Intrinsically colored wholly aromatic polyamides (aramids)

Article abstract of DOI:10.1016/j.dyepig.2015.06.027

This work describes the preparation of intrinsically blue-colored, high-performance aromatic polyamides (aramids). The color was achieved by preparing a diamine monomer containing a chromogenic azadipyrromethene (ADPM) core, which after polymerizing it wi

Full text of DOI:10.1016/j.dyepig.2015.06.027

Dyes and Pigments 122 (2015) 177e183
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Intrinsically colored wholly aromatic polyamides (aramids)
ꢀ
Miriam Trigo-L oꢀ pez, Alvaro Miguel-Ortega, Saúl Vallejos, Asunci oꢀ n Mu n~ oz,
ꢀ
*
Daniel Izquierdo, Alvaro Colina, F eꢀ lix Clemente García, Jos eꢀ Miguel García
Departamento de Química, Facultad de Ciencias, Universidad de Burgos, Plaza de Misael Ba n~ uelos s/n, 09001 Burgos, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
This work describes the preparation of intrinsically blue-colored, high-performance aromatic polyamides
aramids). The color was achieved by preparing a diamine monomer containing a chromogenic azadi-
Received 22 May 2015
Accepted 22 June 2015
Available online 30 June 2015
(
pyrromethene (ADPM) core, which after polymerizing it with commercially available m-phenyl-
eneisophtahlamide and isophthaloyl chloride gave the blue-colored polyisophthalamide copolymers.
These materials were structurally compared to commercialized and high-value-added meta-aramid fi-
Keywords:
Aramids
®
®
bers [poly(m-phenyleneisophthalamide), MPIA, brand names: NOMEX and Teijinconex ]. Blue is the
highest demanded color in the market of protective aramid fibers. The coloration efficiency of this special
monomer is very high. Furthermore, the aramids showed even better thermal properties than com-
mercial MPIA.
Aromatic polyamides
High performance polymers
Inherently colored polymers
Intrinsically colored polymers
Self-colored polymers
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
obtained by wet, dry or dry-jet wet spinning of aramid solutions.
The properties of the fibers are physically enhanced by improving
Wholly aromatic polyamides, usually termed as aramids, are
high value-added high-performance materials. They are prepared
and commercialized as outstanding thermally and mechanically
resistant fibers. The commercial success of the aramids started in
the 1960s with the discovery of the all-para-phenylene oriented
aramid fiber, poly(p-phenylene terphthalamide), by Stephanie
Kwolek at DuPont, which is commercialized under the trade name
the rod-like structure and the percentage and quality of crystalline
microdomains by cold and heat drawing after fiber spinning and
coagulation. This increases the intermolecular packing and also the
cohesive energy even further. The fibers are obtained as flogs,
staple and continuous fibers that are used for preparing yarns and
fabrics for apparel (protective clothing such as fire, chemical and
saw protection suits, and bullet-proof body armor), isolating paper,
composites, and high temperature industrial filters, among others,
covering the broad range of value-added products related with the
following industrial sectors: transport (e.g., automotive and aero-
space), oil and gas, protection and defense, telecom, and civil en-
gineering [7e16].
®
Kevlar . Concomitantly, this put into the market the all-meta-
phenylene oriented aramid fiber, MPIA, under the trade name
®
Nomex . Two decades later, the company Teijin made the only
break-through in 50 years of commercial success of aramids with
®
the commercialization of Technora , the aramid with a copolymer
structure comprising meta- and para-phenylene oriented rings,
poly(p-phenylene-co-3,4 -oxydiphenylene terephthalamide) [1e6].
Although in some industrial applications (e.g., composites and
hot gas filtration) the natural yellowish color of the fibers is used,
generally most of the applications require colored fibers, specif-
ically those related with apparels, carpets, drapes, etc. However, the
aramid fibers have poor dyeing properties for the same reasons that
make them extremely thermal and mechanical resistant, i.e., the
dense molecular packaging, the cohesive energy, the molecular
interactions and the chain orientation and crystallinity. Colored
yarns can be obtained by dying the fibers or by dope dyed. In
conventional dying of aramid fibers, the affinity toward conven-
tional dyes has to be improved by expansion of the amorphous
regions with the combination of swelling agents and extremely
0
Both the remarkable thermal and fire resistance, and the me-
chanical strength of aramids come from their chemical structure.
