Novel self-dyed wholly aromatic polyamide-hydrazides covalently bonded with azo groups in their main chains: 1. structure-property relationships

Article abstract of DOI:10.3390/molecules171213969

Twelve novel intrinsically colored wholly aromatic azopolyamide-hydrazides containing various proportions of para- and meta-phenylene units were successfully synthesized by a low temperature (-10 °C) solution polycondensation reaction of either 4-Amino-3-hydroxybenzhydrazide (4A3HBH) or 3-Amino-4-hydroxybenzhydrazide (3A4HBH) with an equimolar amount of either 4,4'-Azodibenzoyl chloride (4,4'ADBC), 3,3'-Azodibenzoyl chloride (3,3'ADBC), or mixtures of various molar ratios of 4,4'ADBC and 3,3'ADBC in anhydrous N,N-dimethyl acetamide (DMAc) containing 3% (wt/v) LiCl as a solvent. The structures of the polymers were proven by elemental analysis, FTIR, 1H- and 13C-NMR spectroscopy. The polymers' properties were strongly affected by their various structures. The intrinsic viscosities of the polymers were ranged from 0.7 to 4.75 dL g-1 and increased with the para-phenylene units content. The polymers are partially soluble in DMAc, dimethyl formamide (DMF) and N-methyl-2-pyrrolidone (NMP). Their solubility increases with the introduction of meta-phenylene moieties into the polymer chains. The polymers exhibit a great affinity for water sorption. Their hydrophilicity increases as a function of the content of meta-phenylene rings incorporated into the polymer. Mechanical properties of the polymer films are improved markedly by substitution of para-phenylene units for meta-phenylene units. The completely para-oriented type polymer has the best thermal and thermo-oxidative stability relative to those of the other polymers.

Full text of DOI:10.3390/molecules171213969

Shia Molecules 2012, 17, 13969-13988; doi:10.3390/molecules171213969
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Novel Self-Dyed Wholly Aromatic Polyamide-Hydrazides
Covalently Bonded with Azo Groups in Their Main Chains:
1. Structure-Property Relationships
Nadia A. Mohamed 1,*, Mohammad H. Sammour 2 and Ali M. Elshafai 2
1
Department of Chemistry, Faculty of Science, Cairo University, Giza 12613, Egypt
2
Excellence Center of Science and Technology, Elsalam-2, Cairo 3066, Egypt;
E-Mails: mhsamour@yahoo.com (M.H.S.); alielshafaie1@yahoo.com (A.M.E.)
* Author to whom correspondence should be addressed; E-Mail: namadm@hotmail.com.
Received: 4 September 2012; in revised form: 2 November 2012 / Accepted: 12 November 2012 /
Published: 26 November 2012
Abstract: Twelve novel intrinsically colored wholly aromatic azopolyamide-hydrazides
containing various proportions of para- and meta-phenylene units were successfully
synthesized by a low temperature (−10 °C) solution polycondensation reaction of either
4-amino-3-hydroxybenzhydrazide (4A3HBH) or 3-amino-4-hydroxybenzhydrazide (3A4HBH)
with an equimolar amount of either 4,4'-azodibenzoyl chloride (4,4'ADBC), 3,3'-azodibenzoyl
chloride (3,3'ADBC), or mixtures of various molar ratios of 4,4'ADBC and 3,3'ADBC in
anhydrous N,N-dimethyl acetamide (DMAc) containing 3% (wt/v) LiCl as a solvent. The
1
13
structures of the polymers were proven by elemental analysis, FTIR, H- and C-NMR
spectroscopy. The polymers’ properties were strongly affected by their various structures.
The intrinsic viscosities of the polymers were ranged from 0.7 to 4.75 dL g−1 and increased
with the para-phenylene units content. The polymers are partially soluble in DMAc,
dimethyl formamide (DMF) and N-methyl-2-pyrrolidone (NMP). Their solubility increases
with the introduction of meta-phenylene moieties into the polymer chains. The polymers
exhibit a great affinity for water sorption. Their hydrophilicity increases as a function of
the content of meta-phenylene rings incorporated into the polymer. Mechanical properties
of the polymer films are improved markedly by substitution of para-phenylene units for
meta-phenylene units. The completely para-oriented type polymer has the best thermal and
thermo-oxidative stability relative to those of the other polymers.

Molecules 2012, 17
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Keywords: azopolyamide-hydrazides; intrinsic viscosity; solubility; crystallinity;
mechanical properties; thermal characteristics
1. Introduction
Polymeric products are usually dyed with a reactive dye or a disperse dye at a high temperature for
a long time, which will waste energy and time. The use of dispersed pigments for coloring polymeric
products gives rise to some problems such as limited attainable shades, nucleating effect, equipment
cleaning, compatibility with polymeric products, wash fastness, migration, and so on. Thus, the use of
intrinsically colored polymers, which have a chromophoric system (aromatic azo groups) forming part
of their main chains, offers lots of advantages. In addition, the aromatic azo group is of special interest
because of the existence of cis-trans isomerism and its effect on the photochromic and photocontractile
properties of the polymers [1–3]. Further, the azo-para-phenylene moiety in the polymer main chain
can impart rigidity to the polymer [4]. Therefore, polymers that contain the azo group have potential
use in a variety of applications, such as materials with liquid-crystal [5,6], or non-linear optic
properties [7]. Azobenzene-containing materials are photochromic and reversibly switch between two
spectroscopically distinct forms by use of light. Thus azobenzene-based polymers are interesting due
to their photoresponsive behavior which can be utilized in optical switches, optical data recording or
optical information storage [8–10].
Wholly aromatic polyamide-hydrazides are a unique class of materials which combine the
properties of both the aromatic polyamides and the aromatic polyhydrazides [11–15]. These polymers
could be classified as high performance materials because they possess some interesting and potentially
useful properties. They exhibit favorable rheological behavior that allowed for successful preparation
of fibers and films from their solutions in dimethyl acetamide [16] and dimethyl sulfoxide [13]. Their
appropriately drawn fibers showed very high mechanical strength and moduli, which are superior to
that of glass fiber and steel wire [13]. These fibers can be successfully applied in fabrics, rigid
composites, tire-cords, impact-absorbing devices, and for ballistic protection [17]. Polyamide-hydrazide
films are useful as reverse osmosis membranes in water-pollution control, and for concentration of
fruit juices [18], and as semipermeable membranes for desalination of artificial and natural
seawater [19–21]. Further, their modified transition metal chelates have shown good electrical
conductivity [22]. Moreover, these polymers are characterized by high thermal and thermo-oxidative
stability [23,24]. Aromatic polyamide-hydrazides are well known as precursors to polyamide-1,3,4-
oxadiazoles, which are one of the most important classes of chemically and thermally stable
heterocyclic polymers [25,26]. It is well established that all aromatic polymers containing only
benzoxazole [27,28] or 1,3,4-oxadiazole [29,30] moieties as the main structural feature of the polymer
chains exhibit excellent thermal stability and other attractive physical properties.
Thus, our goal will be the synthesis and characterization of several novel self-colored wholly
aromatic azopolyamide-hydrazides with o-hydroxyl groups which would be expected to undergo
thermo-chemical transformation into linear aromatic polymers with alternating 1,3,4-oxadiazole and
benzoxazole structural units within the same polymer backbone in order to classify these materials as a

