Novel flame retardant poly(thiourea-sulfone-imide)s for high temperature applications: Synthesis and characterization

Article abstract of DOI:10.3906/kim-1207-30

The synthesis and polymerization of a novel diamine monomer, 1,1-(sulfonylbis (4,1-phenylene)) bis (thiourea) (SPT), with various aromatic dianhydrides were carried out. In addition, the fabrication of poly(thiourea-sulfone-imide)s (PTSIs) with noble ther

Full text of DOI:10.3906/kim-1207-30

Turk J Chem
Turkish Journal of Chemistry
(2013) 37: 946 – 958
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1207-30
Research Article
Novel flame retardant poly(thiourea-sulfone-imide)s for high temperature
applications: synthesis and characterization
Ghulam WARIS,1,2 Humaira Masood SIDDIQI,1,∗ Lance James TWYMAN,2
Rizwan HUSSAIN,3 Zareen AKHTER,1 Muhammad Saeed BUTT1,3
1Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
2Department of Chemistry, University of Sheffield, UK
3NESCOM, Islamabad, Pakistan
Received: 09.07.2012
•
Accepted: 06.06.2013
•
Published Online: 04.11.2013
•
Printed: 29.11.2013
Abstract:The synthesis and polymerization of a novel diamine monomer, 1,1-(sulfonylbis (4,1-phenylene)) bis (thiourea)
(SPT), with various aromatic dianhydrides were carried out. In addition, the fabrication of poly(thiourea-sulfone-imide)s
(PTSIs) with noble thermal properties and flame retardancy was conducted. The structures of SPT and polymers were
characterized by FT-IR, 1 H NMR, and 13 C NMR spectroscopy along with elemental analysis. Crystallinity, organosolu-
bility, inherent viscosity, and gel permeation chromatographic measurements were taken as well. The amorphous nature
was exhibited by PTSIs containing C=S and –SO2 - moieties in the backbone that were readily soluble in highly polar
organic solvents. The inherent viscosities of PTSIs were 0.89–1.13 dL g−1 and molecular weights were 87,000–96,550
g mol−1 , respectively. Their thermal stability was studied in terms of temperature at 10% weight loss, which ranged
between 478 and 526 ◦ C under inert atmosphere. Polyimides had a glass transition temperature around 253–268 ◦ C
depending on the dianhydrides used. The flame retardant properties of PTSIs were studied in terms of limiting oxygen
index values and were measured in the range of 50–56.
Key words: Poly(thiourea-sulfone-imide)s, crystallinity, flame retardancy, GPC measurements
1. Introduction
Aromatic polyimides have been acknowledged as high performance polymers having excellent potential for use
in electronic devices, aerospace composites, aircraft wire, and cable coatings and materials for gas separation
membranes, thanks to their high inherent thermal and thermo-oxidative and dimensional stability, marvelous
electrical and mechanical properties, and high radiation and solvent resistance.1−5 In contrast, their insoluble
nature and high softening temperatures have often restricted their use for such applications. Chain rigidity,
strong intermolecular interactions, and chain packing ability are the significant reasons for the insoluble and
nonmelting character of solely aromatic polyimides.6,7 The incorporation of bulky substituents,8−13 flexible
linkages, fluoro groups,14 and noncoplanar15 and nonsymmetric moieties16 into the rigid polymer backbone
are the most successful ways to overcome the above-mentioned problems without sacrificing the inherent
characteristics of these polymers.
