1H NMR (400 MHz, DMSO-d6, d, ppm): 10.31 (s, CONH,
2H), 8.22 (d, 2H), 8.07 (d, 4H), 8.00 (d, 2H), 7.78 (d, 2H),
7.72 (d, 4H), 7.62 (d, 4H), 7.23 (d, 4H), 3.91 (s, 2H).
The structures of the resulting polymers I, I0, II, and II0
are shown in Scheme 2.
Thin films of polymers I and I0 were obtained by casting
the polymer solutions of 10–14 % in NMP onto glass
plates, followed by gradual heating from room temperature
up to 210 °C. In the case of polymers II and II0
poly(amidic acid) solutions were used to prepare films with
thickness of 40–60 lm.
The strain resulted by applying a small sinusoidal stress on
polymer films with length of 10 mm, width of 10 mm, and
thickness of 0.05 mm at a frequency of 1 Hz was mea-
sured. The variation of storage modulus (E0), loss modulus
(E00), and loss tangent (tan d) as a function of temperature
were obtained.
Wide-angle X-ray diffraction measurements (WAXD)
were performed on a D8 Advance Bruker AXS diffrac-
tometer. The X-rays were generated using a Ni-filtered Cu
Ka source with an emission current of 30 mA and a voltage
of 36 kV. Scans were collected at room temperature, over
the scattering angle 2h = 2–40° range, using a step size of
0.01° and a count time of 0.5 s per step. Initial samples for
X-Ray measurements were powders obtained directly by
polycondensation.
123

Struct Chem
Scheme 2 Structures of the
polymers I, I0, II, and II0
*
HN
CH
O
NH
C
O
C
O
*
CN
CN
Ia
O
O
CF3
C
N
C
O
*
*
CH
O
NH
C
O
N
HN
CF3
O
O
CN
CN
Ib
O
O
O
N
*
*
CH
N
O
O
O
CN
CN
IIa
O
O
C
O
N
*
*
CH
O
N
O
O
CN
CN
IIb
I’a
*
HN
CH2
NH
C
C *
O
O
O
O
CF3
C
N
C *
*
HN
CH2
NH
CH2
CH2
C
N
CF3
O
O
O
O
I’b
O
O
O
N
*
*
N
O
O
II’a
O
O
C
O
N
*
*
N
O
O
II’b
123

Struct Chem
The densities of polyheteroarylene films were obtained
Therefore, the model of the repeating unit was built in the
molecular editor and the geometry of the corresponding
unit was optimized by the quantum–mechanical method
AM1 [26]. This repeating unit of the polymer was con-
sidered as a system composed of rigid spheres with the
coordinates of centers corresponding to the coordinates of
atoms and the radii equal to the corresponding radius of
each type of atoms [28]. This model was introduced in a
three-dimensional rectangular box having the axes Lx, Ly,
and Lz given by the Eq. (5):
using an electronic analytic balance Ohaus AP 250D from
Ohaus Corp US connected to a computer. This ultrasensi-
tive balance measured the weight change of sample during
the experiment, with a precision of 0.001 g cm-3 in the
value of density. Ethanol was taken as liquid with known
density. All measurements of the density were performed at
23 °C. The density was calculated with the Eq. (1):
Wa
qs ¼
ql
ð1Þ
Wa ꢁ Wl
Lx ¼ xmax þ Rmax ꢁ ðxmin ꢁ RmaxÞ ¼ xmax ꢁ xmin þ 2Rmax
ð5Þ
where qs is density of the sample; Wa is the weight of the
sample in air; Wl is the weight of the sample in liquid; ql is
the density of the liquid. The error of the density mea-
surements was 0.3–0.5 %.