Molecules 2012, 17
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category of heat resistant polymers. 4-Amino-3-hydroxybenzhydrazide (4A3HBH), 3-amino-4-
hydroxybenzhydrazide (3A4HBH), 4,4'-azodibenzoyl chloride (4,4'ADBC) and 3,3'-azodibenzoyl
chloride (3,3'ADBC) were selected as monomers to be used in this study. The second phase of this
work will be extended to study the effect of introducing different predetermined proportions of para-
and meta-oriented phenylene moieties into the backbone chain of the polymers on their properties.
2. Results and Discussion
2.1. Polymer Synthesis
A series of novel wholly aromatic azopolyamide-hydrazides has been synthesized by a low
temperature (at –10 °C) solution [in anhydrous DMAC containing 3% (wt/v) LiCl] polycondensation
reaction of either 4A3HBH or 3A4HBH with stoichiometric amounts of either 4,4'ADBC, 3,3'ADBC
or mixtures of various molar ratios of 4,4'ADBC, and 3,3'ADBC as summarized in Table 1 and
illustrated in Schemes 1 and 2. The content of para- and meta-phenylene moieties was varied within
this series so that the changes in the latter were 10 mole % from polymer to polymer, starting from an
overall content of 0–100 mol %. Thus, all the polymers are structurally similar except for the mode of
linking of phenylene groups in the polymer chain. All the polymers were produced in a quantitative
yield which ranged between 99.38% and 99.95% (Table 1). The basic reaction employed here is the
Schotten-Baumann condensation of an aromatic acid chloride and an aminohydrazide, which is
generally known to be fast and quantitative. It is established that the rate by which hydrazide groups of
aminobenzhydrazide reacted with acid chloride is about seven times greater than that of the amino
groups with the same acid chloride [31,32]. These properties indicate that when this polycondensation
is performed by gradual addition of solid acid chloride in a solution of aminobenzhydrazide monomer,
the hydrazide groups of the latter react first, and the so-called “wholly ordered” polymer with
alternating amide and hydrazide linkages is formed.
Table 1. Syntheses of novel wholly aromatic azopolyamide-hydrazides containing various
proportions of para-phenylene and meta phenylene units.
Expected para/meta-
Intrinsic
viscosity a
(dL g−1)
4.75
Monomer feed composition (mol %)
4A3HBH 3A4HBH 4,4'ADBC 3,3'ADBC
Polymer
code
Yield
(%)
phenylene rings in
polymer (mol %)
100/0
I
II
100
100
100
100
100
100
-
-
-
100
80
60
40
20
-
-
99.85
99.64
99.46
99.38
99.81
99.80
99.77
99.45
99.57
99.83
99.95
99.78
20
40
60
80
100
-
90/10
3.98
III
IV
V
-
80/20
3.51
-
70/30
2.83
-
60/40
2.64
VI
VII
VIII
IX
X
-
50/50
2.26
100
100
100
100
100
100
100
80
60
40
20
-
50/50
2.21
-
20
40
60
80
100
40/60
1.80
-
30/70
1.47
-
20/80
1.22
XI
XII
-
10/90
1.08
-
0/100
0.71
a Determined in DMAc/3%(wt/v) LiCl at 30 °C, polymer concentration was 0.5 g dL−1.