Thus, the rigidity of the main polymer chain due to the wholly ar-
omatic structure conjugated with the amide groups, the high
average bond energy, along with the strong and highly directional
interchain hydrogen bonds between amide moieties, provide the
materials with extraordinary cohesive energy. The fibers are
*
Corresponding author. Tel.: þ34 947 25 80 85; fax: þ34 947 25 88 31.
E-mail address: jmiguel@ubu.es (J.M. García).
ꢀ
high temperatures, over 190 C [17]. For this reason, the dope
URL: http://www2.ubu.es/quim/quimorg/polimeros/polimeros.htm
http://dx.doi.org/10.1016/j.dyepig.2015.06.027
143-7208/© 2015 Elsevier Ltd. All rights reserved.
0

178
M. Trigo-L oꢀ pez et al. / Dyes and Pigments 122 (2015) 177e183
dyeing using pigments is usually preferred, and dope dyed yarns
are usually used for garments woven with aramids. Anyhow, dying
impairs the properties of the fibers and at the same time the
resistance to fading is barely achieved.
Synthesis of (E)-1-(4-aminophenyl)-3-(4-methoxyphenyl)prop-2-
en-1-one (1): A stirred mixture of 5.0 g (37 mmol) of p-amino-
acetophenone, 140 mL of ethanol and 240 mL of aqueous NaOH 4M
ꢀ
solution was brought to 0 C. 5.05 g (37 mmol) of p-anisaldehyde
Ideal color fastness of fibers is achieved by intrinsically, inher-
ently or self-colored polymers [18e21]. Inherently colored macro-
molecules are polymers with chromophores in their structure, i.e.,
polymers that have dye motifs chemically anchored. Accordingly,
we designed and prepared a blue chromophore as an aromatic
diamine monomer, and we used it to synthesize blue meta-aramids.
The color hue was modified by tuning molar content of the chro-
mophore monomer in relation with the conventional diamine
monomer, i.e., m-phenylenediamine. The chromophore motifs
cannot migrate and are evenly distributed along the polymer chains
due random copolymerization. Moreover, the impressive thermal
properties of the aramids are even improved by using this colored
monomer.
was added and the mixture was stirred at room temperature for 4 h.
The flask was kept at 4 C for 12 h, and then filtered off. The
compound was dissolved in 300 mL of boiling methanol. The hot
ꢀ
solution was filtered off and the solvent was evaporated to give
ꢀ
1
compound (1). Yield: 4.8 g (51%). M.p.: 120 ± 1 C. H NMR
(400 MHz, DMSO-d , Me Si): 7.95 (2H, d, J 8.7 Hz, Ph); 7.83 (2H, d, J
8.8 Hz, Ph); 7.77 (1H, d, J 15.5 Hz, CH); 7.63 (1H, d, J 15.5 Hz, CH);
7.03 (2H, d, J 8.8 Hz, Ph); 6.65 (2H, d, J 8.8 Hz, Ph); 6.14 (2H, s, NH );
Si): 186.80,
d
H
6
4
2
13
3 C 6 4
3.85 (3H, s, CH ). C NMR, d (100.6 MHz, DMSO-d , Me
161.74, 154.61, 142.24, 131.87, 131.17, 128.71, 126.46, 120.84, 115.23,
113.62, 56.24. EI-LRMS m/z: 253.11 (Mþ, 100), 252.10 (35), 238.08
(38), 225.11 (18), 210.08 (22), 161.06 (18), 145.05 (15), 120.04 (73),
92.05 (22), 65.04 (18). HRMS calcd. for (C16
Found: 253.1102. FT-IR [wavenumbers (cm )]: nNH2 : 3350;
H
15NO
2
): 253.1103;
ꢂ1
n
]
C O
:
1
640.