The introduction of thiourea moieties into the backbone of polymer functions as indispensable scaffolds
in material chemistry belonging to electroluminescent organic dyes.17,18 Thiourea containing polymers are
∗Correspondence: humairas@qau.edu.pk
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important in agricultural and pharmaceutical chemistry and the presence of coordination sites acts as a support
for immobilized catalysts.19−21 In advance technological applications, they are applied in the field of anion
sensing waste recovery of anions through solid-phase extraction and anion membrane for purification and
synthetic purposes.22,23 Thiourea-based polymers can be used as coordinating resins for selective binding of
heavy metal cations such as Hg(II) and Pb(II) from acidic as well as neutral solutions. Thus, they are promising
materials for the elimination of environmentally toxic Hg(II) and Pb(II) cations.24,25
Several efforts have been made to fabricate fire-resistant polymers by using monomers containing sulfur,
nitrogen, phosphorous, sulfone, and boron units and consequently the flame retardancy of sulfur-containing
groups has been investigated.26,27 An efficient way to improve thermal stability and flame retardancy is to
introduce thiourea and sulfone linkages in the backbone of a polymer. In this connection, we have explored
the nonflammability and thermal stabilization of poly(thiourea-sulfone-imide)s (PTSIs) originating from the
presence of SO2 and C=S linkages. This paper presents the synthesis of polyimides with better processability
and solubility from a novel aromatic monomer, 1,1-(sulfonylbis(4,1-phenylene))bis(thiourea)(SPT), containing
1 sulfone and 2 thiourea groups. It was especially considered a potential monomer for polyimides, imparting
solubility and better thermal stability. The structure property relationship of thiourea substituted polymers
was investigated by synthesizing polyimides from SPT and 4 commercially available dianhydrides through a
2-stage process. Hence SPT was found to be a suitable candidate to produce processable PTSIs with excellent
thermal stability as well as low flammability.
2. Experimental
2.1. Materials
Bis(4-aminophenyl) sulfone, acetic anhydride (Ac2 O; 98%), pyridine (99.8%), pyromellitic dianhydride (PMDA;
97%), 3,3′ ,4,4′ -biphenyltetracarboxylic dianhydride (BPDA; 98%), 4,4′ -oxydiphthalic anhydride (ODPA; 98%),
4,4-(hexafluoroisopropylidene)diphthalic anhydride (6FDA; 99%) and N,N’-dimethylacetamide (DMAc; 99%)
were purchased from Aldrich and were utilized without further purification. Ammonium thiocyanate (98%),
N, N′ -dimethylformamide (DMF; 99%), and tetrahydrofuran (THF; 99.5%) were purchased from Fluka. N -
methylpyrrolidone (NMP; 99%) and dimethylsulfoxide (DMSO; 99%) were provided by Merck.
2.2. Measurements
The recording of NMR spectra was done at room temperature using a BRUKER spectrometer operating at
300.13 MHz for 1 H NMR and at 75.47 MHz for 13 C NMR. Deutrated dimethyl sulfoxide (DMSO-d6) was
used as solvent for analysis with tetramethylsilane as an internal reference. Attenuated total reflectance-
Fourier transform infrared (ATR-FTIR) spectra (400–4000 cm−1) of monomers and polymers were recorded
at room temperature on a Bio-Rad Excalibur FTIR, model FTS 3000 MX. Melting points were recorded on
an Electrothermal 1A9000 series digital melting point apparatus. Inherent viscosities were measured with
a Gilmount falling ball viscometer (Vemon Hills, IL, USA). Thermal and differential scanning calorimetry
(DSC) analyses were carried out with a PerkinElmer TGA-7 (Waltman, MA, USA) and Netzsch DSC-404C
(Selb, Germany). Elemental analysis was carried out with a PerkinElmer CHNS/O 2400. Wide-angle X-ray
diffractograms were obtained with an X’Pert PRO 3040/60 diffractometer (Eindhoven, Netherlands). Moisture
absorption was determined through the weighing of the changes in dried pellets before and after immersion in
distilled water at 25 ◦ C for 24 h. A VISCOTEK TDA MODEL 300 calibrated with a mixture of polystyrene
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WARIS et al./Turk J Chem
standards and GPC software was used to determine the number-average molecular weight (Mn), weight average
molecular weight (Mw), and polydispersity of the polyimides. Dimethylformamide (DMF) with 0.1% lithium
bromide (LiBr) was used as an eluent at a flow rate of 0.5 mL/min at 70 ◦ C. A refractive index detector was
used. Tensile properties of the polyimide films were measured according to ASTM D638 standard. Tensile
testing was performed on an Instron model 4301 universal testing machine having a crosshead speed of 5 mm
min−1 at 25 ◦ C. The dielectric constant of the polyimide films was determined on an Agilent 4291B instrument
at a frequency of 100 MHz at 25 ◦ C.
