M. M. LAKOURAJ et al.: ORGANOSOLUBLE XANTHONE-BASED POLYIMIDES: SYNTHESIS ...
471–478
ORGANOSOLUBLE XANTHONE-BASED POLYIMIDES:
SYNTHESIS, CHARACTERIZATION, ANTIOXIDANT ACTIVITY
AND HEAVY-METAL SORPTION
ORGANSKO TOPNI POLIAMIDI NA OSNOVI KSANTONA:
SINTEZA, KARAKTERIZACIJA, ANTIOKSIDATIVNA AKTIVNOST
IN SORPCIJA TE@KIH KOVIN
Moslem Mansour Lakouraj1, Ghasem Rahpaima2, Razieh Azimi1
1University of Mazandaran, Faculty of Chemistry, Department of Polymer Chemistry, Babolsar, Iran
2Islamic Azad University, Department of Chemistry, Lamerd Branch, Lamerd, Iran
lakouraj@umz.ac.ir
Prejem rokopisa – received: 2013-10-15; sprejem za objavo – accepted for publication: 2015-08-25
doi:10.17222/mit.2013.250
To improve the solubility, thermal properties and processability of polyxanthones, a new class of polyxanthones,
poly(xanthone-imide)s (PXIs), with a high yield was prepared using a two-step chemical imidation of 2,7-diaminoxanthone with
pyromellitic dianhydride (PMDA), 3,3’,4,4’-benzophenone tetracarboxylic dianhydride (BTDA) and 2,2’-bis-(3,4-dicarbo-
xyphenyl) hexafluoropropane dianhydride (6-FDA)). These PXIs were characterized with FT-IR and 1H NMR spectroscopies.
They presented a good solubility in aprotic polar solvents such as N,N-dimethyl acetamide (DMAc), N,N-dimethyl formamide
(DMF), N-methyl pyrrolidone (NMP) and dimethyl sulfoxide (DMSO), and showed inherent viscosities in a range of 0.34–0.58
dL/g. These PXIs exhibited lower glass-transition temperatures than the original polyxanthones and a high thermal stability. The
obtained results of the UV-vis absorption and photoluminescence indicated that the maximum absorption and fluorescence
emission of PXIs were in the range of 300–304 nm and 432–510 nm, respectively. The antioxidant activity of PXIs was
evaluated with a DPPH assay. The antioxidant values for PXIs were greater than for the parent xanthone (X). The polyimides
were investigated for the extraction of environmentally noxious metal ions such as Cr (VI), Co (II), Ni (II), Cu (II), Pb (II) and
Cd (II) from aqueous solutions.
Keywords: poly(xanthone-imide)s, antioxidant activity, fluorescence, organosolubility, heavy metals
Za izbolj{anje topnosti, toplotnih lastnosti in predelovalnost poliksantonov je bila pripravljena nova vrsta poliksantonov,
poli(ksanton-imidov (PXIs)) z visokim izkoristkom, z uporabo dvostopenjske kemijske sinteze 2,7-diaminoksantona z piro-
melitnim dianhidridom (PMDA), 3,3’,4,4’-benzofenon tetrakarboksili~nega dianhidrida (BTDA) in 2,2’-Bis-(3,4-Dikarbofenil)
heksafluorpropan dianhidrida (6-FDA)). Ti PXI-ji so bili karakterizirani s FT-IR in 1H NMR-spektroskopijo. Predstavljajo dobro
topnost v aproti~nih polarnih topilih, kot je N,N-dimetil acetamid (DMAc), N,N-dimetil formamid (DMF), N-metil pirolidon
(NMP) in dimetil sulfoksid (DMSO) in ka`ejo nespremenljivo viskoznost v obmo~ju 0,34–0,58 dL/g. Ti PXI-ji ka`ejo ni`jo
temperaturo prehoda v steklasto stanje kot originalni poliksantoni ter veliko termi~no stabilnost. Dobljeni rezultati UV-absorp-
cije in fotoluminiscence ka`ejo, da je maksimalna absorpcija in emisija fluorescence PXI-jev v obmo~ju 300–304 nm, oziroma
432–510 nm. Antioksidativna aktivnost PXI-jev je bila ocenjena z DPPH preizkusom. Vrednost protioksidativnosti za PXI-je je
bila ve~ja kot pri mati~nem ksantonu. Poliamidi so bili preiskovani za ekstrakcijo okoljsko {kodljivih kovinskih ionov, kot so Cr
(VI), Co (II), Ni (II), Cu (II), Pb (II) in Cd (II) iz vodnih raztopin.