Synthesis
nitrobutan-1-one (2): 1 g (3.95 mmol) of compound (1) and
.041 mL (19.73 mmol) of DEA were mixed in a pressure tube
2
. Experimental
of
1-(4-aminophenyl)-3-(4-methoxyphenyl)-4-
All materials and solvents were commercially available and used
2
as received, unless otherwise indicated. p-aminoacetophenone
together with 10 mL of ethanol. Then, 2.138 mL (39.5 mmol) of
nitromethane was added to the mixture. The solution was refluxed
for 24 h. The solvent was evaporated under vacuum and 20 mL of
diethyl ether was added to the brown oil. A palid yellow solid was
formed, and it was filtered off and air dried. Yield: 0.74 g (60%).
(
99%, Aldrich), NaOH (99.9%, VWR-Prolabo), p-anisaldehyde (ꢁ98%,
Merck), diethylamine (DEA) (98%, Aldrich), nitromethane (ꢁ95%,
Aldrich), ammonium acetate (97%, Alfa Aesar), benzoyl chloride
(
(
(
99%,
TBAPF
Aldrich),
tetrabutylammonium
hexafluorophosphate
6
) (98%, Aldrich), ethanol (99.9%, VWR-Prolabo), methanol
ꢀ
1
M.p.: 99 ± 1 C. H NMR
J 8.7 Hz, Ph); 7.30 (2H, d, J 8.6 Hz, Ph); 6.97 (2H, d, J 8.6 Hz, Ph); 6.58
2H, d, J 8.7 Hz, Ph); 6.11 (2H, s, NH ); 4.95 (1H, dd, J 12.7 Hz, J
.6 Hz, CH eNO ); 4.82 (1H, dd, J 12.6 Hz, J 10.0 Hz, CH eNO ); 3.99
1H, dd, J 9.6 Hz, J 6.1 Hz, CH); 3.74 (3H, s, CH ); 3.35 (1H, dd, J
7.2 Hz, J 7.5 Hz, CH ): 3.23 (1H, dd, J 17.1 Hz, J 6.8 Hz,CH
(100.6 MHz, DMSO-d , Me Si): 195.17, 159.15, 154.74, 132.99,
31.31, 129.73, 125.19, 114.70, 113.40, 80.98, 55.87. EI-LRMS m/z:
14.10 (Mþ, <1), 268.10 (13), 239.10 (12), 134.06 (10), 121.03 (22),
H 6 4
d (400 MHz, DMSO-d , Me Si): 7.69 (2H, d,
ꢁ99.8%, Aldrich), dimethyl sulfoxide (DMSO) (99%, Prolabo),
deuterated dimethyl sulfoxide (DMSO-d
6
) (99.80D, VWR-Prolabo),
(
5
(
1
2
and sulfuric acid (100%, Merck) were used as received. N,N-dime-
thylacetamide (DMA) (ꢁ99%, Aldrich) was vacuum distilled over
phosphorous pentoxide twice and then stored over 4 Å molecular
sieves. m-phenylenediamine (MPD) is commercially available
2
2
2
2
3
13
2
2
). C NMR,
d
C
6
4
(>99%; Aldrich) and was purified by double vacuum sublimation.
1
3
Isophthaloyl dichloride (ICL) (>99%, Aldrich) was purified by dou-
ble crystallization from dry heptane (99.9%, VWR-Prolabo).
ꢂ1
120.03 (100), 106.05 (5). FT-IR [wavenumbers (cm )]: n
: 3364;
NH
2
n ]
C O
: 1622; dNH2 : 1596; nas NO2 : 1580; ns NO2 : 1349.
Synthesis of (Z)-5-(4-aminophenyl)-N-(5-(4-aminophenyl)-3-(4-
2
2
.1. Synthesis
methoxyphenyl)-2H-pyrrol-2-ylidene)-3-(4-methoxyphenyl)-1H-
pyrrol-2-amine (3): Compound (2) (1 g, 3.18 mmol), ammonium
acetate (8.309 g, 107.7 mmol) and ethanol (18 mL) were heated
under reflux for 24 h in a pressure tube. The reaction solvent was
.1.1. Synthesis of intermediates, monomer and model
The synthetic steps carried out to prepare monomer (3) and
model (M1) are depicted in Scheme 1.
