Synthesis, characterization and film preparation of new co-polyimide based on new 3,5-diamino-N-(pyridin-4-ylmethyl)benzamide, ODA and 6FDA

Article abstract of DOI:10.4067/S0717-97072018000404239

This work describes mainly the synthesis and characterization of new co-polyimides obtained from the polycondensation of the dianhydride 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4’-oxydianiline (ODA) and the new diamine 3,5-diamino-N-

Full text of DOI:10.4067/S0717-97072018000404239

J. Chil. Chem. Soc., 63, Nº 4 (2018)
SYNTHESIS, CHARACTERIZATION AND FILM PREPARATION OF NEW CO-POLYIMIDE BASED ON NEW
3,5-DIAMINO-N-(PYRIDIN-4-YLMETHYL)BENZAMIDE, ODA AND 6FDA
ALAIN TUNDIDOR-CAMBA1, CESAR SALDIAS2, LUIS H. TAGLE1, CLAUDIO A. TERRAZA1, DEYSMA COLL1,3,
GERMÁN PÉREZ1, MANUEL AGUILAR-VEGA4, ROMINA L. ABARCA5 AND PABLO A. ORTIZ*1,3
1Research Laboratory for Organic Polymers, Faculty of Chemistry, Pontificia Universidad Católica de Chile, Santiago, Chile.
2Physical-Chemistry Department, Faculty of Chemistry, Pontificia Universidad Católica de Chile, Santiago, Chile
3Núcleo de Química y Bioquímica, Facultad de Estudios Interdisciplinarios, Universidad Mayor, Santiago, Chile.
4Unidad de Materiales. Laboratorio de Membranas. Centro de Investigación Científica de Yucatán A.C., Mérida, México
5Instituto de Ciencia y Tecnología de los Alimentos, Facultad de Ciencias Agrarias, Universidad Austral, Valdivia, Chile.
ABSTRACT
This work describes mainly the synthesis and characterization of new co-polyimides obtained from the polycondensation of the dianhydride
4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4’-oxydianiline (ODA) and the new diamine 3,5-diamino-N-(pyridin-4-ylmethyl)benzamide
(PyDA). It describes as the different compositions of ODA and PyDA present in the polymers, produce variation on thermal and mechanical properties, which are
important characteristics for the development of future nanocomposites derived from these polymers. Both PyDA monomer and polymers were characterized using
FT-IR and NMR (1H, 13C, 19F, dept 135°, COSY, HMBC, HMQC) spectroscopy. The inherent viscosity of polymers is between 0.3 to 1.49 dL/g, they are soluble in
aprotic polar solvents, such as: DMSO, DMF and DMA. In addition, all co-polymers showed thermal decomposition temperature and glass transition temperatures
above 483 °C and 289 °C, respectively. Mechanical tests under tension of co-polymer films showed Young’s modulus between 2.1-3.9 GPa and tensile strength
between 35.5 and 120.0 MPa. On the other hand, an increase in crystallinity and hydrophilicity is observed when increasing the amount of pyridinyl groups.
Keywords: aromatic co-polyimides, pendant amide, pyridinyl pendant group.
All this with the aim of establishing the most suitable relationship between
the various properties in order to prepare future membranes that are capable of
containing nanoparticles.
1. INTRODUCTION
Aromatic polyimides and polyamides have been widely used for several
decades due to their excellent properties, such as thermal and chemical stability,
flame and radiation resistance, mechanical strength and flexibility.1,2 While
polyimidesaretheyhaveusedasphotoresistlayers, alignmentsofliquidcrystals,
gas separation membranes, electroluminescent devices, polyelectrolytes and
fuel cells.3-11 Polyamides have been used in personal protection elements, in
the military and aerospace industry, high temperature insulating paper and to a
great extent for the manufacture of water treatment membranes.12-14 However,
both are often limited by processing difficulties attributed to the high melting
temperature and poor solubility in various organic solvents. Both problems can
be explained by the existence of strong interactions due to the formation of
intra- and intermolecular charge transfer complexes of the polyimides15,16 and
to the formation of hydrogen bonds in the polyamides.