A Stanton Redcroft FTA flammability unit (nitrogen gas, oxygen gas, and precision pressure regulator
systems) was used to determine the limiting oxygen index (LOI) of the PTSIs. LOI values were measured
according to ASTM D2863, which is a method to determine the minimum concentration of oxygen in an
oxygen/nitrogen mixture that will support a flaming burn in the specimen.
2.3. Synthesis of 1,1-(sulfonylbis(4,1-phenylene))bis(thiourea) (SPT)
Bis(4-aminophenyl)sulfone 2.48 g (0.01 mol), 16 mL of concentrated hydrochloric acid, ammonium thiocyanate
1.52 g (0.02 mol), and 120 mL of deaerated water were thoroughly mixed and heated in a porcelain dish for
2–3 h on a steam bath and allowed to cool to room temperature.34 The mixture was then again evaporated
gradually to dryness over 8–10 h. The product obtained was boiled with a small quantity of charcoal in ethanol,
filtered, and cooled. Finally, 1,1-(sulfonylbis(4,1-phenylene))bis(thiourea) thus obtained was recrystallized from
methanol and dried under vacuum at 70 ◦ C for 24 h. Yield: 86%. Melting point: 157–158 ◦ C.
Elemental analysis: C14 H14 N4 O2 S3 (MW =366.48) in wt % Calc. C = 45.88, H = 3.85, N = 15.29,
S = 26.25 and found to be C = 46.12, H = 3.58, N = 15.02, S = 26.68. FT-IR: v¯ (cm−1): 3452–3329 (N-H
stretching), 1598 (N-H bending), 1412 (C-N stretching), 1327 (SO2 asymmetric stretching), 1134 (C=S), 1112
(symmetric stretching), 1 H NMR (DMSO-d6, δ, ppm and J , Hz): 4.1 (s, 4H, Ha) 9.95 (s, 2H, Hb), 7.78 (d,
4H, Hc ,), 7.95 (d, 4H, Hd). 13 C NMR (DMSO-d6, δ, ppm): 164.3 (C1), 144.8 (C2), 120.5 (C3), 130.2 (C4),
135.3 (C5).
2.4. Preparation of polymers
The synthesis of the polyimides was carried out in a 100-mL 2-necked round-bottom flask equipped with a
magnetic stirrer, nitrogen gas inlet tube, and a calcium chloride drying tube charged with 0.01 mol of the
diamine (SPT) and 50 mL of dry NMP. The mixture was stirred at 0 ◦ C for 0.5 h. Subsequently, 0.01 mol
of the dianhydride was added and the mixture was again stirred at 0 ◦ C for 1 h. The stirring was continued
at room temperature for 24 h. Poly (amic acid) was precipitated by pouring the contents of the flask into 200
mL of a water–methanol (3:1) mixture. The precipitate was then filtered, washed with hot water, and dried
overnight under vacuum at 70 ◦ C.
The conversion of poly (amic acid) into polyimide was achieved through chemical cyclization.18 For this
purpose, a 100-mL 2-necked round bottom flask equipped with a magnetic stirrer and a nitrogen gas inlet tube
was charged with 1.0 g of poly (amic acid) and 5 mL of dry DMAc. The mixture was stirred and 5 mL of acetic
anhydride and 2.5 mL of pyridine were added. After that, the mixture was stirred for 0.5 h and then slowly
heated to 80 ◦ C and held for 6 h at the same temperature. The mixture was then cooled and poured into
water. The precipitate thus obtained was filtered, washed with hot water and methanol, and dried overnight
under vacuum at 120 ◦ C.
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The spectral data of synthesized PTSIs validate their expected structures. FTIR: 1786 and 1721 cm−1
(typical of imide carbonyl asymmetric and symmetric stretching), 1361 cm−1 (C-N stretching), 1097 and 723
cm−1 (imide ring deformation). 1 H NMR (DMSO-d6, δ, ppm and J , Hz): 7.75–7.96 (2 doublets corresponding
to the aromatic protons of SPT diamine unit), 10.02–10.1 (singlet assigned to N-H), other signals in the aromatic
region are due to aromatic protons of the anhydride unit.