where xmax and xmin are the maximum and the minimum
values of the coordinates of atoms corresponding to the
repeating unit; Rmax is the maximum value of the radius of
atoms corresponding to the repeating unit. Ly and Lz were
determined in the same manner. The volume of this model
was calculated with Monte Carlo method. Therefore, in the
volume corresponding to the parameters of the box,
random points were generated. Let m be the number of
random points landing in the repeating unit. At the
beginning of calculation, m was equal with zero. For
each random point, the following condition was verified:
Calculations of conformational parameters, van der
Waals, and free volumes
The statistical Kuhn segment (Afr) was determined with
Eq. (2), under the assumption of free rotation [25]:
ꢀ
ꢁ
ꢂ
ꢃ
R2
A ¼ lim
ð2Þ
n!/
nl0
ꢀ
ꢁ
where R2 is the square distance between the ends of the
chain calculated for all possible conformations; n is the
number of repeating units; l0 is the contour length of a
repeating unit; L = nl0 is the contour length of the chain, a
parameter that does not depend on the chain conformation.
For polyheteroarylenes in which the macromolecular unit
contains virtual bonds with different length and different
angles between them, the length of the zig-zag line con-
necting the mid-points of the virtual bonds is taken as the
contour length. The Kuhn segment lengths were calculated
by Monte Carlo method while the geometry of repeating
unit was assigned by quantum-method AM1 [26].
The characteristic ratio (C?), which shows how many
repeating units are in Kuhn segment, was calculated with
Eq. (3):
jrd ꢁ rij ꢂ Ri; i ¼ 1; N
ð6Þ
where N is the number of atoms in the repeating unit; jrd ꢁ rij
is the distance between a given point and any other point in
the repeating unit. If this condition was accomplished for at
least one atom, the procedure of verification was stopped, the
number of successful events began with m ? 1 and the next
random point was generated.Van der Waals volume (VW)
was calculated with the Eq. (7):
m
VW
¼
Vbox
ð7Þ
M
where M is the total number of all points and Vbox is the
volume of the box.The free volume (Vf) was obtained using
Eq. (8):
A
1
NAVW
M0
C ¼
/
ð3Þ
Vf ¼
ꢁ
ð8Þ
l0
q
The parameter of conformational rigidity (p) takes into
consideration the conformational flexibility of the polymer
and the interactions between the aromatic rings. From the
mathematical point of view, this parameter is defined by
the Eq. (4):
where NA is the number of Avogadro; q is the density of
polymer; M0 is the molecular weight of the repeating unit.
The value Vf thus calculated represents the volume which
is not occupied by the macromolecular chains in 1 cm3 of
polymer film.
A
p ¼
k
ð4Þ
l0
Results and discussion
where k is the number of aromatic rings in a repeating unit.
The van der Waals (VW) and free volumes (Vf) were
The polymers studied here contain 4-phenoxyphthalonitrile
units linked to the polymer backbone (Scheme 2).
calculated by
a method described previously [27].
123

Struct Chem
Polycondensation reaction of equimolar amounts of aro-
matic phthalonitrile-diamine 1 with two aromatic diacids
chloride (terephthaloyl chloride, 3a, and 2,2-bis[N-(4-chlo-
roformylphenyl)phthalimidyl]hexafluoropropane, 3b) or
with various aromatic dianhydrides, namely 4,40-oxydiph-
thalic anhydride, 4a, and benzophenone-3,30,4,40-tetracarb-
oxylic dianhydride, 4b, in NMP, yielded polymers I and II,
with the formation of intermediate poly(amidic acid)s in the
case of the polyimides II. The resulting polymers were
isolated by precipitation in water, followed by washing and
drying. The physical properties of these polymers (solubil-
ity, glass transition temperature, and initial decomposition
temperature) were discussed in connection with conforma-
tional parameters of the macromolecules.
characteristic absorption bands for carbonyl bond of the imide
ring at about 1,775–1,785 cm-1 (imide I), which correspond
to C=O asymmetric stretching, and 1,712–1,720 cm-1 (imide
II) corresponding to C=O symmetric stretching. The absorp-
tion peaks at 1,363–1,375 cm-1 (imide III, C–N stretching
vibration) and 716–746 cm-1 (imide IV, C–N bending
vibration) were also observed. For polymers Ib and I’b
characteristic absorptionbands ofC–Flinkage appearedinthe
range 1,188–1,209 cm-1. The polyimides IIb and II’b
derived from benzophenone-3,30,4,40-tetracarboxylic dian-
hydride presented characteristic absorption bands of carbonyl
bond stretching at 1,672–1,684 cm-1. The polymers con-
taining bulky side substituents (I and II) showed absorption
peaks at 2,231–2,232 cm-1 corresponding to nitrile bonds.