Molecules 2012, 17
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Scheme 1. Typical synthetic scheme for novel wholly aromatic azopolyamide-hydrazides I-VI.
O
O
N = N
C - Cl
+ n Cl - C
4,4`-Azodibenzoyl chloride
O
4,4`ADBC
O
H
H O
H
N
C -
N = N
N = N
C - N - N - C
+ 2n HCl
n
HO
I
O
O
H
H
H
O
O
H
H
O
O
H
H
H O
H
N
+x 4,4`ADBC
+ (n-x) 3,3`ADBC
C -
N = N
+ 2n HCl
nH-N
C - N - N - H
C - N-
x
C - N - N - C
C - N - N - C
(n-x)
n/2
HO
HO
HO
4-amino-3-hydroxybenzhydrazide
4A3HBH
II: x = 0.8 and (n-x) = 0.2 ;
IV: x = 0.4 and (n-x) = 0.6 ;
III: x = 0.6 and (n-x) = 0.4
V: x = 0.2 and (n-x) = 0.8
O
O
H
H
O
H
O
C -
N = N
O
C - N - N - C
N
+ 2n HCl
C - Cl
N = N
Cl - C
+ n
HO
n
VI
3,3`-Azodibenzoyl chloride
3,3` ADBC
Scheme 2. Typical synthetic scheme for novel wholly aromatic azopolyamide-hydrazides VII-XII.
O
O
N = N
C - Cl
+ n Cl - C
O
O
H
H
O
H
N
4,4`-Azodibenzoyl chloride
4,4`ADBC
C -
N = N
+ 2n HCl
C - N - N - C
n
HO
VII
O
O
H
H
O
H
O
H
H
O
O
H
H
H O
H
N
+ x 4,4`ADBC
+ (n-x) 3,3`ADBC
C -
+ 2n HCl
N = N
C - N - N - H
nH-N
C - N-
x
N = N
C - N - N - C
C - N - N - C
n/2
HO
HO
HO
3-amino-4-hydroxybenzhydrazide
3A4HBH
VIII: x = 0.8 and (n-x) = 0.2
X: x = 0.4 and (n-x) = 0.6
IX: x = 0.6 and (n-x) = 0.4
XI: x = 0.2 and (n-x) = 0.8
O
O
H
H O
H
N
C -
n
N = N
C - N - N - C
+ 2nHCl
O
O
C - Cl
N = N
HO
Cl - C
+ n
XII
3,3`-Azodibenzoyl chloride
3,3` ADBC
The liberation of HCl by-product has multifunctional effects on this reaction. First, HCl acts as a
reaction catalyst. Second, it reacts with the solvent (DMAc) to form a salt (DMAc-HCl) [33].
Although, it could be expected that there is a competition between the growing polymer molecules and
the solvent (DMAc) for HCl, solvent is more basic than the polymeric product, thus associating with
solvent carbonyl oxygen predominates. This salt is useful in making and keeping in solution
azopolyamide-hydrazides through enhancing the solubility of the growing polymer, thereby assisting
in molecular weight build up and should direct the polycondensation equilibrium towards completion
and be an additional driving force for the process. Consequently, since the basic reaction is known as
rapid even at low temperatures, and, since one of the polycondensation products is continuously being
withdrawn from the equilibrium, this condensation polymerization reaction is expected to proceed to

Molecules 2012, 17
13973
acceptably high conversions and thus satisfy one of the mentioned requirements for preparation of high
molecular weight products imposed by the nature of this step-growth process. Addition of LiCl to the
polymerization medium enhances the solubility of the polymers. Although there is a competition
between the polymer and solvent (DMAc) for the Li+ cation; the solvent is more basic than the
polymeric product. Binding by the solvent predominates and the Li+ cation associates with the solvent
carbonyl oxygen; this is useful for making and retaining the polymer in solution [34].
2.2. Polymer Identification
The structure of the prepared polymers is proposed on the basis of their elemental analyses, FTIR,
13
1H- and C-NMR spectra. The elemental analysis values are in good agreement with the theoretical
ones calculated for the expected repeating unit (C21H15N5O4) as shown in Table 2. On the other hand,
all the FTIR spectra of these polymers showed common absorption stretching vibration bands at the
following wave numbers: 3650–3150 cm−1 (intense and broad) assigned to the overlapped =NH, -OH
(phenolic) and the possible interchain hydrogen bonding; 2300–2180 cm−1 (weak) is attributed to
possible enol-type configuration of hydrazide and amide groups; 1660–1650 cm−1 (strong)
corresponded to the amide carbonyl group; 1580–1600 cm−1 indicated the aromatic carbon-carbon
double bonds; 1525–1515 cm−1 is due to =NH of amide II; 1500 cm−1 indicated carbon-carbon single
bond in ring; 1420 cm−1 is attributed to carbon-oxygen (phenolic); 1340, 1280 and 1250 cm−1
corresponded to carbon-hydrogen combined with =NH of amide III.
Table 2. Elemental analysis data of novel wholly aromatic azopolyamide-hydrazides
containing various proportions of para-phenylene and meta-phenylene units.
Elemental analyses
Polymer code
% C
% H
% N
% O a
15.96
Calculated for the expected
62.84
3.74
17.46
repeating unit, C21H15N5O4
I
II
62.83
62.79
62.81
62.78
62.76
62.72
62.80
62.73
62.75
62.70
62.74
62.71
3.76
3.75
3.78
3.76
3.79
3.77
3.80
3.80
3.79
3.81
3.81
3.82
17.41
17.43
17.40
17.42
17.39
17.44
17.38
17.40
17.37
17.34
17.33
17.36
16.00
16.03
16.01
16.04
16.06
16.07
16.02
16.07
16.09
16.15
16.12
16.11
III
IV
V
VI
VII
VIII
IX
X
XI
XII
a Calculated as % O = 100 − (% C + % H + % N).
Specific assignment of the band due to -N=N- stretching was not possible because of the overlap by
the band corresponding to C=C stretching in aromatic rings [35,36]. Further, it would be expected that
all of these polymers have the same structural formula except for the mode of linking of phenylene

Molecules 2012, 17
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rings in the polymer chains. This expectation was evidenced experimentally by comparing the FTIR
spectra of the polymers in the region of 900–800 cm−1 which is characteristic for aromatic C-H
out-of-plane bending vibration. In this experiment, thin polymer films having similar thickness of
3 μm were tested in order to obtain a fair detection of any change in the spectra. Figure 1 illustrates the
variations of the intensities of FTIR peaks of these polymers in the aforementioned region as a
function of their meta- to para-phenylene moieties content. It can be noted that the meta- to
para-phenylene unit compositions of the polymers changed quantitatively in proportion to their
compositions in feed. FTIR spectra showed a gradual increase in the intensity of the band
corresponding to the meta-phenylene moiety (at 825 cm−1), together with a reduction of that of the
para-phenylene ring (at 865 cm−1) from polymer I to XII.
Figure 1. Changes of FTIR peak intensities of novel wholly aromatic azopolyamide-
hydrazides as a function of their para-phenylene and meta-phenylene rings content.
The molecular structures of the evaluated polymers were also confirmed by NMR analysis. The
1H-NMR and 13C-NMR spectra of polymer XII (Figure 2), as an example, are shown in Figures 3 and 4,
1
respectively. In the H-NMR spectrum (200 MHz, DMSO-d6), the polymer showed the following
characteristic peaks:  = 10.73 (s, 1H, OH, D2O exchangeable), 10.47 (s, 1H, NHb, D2O exchangeable),
10.01 (s, 1H, NHa, D2O exchangeable), 7.08–8.59 ppm (m, 13H, 12 Ar. H+NHc). In the 13C-NMR
(300 MHz, DMSO-d6) spectrum, the characteristic resonances of the carbon atoms are as follows:
 = 169.4 (C1=O), 164.9 (C8=O), 153.5 (C6), 151.5 (C12), 135.4 (C2), 133.8 (C3), 130.4 (C4), 129.6
(C13), 125.1 (C9), 123.0 (C5), 121.6 (C10), 121.1 (C7), 115.1 (C14), 113.0 ppm (C11).