Klju~ne besede: poli(ksanton-imidi), antioksidacijska aktivnost, fluorescenca, organska topnost, te`ke kovine
1 INTRODUCTION
Xanthones have a conjugated planar ring system con-
sisting of two benzene rings bridged through a carbonyl
group and an oxygen atom. This unique structure shows
an admirable thermo-oxidative and hydrolytic stability
with numerous potential beneficial properties such as
antioxidant,1,2 antihistamine,3,4 antiinflammatory,5,6 anti-
bacterial,7,8 antifungal,9 antiviral10,11 and anticancer12,13
effects. Consequently, it seems to have a great potential
as a structural pattern in high-performance polymers.
Although several different synthetic and natural xan-
thones were introduced14–18 few of such biologically
active compounds were investigated in a polymer syn-
thesis.19
Polyxanthones can be used for high-temperature
applications and as electrical insulating materials due to
their excellent solvent and chemical resistance, good
physical and electrical properties even at high tempe-
ratures. However, a poor processability of the polymers
is the main drawback as they have high glass or melting
temperatures and are often insoluble in common organic
solvents.20,21
On the other hand, due to oxidative-degradation
reactions that may occur during various stages of the
polymer lifecycle including the manufacturing, process-
ing and end-use stages, it is essential to place antioxidant
building block in polymer matrices. These polymers,
whose antioxidant moieties are covalently attached to the
backbone of a polymer, have unique advantages: they are
Materiali in tehnologije / Materials and technology 50 (2016) 4, 471–478 471
UDK 678.7:678-13 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(4)471(2016)
non-volatile and they do not penetrate into the skin and
tissue.
Aromatic polyimides are well-known high-perfor-
mance polymers that have a high thermal stability,
excellent mechanical and chemical properties. Due to
these advantageous properties, polyimides are widely
used as adhesives, films, composite matrices, coatings,
membranes, and in the electronic-packaging industry.22–25
Therefore, in continuation of our ongoing work on
polyxanthones26 and also in order to improve the phy-
sical properties of polyxanthones, in the present study,
we attempted to insert a xanthone unit into the backbone
of a polyimide to develop new organosoluble polyxan-
thones. For this purpose, at first, 2,7-diaminoxanthone
was prepared and then used for a polymerization with
pyromellitic dianhydride (PMDA), 3,3’, 4,4’-benzo-
phenone tetracarboxylic dianhydride (BTDA) and
2,2’-bis-(3,4-dicarboxyphenyl) hexafluoropropane
dianhydride (6-FDA). The structures of the resulting
poly(xanthone-imide)s were characterized with NMR
and FT-IR spectroscopies. The physical properties of
these PXIs, including their viscosity, solubility, thermal
stability, morphology, and their spectroscopic properties
such as ultraviolet-visible absorption and fluorescence
emission were studied. In addition, PXIs were also eva-
luated for their antioxidant activities with a 2,2-diphe-
nyl-1 picrylhydrazyl (DPPH) assay. Furthermore, their
ability to eliminate environmentally toxic heavy-metal
ions such as Pb (II), Cd (II), Co (II), Ni (II), Cu (II) and
Cr (VI) were studied in aqueous media.