1H NMR spectra of I, II, and I’b were recorded in order
to confirm the structure of the polymers. Thus, the signals
corresponding to the aromatic protons appeared in the
range 8.24–7.17 ppm, while in the aliphatic region, the
polymers exhibited singlets due to the CH, respectively,
CH2 protons of diamine units, as follows: Ia: 5.66 ppm; Ib:
5.65 ppm; IIa: 5.86 ppm; IIb: 5.87 ppm; I’b: 3.91 ppm. It
can be noted that the peaks corresponding to the aliphatic
protons of polymer I’b were shifted upfield as compared to
those of Ib as a result of higher electron density around the
methylene hydrogens. Amidic protons displayed low field
signals, at 10.40 ppm for Ia, at 10.35 ppm for Ib, and at
For comparison, the related polymers based on 4,40-
diaminodiphenylmethane, 2, were prepared using the same
synthetic procedure with some observations: in the case of
polyamide I’a, CaCl2 was added to the reaction mixture in
order to achieve better solubility by reducing the strength
of the interchain hydrogen bonds between amide groups
[29, 30]; polyimides II’ precipitated during imidization.
The structures of these polymers are shown in Scheme 2.
The chemical structures of the synthesized polymers were
1
confirmed by FTIR and H NMR spectroscopy. The most
important signals in FTIR spectra and their attribution are
given in Table 1. The polymers I and I0 showed a broad IR
absorption band at 3,319–3,363 cm-1 characteristic of N–H
1
10.31 ppm for I’b. A typical H NMR spectrum is shown
amide and
a
narrow strong absorption peak at
in Fig. 1. In the case of I’a and II0, 1H NMR spectroscopy
could not be performed because of the lack of solubility of
these polymers in deuterated solvents.
1,645–1,670 cm-1 due to C=O bond in amide linkage. The
FTIR spectra of polymers Ib, I’b, II, and II0 exhibited
Table 1 FTIR (KBr pellets, cm-1) absorption bands of the synthesized polymers
Band assign
Ia
Ib
IIa
IIb
I’a
I’b
II’a
II’b
Ar. C–H str.
3,060
2,872
3,060
2,860
3,069
2,862
3,072
2,875
3,033
3,070
3,057
3,071
Aliph. C–H str.
2,918 asym. 2,921 asym. 2,922 asym. 2,923 asym.
2,873 sym.
2,851 sym.
2,879 sym.
2,853 sym.
Amide, N–H
3,363
1,660
2,231
–
3,350
1,665
2,232
–
–
–
3,319
3,357
1,670
–
–
–
–
–
–
Amide, C=O
–
–
1,645
–
Ar. C:N
2,232
–
2,231
1,672
–
–
–
–
Ar. C=O (benzophenone unit)
Imide, C=O
–
1,684
–
1,785 asym. 1,777 asym. 1,779 asym.
1,785 asym. 1,777 asym. 1,775 asym.
1,720 sym.
1,369
1,720 sym.
1,375
1,716 sym.
1,368
720
1,719 sym.
1,365
716
1,713 sym.
1,367
1,712 sym.
1,363
742
Imide, C–N str.
Imide, C=O bend.
Ar. C=C
–
–
–
719
746
–
718
1,600
1,500
1,600
1,592
1590
1596
1511
–
1604
1511
–
1600
1607
1511
–
1,508
1,502
1503
1511
Ar. C–O–C
1,248 asym. 1,254 asym. 1,249 asym. 1,247 asym.