To overcome these problems, researchers have worked on designing
processable polyimides and polyamides soluble and reducing stiffness or
regularity of the chain and minimizing the density of the imide rings or amide
groups along the chain. Therefore, it has been proposed to use dianhydrides
containing fluorine, such as 4,4’- (hexafluoroisopropylidene)diphthalic
anhydride, the incorporation of atoms or groups “hinges”, as the oxygen atom
in the diamine monomer (for example, 4,4’-oxidianiline) or the use of meta-
oriented or asymmetric diamines could represent a plausible route.17-22
On the other hand, several authors have reported that the incorporation of
pyridine and its derivatives into polymer chains increases the thermal stability,
electron affinity, electron-transport and offers the possibility of protonation or
alkylation site as a way of modifying their properties. The above is due to
pyridine is an electron-deficient aromatic heterocycle having a highly localized
sp2 lone-pair electrons into nitrogen atom.23-27 Moreover, it has been reported
that polymers functionalized with nanoparticles containing pyridine groups
can be used in technological applications such as absorption and conversion
of CO2, homogeneous catalysis, pH sensors, opto-biodetection and biomedical
applications, among others.28-32
2.EXPERIMENTAL
2.1. Materials
Anhydrous acetic anhydride, anhydrous N,N-dimethylacetamide
(DMA),
(DMSO-d6),
N,N-dimethylformamide
N-methyl-2-pyrrolidinone
(DMF),
(NMP),
dimethylsulfoxide-d
3,5-dinitrobenzoy6l
chloride, 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA),
4,4’-oxydianiline (ODA), Pd/C (10% w/w), anhydrous pyridine, pyridin-4-
ylmethanoamine, tetrahydrofuran (THF) and anhydrous triethylamine (TEA)
were obtained from Sigma-Aldrich. Ethanol, ethyl ether and hydrazine 80%
were obtained from Merck KGaA. 4,4’-(hexafluoroisopropylidene)diphthalic
anhydride and 4,4’-oxydianiline were purified by sublimation previous to its
use.
2.2. Instrumentation and measurements
FT-IR spectra (KBr pellets) were recorded on a Perkin-Elmer (Fremont
CA) 1310 spectrophotometer over the range of 4000-400 cm-1. FT-IR-ATR
(ZnSe) were recorded on a Thermo Scientific (Nicolet IS10) spectrophotometer
over the range of 4000-400 cm-1. Melting points were obtained on a SMP3
Stuart Scientific melting point apparatus. 1H, 13C, 19F, dept 135°, COSY,
HMBC and HMQC NMR spectra were carried out on a 400 Hz instrument
(Bruker Avance III HD-400) using DMSO-d6 as solvent and TMS as internal
standard. Viscosimetry measurements were made in a Desreux-Bischof type
dilution viscosimeter at 30 °C (c = 0.5 gdL-1). Glass transition temperature (Tg)
values were obtained with a Mettler-Toledo (Greifensee, Switzerland) DSC
821 calorimetric system from the second heating run (10 °C min-1 under N2
flow). Thermogravimetric analysis were carried out in a Mettler (Switzerland)
TA-3000 calorimetric system equipped with a TC-10A processor, and a TG-50
thermobalance with a Mettler MT5 microbalance (temperature range between
25 °C and 900 °C at 10 °C min-1 under constant N2 gas flow). Tensile test
was performed under uniaxial tension on a Shimadzu AGS-X universal testing
machine with a 100 N load cell at a crosshead speed of 1 mm min-1. The films
were cut in strips 0.5 cm long x 2 cm wide with thickness between 100 and
60 mm. The contact angle was determined by the sessile drop method at room
temperature using a Dataphysics OCA 20 system. A syringe connected to a
Teflon capillary with an internal diameter of approximately 2 mm was used to
introduce the liquids and deposit the respective drop on the films. The surface
free energies were calculated by the Owens-Wendt-Kaelble method. Tapping
mode AFM images were recorded by using a Bruker atomic force microscope,
model Innova.
Considering these backgrounds, in this work we report the synthesis,
characterization and preparation of films from a series of soluble aromatic co-
polyimides derived from 4,4’-(hexafluoroisopropylidene)diphtalic anhydride
(6FDA), 4,4’-oxydianiline (ODA) and the new diamine 3,5-diamino-N-
(pyridin-4-ylmethyl)benzamide (PyDA). These polymers have imide groups
which provide both thermal and mechanical stability, partially incorporate
a pendant pyridinyl group that will allow functionalization with several
nanoparticles in the future, as well as the presence of amide groups that may
facilitate water permeability through the membrane. The research explores the
effect of the ODA and PyDA proportions used in polymerizations and how the
structural composition affects the properties of the synthesized co-polymers.
e-mail: pablo.ortiza@mayor.cl
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2.3 Monomer synthesis and characterization
new solution was kept under stirring for 2 hours at room temperature. Later, the
solution was heated to 60 °C for one hour and after completion of the reaction
the solution was slowly poured into 600 mL of distilled water. The formed
solid was filtered and dried at 150 °C overnight. To purify the polymer, this
was dissolved in DMF and subsequently re-precipitated with ethanol. To be
extracted later with acetone in a soxhlet.
The five synthesized polymers are referred to as PO6-Z/X, where P
corresponds to the 3,5-diamino-N-(pyridin-4-ylmethyl)benzamide (PyDA), O
to 4,4’-oxydianiline (ODA) and 6 to 4,4’-(hexafluoroisopropylidene)diphthalic
anhydride (6FDA). Z/X label corresponds to the feed PyDA/ODA monomers
molar ratio considering that 10 mmol of 6FDA monomer were used in each
polymerization.