2 EXPERIMENTAL WORK
2.1 Materials
N-methyl-2-pyrrolidone (NMP), N,N-dimethylace-
tamide (DMAc), acetic anhydride and pyridine were
purchased from Merck and purified with distillation
under a reduced pressure over calcium hydride and
stored above a 4° A molecular sieve. Pyromellitic
dianhydride (PMDA), 3,3’,4,4’-benzophenone tetracar-
boxylic dianhydride (BTDA) and 2,2’-bis-(3,4-dicarbo-
xyphenyl) hexafluoropropane dianhydride (6-FDA) were
dried in a vacuum oven at 110 °C for 5 h. All the other
materials and solvents, such as DMF, pyridine and
DMSO were purchased from TCI Chemical Co., Fluka
Chemical Co. (Buchs, Switzerland) and Merck Chemical
Co. and used as received.
2.2 Measurements
A Bruker Tensor 27 spectrometer (Bruker, Karlsruhe,
Germany) and a 400-MHz Bruker Avance DRX spec-
trometer in DMSO-d6 were used for recording FT-IR, 1H
and 13C NMR spectra, respectively Inherent viscosities
(hinh) of the polymers were determined in NMP at 0.5 g
per 100 mL concentration, with an Ubbelohde
viscometer (Schott-Gerate, Hofheim, Germany) at 25 °C.
A thermogravimetric analysis (TGA) was conducted
with a TA Instruments TGA-50 (Shimadzu, Kyoto,
Japan) in a temperature range of 50–650 °C at a heating
rate of 10 °C/min under nitrogen atmosphere.
Glass-transition temperatures (Tg) of the polymers
were determined with a Perkin-Elmer Pyris 6 differential
scanning calorimeter at a heating rate of 10 °C/min
under nitrogen atmosphere. The UV-visible absorption
and fluorescence emission spectra were recorded on
Cecil 5000 and Perkin-Elmer LS-3B spectrophotometers
in an NMP solution, respectively. X-ray powder
diffraction patterns were performed at room temperature
(about 25 °C) with an X-ray diffractometer (GBC MMA
instrument) with Be-filtered Cu-K (0.15418 nm) ope-
rating at 35.4 kV and 28 mA. The 2 scanning range was
set between 4° and 50° at a scan rate of 0.05° per s. The
concentration of metal cations in the liquid phase was
determined with an atomic-absorption instrument
(BRAIC WFX-130 AA).
2.3 Monomer synthesis
As illustrated in Figure 1, the 2,7-dinitroxanthone
(DNX) and 2,7-diaminoxanthone (DAX) were prepared
according to our published article.22
2.4 Polymer synthesis
The general synthetic route used to produce the PXIs
was as follows. A 100-mL two-necked, round-bottomed
flask equipped with a magnetic stirrer bar, nitrogen-gas
inlet tube and calcium chloride drying tube was charged
with 0.226 g (1.0 mmol) of diamine (DAX) and 10 mL
of dry NMP. The mixture was stirred at room tempera-
ture for 0.5 h. Then 1.0 mmol of a dianhydride was
added and the mixture was again stirred at room tem-
perature for 24 h, forming a viscous solution of a poly
(amic acid) (PAA) precursor in NMP. The PAA was
converted into polyimide with the chemical-imidization
process.27 The chemical imidization was carried out by
adding 3 mL of a mixture of acetic anhydride/pyridine
(6:4, v/v) into the PAA solution, while stirring it at room
temperature for 1 h. Then the mixture was stirred at
130 °C for 12 h to yield a homogeneous solution. The
polymer solution was slowly poured into methanol to
form a precipitate. The precipitate was then filtered,
washed thoroughly with hot methanol and dried over-
night under vacuum at 80 °C.
PXI-a. A yield of 97 %, FTIR (KBr, cm–1): 3020
(aromatic C-H stretching), 1775 (C=O asymmetric stre-
tching), 1714 (C=O symmetric stretching), 1658 (C=O
stretching of the carbonyl group), 1466 (C=C stretching),
1375 (C-N stretching), 1145 (C-O stretching). 1H NMR
(400 MHz, DMSO-d6): d 7.52–8.84 (8 H, aromatic pro-
tons).