1,250 asym.
1,090 sym.
–
1,100 sym.
–
1,104 sym.
1,209
1,064 sym.
–
1,089 sym.
–
C–F (6F unit)
–
1,208
1,191
–
1,188
Ar aromatic, str stretching, aliph aliphatic, asym asymmetric stretching, sym symmetric stretching, bend bending
123

Struct Chem
Fig. 1 1H NMR spectrum of
poly(amide imide) I’b
All these polymers, except I’a and II0, were easily sol-
uble in polar aprotic solvents such as NMP, DMF, N,N-
dimethylacetamide, or dimethylsulfoxide (DMSO), and
even in less polar solvents such as tetrahydrofurane which
is a convenient and easily accessible solvent. This
improved solubility can be explained by the presence of
sp3–carbon atom of diamine monomer, combined with
ether, carbonyl, or flexible hexafluoroisopropylidene
groups of the comonomer units, which disturbs the tight
packing of macromolecules and generates deviation from
the linear rigid-rod form characteristic to completely aro-
matic insoluble structures. The incorporation of bulky
pendant phenoxyphthalonitrile units resulted in an increase
in free volume, and thus facilitated the diffusion of small
molecules of solvent among the polymer chain. Therefore,
this enhancement in solubility makes the present polymers
potential candidates for practical applications in spin-
coating and casting processes.
introduction of phenoxyphthalonitrile bulky moieties into
the polymer backbone (I and II) led to lower decomposi-
tion temperatures as compared with those obtained for the
polymers without side groups.
The glass transition temperature of the polymers was
evaluated by DSC. All these polymers, except I0, exhibited
glass transitions, in the range of 197–282 °C (Table 2).
The polymers I0 did not show a Tg in DSC experiments
when the samples were heated up to 300 °C, which means
that the Tg may be higher than 300 °C. The polymers II
exhibited lower Tg values as compared with polymers II0
due to the presence of bulky side substituents connected to
a kinking sp3-carbon atom that restricted dense packing,
reduced the crystallinity, and allowed the segmental
movement in the polymer chain. It can be seen that there
was a large interval between Tg and decomposition tem-
perature of these polymers, which can be advantageous for
their processing by thermoforming technique. A typical
DSC curve is shown in Fig. 3.
Model molecules for four repeating units of polymer
were obtained by molecular mechanics by means of the
Hyperchem software version 7.51 [24] to better understand
the chain conformation. As can be seen from Fig. 2, in the
case of polymer Ia, the arrangement of phenoxyphthalo-
nitrile units provided a distance between the macromole-
cules, and thus preventing dense packing. By contrast,
polymer I’a tended to orient in the extended form (rigid-
rod), and consequently, the polyamide based on diamine 1
exhibited better solubility compared with that obtained
from 4,40-diaminodiphenylmethane, 2.
The thermal stability of these polymers was investigated
by TGA. The polymers were highly thermostable, without
significant weight loss below 400 °C (Table 2). The initial
decomposition temperatures (Td) as determined from TG
curves were in the range 400–533 °C. As expected, the
Fig. 2 Models of polyamides Ia and I’a
123

Struct Chem
Table 2 Thermal properties and conformational parameters of the synthesized polymers
3
VW (A )
Tag (°C)
Tbd (°C)
q (g cm-3
)
Vf (cm3 g-1
)
˚
l0 (A)
˚
Afr (A)
˚
Polymer
C?