3,5-Dinitro-N-(pyridin-4-ylmethyl)benzamide (PyDN). 15 mL (0.148
mol) of pyridin-4-ylmethanoamine, 100 mL of THF and 45 mL of TEA
were placed into 500 mL flask, equipped with a magnetic bar and condenser
system. The solution formed was cooled to 0 °C with an ice water bath and
stirred vigorously, then a 34.06 g (0.148 mol) solution of 3,5-dinitrobenzoyl
chloride in 200 mL of THF was slowly added. After the addition, the reaction
mixture was kept under stirring at 0 °C for one hour, then it was warmed to
room temperature and held therein for an additional hour, and finally refluxed
at 100 °C for another hour to complete the reaction. Upon completion of the
reaction, the resulting mixture was poured into 600 mL of a NaOH 5 % aq.
solution, resulting in a brown precipitate, which was filtered and dried at 120
°C overnight, finally obtaining 34.2 g of PyDN, which were subsequently
recrystallized from an ethanol:DMF (9:1 vol/vol) solvent mixture with a 80
% yield.
Yield: 85 % (crude). M.p.: 200-201 °C. IR (KBr, n, cm-1): 3318 (N-H);
3109, 3078, 3055 (C-H, arom.); 2930, 2872 (C-H, aliph); 1674 (C=N); 1660
(C=O); 1606, 1624 (C=C); 1536, 1335 (N-O); 720 (mono-subst). H NMR
1
(DMSO-d6, δ, ppm): 9.82 (t, J = 5.1 Hz, 1H, 6); 9.11 (s, 2H, 3); 8.99 (s, 1H, 1);
8.53 (d, J = 4.5 Hz, 2H, 10); 7.37 (d, J = 4.6 Hz, 2H, 9); 4.59 (d, J = 5.6 Hz, 2H,
7). 13C NMR (DMSO-d6, δ, ppm): 162.5 (5); 149.6 (10); 148.2 (2); 147.6 (8);
136.5 (4); 127.6 (3); 122.3 (9); 121.0 (1); 42.2 (7).
PO6-10/0. FT-IR-ATR (ZnSe, n, cm-1): 3080, 3060, 3015 (C-H, arom);
2980, 2905 (C-H, aliph); 1783, 1718 (C=O); 1590, 1542, 1498 (C=C); 1244
(C-N), 740, 719 (p-subst). 1H NMR (DMSO-d , δ, ppm): 9.33 (s, 1H, 6); 8.50
(s, 2H, 10); 8.23 (d, 2H, 17); 8.14 (s, 2H, 3); 8.061 (d, 2H, 18); 7.82 (s, 3H, 1,14);
7.34 (s, 2H, 9); 4.54 (s, 2H, 7). 13C NMR (DMSO-d6, δ, ppm): 165.9 (20); 165.7
(19); 164.8 (5); 149.4 (10); 148.4 (8); 137.5 (13); 136.0 (18), 135.6 (2); 133.0
(16); 132.6 (4); 132.4 (15); 128.9 (1); 126.3 (3); 124.6 (17); 123.8 (14); 122.1
(9); 121.9 (11); 64.7 (t, 12); 42.0 (7). 19F NMR (DMSO-d6, δ, ppm): -62.9.
PO6-7/3. FT-IR-ATR (ZnSe, n, cm-1): 3065, 3041, 3015 (C-H, arom);
2986, 2966, 2905 (C-H, aliph); 1784, 1717 (C=O); 1629, 1600, 1539, 1498
3,5-Diamino-N-(pyridin-4-ylmethyl)benzamide (PyDA). 9.9
g of
PyDN, 0.5 g of Pd/C and 200 mL of ethanol were placed into 250 mL flask
equipped with a magnetic bar and a condenser system. The reaction mixture
was vigorously stirred and heated up 70 °C. When the mixture reached that
temperature 10 mL ofhydrazine was added dropwise. After the addition, the
mixture was refluxed at 100 °C and held there to for up to three hours. The
resultant mixture was rapidly filtered over celite and the solution obtained is
concentrated to dryness. Subsequently, 200 mL of ethyl ether were added to
the obtained solid. This new mixture was stirred for a two hours and filtered,
obtaining 7.53 g of PyDA in 95 % yield. Finally, PyDA was purified by
sublimation at 180 °C with an 80 % efficiency.
1
(C=C); 1349 (C-N), 1240 (C-O); 740, 719 (p-subst). H NMR (DMSO-d6, δ,
ppm): 9.33 (s, 7H, 6); 8.51 (s, 14H, 10); 8.21 (m, 20H, 17); 8.14 (s, 14H, 3);
7.99 (m, 20H, 18); 7.80 (m, 27H, 1,14); 7.51 (d, 12H, 22); 7.34 (s, 14H, 9); 7.26
(d, 12H, 22); 4.54 (d, 14H, 7). 13C NMR (DMSO-d6, δ, ppm): 166.2, 166.1 (20);
165.9, 165.7 (19); 164.9 (5); 156.1 (24); 149.4 (10); 148.6 (8); 137.5, 137.4
(13); 136.0, 135.9 (18), 136.0 (2); 133.1, 133.0 (16); 132.6 (4); 132.6, 132.4
(15); 129.4 (22); 129.0 (1); 127.2 (21); 126.3 (3); 124.6, 124.4 (17); 123.8,
123.6 (14); 122.3 (9); 122.0 (11); 119.2 (23); 64.7 (t, 12); 42.0 (7). 19F NMR
(DMSO-d6, δ, ppm): -62.9.