PXI-b. A yield of 96 %, FTIR (KBr, cm–1): 3050
(aromatic C-H stretching), 1780 (C=O asymmetric stre-
tching), 1705 (C=O symmetric stretching), 1660 (C=O
stretching of the carbonyl group), 1470 (C=C stretching),
M. M. LAKOURAJ et al.: ORGANOSOLUBLE XANTHONE-BASED POLYIMIDES: SYNTHESIS ...
472 Materiali in tehnologije / Materials and technology 50 (2016) 4, 471–478
1370 (C-N stretching), 1155 (C-O stretching). 1H NMR
(400 MHz, DMSO-d6): d 7.61–8.52 (12 H, aromatic pro-
tons).
PXI-c. A yield of 93 %, FTIR (KBr, cm–1): 3010
(aromatic C-H stretching), 1785 (C=O asymmetric stre-
tching), 1755 (C=O symmetric stretching), 1658 (C=O
stretching of the carbonyl group), 1475 (C=C stretching),
1385 (C-N stretching), 1140 (C-O stretching). 1H NMR
(DMSO-d6): d 7.29-7.79 (12H Aromatic).
2.5 Antioxidant activities
The antioxidant activity of the compounds (PXI-a, b
and c) was determined spectrophotometrically, using a
stable 2,2-diphenyl-1 picrylhydrazyl (DPPH) radical
according to the already reported method, with a slight
modification.28 A stock solution of the PXIs (0.5 mg/mL)
was prepared in DMSO, and 50 μL of the prepared PXI
solution was added to 5 mL of a 0.004 % ethanol so-
lution of the DPPH radical. After 30 min of incubation in
dark at room temperature, the absorbance was observed
against a blank at 517 nm. The assay was carried out in
triplicate and the percentage of inhibition was calculated
using the following formula:
% inhibition = (AB-AA)/AB×100
where AB = absorption of the blank and AA = absorp-
tion of the test.
2.6 Water absorption
Two examination methods were investigated for the
water-absorption capacity of poly(xanthone-imide)s.
Method I: the polymer (0.2 g) was poured in 30 mL
water at 25 °C for 24 h, followed instantly by weighing.
Method II: a polymer powder was boiled in water at 100
°C for 30 min, and its weight difference was determined
with the measurements before and after the insertion.
2.7 Adsorption capability
Solid-liquid extractions of Cd (II), Cu (II), Co (II)
and Ni (II) as their chloride salts, Pb (II) as nitrate salt
and Cr (VI) as Cr2O72–(K2Cr2O7) were carried out either
individually or in the mixture. Approximately 10 mg of
the appropriate polymer powder was shaken with 10 mL
of an aqueous solution of the metal salt for 3 d at 25 °C.
The initial concentration of the salts was 10 mg L–1.
After filtration, the concentration of each metal cation in
the liquid phase was determined with the atomic-absorp-
tion technique, and direct information regarding the
extraction percentage of metal ions by the polymer was
obtained using a calibration curve, made for each metal
ion from the standard solutions of (5, 10, and 20) μg/g.
3 RESULTS AND DISCUSSION
3.1 Polyimide synthesis and characterization
Three polyimides containing a xanthone group were
prepared in high yields (90–97 %) with a polyconden-
sation of equal molar amounts of the diamine with
commercially available aromatic dianhydrides, such as
PMDA, BTDA and 6-FDA, as shown in Figure 1. The
polycondensation was carried out in NMP at room
temperature for 24 h to form poly(amic acid)s, followed
by chemical imidization with acetic anhydride and pyri-
dine. The inherent viscosity of the polymers, as a suit-
able criterion for evaluating the molecular weight, was
measured at a concentration of 0.5 g/dL in NMP at
25 °C. The inherent viscosities of the polyimides were in
a range of 0.34–0.58 dL/g, indicating moderate mole-
cular weights. All the polymers were characterized using
the FT-IR and 1H NMR techniques. In Figure 2, the
FT-IR spectrum of the PXI-b is shown as a representative
polyimide. The FT-IR spectra of the PXI-b exhibited
characteristic absorption bands of the five-membered
imide ring at 1780 and 1705 cm–1 (typical of the imide
carbonyl asymmetric and symmetric stretching), 1660
cm–1 (C=O stretching of the xanthone carbonyl group),
1470 cm–1 (C=C stretching), 1370 cm–1 (C-N stretching),
together with a strong absorption band at 1155 cm–1
(C-O stretching). 1H NMR spectra for the poly(xan-
thone-imide), PXI-b, is shown in Figure 3. The spectrum
showed characteristic resonance signals of aromatic
protons in the region of 7.61–8.52 μg/g.