3. Results and discussion
3.1. Synthesis of monomers and polymers
The main objective of this study was the preparation of highly soluble and processable polyimides without
sacrificing their thermal and physical properties. A novel monomer containing their linkage and its polymers
was designed and synthesized successfully. Characterization of the structure of SPT was done by elemental
analysis, and FTIR, 1 H NMR, and 13 C NMR spectroscopy. Figure 1 presents the FTIR spectrum of SPT,
where the absorption bands in the region of 3452–3329 cm−1 show the presence of NH2 (N-H asymmetric
and symmetric stretching); those at 1598 cm−1 (N-H bending), 1412 cm−1 (C-N stretching), and 1134 cm−1
(C=S) confirm the presence of a thioamide group.
4000
3000
2000
1000
Wavenumber cm-1
Figure 1. FTIR spectrum of SPT.
The 1 H NMR spectrum of SPT (Figure 2) showed 2 singlets at 4.1 and 9.95 ppm, which were assigned
to the -NH- group attached to the aromatic ring and the –NH2 group of thioimide, respectively. Two doublets
at 7.95 and 7.78 ppm were assigned to aromatic protons directly attached to adjacent ipso carbon to sulfone
and thioamide group, respectively. The 13 C NMR spectrum of SPT also supported the proposed structure as
presented in Figure 3. A signal at 164.3 ppm was assigned to thioamide carbon and another downfield signal at
144.8 ppm was assigned to the aromatic carbon directly attached to the nitrogen atom. The structure of SPT
was also confirmed by elemental analysis, which was in good agreement with the calculated values.
A good agreement of all the spectroscopic data was noted with the expected structure. Thus, the
diamine building blocks were successfully transmitted to the polyimides, resulting in the design and synthesis
of an entirely unique aromatic diamine with built-in thiourea and sulfone units.
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S
S
O
c
d
a
b
NH2
N
H
S
N
H
H2N
O
c
d
a
b
Figure 2. 1 H NMR spectrum of SPT.
S
1
S
O
3
4
2
5
NH2
N
H
S
N
H
H2N
4
3
O
5
2
1
Figure 3. 13 C NMR spectrum of SPT.
Highly soluble PTSIs were prepared from SPT and 4 commercially available dianhydrides, i.e. BPDA,
PMDA, ODPA, and 6FDA, through a 2-step chemical imidization as shown in the Scheme. A chemical method of
polymerization was followed because of its advantages over thermal imidization. Thermally imidized polyimides
show poor solubility and the reason might be the presence of intermolecular crosslinking produced during the
thermal imidization process.41 Hence polyimides were synthesized by reacting stoichiometric amounts of SPT
monomer with aromatic dianhydrides in NMP. Stirring at 0–5 ◦ C for 24 h yields poly(amic acid)s and then
heating at 80 ◦ C in a mixture of Ac2 O/pyridine leads to the corresponding polymers.
The chemical structures of the polyimides were confirmed by spectroscopic techniques and elemental
analysis; the results are reported in Table 1. Inherent viscosities, yields, Mw , Mn , and polydispersities of
PTSIs were also investigated. The FTIR spectra of the polyimides shown in Figure 4 exhibit characteristic
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WARIS et al./Turk J Chem
Scheme. Synthesis of SPT and PTSIs.
SPT-BPDA
SPT-PMDA
SPT-ODPA
SPT-6FDA
4000
3000
2000
1000
500
cm–1
Wavenumber
Figure 4. FTIR spectra of PTSIs.
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WARIS et al./Turk J Chem
absorption bands of the 5-membered imide ring around 1783–1777 cm−1 , 1724–1716 cm−1 (typical of imide
carbonyl asymmetric and symmetric stretching), 1363–1359 cm−1 (C-N stretching), 1103–1097 cm−1 , and 734–
723 cm−1 (imide ring deformation). In the 1 H NMR spectra, the characteristic singlet of the amino group at 4.1
ppm disappeared and the singlet corresponding to the thioamide proton of SPT at 9.95 ppm shifted downfield
around 10.01–10.08 ppm due to the electron-withdrawing effect of carbonyl functionality of dianhydride. These
observations authenticated the successful synthesis of polyimides.