Yield: 95 % (crude). Mp: 167-169 °C. IR (KBr, n, cm-1): 3412, 3339, 3201
(N-H); 3078, 3029 (C-H, arom.); 2988, 2966, 2918 (C-H, aliph); 1632 (C=O);
1
1592, 1535 (C=C); 709 (mono-subst). H NMR (DMSO-d6, δ, ppm): 8.67 (t,
J = 5.7 Hz, 1H, 6); 8.49 (d, J = 4.8 Hz, 2H, 10); 7.26 (d, J = 4.8 Hz, 2H, 9);
PO6-5/5. FT-IR-ATR (ZnSe, n, cm-1): 3072, 3039 (C-H, arom); 2970,
2909 (C-H, aliph); 1786, 1719 (C=O); 1622, 1593, 1539, 1498, 1452 (C=C);
6.30 (s, 2H, 3); 5.99 (s, 1H, 1); 4.89 (s, 4H, 11); 4.41 (d, J = 5.9 Hz, 2H, 7). 13
C
NMR (DMSO-d6, δ, ppm): 168.2 (5); 149.5 (10); 149.2 (2); 149.1 (8); 136.0
(4); 122.1 (9); 102.3 (1); 102.1 (3); 41.6 (7).
1
1355 (C-N), 1234 (C-O); 743, 714 (p-subst). H NMR (DMSO-d6, δ, ppm):
9.32 (s, 1H, 6); 8.50 (s, 2H, 10); 8.21 (m, 4H, 17); 8.13 (s, 2H, 3); 7.98 (m,
4H, 18); 7.79 (s, 5H, 1,14); 7.51 (m, 4H, 22); 7.34 (s, 2H, 9); 7.25 (d, 4H, 23);
4.54 (s, 2H, 7). 13C NMR (DMSO-d6, δ, ppm): 166.2, 165.9 (20); 166.1, 165.7
(19); 164.8 (5); 156.1 (24); 149.3 (10); 148.5 (8); 137.5, 137.3 (13); 135.8 (18),
135.6 (2); 133.0, 132.9 (16); 132.6 (4); 132.6, 132.4 (15); 129.3 (22); 129.0
(1); 127.2 (21), 126.3 (3); 124.5, 124.4 (17); 123.8, 123.6 (14); 122.3 (9); 121.9
(11); 119.1 (23); 64.6 (t, 12); 42.0 (7). 19F NMR (DMSO-d6, δ, ppm): -62.9.
PO6-3/7. FT-IR-ATR (ZnSe, n, cm-1): 3068, 3039 (C-H, arom); 2934,
2912 (C-H, aliph); 1780, 1716 (C=O); 1624, 1593, 1500, 1455 (C=C); 1375
(C-N), 1234 (C-O); 744, 714 (p-subst). 1H NMR (DMSO-d , δ, ppm): 9.32 (s,
1H, 6); 8.51 (s, 2H, 10); 8.20 (m, 4H, 17); 8.13 (s, 2H, 3);67.97 (m, 4H, 18);
7.77 (s, 5H, 1,14); 7.51 (m, 4H, 22); 7.34 (s, 2H, 9); 7.25 (d, 4H, 23); 4.54 (s,
2H, 7). 13C NMR (DMSO-d6, δ, ppm): 166.2, 165.9 (20); 166.1, 165.7 (19);
164.8 (5); 156.1 (24); 149.3 (10); 148.6 (8); 137.5, 137.3 (13); 135.8 (18),
135.6 (2); 133.1, 133.0 (16); 132.7 (4); 132.6, 132.4 (15); 129.3 (22); 129.0
(1); 127.2 (21), 126.3 (3); 124.5, 124.4 (17); 123.8, 123.6 (14); 122.3 (9); 122.0
(11); 119.1 (23); 64.6 (t, 12); 42.0 (7). 19F NMR (DMSO-d6, δ, ppm): -62.9.
PO6-0/10. FT-IR-ATR (ZnSe, n, cm-1): 3061, 3039, 3015 (C-H, arom);
2977, 2962 (C-H, aliph); 1784, 1714 (C=O); 1623, 1598, 1541, 1436 (C=C);
1369 (C-N), 1229 (C-O); 744, 711 (p-subst). 1H NMR (DMSO-d , δ, ppm): 8.9
(d, J = 7.9 Hz, 2H, 17); 7.98 (d, J = 7.4 Hz, 2H, 18); 7.77 (s, 2H, 614); 7.50 (d, J
= 8.2 Hz, 4H, 22); 7.25 (d, J = 8.2 Hz, 4H, 23). 13C NMR (DMSO-d6, δ, ppm):
166.2 (20); 166.1 (19); 164.8 (5); 156.1 (24); 137.3 (13); 135.8 (18), 133.0
2.4 Polymers synthesis and spectroscopy characterization
10 mmol of the diamines and 10 mL of DMA were placed into a 50
mL three-necked flask equipped with a mechanical stirrer. The mixture was
vigorously stirred under a nitrogen atmosphere until complete dissolution of
the diamines. Thereafter, the solution was cooled to 0 °C in a water-ice bath,
then 10 mmol of 6FDA and 10 mL of DMA were added. This reaction mixture
was stirred until completing the dissolution of 6FDA. Then, the bath ice-bath
was then removed and the reaction mixture was allowed to stand overnight
(~12 hours) at room temperature. Subsequently, to this solution was added 50
mmol of pyridine and a determined volume of anhydrous acetic anhydride. The
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(16); 132.6 (15); 129.3 (22); 127.1 (21); 124.4 (17); 123.6 (14); 121.9 (11);
119.1 (23); 64.6 (t, 12). 19F NMR (DMSO-d6, δ, ppm): -62.8.