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Materiali in tehnologije / Materials and technology 50 (2016) 4, 471–478 473
Figure 2: FT-IR spectrum of PXI-b
Slika 2: FT-IR spekter PXI-b
Figure 1: Synthesis and designations of poly(xanthone-imide)s (PXIs)
Slika 1: Sinteza in oznaka poli(ksanton-imidov) (PXI-jev)
3.2 Solubility and viscosity
The solubility behavior of the poly(xanthone-imide)s
depended on their chain-packing ability and intermole-
cular interactions, which were affected by the rigidity,
symmetry and regularity of the molecular backbone. The
solubility behavior of the PXIs was experienced in
several organic solvents using a 5 % (w/v) concentration
at room temperature, and the results are tabulated in
Table 1. These poly(xanthone-imide)s had a higher solu-
bility than the earlier reported polyxanthones.21 The
solubility of these polyimides varies depending on the
dianhydride used. A comparison of the solubility values
for PXIs shows that the presence of carbonyl and
hexafluoroisopropylidene groups in the PXI-b and PXI-c
enhances their solubility at room temperature. However,
the solubility of these poly(xanthone-imide)s could be
improved by attaching bulky trifluoromethyl (-CF3)
groups onto the polymer chain.
The enhancement in solubility is clearly attributed to
the additional effect arising from the bulky -CF3 unit,
which increased the disorder in the chains, causing a less
close chain packing, thus facilitating the distribution of
solvent molecules among the macromolecule chains.
These PXIs showed a good solubility in polar aprotic
solvents such as DMF, DMAc, DMSO and NMP at room
temperature, while the corresponding polyxanthone
homopolymers have no solubility in such solvents at
ambient temperature. This suggests that these polymers
have potential applications in the areas such as film
formation or casting process where the temperature is a
determining factor.
3.3 Optical properties
Several classes of xanthone compounds were
successfully used for protecting light-sensitive materials,
controlling the harmful influence of light, especially of
UV irradiation. The photophysical properties of the
monomer and PXIs were examined with UV-visible and
fluorescence spectroscopies in an NMP solution. The
UV-vis absorption of the monomer and poly(xanthone-
imide)s (Figure 4a) showed a strong absorption at 355
and 300–304 nm, respectively, in the NMP solutions due
to the -* transition of the aromatic chromophores of a
xanthone ring. In comparison with the parent xanthone,
M. M. LAKOURAJ et al.: ORGANOSOLUBLE XANTHONE-BASED POLYIMIDES: SYNTHESIS ...
474 Materiali in tehnologije / Materials and technology 50 (2016) 4, 471–478
Figure 3: 1H NMR spectrum of PXI-b
Slika 3: 1H NMR spekter PXI-b
Figure 4: a) UV-vis absorption spectra and b) fluorescence emission
spectra of the DAX and PXIs in NMP solution
Slika 4: a) UV absorpcijski spekter in b) emisijski spekter fluores-
cence DAX in PXI-jev v NMP raztopini
Table 1: Solubility behavior and inherent viscosities of poly(xanthone-imide)s
Tabela 1: Topnost in nespremenljiva viskoznost poli(ksanton-imidov)
PXIs code NMP DMF DMSO DMAc THF Acetone ETOH Inherentviscosity (dL/g)
PXI-a + + + + - - - 0.58
PXI-b ++ ++ ++ ++ - - - 0.43
PXI-c ++ ++ ++ ++ - - - 0.34
(DMAc: N,N-dimethyl acetamide; DMF: N,N-dimethyl formamide; NMP: N-methyl pyrrolidone; DMSO: dimethyl sulfoxide; THF:
tetrahydrofuran; ETOH: ethanol); ++: soluble at room temperature; +: soluble during heating at 60 °C; -: insoluble)
the UV spectra of the corresponding poly(xanthone-
imide)s were slightly blue shifted and broadened. The
fluorescence emission spectra of the PXIs exhibited peak
positions with the maxima at 413–510 nm (Figure 4b).