Scheme 1. Synthesis of monomer (3) and model (M1).

M. Trigo-L oꢀ pez et al. / Dyes and Pigments 122 (2015) 177e183
179
evaporated under vacuum, and 50 mL of distilled water was added
to the crude product. The mixture was stirred at room temperature
for 10 min, and the green solid was filtered off and air dried. Yield:
A three necked flask with a nitrogen inlet and a mechanical
stirrer was charged with 4.3 mL of DMA under a blanket of nitrogen
at room temperature. Then, 0.23 g (0.426 mmol) of diamine 1 and
0.41 g (3.834 mmol) of MPD were added. The solution was stirred at
room temperature until the solution of the diamine. The system
ꢀ
1
0
.67 g (80%). M.p.: 235 ± 1 C (decomp.). H NMR
DMSO-d , Me Si): 12.87 (1H, s, NH); 8.10 (4H, d, J 8.7 Hz, Ph); 7.78
4H, d, J 8.6 Hz, Ph); 7.36 (2H, s, CH); 7.07 (4H, d, J 8.8 Hz, Ph); 6.78
H
d (400 MHz,
6
4
ꢀ
(
(
(
was then cooled to 0 C, and 0.86 g (4.260 mmol) of ICL was added
13
4H, d, J 8.6 Hz, Ph); 6.02 (4H, s, NH
100.6 MHz, DMSO-d , Me Si): 159.99, 154.40, 152.30, 149.26,
40.51, 130.60, 128.98, 127.44, 119.77, 115.08, 114.72, 113.66, 56.13.
EI-LRMS m/z: 539.23 (Mþ, 100), 540.23 (40), 524.21 (8), 280.15 (6),
79.15 (26), 269.63 (16), 264.14 (20), 249.15 (6), 120.07 (6), 69.02
5). HRMS calcd. for (C34 ): 539.2321; Found: 539.2310. FT-
NH: 3466, 3320, 3210; : 1627,
2
); 3.88 (6H, s, CH
3
). C NMR,
d
C
portionwise (four equal amounts) over 5 min. The mixture was
ꢀ
6
4
allowed to react under nitrogen at 0 C for 30 min, and then at room
1
temperature for additional 3.5 h. The final solution was slowly
poured into distilled water, forming a dark blue, fibrous, swollen
polymer precipitate that was filtered, washed thoroughly with
water and acetone. The yield was quantitative.
2
(
29 5 2
H N O
ꢂ1
IR [wavenumbers (cm )]:
594; dNH2 : 1565, 1544.
Synthesis of model (M1): To a solution of benzoyl chloride
n
C N
n ]
1
2.2. Measurements
ꢀ
1H and 13C NMR spectra were recorded with a Varian Inova 400
spectrometer operating at 399.92 and 100.57 MHz, respectively,
(
0.80 mmol, 0.112 g) in 0.6 mL of DMA at 0 C, 0.178 g (0.33 mmol)
of compound 3 was added portionwise keeping vigorous stirring
for 30 min under nitrogen, and then at room temperature for an
additional 3.5 h. The final solution was slowly poured into distilled
with deuterated dimethyl sulfoxide (DMSO-d ) as solvent.
6
Infrared spectra (FT-IR) were recorded with an FT/IR-4200 FT-IR
water, forming a dark blue solid that was filtered and washed
Jasco Spectrometer with an ATR-PRO410-S single reflection acces-
sory. Low-resolution electron impact mass spectra (EI-LRMS) and
high resolution electron impact mass spectra (EI-HRMS) were ob-
tained at 70 eV on an Aligent 6890N mass spectrometer and on a
Micromass AutoSpec mass spectrometer respectively.