3. RESULTS AND DISCUSSION
3.1 Monomer synthesis
2.5 Films preparation
The synthesis of the PyDA monomer was carried out using the procedure
described in scheme 1. Starting from 3,5-dinitrobenzoyl chloride and pyridin-
4-ylmethanoamine was possible to obtain the dinitro PyDN intermediate
with an 85 % yield. The reaction was carried out in THF, in order to dissolve
precursors and favour the PyDN precipitation. In this case, TEA was used as
a base to neutralize the protons released during the substitution reaction in the
acyl, those that considerably reduce the yield of the reaction. Once finished
the reaction, it was poured into 5 % NaOH aq. solution to allow total PyDN
precipitation. Note that once the synthesis of these types of compounds has
been completed, they are usually poured into acidic water, which for this
case is unfeasible due to the presence of the pyridinyl group, which would be
protonated by making PyDN soluble in the acid solution.
Films were prepared from a solution with 400 mg of co-polymer in 10 mL
of DMF. The solution was filtered through on glass fiber of 3.1 μm porosity
and poured into a 5.5 cm in diameter glass ring, which, in turn, was on a 20 x
20 cm glass square. This system was put on a heating plate and heated to 40
°C for 12 hours. Subsequently, the formed film was removed and placed on
a stainless-steel mesh and dried for 24 hours at 200 °C. Resulting films have
thickness between 35.8 to 85.9 μm.
Figure 1. Digital camera images of polyimides films.
Scheme 1. Synthesis of 3,5-diamino-N-(pyridin-4-ylmethyl)benzamide (PyDA).
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J. Chil. Chem. Soc., 63, Nº 4 (2018)
1
The structure of PyDN was verified by means FT-IR, H and 13C NMR
spectroscopy, and the 135° dept, COSY, HMBC, HMQC techniques. The first
one reveals the existence of the most important groups, such as: the amide
group, by the bands in 1675 (C=O) and 3329 cm-1 (N-H), the nitro groups with
signals at 1540 and 1335 cm-1, and the presence of the aromatics C=C in 1606
cm-1 and 1624 cm-1. On the other hand, the NMR is clear in determining the
nature of PyDN. The figure 2a shows the 1H NMR spectrum where is possible
to observe signals for all hydrogen expected for this dinitro derivative with
their respective assignments. Within these signals, those observed for 6 and
7 stand out, which show a coupling due to a lower exchange of the amide
hydrogen with the solvent.
Figure 2. 1H NMR spectrum (400 MHz, DMSO-d ) a) 3,5-dinitro-N-(pyridin-4-ylmethyl)benzamide and
b) 3,5-diamino-N-(pyridin-4-ylmethyl)benzamide.6
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In the second stage to produce PyDA, hydrazine and Pd/C were used to
selectively reduce the amino groups in ethanol as solvent. Unlike classical
technique for this type diamine synthesis, once the remaining Pd/C was
filtered, the solution was concentrated to dryness. Diethylether 200 mL were
poured over the diamine to eliminate the remains of ethanol present. This was
done since the diamine is soluble in water. On the other hand, recrystallization
or sublimation are generally used in the purification of this type of monomers.
In this case, sublimation was used, because in the recrystallization, which can
be done in water or ethanol, it generates the coloration of the obtained crystals.
Obtaining the PyDA using IR and NMR spectroscopy was also
corroborated. From IR analysis, the appearance of the N-H signals (3412 cm-1
y 3339 cm-1) of the formed amines and the disappearance of the signals at 1335
cm-1 and at 1536 cm-1 of the nitro groups of PyDN were observed. In NMR
analysis, the identification and assignment of the different nuclei was carried
out using all the mentioned techniques. In the figure 2b, spectrum shows nitro
groups conversion to amino generates a displacement of the signals of the ring
hydrogens towards high magnetic fields. This fact was due to the shielding
generated by the amino groups, an effect that can also be seen on the hydrogen
of the amide group. In addition, Figure 3 shows the NMR spectrum of 13C and
HMQC of PyDA which would corroborate the performed assignation.