3.4 Thermal properties
The thermal behavior of the poly(xanthone-imide)s
was evaluated with a thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) at a heating
rate of 10 °C min–1 and the results obtained from these
thermograms are summarized in Table 2. DSC thermo-
grams of the polyimides are shown in Figure 5. As
indicated in Figure 5, the Tg values of these PXIs are in a
range of 213–285 °C, while the reported values of Tg for
the bare polyxanthones were found to be higher than
370 °C.21 The results showed that the Tg values of these
PXIs depend on the structure of the dianhydride com-
ponent and they decrease with the increasing flexibility
of the dianhydride structure.
Table 2: Characteristic thermal data of poly(xanthone-imide)s
Tabela 2: Podatki o toplotni zna~ilnosti poli(ksanton-imidov)
Compound Tg (°C) T5 (°C) T10 (°C) Char. yield(%)
PXI-a 285 415 460 60
PXI-b 218 380 400 61
PXI-c 213 370 394 46
Tg: glass-transition temperature; T5: temperature for a 5% weight loss
T10: temperature for a 10 % weight loss; Char. yield: weight of the
polymer, kept at 700 °C
The ployimide derived from PMDA exhibits the
highest Tg because of a rigid backbone, while the PXI-c
obtained from 6-FDA showed the lowest Tg owing to the
bulky CF3 groups between the phthalimide units, which
might be a result of reduced chain-to-chain charge-trans-
fer interactions and poor chain packing of the bulky
pendant -CF3 groups. Indeed, the difference in Tg of the
PXIs can be attributed to the rigidity and close packing
of the polymer chains. No melting endotherm or sign of
a crystalline formation was observed in the DSC traces
of the poly(xanthone-imide)s as confirmed by their
WAXD patterns (Figure 6).
These observations disclose an amorphous nature of
the polymers. The thermal stabilities of these poly(xan-
thone-imide)s were evaluated with TGA (Figure 7). The
TGA data indicated a good thermal stability of the PXIs
up to 460 °C with a weight loss of 10 %. Also, the char
yields of the poly(xanthone-imide)s at 600 °C were in a
range of 46–67 % implying that these polymers possess
a good thermal stability. Summing up, it can be deduced
that the PMDA-derived polyimide (PXI-a) has the
highest thermal stability among those investigated,
which can be related to the incorporation of rigid PMDA
units. The introduction of hexafluoroisopropylidene units
appears to reduce the packing density of molecular
chains, which is strongly affected by the intermolecular
interactions.
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Materiali in tehnologije / Materials and technology 50 (2016) 4, 471–478 475
Figure 7: TGA thermograms of PXIs
Slika 7: TGA-krivulja PXI-jev
Figure 5: DSC thermograms of PXIs
Slika 5: DSC-krivulja za PXI-je
Figure 6: X-ray diffraction patterns of PXIs
Slika 6: Rentgenska difrakcija PXI-jev
3.5 Antioxidant activity
Antioxidant polymers, both externally doped and
chemically bounded, are extensively utilized in the field
of polymeric materials showing antioxidant properties.