Molecules 2012, 17
13975
Figure 2. Polymer XII.
O
C
c
O
C
O
7
4
NH
13
1
14
11
a
b
9
2
H
N
H
6
8
N
C
N
N
12
10
5
3
HO
n
Figure 3. 1H-NMR spectrum of polymer XII in DMSO-d6.
Figure 4. 13C-NMR spectrum of polymer XII in DMSO-d6.

Molecules 2012, 17
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1
The results of elemental analyses and FTIR spectra coupled with the results of H-NMR and
13C-NMR spectra seemed to be in good agreement with the expected structures of the prepared
polymers as illustrated in Schemes 1 and 2.
2.3. Polymer Characterization
2.3.1. Intrinsic Viscosity
Table 1 lists the intrinsic viscosity values for the prepared azopolyamide-hydrazides. These values
reflect the high molecular weight of the polymers. It can be noted that the intrinsic viscosity of these
polymers [measured in DMAc/3% (wt/v) LiCl at 30 °C with a concentration of 0.5 g dL−1] increases by
increasing the para-oriented phenylene rings content and ranges between 0.71 and 4.75 dL g−1 when the
para-phenylene rings content increases from 0 to 100 mol %. This indicates that the polymers’ chain
rigidity increases as a function of their para-phenylene linkages. The polymer with all meta-linkages
has the lowest intrinsic viscosity, whereas that with all para-linkages has the highest one. The
polymers with various proportions of meta- to para-linkages have intermediate intrinsic viscosities in
between these two extreme values. Although the intrinsic viscosity values are generally higher for
polymers having high para-phenylene rings content, it is expected that the molecular weight of these
polymers are generally comparable since the purity level of the two acid chlorides used are similar. The
higher intrinsic viscosity value of wholly para-oriented type of polymer can be attributed to the higher
rigidity, large interchain hydrogen-bonding, associated with higher chain symmetry and packing
efficiency. This observation seems to support the results which were previously reported [20,37,38].
2.3.2. Solubility
All the evaluated polymers are found to be partially soluble in polar aprotic solvents like DMAc,
DMF and NMP at room temperature. Addition of lithium chloride enhances their solubility. The
maximum solubility (%wt/v) of the polymers at 25 °C in these solvents containing various amounts of
lithium chloride is given in Table 3. It can be noted from this table that polymer with all para-linkages
appears to be less soluble relative to other polymers containing different proportions of meta-linkages
and forms high viscous solutions even at lower concentrations; however, at higher concentrations
these solutions appear as gels. Generally, the solubility characteristic of the polymers is in the order:
XII > XI > X > IX > VIII > VII > VI > V > IV > III > II> I.
An increase in meta-phenylene moieties content of the polymers from 0 to 100 mol % resulted in an
increase in its solubility from 10 to 28 wt/v % in DMAc/5% (wt/v) LiCl. This behavior provides
additional evidence for the increased rigidity, packing efficiency and hydrogen bonding of wholly
para-oriented type of polymer. This observation would seem to be supported by the conclusion made
earlier by Satre et al. [38].

Molecules 2012, 17
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Table 3. Maximum solubility and percent moisture regain of the novel wholly
aromatic azopolyamide-hydrazides containing various proportions of para-phenylene and
meta-phenylene units.
Maximum solubility (%wt/v)
Moisture
regain
(%) a
18.67
19.11
20.22
21.78
22.48
23.58
23.83
24.52
24.93
25.28
25.93
26.29
Polymer
code
DMAc/LiCl(%wt/v) DMF/LiCl(%wt/v) NMP/LiCl(%wt/v)
1
2
3
5
1
1
3
3
5
1
1
3
3
5
I
II
5
10
12
14
17
21
22
22
24
25
25
26
28
6
7
2
6
2
3
7
2
3
9
III
IV
V
3
6
2
4
8
3
5
9
5
9
3
6
11
12
14
14
17
19
23
23
25
5
7
13
15
18
19
21
24
24
27
29
6
10
13
13
15
16
17
18
20
4
7
6
8
VI
VII
VIII
IX
X
8
6
10
9
7
11
12
13
15
17
20
22
9
7
6
10
11
13
15
15
8
11
12
14
17
19
9
10
12
14
14
11
14
17
19
XI
XII
a Determined at 23 °C and relative humidity 85%.
2.3.3. Percent Moisture Regain
All the prepared azopolyamide-hydrazides showed the percent moisture regain (%MR) between
18.67% and 26.29% (at 23 °C, relative humidity, RH 85%) as represented in Table 3. This high
moisture uptake reflects the hydrophilic/polar character of these polymers. The presence of a hydrazide
group, a hydroxyl group, an amide group in addition to an azo group enhance the moisture uptake
relative to Nomex [poly(meta-phenylene isophthalamide)] (12% MR, at 25 °C, RH 85%) [39]. In the
Nomex repeating unit, there is one amide group compared to the aforementioned four functional
groups of the evaluated polymers. As most of the polymers’ polarity is due to functional groups, the
azopolyamide-hydrazides are more polar and hence attract larger water sorption. Further, the
hydrophilicity is increased with increasing the meta-oriented phenylene rings content in the
investigated polymer. This observation may be explained by the wholly para-oriented type polymer I
being more strongly hydrogen bonded than those containing meta-phenylene rings as a result of its
greater chain symmetry and shorter interchain distance, while the separation between the
macromolecular segments of meta-phenylene containing polymers would be further expected,
particularly in the presence of water, and consequently the inter- and intra-hydrogen bonds would be
partially sterically prevented. Thus they are free to interact with water.
2.3.4. X-ray Diffraction
The wide-angle X-ray diffraction patterns of the evaluated polymers over the 2θ range of 5–40° are
shown in Figure 5. The polymer I derived from 4A3HBH and 4,4'ADBC (completely para-oriented
type polymer) revealed a higher level of crystallinity consistent with its lower solubility and indicating
more order structure and efficient packing. At the molecular level, it is expected that an increase in the