Figure 3 a:
Figure 3 b:
Figure 3. 13C NMR and HMQC spectrum (400 MHz, DMSO-d6) of 3,5-diamino-N-(pyridin-4-ylmethyl)
benzamide
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J. Chil. Chem. Soc., 63, Nº 4 (2018)
3.2 Polymers synthesis
for around 12 hours to favour the nucleophilic attack of PyDA, which shows
a lower reactivity due to the deactivating effect of the amide group and the
second stage is performed in a usual way for this type of polymerization. The
co-polyimides were precipitated in water and dried at 150 °C overnight to
remove the DMA, giving a 99 % yield in each reaction.
The synthesis of the co-polymers was carried out in two stages as described
in scheme 2. The first stage produces the opening of the anhydrous groups by
action of the amino groups of both diamines to obtain the co-polyamic acid.
In the second stage, using acetic anhydride and pyridine the cyclization and
formation of the imide groups was generated. The first stage was carried out
Scheme 2. Synthesis of co-polyimide (PO6-Z/X).
To remove traces of occluded monomer, the polymers were purified by
re-precipitation from a polymer solution in DMF with ethanol and subsequent
extraction with acetone.
3.3 Viscosity and solubility
Table 1 shows the values of inherent viscosity and solubility tests for each
polymer synthesized, together with the values obtained by Bong et al. for PO6-
0/10 (polyimide derived from 6FDA and ODA),33 which has a viscosity similar
to that we prepared. These values describe how increasing the amount of ODA
in the polymer chain increases the viscosity, which results in an increase in
molecular weight. As is known, the viscosity of a polymer, among other factors,
depends directly on the size of the chain and based on this fact, it is expected
that polymers having a higher ODA content have longer chains. This tendency
was expected due to the greater reactivity of the ODA monomer compared to
the PyDA, an effect that favours the growth of the polyimide chains.
The solubility tests carried out at room temperature and under heating
show that the polymers are soluble in polar aprotic solvents such as DMSO
or NMP among others, while they are insoluble in protic polar solvents such
as methanol and ethanol. A special behaviour is present, in the case of PO6-
10/0, which shows swelling due to the high content of pyridinyl groups in
protic polar solvents. This same effect is seen with solvents such as acetone
and chloroform, which show a tendency to swell depending on pyridinyl group
content. In the case of acetone, the presence of this moiety favours the swelling
and in the case of chloroform it decreases. It is also observed that a similar
distribution of fragments of both diamines (PO6-5/5) show solubility in THF
and that the polymers are insoluble in apolar solvent as Et2O.
The structural determination of the repetitive unit of the co-polymers was
carried out using the spectroscopic techniques previously described for the
monomers, in which the clear presence of the desired polymers was observed.
FT-IR spectroscopy allows to distinguish the presence of the imide groups
formed during the polymerization with the presence of two bands at 1714 and
1784 cm-1, which indicate the presence of amidic C=O. The rest of the bands
agree with those presented in the monomers, indicating that the structure of
them did not undergo modifications during the polymerization process.
On the other hand, the NMR spectroscopies account for the expected
nuclei for the polymers allowing the assignment of each of them within the
1
chains formed. In figure 4, an example of such assignments in the H and 13
C
NMR spectrum of PO6-5/5 can be seen, which were corroborated using the
dept 135 °, COSY, HMBC and HMQC spectra. This was also done considering
that PO6-10/0 and PO6-0/10 only present the monomers diamine PyDA or
ODA; respectively, which allows to accurately distinguish the presence of each
of the fragments contributed by each monomer in the three co-polymers.
To corroborate that the ODA/PyDA ratio used was transferred to the co-
polymers, the 1H NMR spectrum was recovered after the purification process,
which eliminates the smaller chains. For this, the methylenic hydrogen (H7)
are used as a reference, because it is only in fragment derived from PyDA and
maintains the expected relationship with each of the hydrogens, both in its
fragment and that coming from the monomers ODA and 6FDA.
On the other hand, in the 13C spectrum the signals belonging to the carbons
that are part of the dianhydride moiety have a different magnetic environment,
depending on the fragment of the diamine monomer that is at their side, causing
them to present two signals for each one of these nuclei.
4244

J. Chil. Chem. Soc., 63, Nº 4 (2018)
Figure 4 a:
Figure 4 b:
Figure 4 c:
Figure 4. 1H and 13C NMR spectrum (400 MHz, DMSO-d6) of PO6-5/5.
4245

J. Chil. Chem. Soc., 63, Nº 4 (2018)
Table 1. Inherent viscosity and solubility of PO6-Z/X.
Z/X
10/0
7/3
5/5
3/7
0/10
0/10c
h
inh (dL/g)a
0.30 0.33 0.57 0.53 1.49
1.42
DMSO
DMA
+
+
+
+
*
*
*
*
*
-
+
+
+
+
*
-
+
+
+
+
*
-
+
+
+
+
-
+
+
*
+
-
+
+
+
+
-
DMF
NMP
(CH3)2CO
MeOH
EtOH
THF
-
-
-
-
-
-
-
-
-
+
*
-
+
*
-
*
++
-
+
+
-
Figure 6. DTG curves of PO6-Z/X.