In recent years, this research area has grown rapidly
because antioxidant polymers have many industrial
applications including materials science, biomedicine,
pharmaceuticals, cosmetics and food packaging. Gener-
ally, chemically bounded polymeric antioxidants have
unique advantages: they are non-volatile and do not
penetrate into the skin and tissue. Consequently, the use
of polymeric antioxidants, in which the antioxidant spe-
cies are covalently bonded to the backbone of the
polymer, especially in biocompatible materials, can be a
key step toward a healthy life. The antioxidant activities
of the parent xanthone (X) and PXIs were evaluated with
a DPPH assay and the results are given in Table 3. The
inhibition percentages determined with the DPPH assay
for the monomeric diaminoxanthone was 27.07 %,
whereas those of the PXIs were in a range of
47.80–61.30 %. It is evident that the polyxanthones ex-
hibit a higher antioxidant activity than the xanthone
itself. As illustrated in Figure 8, this behavior can be re-
lated to a more extended -conjugation of the xanthone
ring with imide nitrogen of poly(xanthone-imide).
Table 3: Antioxidant inhibition percentages of X and PXIs
Tabela 3: Protioksidativni odstotek inhibicije X in PXI-jev
Compound Inhibition percentage
X
PXI-a
PXI-b
PXI-c
27.0
49.4
61.3
47.8
The PXI-b, obtained from BTDA, exhibited the
highest antioxidant activity because of the presence of
the carbonyl group between two phenyl rings, which
increases the stability of radicals due to further
conjugation. On the contrary, the PXI-c, obtained from
6-FDA, exhibited the lowest antioxidant activity because
of the saturated carbon in the C(CF3)2 groups between
the phthalimide units, which can effectively interrupt the
conjugation. Thus, placing a xanthone unit into the
polymer backbone is a simple way of preparing macro-
molecular systems with a high antioxidant power that
may have the potential for use as biomaterials in
biological media.
3.6 Water absorption
The water uptake affects the final application of these
high-performance materials, especially in the sorption of
heavy metals in water. Though the absorbed water
reduces the physical properties such as the mechanical,
electrical and dielectrical properties and Tg, it also
provides for a better presentation in the other high-tech
fields such as the membrane technology. The polymers
dealt with in this study have two imide groups and one
xanthone moiety per repeating unit; the polar groups
interact with water, which leads to partly hydrophilic
materials. Two methods described in the literature29 were
used for the water-absorption measurement of the PXIs:
at 25 °C for 24 h and at 100 °C for 30 min. As can be
seen in Table 4, the PXI-b derived from BTDA showed a
higher percentage of water absorption (up to 5.4 %) in
comparison with the water absorption of the PXI-a and
PXI-c based on PMDA and 6FDA (up to 5.0 and 4.8 %,
respectively). This is probably due to the existence of the
carbonyl group in the backbones of BTDA-based
polymers.
Table 4: Water absorption of PXIs
Tabela 4: Absorpcija vode PXI-jev
Compound Method 1(%) Method 2(%)
PXI-a
PXI-b
PXI-c
4.7
5.1
4.4
5.0
5.4
4.8
3.7 Adsorption capability
The PXI-b (10 mg) was dissolved in 10 mL of an
aqueous solution of heavy metals such as Co (II), Pb (II),
Cd (II), Ni (II), Cu (II) and Cr (VI). The mixtures were
agitated magnetically at reasonable min–1 for 3 d and the
solids were separated with filtration. The concentration
of metal ions in the filtrate was analyzed via atomic
absorption spectroscopy. The quantity of the adsorbed
metal ions was calculated with the following equation:
Qt = (C0 – CA) × V/w
where Qt is the amount of metal ions adsorbed into the
unit of the composites (mg g–1), C0 and CA are the con-
centrations of metal ions in the initial solution and in the
aqueous phase after the adsorption, respectively
(mg mL–1). V is the volume of the aqueous phase (mL)
and w is the weight of the polymer (mg). The effective-
ness of the metal-ion adsorption from the solution (% R)
was calculated using the following equation:
R = (Ci – Ce) / Ci × 100
M. M. LAKOURAJ et al.: ORGANOSOLUBLE XANTHONE-BASED POLYIMIDES: SYNTHESIS ...
476 Materiali in tehnologije / Materials and technology 50 (2016) 4, 471–478
Figure 8: Sorption of transition-metal cations in water using PXIs
Slika 8: Sorpcija kationov prehodnih kovin v vodi z uporabo PXI-jev
where Ci and Ce are the initial concentration and the
concentration at the equilibrium of metal ions in the
solution, respectively.