Table 1. Polymer characterization.
Elemental analysis
Polymer
Composition of
repeating unit
–1
Frequency (cm ) 1H NMR(DMSO-d6, ppm)
C
H
N
S
Calc.
57.68
2. 58
8.97
15.40
1783, 1724, 1363,
1097, 731
7.79/4H, 7.96/4H, 8.06/2H,
8.1/2H,8.2/2H,10.04/2H
SPT-BPDA
C30H16N4O6S3
C24H12N4O6S3
C30H16N4O7S3
Found
57.84
2.77
9.17
15.21
Calc.
52.55
2.20
2.57
2.52
10.21
10.68
8.57
17.54
17.91
15.01
1778, 1719, 1359,
1101, 723
7.77/4H, 7.95/4H, 8.87/2H,
10.08/2H
SPT-PMDA
Found
52.67
Calc.
56.24
1777,1716,
1360,1103, 734
7.75/4H, 7.94/4H,8.05/2H,
8.11/2H, 8.21/2H, 10.02/2H
SPT-ODPA
SPT-6FDA
Found
56.69
2.57
8.45
15.18
Calc.
51.16
Found
51.48
2.08
2.14
7.23
7.39
12.42
12.27
7.76/4H,7.92/4H, 8.02/2H,
8.12/2H, 8.2/2H
10.1/2H
1781, 1722, 1361,
1097, 733
C33H16FN4O6S3
6
3.2. Organosolubility
The solubilities of the PTSIs synthesized by chemical imidization were investigated in various organic solvents
by dissolving 30 mg of the polymer in 1 mL of solvent (3% w/v) at different temperatures. The results are
reported in Table 2. The PTSIs are soluble in polar aprotic solvents (DMSO, DMF, DMAc, and NMP) at room
temperature. However, they are soluble on heating in less polar solvents like m-cresol and are slightly soluble in
common solvents such as THF but not SPT-BPDA. The solubility of these polyimides depends on their chemical
structure and is in the order SPT-6FDA > SPT-ODPA > STP-PMDA > SPT-BPDA. PTSIs obtained from
ODPA and 6FDA are highly soluble due to the presence of a more flexible and bent chain structure. The
presence of ether and hexafluoroisopropylidine units in SPT-ODPA and SPT-6FDA, respectively, increases
their solubility at room temperature. This improvement in solubility might be attributed to the presence of a
bulky trifluoromethyl group, which increased the disorder in the chains, hindered the dense chain packing, and
increased the free volume, reducing the interchain interactions and causing better solvent molecule penetration
into the polymer chain. Moreover, the solubility behavior of 6FDA- and ODPA-based polyimides may also be
credited to the capability of hydrogen bonding.35 However, PTSIs with BPDA and PMDA have lower solubility,
which may be due to the rigidity of their aromatic structure and dense chain packing.
In general, the superior solubility of PTSIs is due to the presence of C=S and –SO2 -, because sulfone
groups have very different bond angles (around 106◦) as compared to aromatic ether (around 121◦) groups.
Thus the presence of a kinked sulfone group in the polymer backbone might increase the intermolecular freedom
and polymer polar solvent interactions, resulting in improved solubility in organic polar solvents.36
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Table 2. Solubility of the polymers.
Polymer
DMSO DMF
NMP
DMAc m-Cresol THF H2SO4
SPT-BPDA + + + + + + + + + + + +
SPT-PMDA + + + + + + + + + + + +
SPT-ODPA + + + + + + + + + + + + ++
SPT-6FDA + + + + + + + + + + + + ++
–
+
–
++
++
+ + +
+ + +
+
+
+
Qualitative solubility was determined at 3.0% (w/v) + + + = soluble at room temperature; ++
soluble on heating; + = slightly soluble on heating; – = insoluble
=
3.3. Moisture absorption of polyimides
The moisture absorption behavior of polyimides is dependent on their chemical structures, which increases the
dielectric constant and decreases the mechanical strength.28 In addition, the corrosion of integrated circuits
and ionization of ionic impurities are due to the presence of moisture content. The moisture uptake of PTSIs
was determined by using the following equation:
Moisture absorption (%) = (W/Wo – 1) × 100,
where W is the weight of the sample after immersion in water at 25 ◦ C for 24 h and Wo is the weight of
the sample dried in a vacuum oven for 24 h. PTSIs exhibited relatively low moisture absorption due to the
hydrophobic nature of thiourea moiety. The moisture absorption behavior of PTSIs is illustrated in Table 3,
which shows that SPT-6FDA absorbs the least moisture due to synergy in the hydrophobic nature of thiourea
and fluorine moieties.