1
thoroughly with water. Yield: 0.21 g (83%). H NMR
d
H
(400 MHz,
6 4
DMSO-d , Me Si): 12.76 (1H, s, NH); 10.62 (2H, s, NH); 8.11e8.04
(
14H, m, Ph); 7.69e7.57 (10H, m, Ph, CH); 7.05 (4H, d, J 8.8 Hz, Ph);
13
3
.88 (6H, s, CH
59.62, 153.85, 141.54, 134.76, 131.97, 130.27, 128.72, 128.60, 127.91,
27.78, 127.55, 126.17, 125.97, 120.54, 114.07, 113.93, 55.31. EI-LRMS
m/z: 747.28 (Mþ, 3), 628.26 (5), 384.20 (30), 383.20 (97), 368.19
100), 353.17 (5), 279.17 (10), 278.17 (37), 263.16 (28), 181.04 (8),
69.04 (9), 131.04 (11), 119.04 (12), 105.09 (77). HRMS calcd. for
): 747.2846; Found: 747.2841. FT-IR [wavenumbers
NH: 3283; : 1654; C-N: 1243.
3 C 6 4
). C NMR, d (100.6 MHz, DMSO-d , Me Si): 165.96,
1
1
Thermogravimetric analysis (TGA) data were recorded on a 5 mg
sample under a nitrogen or oxygen atmosphere on a TA Instrument
ꢀ
ꢂ1
Q50 TGA analyzer at a scan rate of 10 C min . LOI was estimated
(
1
(
(
using the experimental Van Krevelen equation, LOI ¼ 17.5 þ 0.4 CR,
ꢀ
where CR is the char yield weight percentage at 800 C, which was
C
48
H
37
N
n
5
O
4
obtained from TGA measurements in a nitrogen atmosphere [22].
Melting points (m.p.) were visually determined with a Gallen-
kamp melting point apparatus.
ꢂ1
cm )]:
n
]
C O
n
UV/Vis spectroelectrochemical measurements were carried out
using an SPELEC (DropSens), a commercial fully integrated syn-
chronized spectroelectrochemical device. The light beam, supplied
by a 360e2500 nm tungsten-halogen light source was both con-
ducted to and collected from the spectroelectrochemical cell by a
2
.1.2. Synthesis of polymers
The preparation of copolyamides I, II, and III is schematically
shown in Scheme 2. The synthesis of copolyamide I is described
below as an illustrative example.
Scheme 2. Synthesis of copolyamides I, II and III.

180
M. Trigo-L oꢀ pez et al. / Dyes and Pigments 122 (2015) 177e183
2
00
mm reflection probe (RPROBE, DropSens). A standard three-
3.1. Polymer synthesis and characterization
electrode cell was used in all experiments, consisting of a com-
mercial Pt working electrode, a Pt wire as auxiliary electrode and a
homemade Ag/AgCl/KCl (3 M) reference electrode. The polymer
was deposited on the Pt working electrode by drop casting.
Reflection UV/Vis data were recorded using a spectrometer QE-
The monomer (3) was prepared in a straightforward manner
from commercially available materials, as shown in Scheme 1. The
synthesis of model M1 (Scheme 1) was conducted according to a
synthetic procedure similar to the one used in the preparation of
the polymers (Scheme 2).
6
5000 (Ocean Optics) with a light beam supplied by an Avalight-
DH-s-BAL Deuterium-halogen light source(Avantes), guided and
collected by a reflection probe FCR-7IR200-2-2.5 ꢃ 100 (Avantes).
The polymer solubility was determined by mixing 10 mg of the
Thus, the preparation of the model containing the chromophore,
M1, was easily achieved from (3) and benzoyl chloride. The FT-IR
and H and C NMR spectra of the model confirm its structure
and purity (Supporting information, Fig. S4).
As previously mentioned, aromatic copolyamides with a low
percentage of modification of the structural units from the com-
mercial aramids are interesting materials that can impart new
properties and characteristics without penalizing their outstanding
mechanical and thermal properties. The structure of the co-
polymers was in agreement with the NMR data (Fig. 1) and the
content of structural units corresponded with the feed molar ratio
1
13
polymer with 1 mL of a solvent, followed by stirring for 24 h at
ꢀ
2
0 C. The polymer was considered soluble at room temperature if a
homogeneous solution was obtained. If the polymer was not sol-
uble, the system was heated to reflux to 2 h, and the polymer was
considered soluble on heating if a homogeneous solution was ob-
tained. Otherwise, the polymer was considered insoluble. The
polymer inherent viscosities were measured with a Ubbelohde
viscometer using N-methyl-2-pirrolidone and sulfuric acid (100%)
ꢀ
ꢂ1
as solvents at 25 ± 0.1 C and a polymer concentration of 0.5 g dL
.