Figure 9 depicts the solid-liquid extraction results for
the elimination of individual metal cations from the
aqueous solution at 25 °C. As can be seen in Figure 9,
both parameters, Qt and %R, increased in the order of Cd
(II) > Pb (II) > Cu (II) > Ni (II) > Co (II) > Cr (VI),
which is comparable with the order of their ionic radia.
In Figure 9, the extraction for the smallest metal ion Cr
(VI) is about 39 % and for the largest metal ion Pb (II), it
increases up to 47 %, while the extraction for Cd (II) has
the highest quantity; these results were compared with
those of poly(azoxanthone-esters)30 in Table 5.
Therefore, it can be suggested that the carbonyl
functional groups along the polymer chains provide a
coordinating site to chelate with the metal ions (Figure
10).
Table 5: Heavy-metal sorption capability (%) of PXI-b in comparison
with poly(azoxanthone-ester)
Tabela 5: Sposobnost sorpcije te`kih kovin (%) PXI-jev v primerjavi s
poli(azoksanton-estra)
Compound Cd2+ Pb2+ Cu2+ Ni2+ Co2+ Cr(VI)
PAXEO 59.7 48.8 45 44.2 43.6 40
PXI-b 57.3 47 45.2 42 41.1 39
4 CONCLUSION
A series of organosoluble poly(xanthone-imide)s
with an antioxidant activity was prepared through a
two-step chemical imidation of 2,7-diaminoxanthone
with commercially available dianhydrides and they it
was characterized using different spectroscopic tech-
niques. These polyxanthones showed an amorphous
nature with lower glass-transition temperatures (Tg) and
a higher solubility than the previously reported polyxan-
thones. These polymers indicated inherent viscosities in
a range of 0.34–0.58 dL/g, showing a moderate mole-
cular weight and being soluble in polar aprotic solvents
such as DMF, NMP, DMSO and DMAc at room tempe-
rature. The T10 values and high char yields for the
poly(xanthone-imide)s indicate their good thermal
stability. The introduction of a xanthone ring to the main
chains of the polyimides imparts advantageous proper-
ties of the xanthone nucleus to the polyimides.
The UV-vis absorption spectra of the monomer and
poly(xanthone-imide)s exhibited a strong absorption at
355 nm and 300–304 nm, respectively, and the fluore-
scence emission spectra of the PXIs exhibited peak
positions with the maxima at 413–510 nm in the NMP
solutions.
The antioxidant capacities of the PXIs were investi-
gated. Good antioxidant activities were obtained for all
the poly(xanthone-imide)s. Interestingly, the results
showed that antioxidant activities were enhanced after an
insertion of xanthone into the polymeric chain.
Also, the solid-liquid extractions were carried out in
an aqueous solution and the results showed that the PXIs
are effective for eliminating risky heavy metals such as
Co (II), Pb (II), Cd (II), Ni (II), Cu (II) and Cr (VI) from
wastewaters.
Summing up, an insertion of the xanthone structure
into the polymer backbone provides for a high antioxi-
dant activity of polyxanthone imides that may have a
potential for application in pharmaceutical and food
industries, and also in the solid-phase extraction of
environmentally risky cations.
Acknowledgements
We wish to express our gratitude to the Research
Council of the University of Mazandaran for partial
financial support of this work.
M. M. LAKOURAJ et al.: ORGANOSOLUBLE XANTHONE-BASED POLYIMIDES: SYNTHESIS ...
Materiali in tehnologije / Materials and technology 50 (2016) 4, 471–478 477
Figure 10: Chelation of PXIs with metal cations
Slika 10: Kelacija PXI-jev s kovinskimi kationi
Figure 9: Proposed DPPH radical-scavenging mechanism
Slika 9: Predlagani radikalni splakovalni mehanizem DPPH
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478 Materiali in tehnologije / Materials and technology 50 (2016) 4, 471–478