3.4. Viscometry and molecular weight analysis
The inherent viscosities of thiourea-based polyimides were measured in NMP at 30 ◦ C and the results are
summarized in Table 3. The inherent viscosities were found to be in the range of 0.89–1.13 dL/g, indicating the
formation of medium to high molecular weight polymers.37,38 As expected, SPT-6FDA having a hexafluoroiso-
propylidene group displayed the lowest ηinh among all the polyimides. This is because the presence of these
bulky groups in the polymer backbone hinders the close packing of the polymer chain. PTSIs with rigid BPDA
and PMDA units presented higher inherent viscosity due to higher chain stacking efficiency. GPC measurements
were carried out to determine weight average molecular weight (Mw), number-average molecular weight (Mn),
and polydispersity values. Mn and Mw values were found in the range of 54,037–61,497 and 87,000–96,550
with respect to polystyrene standards, respectively. The ratio of Mw /Mn was about 1.57–1.61. BPDA-derived
polyimides showed the highest values of Mn and MW among these polymers and 6FDA-derived polyimides had
the lowest ones. GPC results validated that the diamine monomer, SPT, bearing thiourea and sulfone linkages
had very good polymerization activity to form polyimides with reasonable ηinh Mn and Mw .
3.5. X-ray diffraction
The crystallinity of PTSIs was examined by X-ray diffraction in the region of 2θ = 5–50◦ , using the polyimide
powdered samples. The results are shown in Figure 5. Regardless of the anhydrides employed, all of the PTSIs
exhibited amorphous patterns in the WAXD analysis. The X-ray diffraction curve of polyimide based on
SPT-BPDA and SPT-PMDA exhibit a broad peak at around 20◦ . This might be interpreted as the region
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WARIS et al./Turk J Chem
of crystallinity. Polyimides based on SPT-ODPA and SPT-6FDA appear amorphous as expected, because
the presence of flexible linkages induces looser chain packing. Furthermore, the improved solubility of these
polyimides is a reflection of their entirely amorphous nature.
Table 3. Inherent viscositya , Mn , Mw , polydispersity, and yield of polymers.
Mn
Mw
Mw/Mn Water
Yield
Polymer
ηinhdL/g
(g/mol) (g/mol)
uptake (%) (%)
SPT-BPDA 1.13
SPT-PMDA 1.1
SPT-ODPA 0.93
61,497
59,248
56,062
54,037
96,550
93,612
89,700
87,000
1.57
1.58
1.6
0.19
0.17
0.22
0.11
83
87
87
92
SPT-6FDA
0.89
1.61
aMeasured in NMP at a concentration of 0.3 g dL−1 at 30 C.
◦
SPT- BPDA
SPT - PMDA
SPT - ODPA
SPT - 6FDA
10
20
30
2θ(°)
40
50
Figure 5. XRD pattern of PTSIs.
3.6. Mechanical and electrical properties
All the polyimides resulted in clear, flexible, and tough films after processing. The mechanical properties such
as tensile strength and tensile modulus as well as elongation at break for these polyimides films were notable.