Molecules 2012, 17
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polymer chain rigidity is associated with a higher para–phenylene rings content which develops denser
packing of the macromolecular segements. This dense packed polymer segements are expected to be
held together by short-range intermolecular hydrogen bonds. Gradual changing of the 4A3HBH
moiety to 3A4HBH and 4,4'ADBC to 3,3'ADBC (polymers III, V, VIII, and X) reduced the polymer
crystallinity. The polymer XII derived from 3A4HBH and 3,3'ADBC (completely meta-oriented type
polymer) reveals a nearly amorphous pattern. This may be attributed to the fact that the regularity of
the repeat units in polymer XII is disrupted by the meta-oriented phenylene rings, thus leading to a
lower crystallinity.
Figure 5. Wide-angle X-ray diffractograms of the novel wholly aromatic azopolyamide-
hyrdrazides containing various proportions of para-phenylene and meta-phenylene rings.
2.3.5. Mechanical Properties
Flexible and tough films were obtained in all cases by evaporation of solvent (DMAc/3% (wt/v)
LiCl) from the cast polymers solutions. Mechanical properties of these films before and after being
drawn at various temperatures are shown in Table 4. The unoriented films showed good mechanical
properties. In general, elastic modulus as well as tensile strength are increased as a function of
para-phenylene units content in the tested polymer. An increase in para-phenylene rings content from
0 to 100 mol % resulted in an increase in the elastic modulus and tensile strength from 0.74 to
5.81 GPa and from 23.42 to 287.56 MPa, respectively. On the other hand, ductility of these polymers

Molecules 2012, 17
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is increased by increasing the meta-phenylene units content in the prepared polymer as displayed by
the values of elongation-to-break. Several attempts were made to improve the mechanical properties of
these films by heat drawn as shown in Table 4, but their success was limited.
Table 4. Mechanical properties of novel wholly aromatic azopolyamide-hydrazides
containing various proportions of para-phenylene and meta-phenylene units.
Mechanical properties a
Heat drawn b
Polymer
As cast
code
At 140 °C
E
At 160 °C
E
At 180 °C
M
E
T
M
T
M
14.9
12.8
10.5
9.1
7.6
6.7
5.5
4.2
3.7
3.0
2.4
1.6
T
M
E
T
I
II
5.8
4.8
3.9
3.5
2.8
2.5
2.0
1.8
1.5
1.3
1.1
0.7
47.3
287.6
249.6
213.3
181.4
147.2
116.7
87.8
9.3
7.9
6.5
5.4
4.2
4.1
3.9
3.2
2.3
1.9
1.6
1.1
26.3
378.1
326.9
271.4
220.6
168.8
148.6
126.9
118.2
108.1
91.7
16.3
19.8
23.8
29.1
34.7
36.6
38.9
42.6
46.6
53.7
60.7
67.9
477.9
408.7
336.2
215.4
191.7
179.1
166.1
155.9
144.4
120.9
98.1
30.9
26.1
6.2
9.3
885.6
739.2
595.3
555.6
515.9
449.6
383.5
312.3
237.6
198.1
158.4
138.6
68.5
34.2
III
IV
V
91.8
42.1
20.1 12.3
17.6 13.5
15.0 14.5
13.2 17.3
11.2 20.9
10.1 22.5
100.2
108.2
121.4
136.4
147.2
157.9
167.7
177.5
181.2
51.0
60.2
VI
VII
VIII
IX
X
63.9
68.6
62.3
80.1
36.5
92.5
8.9
7.4
5.8
4.4
24.7
29.4
32.9
35.2
31.1
100.2
108.5
120.3
XI
26.1
72.4
XII
23.4
56.6
74.7
a
Measured at room temperature, M is the elastic Yong’s modulus (GPa), T is the tensile strength (MPa), and E is the
elongation-at-break (%); b Achieved by hand at a preselected temperature till break (maximum stretching).
2.3.6. Thermal Properties
The thermal properties of all the azopoly(amide-hydrazide)s were evaluated by thermogravimetry
(TG) in nitrogen and in air atmospheres at a heating rate of 10 °C min−1 and the typical TG curves thus
obtained are shown in Figures 6 and 7, respectively. In both degradation atmospheres, all the polymers
showed characteristic similar thermal behavior and their weight losses could be observed in three
distinctive steps. First, a small weight loss in the range of 1–3 wt% (based on the original polymer
weight) occurred starting at 80 °C up to 130 °C, which may be attributed to loss of absorbed moisture.
Second, a considerable weight loss (15–17.5 wt%, based on the weight of the perfectly dried polymer)
was observed at various temperature ranges for different polymers in nitrogen and in air atmospheres.
This weight loss is attributed to loss of water during the thermally induced conversion of the hydrazide
and o-hydroxybenzamide groups to 1,3,4-oxadiazole and benzoxazole rings, respectively [12,23,40].
Along with this cyclodehydration reaction, the polymers may lose the azo group as molecular
nitrogen [41,42]. Third, a sharp weight loss was obtained which indicated the decomposition of the
poly(1,3,4-oxadiazolyl-benzoxazole)s which were formed in situ. The almost identical nature of the
curves in both atmospheres indicates that the decompositions occur by the same mechanism. The
improved resistance to high temperature was always associated with increased para-phenylene rings
content of the evaluated polymers. In both degradation atmospheres and at all applied temperatures,
polymer I showed the highest thermal stability as judged by its highest initial decomposition