Water sorption experiments were conducted gravimetrically at
ꢀ
room temperature. 200 mg of the sample was dried at 60 C for 24 h
over phosphorus pentoxide, and the sample was placed in a closed
ꢀ
box containing a saturated aqueous solution of NaNO
2
at 25 C,
which provided a relative humidity of 65%. The samples were
weighed periodically over a period of 8 days until they equilibrated
with their surroundings and presented no further changes in
weight.
Polyamide films were prepared by evaporation of cast solutions
of 12% by polymer weight in DMA. The solvent was eliminated by
ꢀ
heating at 80 C overnight. To determine the tensile properties of
the polyamide, strips (5 mm in width and 30 mm in length) were
cut from a polymer film of 551
EZ Test Compact Table-Top Universal Tester at 20 C. Mechanical
clamps were used and an extension rate of 5 mm min
m
m thickness for I on an SHIMADZU
ꢀ
ꢂ1
was
applied using a gauge length of 9.44 mm. At least 6 samples were
tested, and the data was then averaged.
3
. Results and discussion
The preparation of inherently colored aromatic polyamides is a
promising methodology for the exploitation of the high-
performance thermal and mechanical properties of the aramids
in the preparation of fibers for protective clothes. In this regard, it is
advisable to modify as little as possible the aromatic nature of the
main chain to maintain the outstanding properties of the aramids
and at the same time achieving the new functionalities of the
materials, i.e., permanent color in the development we are
reporting, that increases even more their value added. In our case,
we prepared a diamine monomer as novel blue chromophore for
the preparation of a set of inherently blue copolyamides I, II and III,
in which the color hue can be easily tuned by adjusting its feed
percentage in the synthesis of the aramids. In our study, we have
prepared aromatic copolyamides as MPIA derivatives with 0.1, 1.0
and 10.0% of structural units containing the chromophore (3)
(Scheme 2).
In an initial approach to the preparation of the inherently
colored polyamides we modeled the chemical modification re-
actions using a polyamide model to assure that the condensation
reaction is carried out in a clean manner, i.e., without side reactions
or any modification of the chromophore core.
For comparative purposes, some of the properties of the colored
aramids are compared with those of the commercial m-aramid,
MPIA, which we prepared following the standard procedure for
aramid synthesis depicted in Scheme 2.
Fig. 1. 1H NMR spectra of copolyamides: a) I; b) II; and c) III.

M. Trigo-L oꢀ pez et al. / Dyes and Pigments 122 (2015) 177e183
181
of the chromophore monomer (3), as per the integral ratio of the
methyl protons of methoxy group (~3.9 ppm) to the aromatic re-
1
gion (7e9 ppm) in H NMR spectra.
The chemical constitution limits the upper performance of
materials. However, correct chemical structures do not guarantee
the achievement of the highest properties, which are affected by
different concomitant factors, such us molecular weight. This
parameter is related with viscosity and in the field of high perfor-
mance materials the inherent viscosity (
comparative purposes.
hinh) is usually studied for
The viscosity is solvent dependent, and for this reason the
inherent viscosity was measured in two different solvents (Table 1).
Aramids with hinh higher than 0.5 in NMP are usually considered as
high molecular weight polymers, and our data are consistent with
this approach.
Polymers with polar groups absorb water from the environ-
ment, influencing their performance. The amide linkage of aramids
is highly polar and its interaction with water molecules is highly
favorable. The water sorption lowers the thermal transitions by
weakening the amideeamide strong interactions impairing the
electrical insulation, the mechanical, and the thermal properties.