p
Ia
17.90
35.63
22.42
22.49
17.90
35.63
22.42
22.49
40.15
36.80
27.54
26.03
40.15
36.80
27.54
26.03
2.2430
1.0328
1.2284
1.1574
2.2430
1.0328
1.2284
1.1574
11.22
10.33
9.83
9.26
6.73
8.26
7.37
6.94
243
400
427
420
425
420
485
533
525
1.284
1.463
1.299
1.230
1.589
1.710
1.339
1.346
507.22
897.11
613.29
623.22
317.31
688.91
417.44
427.30
0.2202
0.1754
0.2351
0.2792
0.0503
0.0950
0.2163
0.2133
Ib
229
IIa
IIb
I’a
I’b
II’a
II’b
a
211
197
Not detected
Not detected
282
265
Glass transition temperature measured by DSC
b
Td = temperature of 5 % weight loss on TG curve
In order to be assured that the value of Tg is unequiv-
denser molecular packing. In the wide-angle region, two
sharp strong peaks were observed at 9.8° and 16.2°, and
relatively broad reflections at 17.8°, 21°, 22.7°, and 27.7°
(associated with the lateral packing). This behavior is
consistent with the results obtained by other researchers.
Cristea and co-workers [31] found a degree of crystallinity
equal to 77 % for polyimide II’b in powder form.
ocally defined, DMA experiments were also conducted.
Thus, in the case of polyamide Ia, the variation of loss
tangent (tan d) on temperature revealed a well-defined peak
corresponding to a sharp drop in storage modulus of
approximatively three orders in magnitude at around
273 °C which was attributed to the glass transition tem-
perature. A difference of 30 °C between the Tg values
obtained by DSC and DMA measurements that can mainly
be explained by the use of different types of materials
(powder, in DSC experiments and polymer films, in DMA
experiments) can be noticed. The thermal treatment applied
to the polymer films has also an important influence.
WAXD experiments confirmed the assumption that the
bulky substituents reduced the intermolecular interactions
between polymer chains, and thereby, the supramolecular
architecture was modified and a less organized structure
was obtained. Figure 4 presents comparatively the XRD
diffraction patterns at room temperature of polyimides IIb
and II’b. Polymer IIb showed only a broad halo centered at
around 2h = 15.9° indicating an amorphous behavior. By
contrast, the polyimide II’b displayed a sharp strong dif-
fraction peak at medium angle (2h = 4.7°) suggestive for a
The Kuhn segment (Afr), the characteristic ratio (C?),
and the number of aromatic rings in the Kuhn segment
(p) were chosen as the conformational parameters and were
discussed in correlation with thermal properties of the
present polymers [32]. The parameter p was included
because in previous works the correlation of physical
properties with the number of aromatic rings in Kuhn
segment was shown [33–35].
The Kuhn segment values of polymers were calculated
under the assumption of free rotation. Despite the large
volume of the lateral substituents, the rotation around the
CH–Ph bond was not hindered because the distances
Fig. 4 XRD diffraction patterns at room temperature of polyimides
IIb and II’b
Fig. 3 DSC curve of polymer IIb
123

Struct Chem
between unbounded hydrogen atoms in phenyl rings were
longer than the sum of van der Waals radii for all three
phenyl rings. However, the existence of such bulky sub-
stituents should influence the polymer packing in the glassy
state. In order to prove this hypothesis, the polymers
without lateral substituents were also considered.
The calculated conformational parameters of the present
polymers are given in Table 2. The interdependence of
glass transition temperature (Tg) as determined from DSC
curves on the length of Kuhn segments (Afr) for the syn-
thesized polymers is shown in Fig. 5.
Using the least-squares method, the dependence of glass
transition temperature of the polymers containing bulky
moieties on Kuhn segment (Fig. 5) can be described by a
linear equation: Tg = 2.85 Afr ? 127.14 with a very good
factor of convergence (R = 92.79 %). From physical point
of view, this dependence showed that the glass transition
temperature increased by increasing the rigidity.
Fig. 6 The dependence of glass transition temperature (Tg) on the
number of aromatic rings in the Kuhn segment (p) of polymers I and
II
a linear equation: Tg = 23.94 p—23.18 with a very good
factor of convergence (R = 95.91 %). The polymer Ia,
with the higher value of rigidity parameter (11.22) had the
highest Tg (243 °C). The introduction of flexible linkage
such as ether or hexafluoroisopropylidene led to a decrease
of Tg values opposed to the polymer Ia. Comparing the
polymers IIa with II’a and IIb with II’b, a reversed
dependence can be noted: by increasing the Tg values, the
p values decreased for both cases. This behavior proved
that the introduction of bulky side substituents into the
polymer backbone diminished the interaction between the
macromolecules and led to lower Tg values.