Molecules 2012, 17
13980
temperatures as well as its highest residual weight at a particular temperature, whereas the reverse was
true for polymer XII. The other polymers of this series aligned themselves in between these two
extreme cases so that with respect to their weight remaining at any particular temperature their order of
stability was: I > II > III > IV > V > VI > VII > VIII > IX > X > XI > XII. Thus, substitution of
para-phenylene nuclei for meta-phenylene improved the thermal stability of the polymer. This should
also be associated with regularity of supermolecular packing within the bulk of the evaluated polymers
wherein the collinear arrangement of the para-phenylene units should allow for establishment of
stronger intermolecular hydrogen bonds which would be more difficult to break and therefore more
resistance to elevated temperature. The resulting poly(1,3,4-oxadiazolyl-benzoxazole)s start to degrade
in the temperature range above 340–560 °C in nitrogen and 330–550 °C in air without weight loss at a
lower temperature. The percentage residues of poly(1,3,4-oxadiazolyl-benzoxazole)s at 750 °C in
nitrogen were 64.5–80% of their original weights. On the other hand, smaller residues of 24.5–39%
were observed at the same temperature in air. This indicates that the degradation rate of poly(1,3,4-
oxadiazolyl-benzoxazole)s in air is faster than in nitrogen supported by the residual weight of the
polymer at 750 °C as well as at particular temperatures. These observations confirm that azopoly(amide-
hydrazide)s can be used as precursor polymers for the preparation of thermally stable poly(1,3,4-
oxadiazolyl-benzoxazole)s. The detailed thermal and thermo-oxidative characterization of these
polymers is in progress.
Figure 6. Typical TG thermograms patterns of novel wholly aromatic azopolyamide-
hydrazides containing various proportions of para-phenylene and meta-phenylene rings.
All the thermograms were recorded in nitrogen atmosphere at a heating rate of 10 °C min−1
and under a gas flow rate of 30 mL min−1.

Molecules 2012, 17
13981
Figure 7. Typical TG thermograms patterns of novel wholly aromatic azopolyamide-
hydrazides containing various proportions of para-phenylene and meta-phenylene rings.
All the thermograms were recorded in air atmosphere at a heating rate of 10 °C min−1 and
under a gas flow rate of 30 mL min−1.
2.3.7. Ultraviolet-Visible Spectroscopy
All the azopolyamide-hydrazides display orange colorations of varying intensity owing to the
presence of the azo groups. The ultraviolet-visible absorption spectra of selected examples of
azopolyamide-hydrazides I, IV, IX, and XII, which were measured in NMP solution (3 × 10−4 g/mL)
are shown in Figure 8. It was found that the shape of the absorptions of all the investigated polymers
was almost the same. The spectra showed a broad and intensive absorption between 300 and 400 nm
corresponding to the π-π* transition of the trans form of the azo group present in the polymer main
chain. A weak and broad absorption between 400 and 500 nm due to the n-π* electronic transition of
the cis form of the azo group was also observed. The spectra showed absorption maxima at 355 and
460 nm for polymer I, at 345 and 445 nm for polymer IV, at 340 and 440 nm for polymer IX and 335
and 430 nm for polymer XII. It can be noted that the absorption maxima of the polymers were shifted
toward a longer wave length with increasing their para-oriented phenylene units content. This may be
attributed to extension of the conjugation through the azo group in the all-para-linked polymer relative
to the other polymers. Moreover, all azopolyamide-hydrazides exhibited some peaks in the ultraviolet
region which are the benzenoid bands for the various types of benzene rings derived from either
4A3HBH (at 284 ± 1 nm) or 3A4HBH (at 278 ± 2 nm) and 4,4'ADBC (at 270 nm) and/or 3,3'ADBC
(at 250 nm). The spectrum of 4,4'ADBC shows absorption maxima at 445 (n-π*, azo), 330 (π-π*, azo)
and 260 nm (benzenoid band), whereas, the spectrum of 3,3'ADBC shows absorption maxima at 425
(n-π*, azo), 320 (π-π*, azo) and 250 nm (benzenoid band). The red shift for the π-π* transition in the
polymers is due to the extension of the conjugation of the azo group by the hydrazide group in the polymer
main chain.

Molecules 2012, 17
13982
Figure 8. The ultraviolet-visible absorption spectra of some novel wholly aromatic azo
polyamide-hydrazides containing various proportions of para-phenylene and
meta-phenylene units.
The spectra of 4A3HBH and 3A4HBH showed benzenoid bands at 300 and 290 nm, respectively.
The blue shift of these bands in the polymers is due to less availability of the lone pair of electrons on
hydrazide nitrogen for conjugation with the benzene ring as it also can undergo conjugation with the
carbonyl group.
3. Experimental
3.1. Synthesis of the Monomers
4-Amino-3-hydroxybenzhydrazide (4A3HBH) and 3-amino-4-hydroxy-benzhydrazide (3A4HBH)
were synthesized from the corresponding acids by two-step procedure in which the acid was esterified
to its methyl ester followed by reaction of the produced ester with hydrazine hydrate. A detailed
description of this procedure is given in our previous work [43].
4-Amino-3-hydroxybenzhydrazide (4A3HBH, Figure 9) separated as felted needles (45% yield) m.p.
223–224 °C; FTIR: max 3460 (OH str.), 3356, 3252 (N-H str.), 3022 (C-H str.), 1667 (C=O str.),
1
1604 cm−1 (Ar. C=C str.); H-NMR (200 MHz, DMSO-d6):  = 9.25 (s, 1H, OH), 7.19 (s, 1H, Hc),
7.13 (d, 1H, Hb, J = 7.6 Hz), 6.56 (d, 1H, Ha, J = 7.6 Hz), 5.00 (s, 1H, NH), 4.33 (s, br., 2H, NH2,d),
3.49 ppm (s, br., 2H, NH2,e); 13C-NMR (300 MHz, DMSO-d6):  = 166.77 (C=O), 143.06 (C3), 140.11
(C4), 120.9 (C1), 119.10 (C6), 113.62 (C5), 112.76 ppm (C2); MS m/z (%): 167 (9.7, M+), 136 (100.0),
108 (14.2), 80 (42.8), 53 (43.5); Anal. Calcd. for C7H9N3O2: C, 50.30%; H, 5.39%; N, 25.15%; O,
19.16%. Found: C, 50.35%; H, 5.4%; N, 25.12%; O, 19.13%.