Conversely, a high water uptake is interesting for other techno-
logical fields in which aramids play an important role, such as
membrane technology for water desalination, i.e., reverse osmosis
membranes. The water sorption is limited by the aramid crystal-
linity. The prepared aramids absorb 7% by weight of water, a data
fully comparable with laboratory prepared MPIA samples (the re-
ported water uptake for commercial MPIA fibers is 5.2%) [7].
The polymer II was characterized by time-resolved UV/Vis ab-
sorption spectroelectrochemistry. This multiple response tech-
nique allows us to obtain electrochemical and molecular
information simultaneously. Polymer oxidation was studied using
cyclic voltabsorptometry by scanning the potential between 0.00 V
ꢂ1
and þ1.60 V at a scan rate of 0.02 V s in a 0.1 M TBAPF
6
aceto-
nitrile solution. The cyclic voltammograms (Fig. 2a) shows an ill-
defined response providing little information on the oxidation
process. However, the optical response is much more informative.
Fig. 2b shows the UV/Vis spectra in which a band centered at
7
75 nm evolves during the oxidation process. This band is related to
the oxidation of the ADPM [23]. The corresponding voltabsorpto-
gram at 775 nm is shown in Fig. 2c. At lower potentials there is no
change in the absorbance, but when the potential reaches a value
of þ0.70 V, the band centered at 775 nm starts to grow. At higher
potentials the intensity of this band increases until the oxidation of
the polymer is completed. As can be observed in Fig. 2c, absorbance
does not decrease in the backward scan and, therefore, the initial
value of absorbance is not recovered after the oxidation, indicating
that an irreversible oxidation of the polymer is taking place.
Fig. 2. Electrochemical analysis of polymer II (film obtained by casting, 13
m
m thick-
ness): (a) Cyclic voltammogram obtained in 0.1 M TBAPF
6
acetonitrile solution. Po-
ꢂ1
tential was scanned between 0.00 V and þ1.60 V at 0.020 V s ; (b) Evolution of the
UV/Vis absorption spectra during the oxidation of the polymer; (c) Cyclic voltab-
sorptogram at 775 nm.
solvents, such as NMP, DMSO, and DMA (Table 2). At this point, it is
important to comment that the commercial MPIA is transformed
into fibers by wet, dry or dry-jet wet spinning from a DMA solution
of approximately 19.2% MPIA. As expected, the low-molecular-
weight model is soluble in conventional organic solvents.
Regarding the water uptake, the absorption of 7% by weight of
water is similar to that of MPIA. This indicates that water affinity of
the structural units containing the chromophore (3) of aramids I, II
and III are similar to those of MPIA. Accordingly, this parameter
will not affect the applicability of the materials as fibers in com-
parison with the commercial aramid fabrics.
3
.2. Solubility and water uptake
The lack of solubility has always been a drawback of polyamides
and has limited their technological applications. However, the co-
polymers described in this work are highly soluble compared with
aramids and can be dissolved at room temperature in polar aprotic
Table 1
Inherent viscosity and water sorption of the polymers.
Polymer
h
inh (dL/g)a
Water uptake (%)b
I
II
III
0.76 (0.48)
1.15 (0.47)
1.03 (0.53)
7
7
7
3.3. Thermal and mechanical properties
a
Polymer concentration ¼ 0.5 g dLꢂ1; temperature ¼ 30 ꢀC; solvents: sulfuric acid
The thermal resistance of the materials was considered to be
related with the decomposition temperatures, in terms of weight
1
00% and NMP, the data obtained in NMP between brackets.
b
Water sorption at 65% RH.