CHCl3
Et2O
*
-
All polymers have a thermal decomposition (TDT10%) above 480 °C and
an onset of thermal degradation (OTD) over 370 °C. These values indicate that
the synthesized polymers are thermally stable, which was expected due to the
high content of aromatic rings and cyclic imide groups in the chains. Likewise,
both the TDT10% and the OTD decrease when the number of pyridinyl groups
increases, which is attributed to the breakdown of the aliphatic portion that
links this lateral moiety with the main chain. This fact is ratified by looking at
the values of the maximum degradation temperature (MDT1) showed in Table
2. MDT1 shows the same trend as the OTD values, decomposition that only the
polymers that contain the pyridinyl group show. In this sense, Table 3 shows
the direct relationship between the decrease in the percentages of pyridinyl
groups in the polymer chains and the decrease in the percentage of mass lost
for the first thermal degradation.
a) c= 50 mg in 10 mL of NMP, at 30 °C, b) c= 10 mg in 1 mL of solvent.
+ soluble at RT, ++ soluble at 40 °C, - insoluble, * swell. c) viscosity and
solubility values described by Bong et al..33
3.4 Thermal properties
Table 2 shows the degradation temperature values obtained from the TGA
thermograms (Figure 5) and DTG analysis (Figure 6), together with the values
of TDT10%, Tg and char yield obtained by Bong et al. for PO6-0/10.33 It is
clear that as the pendant pyridinyl groups increase or decrease the different
degradation temperatures are modifying for co-polymers.
Table 2 shows the degradation temperature values obtained from the
TGA thermograms (Figure 5) and DTG analysis (Figure 6). It is clear that
as the pendant pyridinyl groups increase or decrease the different degradation
temperatures are modifying for co-polymers.
Table 2. Summary of thermal properties of PO6-Z/X.
X/Y
10/0
7/3
5/5
3/7
0/10
0/10f
TDT10% [°C]a
483
509
541
549
565
581
OTD [°C]b
MDT1 [°C]c
MDT2 [°C]c
Char yield [%]d
Tg [°C]e
390
442
564
53
374
439
564
53
378
446
572
56
416
461
572
55
509
-
-
-
564
57
-
57
307
305
299
289
305
322
a) Thermal decomposition at which 10 % weight loss, b) Onset of thermal
degradation, c) Maximum degradation temperature, d) Residual weight when
heated to 900 °C, e) Glass transition temperature, f) TDT10%, Tg and char yield
values described by Bong et al.33
The second thermal degradation observed for the polymers, MTD2, in the
range of 564 to 572 °C, quite similar in degradation temperature, accounts for
the rupture of the main chain, which is comprised mainly of aromatic rings and
cyclic imide groups, the latter being the most prone to degradation.
The last line of Table 2 shows the Tg values for PO6 polymers, where
they show two trends. The first is the increase of the values from PO6-5/5
to PO6-10/0, which is a consequence of increase in the density of imide
groups, higher content of aromatic groups and the greater number of amide
groups present. These originate attractive interactions such charge transfer
complexes, p-stacking and the formation of hydrogen bonds. Figure 7 shows
the interactions contributed by the portion derived from PyDA. The second
trend is the increase in the Tg from PO6-5/5 to PO6-0/10, probably attributable
to the greater degree of packaging that the chains produce due to the lower
number of pyridinyl groups as shown by PO6-0/10 that has the highest Tg of
all the polymers.
Figure 5. TGA thermograms of PO6-Z/X recovered under nitrogen
atmosphere.
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J. Chil. Chem. Soc., 63, Nº 4 (2018)
Table 3. Lost mass and heavy pyridinyl group.
due to the presence of the ether group. Figure 8 shows the diffractograms of
the co-polymers where the amorphous halo observed for all co-polymers with
a maximum at 17 degrees 2q indicates that they do not crystallize. This value is
associated to an average distance, d-spacing, value between neighboring chains
using Bragg`s equation (d = nl /2 sin q). In this particular case l = 1.54 A and d
= 5.2 A represents the average distance between polymer chains.
Then, from the diffractograms it can be concluded that from PO6-5/5 to
PO6-0/10 the polymers have similar d-spacing and that this parameter is not
affected by the PyDA amount.
PO6
10/0
7/3
LM1 (%)a
PG (%)b
9,0
7,5
4,4
2,9
0
14
10
7
5/5
3/7
4
0/10
0
a) Mass lost in the first thermal degradation.
b) Content of pyridinyl groups in the sample.
Figure 8. Diffractograms of PO6-X/Z polymers.
Figure 7. Description of p-stacking and hydrogen bonds between PyDA
fragments
3.6 Surface properties
In order to study the behaviour of the surface of the samples, analyses on
PO6-10/0, PO6-5/5 and PO6-0/10 were carried out.