Molecules 2012, 17
13983
Figure 9. 4A3HBH.
e
H2N
5,a
6,b
NH
d
4
1
H2N
C
3
2,c
O
HO
3-Amino-4-hydroxybenzhydrazide (3A4HBH, Figure 10) separated as felted needles (45% yield) m.p.
227 °C; FTIR: max 3390 (OH str.), 3317, 3249 (N-H str.), 3008 (C-H str.), 1662 (C=O str.), 1598 cm−1
1
(Ar. C=C str.); H-NMR (200 MHz, DMSO-d6):  = 9.31 (s, 1H, OH), 7.13 (s, 1H, Hc), 6.94 (m, 2H,
NH, Hb), 6.64 (d, 1H, Ha), 4.58 (s, 2H, NH2,d), 3.44 ppm (s, 2H, NH2,e); 13C-NMR (300 MHz, DMSO-d6):
 = 166.93 (C=O), 146.82 (C4), 136.30 (C3), 124.76 (C1), 115.80 (C6), 113.64 (C5), 113.55 ppm (C2);
MS m/z (%): 167 (21.2, M+), 136 (100.0), 108 (34.5), 80 (43.2), 53 (39.3); Anal. Calcd. for C7H9N3O2:
C, 50.30%; H, 5.39%; N, 25.15%; O, 19.16%. Found: C, 50.37%; H, 5.38%; N, 25.09%; O, 19.19%.
Figure 10. 3A4HBH.
5,a 6,b
O
e
H
4
1
HO
C
N
NH2
3
2,c
d
H2N
4,4'-Azodibenzoyl chloride (4,4'ADBC, Figure 11) and 3,3'-azodibenzoyl chloride (3,3'ADBC, Figure 12)
were prepared according to the methods previously reported in details [42,44]. 4,4'ADBC separated as
red needles (47% yield), mp: 164 °C; FTIR: max 2925 (C-H str.), 1776 (C=O str.), 1595 (Ar. C=C str.),
1
1151, 885 cm−1 (C-Cl); H-NMR (200 MHz, DMSO-d6):  = 8.18 (d, 4H, Ha, J = 8.6 Hz), 8.00 ppm
(d, 4H, Hb, J = 8.6 Hz); 13C-NMR (300 MHz, DMSO-d6):  = 166.60 (C=O), 154.25 (C1), 133.43 (C4),
130.74 (C2), 122.86 ppm (C3); MS m/z (%): 306 (12.08, M+−1), 271 (25.09), 167 (24.03), 139 (55.59),
104 (65.44), 76 (100.00), 50 (49.77); Anal. Calcd. for C14H8N2Cl2O2: C, 54.72%; H, 2.62%; N, 9.12%;
Cl, 23.09%; O, 10.45%. Found: C, 54.69%; H, 2.64%; N, 9.09%; Cl, 23.11%; O, 10.47%.
Figure 11. 4,4'ADBC.
3,b 2,a
O
Cl
O
1
4
C
N
N
C
Cl
3,3'ADBC separated as orange crystals (51% yield), mp: 97–98 °C; FTIR: max 2925 (C-H str.), 1776
1
(C=O str.), 1595 (Ar.C=C str.), 1151, 885 cm−1 (C-Cl); H-NMR (200 MHz, DMSO-d6):  = 8.4

Molecules 2012, 17
13984
(s, 2H, Ha), 8.12–8.24 (m, 4H, Hb, Hd), 7.71–7.83 ppm (m, 2H, Hc); 13C-NMR (300 MHz, DMSO-d6):
 = 166.60 (C=O), 151.80 (C1), 132.28 (C6), 130.03 (C3), 129.53 (C4), 127.53 (C5), 122.49 ppm (C2);
MS m/z (%): 306 (18.76, M+-1), 271 (39.29), 167 (45.13), 139 (100.00), 103 (45.01), 76 (78.97), 50
(27.57); Anal. Calcd. for C14H8N2Cl2O2: C, 54.72%; H, 2.62%; N, 9.12%; Cl, 23.09%; O, 10.45%.
Found: C, 54.75%; H, 2.63%; N, 9.04%; Cl, 23.08%; O, 10.50%.
Figure 12. 3,3'ADBC.
O
O
Cl
C
C
Cl
2,a
1
3
6,b
N
N
4,d
5,c
3.2. Synthesis of the Polymers
All the polymers were prepared by essentially the same experimental procedure, which will be
given here for preparation of polymer I. A dry 1 L four-necked round bottom flask equipped with a
Teflon coated halfmoon mechanical stirring bar, a thermometer and a drying tube was placed in a dry
box. 4A3HBH, 16.7 g (0.1 mol), was charged into the reaction flask followed by 400 mL of anhydrous
DMAc containing 3% (wt/v) LiCl. Stirring was started until complete dissolution. The reaction flask
was placed in a crushed ice- salt water bath and cooled at −10 °C for 15 min. When this step was
completed, 30.7 g (0.1 mol) solid 4,4'ADBC was added slowly under constant stirring over a period of
1 h by means of a funnel with weighing paper that was positioned just slightly above the surface of the
stirred polycondensation reaction solution. After this period, stirring was continued for another 2 h at
the aforementioned temperature. Then the cooling bath was removed and the temperature of the
polymerization reaction was allowed to rise gradually to room temperature and maintained for 24 h with
stirring. Afterwards, a clear, orange, highly viscous solution was obtained. Finally, the polymer solution
was slowly poured into 3 L of rapidly stirred methanol upon which a fibrous orange precipitate of
polymer I immediately formed. The polymer was isolated by filtration, washed successively with
methanol, acetone and dried in a vacuum oven at room temperature to constant weight.
3.3. Film Preparation
Films were prepared by casting 5% (wt/v) polymer solutions (in DMAc containing 3% (wt/v)
anhydrous LiCl) onto dry clean Pyrex glass plates to a uniform thickness. Solvent evaporation was
performed in a vacuum oven at room temperature. The resulting films were kept in the oven until no
change in weight could be observed. The films were then immersed in deionized water overnight to
remove any residual solvent. Finally, the films were dried in a vacuum oven at room temperature to
constant weight. The thickness of the films was 0.3 ± 0.02 mm.