In the case of polymers derived from diamine 2, such
dependence could not be drawn due to the insufficient
number of Tg values determined experimentally. Thus,
the theoretical value of 360 °C obtained by applying the
method described by Askadskii [36] was introduced, as the
glass transition temperature corresponding to the polyam-
ide I’a. As it is mentioned in literature [6, 37], if the
dependence is linear with a good factor of convergence,
that equation can be used for the estimation of glass tran-
sition temperature and initial decomposition temperature in
the case when their experimental determination is difficult.
Consequently, the glass transition temperature of the
poly(amide imide) I’b was calculated with the equation:
Tg = 6.52 9 36.8 ? 98.73 & 334 °C.
Figure 7 shows the dependence of the glass transition
temperature of polymers I and II on free volume. With the
increase of free volume, the possibility of conformational
transition increased with the increase of temperature, when
the sample was heated, and the glass transition temperature
decreased. It can be observed that the point corresponding
to the polymer Ia was clearly situated far away from the
common dependence. This behavior could be due to the
lower distance between the bulky pendant substituents as
compared to the other polymers that resulted in an
increased free volume, and to the lack of flexible linkages
in the comonomer unit. In all cases the free volume values
were higher in the polymers with side substituents as
compared to those corresponding to the polymers without
lateral moieties.
As can be seen in Fig. 6, the dependent nature of glass
transition temperatures of polymers containing pendant
phenoxyphthalonitrile substituents on the number of aro-
matic rings in the Kuhn segment (p) was also described by
The dependence of the initial decomposition tempera-
ture (Td) of the synthesized polymers as determined from
TG curves on the Kuhn segment is depicted in Fig. 8.
There was a significant linear relationship between thermal
stability of most of these polymers and their conforma-
tional rigidity, described by the equations: Td = -1.61
Afr ? 464.70 with a very good factor of convergence
(R = 99.87 %) for the polymers with pendant units and
Td = -8.03 Afr ? 743.46 (R = 94.86 %) for the polymers
Fig. 5 The dependence of glass transition temperature (Tg) on the
length of Kuhn segments (Afr) of polymers I, II, I0, and II0
123

Struct Chem
with the corresponding comonomers. These polymers
exhibited improved solubility and processability as com-
pared to the related polymers without side substituents,
allowing them to be processed in thin films and coatings.
All the synthesized polymers were highly thermostable,
their initial decomposition temperature being above
400 °C. It was observed a large window between glass
transition and decomposition temperature which may be
useful for their processing by thermoforming techniques.
The values found experimentally for Tg corresponding to
the polymers with bulky lateral substituents correlated well
with those calculated using the conformational rigidity
parameters. Similarly, an increase of decomposition tem-
perature was found with the increase of conformational
rigidity parameters. The dependence of glass transition and
decomposition temperatures of these nitrile-containing
polymers on Kuhn segment can be described by linear
equations, with a very good factor of convergence. The use
of correlations established here by Monte Carlo method
could be used to design similar polymers with tailored
physico-chemical characteristics.
Fig. 7 The dependence of glass transition temperature (Tg) on the
free volume (Vf) of polymers I and II
Acknowledgments The authors thank to Dr. Mariana Cristea for
DMA measurements. Warm thanks are given to European Social
Fund—‘‘Cristofor I. Simionescu’’ Postdoctoral Fellowship Pro-
gramme (ID POSDRU/89/1.5/S/55216), Sectoral Operational Pro-
gramme Human Resources Development 2007–2013, for the financial
support offered to one of the authors (Dr. Tachita Vlad-Bubulac).
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