3.5 Mechanical properties and Wide-angle X-ray diffraction (WAXD)
Table 4 summarizes the values obtained from the tensile tests carried out
on the films manufactured from the synthesized co-polymers, except for the
PO6-10/0 film which was not able to maintain its shape for the analysis. In
addition, the values of Young’s module, tensile strength and elongation at
break of PO6-0/10 obtained by Huang et al. are included as reference.34 Co-
polymers PO6-5/5 to PO6-0/10 show a greater resistance to deformation and
rupture, as well as a greater elongation prior to rupture. The first result is due
to the greater number of interactions resulting from the increase in the size of
the chains, while the elongation of the co-polymers is favored by the greater
number of ether units, which increase the flexibility of the chains.35
3.6.1 Contact angle and surface energy
Table 5 shows the contact angle determination results in the polymers and
the calculations of surface energy (SE) based on the Owens-Wendt-Rabel-
Kaelble method,36-38 using water and methanol as a solvent. The latter was used
because solvents such as n-heptane and toluene were not able to form stable
drops on the surface of the films, product of the strong interaction between the
samples and them.
Table 5. Contact angle and surface energy.
PO6-
H2O
θY (°)a
SE b
Dispersionb
Polarb
Table 4. Summary of mechanical properties of PO6-Z/X.
106.3 ± 2.7
10/0
5/5
15.7
14.0
1.6
Young’s
modulus
(Gpa)
Yield
strength
(Mpa)
Tensile
strength
(Mpa)
Elongation
at break
(%)
MeOH
H2O
54.8 ± 0.7
108.6 ± 2.5
48.8 ± 0.8
108.7 ± 0.1
42.9 ± 1.9
Sample
22.4
25.8
22.1
25.7
0.2
0.1
PO6-7/3
PO6-5/5
PO6-3/7
PO6-0/10
PO6-0/10a
3.9 ± 1.1
2.1 ± 0.9
2.2 ± 0.3
2.5 ± 0.2
1.5 ± 0.0
85.4 ± 3.1
35.5 ± 3.9
65.2 ± 2.1
67.0 ± 3.4
-
120.4 ± 2.4
35.5 ± 3.9
88.8 ± 3.6
75.2 ± 4.3
91.2 ± 0.0
4.5 ± 1.0
1.7 ± 0.4
5.3 ± 1.1
6.8 ± 0.7
15.3 ± 0.0
MeOH
H2O
0/10
MeOH
a Contact angle recorded at rate of 10 μL/s at 25 °C, b Surface energy and
its components in mN/m.
a) Young’s module, tensile strength and elongation at break values of
PO6-0/10 obtained by Huang et al.34
The contact angles recorded for water show that all tested samples have
a hydrophobic surface with values greater than 90 ° (Figure 9). This is due
to the scarce presence of polarized bonds such as O-H or N-H, which would
form hydrogen bonds with water and also to the greater amount of dispersion
forces contributed by the high aromatic content. The comparison of the values
of surface energy and its components obtained for the water-surface systems,
show that a higher content of pyridinyl groups along the chain increases the
hydrophilic character of the surfaces, which is attributed to the hydrogen-bond
type interactions contributed by the amide and pyridinyl groups.
The obtained values for PO6-7/3 indicate that this material is resistant to
deformation. This would be associated to the capacity of p-stacking, hydrogen
bonds and charge transfer interactions in comparison with the other co-
polymers, while PO6-5/5 shows a minimum value for all mechanical properties
indicating a dilution of interactions and a possible formation of two separated
phases. Lower concentrations of pyridinyl groups increases yield strength,
tensile strength and deformation ability since they are rich in the ODA diamine
which dilutes further the pyridinyl interactions and imparts some flexibility
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J. Chil. Chem. Soc., 63, Nº 4 (2018)
Figure 9. Example of droplet profiles on polymers. a) Water on PO6-5/5, b) Methanol on PO6-5/5
3.6.2 AFM and FE-SEM microscopy
that are typical of the manufacturing process used and the usual behaviour for
these type of polymers.39-41 Furthermore, it is possible to see small cavities
on the surface of the films due to the formation of solvent droplets during the
evaporation process.42
To know the surface morphology, FE-SEM and AFM microscopy was
performed. Images obtained in these analyses are shown in figures 10 and
11 respectively. In Figure 10, it is seen that all samples form uniform films,
Figure 13. FE-SEM images obtained at 5 μm, supported by silicon wafer. a) PO6-10/0, b) PO6,-5/5 c) PO6-0/10
The observed behaviour by FE-SEM is confirmed by the profiles obtained from AFM analysis (Figure 11) where the resulting images for the topography of
PO6-5/5 is shown as an example. These images ensure that the films formed will be able to offer uniform contact surfaces and little rough.
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J. Chil. Chem. Soc., 63, Nº 4 (2018)
Figure 14. 2D and 3D images of the surface of PO6-5/5 obtained by AFM
13. J.M. Gohil, P. Ray, Sep Purif Technol, 181, 159 (2017).
4. CONCLUSIONS
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A set of co-polyimides derived from the 6FDA dianhydride having varying
amounts of ODA and PyDA diamines were synthesized and characterized.
The new materials showed a dependence of their specific properties on the
proportion of ODA and PyDA used in the synthesis process. Among which
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5. AGRADECIMIENTOS ACKNOWLEDGMENT(S)
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In memory to the Dr. José Luis Santiago-Garcia, Rest in peace.
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