Molecules 2012, 17
13985
3.4. Measurements and Instruments
Fourier transform infrared spectra were recorded on a Perkin-Elmer infrared spectrophotometer
(FTIR 1650) using KBr pellets. All spectra were recorded within the wave number range of
4000–400 cm−1 at 25 °C. 1H- and 13C-NMR spectra were recorded on a 200 MHz and 300 MHz Varian
Gemini NMR spectrometer, respectively. Deuterated dimethyl sulphoxide (DMSO-d6) was used as the
solvent in all cases at 25 °C. Tetramethylsilane (TMS) was used as an internal standard. Mass spectra
were recorded under electron impact at 70 eV on a GCMS Analysis Shimadzu GP 2010 Plus Japan
instrument. Elemental analyses were performed at the Microanalytical Unit, Cairo University, Giza,
Egypt. Intrinsic viscosity measurements were determined for all of the polymers in DMAc/3% (wt/v)
LiCl at 30 °C. Solutions were prepared at room temperature in all cases. Their concentration was
0.5 g dL−1. Flow times were determined for each polymer at five different concentrations using a
cannon viscometer which was selected so as to give solvent flow times in excess of 100 seconds. All
flow times encountered were sufficiently long to justify neglect of kinetic energy corrections, and the
lack of shear- rate dependence was verified. The plots obtained were linear in all cases. Intrinsic
viscosities were determined by the usual double extrapolation of ηsp/C and (ln ηrel)/C to zero
concentration and expressed in deciliter per gram (dL g−1).
The solubility of polymers in various amide solvents namely DMAc, DMF, and NMP, containing
various amounts of LiCl, was determined at room temperature (25 °C). The saturated solutions of the
polymers in solvent were prepared at room temperature by equilibrating for 24 h. The solubilities of
the polymers were recorded as percent weight dissolution per hundred mL of solvent (%wt/v).
A dried powder polymer sample was allowed to absorb moisture for 24 h in a humidity chamber
maintained at 85% relative humidity (RH) at 23 °C, then a weighed amount of the moisture-absorbed
sample was dried in an air oven at 110 °C to constant weight. The percent moisture regain by the
polymer sample was calculated on the basis of weight loss.
Wide-angle X-ray diffraction measurements were performed at room temperature on a Rigaku
Denki Co. Ltd. (Tokyo, Japan), RAD-B system X-ray diffractometer, using Ni filtered Cukα radiation
(operating at 40 kV and 20 mA). The sample was maintained stationary, while scattering angles from
5 to 90° were scanned in the reflection mode at scanning rate of 1° min−1.
Tensile strength (T), elastic modulus (M) and elongation-to-break (%) (E) of the polymers films
were measured on Shimadzu Autograph in air at room temperature.
Thermogravimetric measurements of all the polymers were carried out in nitrogen and in air
atmospheres. A Shimadzu TGA-50H thermogravimetric analyzer was used. The samples ranging from
4 to 6 mg in weight were heated from 25 to 800 °C at a heating rate of 10 °C min−1 and under a gas
flow stream of 30 mL min−1 in all cases. Ultraviolet-visible absorption spectra of the prepared
azopolymers were recorded in NMP solution (3 × 10−4 g/mL) using a Shimadzu UV-1600 series
spectrophotometer in the range 200–800 nm.
4. Conclusions
High molecular weight easily processed novel self-dyed wholly aromatic azopolyamide-hydrazides
have successfully been synthesized as highly viscous film forming solutions. They show orange colors

Molecules 2012, 17
13986
of varying intensity owing to the presence of the azo groups. They were structurally characterized by
various para- to meta-phenylene moieties contents. These polymers exhibited some desirable properties
which mark them as promising candidates for corresponding engineering applications and perhaps also
for some new ones. Substitution of para-phenylene units for meta-phenylene ones within these
polymers greatly affected their physical, mechanical and thermal properties. The intrinsic viscosity,
tensile strength, crystallinity, thermal and thermo-oxidative stability of the polymers increased as a
function of para-phenylene units content in the polymer. These properties indicate the regularity of
supermolecular packing occurred with the bulk of the polymer in addition to the collinear arrangement of
para-phenylene units which permit the establishment of stronger inter-chain hydrogen bonding. The
latter would also increase the rigidity of the polymer. On the other hand, the solubility and the hydrophilic
character of the polymers increased as a function of meta-oriented phenylene rings content incorporated
into the polymer chains. The gradual substitution of para-phenylene rings in the chains of the polymer
by the more flexibilizing 4meta-phenylene units gradually increases the overall flexibility of the resulting
polymer chains. Thus, it should become possible to prepare materials with the most favorable properties
required for any given engineering application or desired processing conditions by carefully designing
relative ratios of the two types of these aromatic units during the polymer synthesis.
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