182
M. Trigo-L oꢀ pez et al. / Dyes and Pigments 122 (2015) 177e183
Table 2
Solubility of the polymers.a
Polymer
Solvents
DMSO, DMA
NMP, DMF
THF
Acetone
CHCl
3
, CH
2
Cl2, EtOH
Cyclohexanone
I
II
III
MPIA
M1
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
þ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
þþ
þþ
ꢂ
ꢂ
ꢂ
þþ
þꢂ
þþ
a
Essay: 10 mg of polymer in 1 mL of solvent. Solubility: þþ ¼ soluble at room temperature, þ ¼ soluble on heating, þꢂ ¼ partially soluble, ꢂ ¼ insoluble.
loss, and with the char yield at a given temperature. The thermal
resistance was evaluated using TGA. The TGA data, summarized in
Table 3, confirmed the good thermal stability of copolyamides,
which surprisingly exhibited better thermal properties compared
with the commercial aramid MPIA, which displayed 5% (T
5
) and
ꢀ
10% (T10) weight loss under nitrogen atmosphere of 432 and 452 C,
respectively.
Even more important than the weight loss at a given tempera-
ture is the minimum fraction of oxygen in a mixture of oxygen and
nitrogen that will support combustion after ignition, that is, the LOI
(
limiting oxygen index), which is related to the char yield. The
ꢀ
higher the char yield at 800 C, the higher the LOI, and the better
the flame resistance properties of the materials. LOI values higher
than 21%, the oxygen content of air, will most likely not burn in an
open-air situation. All LOI values are significantly higher than 21,
and even higher than the LOI of MPIA calculated using the same
method, which is 38. Considering this, the prepared materials have
very good fireproof characteristics and must be considered as self-
extinguishing materials.
ꢀ
Fig. 3. Weight loss of MPIA and aramid I after heating the polymers at 350 C for 24 h
in oxidizing (synthetic air) atmosphere.
In short, the results of the thermal behavior of the prepared
aramids are quite shocking. We expected thermal property loss
compared to commercial MPIA instead of an improvement.
Remarkably, the improvement is more evident in oxidizing atmo-
sphere (synthetic air) and even more striking in static thermogra-
ꢀ
vimetric analysis at high temperature (350 C) (Fig. 3).
The mechanical properties were similar to those of the reference
m-aramid, MPIA, with tensile strength and Young's modulus of 51
and 1530 MPa, respectively, with elongation of 5% [9]. At this point,
it is important to take into account that the measurements were
carried out with lab-made un-oriented and thermally un-treated
films prepared by casting.
3.4. Applicability of the intrinsically colored aramids
Colored commercial aramid fibers are highly demanded for
protected apparels and are extremely expensive materials. How-
ever, the loss of color upon use is usual in all kind of fibers dyed
using standard dying procedures. We have worked to solve this to
Table 3
Thermal behavior of polymers I, II and III compared to MPIA.
Polymer
N
2
atmosphere
O
2
atmosphere (synthetic air)
a
b
CRc
(%)
a
b
CRc
(%)
LOId
T
(
5
ꢀ
T
10
ꢀ
T
5
ꢀ
T
10
ꢀ
C)
( C)
( C)
( C)
I
II
III
MPIA
416
444
449
432
448
463
472
452
58
56
60
50
427
451
451
419
462
482
480
445
e
1
41
40
42
38
e
9
a
T
T
5
: temperature at 5% weight loss is observed.
Fig. 4. Color hue of the aramids as obtained upon precipitation after synthesis and
films (a), and reflection UV/vis spectrum (b) of polyamide II. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this
article.)
b
c
10: temperature at 10% weight loss is observed.
ꢀ
CR: char yield (%) at 800 C.
Limiting oxygen index, calculated from the TGA data (LOI ¼ 17.5 þ 0.4 CR).
d

M. Trigo-L oꢀ pez et al. / Dyes and Pigments 122 (2015) 177e183
183
has been demonstrated that the hue of the polyamide color can also
be tuned by a redox process, and the evaluation of the application
potential of this process is now under evaluation. Research is in
progress for preparing yellow and red diamine monomers, and
with the correct copolymerization of the described blue, and the
forthcoming red and yellow chromophore monomers, the full color
range will be available.
Acknowledgments
We gratefully acknowledge the financial support provided by
the Spanish Ministerio de Economía y Competitividad-Feder
(
MAT2014-54137-R) and by the Consejería de Educaci oꢀ n e Junta de
Castilla y Le oꢀ n (BU232U13).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.06.027.
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4
. Conclusions
We have synthetized a chromophore monomer that copoly-
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