The tensile strength of polyimide films varied between 119 and 127 MPa, elongation at breakage was found to
be 7%–13%, and tensile moduli ranged between 2.31 and 2.67 GPa. These values are comparable to those of
commercially available polyimides including Ultem 1000 (tensile strength, 105 MPa; tensile modulus, 3.0 GPa;
elongation at break, 60%) and Ultem 6000 (tensile strength 103 MPa; tensile modulus, 3.03 GPa, elongation at
break, 30%).39
The dielectric constants values of the polyimide films were also determined and the results are summarized
in Table 4. The dielectric constant values are dependent on chemical structure and might also be affected by
molecular packing efficiency. Polyimide films with low molecular packing lead to a decrease in the number
of polar groups in a unit volume. Polyimide films based on SPT-6FDA and SPT-ODPA have the lowest
dielectric constant due to the presence of bulky –C(CF3)2 - groups, which cause variation in hydrophobicity,
free volume, and total polarizability.29 SPT-ODPA has ether linkages leading to a decrease in dipole number
density. Furthermore, dielectric constant varies with frequency and it can be attributed to the polarization
mechanism.
Water absorption has a very strong effect on the dielectric constant of polyimides. The absorbed water can
ionize the ionic impurities and deteriorate the mechanical properties of polymeric materials. As discussed earlier
(Table 3), the water uptake values ranged between 0.11 and 0.22 in a decreasing order of SPT-6FDA < SPT-
PMDA < SPT-BPDA < SPT-ODPA. SPT-6FDA has the smallest value because the hexafluoroisopropylidene
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group acts as hydrophobic hindrance in the polyimide. The low water uptake and dielectric constant of SPT-
based polyimides suggest a potential utility of these materials in the microelectronics industry.
Table 4. Mechanical and electrical properties of polyimides.
Tensile strength Tensile modulus Elongation at Dielectric constant
Polymer
(MPa)
119
SPT-ODPA 120
SPT-PMDA 125
SPT-BPDA 127
(GPa)
2.31
2.55
2.63
2.67
breakage (%) at 100 MHz
SPT-6FDA
11
9
6.2
7
2.89
3.04
3.1
3.21
3.7. Thermal stability of polymers
Thermal properties of the PTSIs were investigated using TGA and DSC at a heating rate of 10 ◦ C min−1
.
The results are shown in Table 5. The thermal stability of the polyimides was determined in terms of
temperature at 10% weight loss (T10), maximum degradation temperature (Tmax), and residual weight at
800 ◦ C. Thermograms of the polymers are similar to each other and no significant weight loss below 480 ◦ C
was observed (Figure 6a). The PTSIs exhibit T10 in the range 478–526 ◦ C, Tmax of 571–605 ◦ C, and char
yields of polyimides at 800 ◦ C were about 48%–54%. A comparison of maximum degradation temperature
and residual weight at 800 ◦ C of the synthesized polyimides revealed that the polyimide derived from BPDA
had the maximum degradation temperature and residual weight at 800 ◦ C compared to those derived from
PMDA, ODPA, and 6FDA. A very rigid dianhydride monomer and impenetrable polymer chain packing might
be the reasons. The 10% weight loss above 450 ◦ C showed the effectiveness of the introduction of diacid BPDA
for improving the thermal stability of the polyimides. The rigid BPDA unit and increased chain stiffness can
be credited for this increase in thermal stability. On the other hand, diverse packing density of the polymer
aggregation and the interaction of the polymer chain can be the causes for the lower thermal stability of the
polyimides containing the flexible units than those with the BPDA and PMDA unit.26 Reduction in the chain
interactions, poor packing of the polymer chain, and lower thermal stability are due to the presence of bulky
CF3 groups.
The glass transition temperatures of PTSIs were about 253–268 ◦ C (Table 5). Tg was taken as the
midpoint of the change in the slope of the baseline in the DSC curve (Figure 6b). The Tg values of the
polyimides depend on the structure of the polymer and decrease with the increase in flexibility of the polyimide
backbone.25 ODPA- and 6FDA-based polyimides showed lower Tg values than those derived from BPDA and
PMDA due to the presence of flexible linkages between the phthalimide units, leading to intrasegmental mobility
and also disrupting intersegmental chain packing. SPT-BPDA showed the highest tag because of the presence
of the highly rigid biphenyl unit.
The flame retardant properties of the PTSIs were studied in terms of char yields and LOI values (Table
5). It is interesting to note the dependence of thermal stability on the polymer structure and the introduction of
sulfone and thiourea linkages into the main chain. It has also been reported that the weight of thermal residue up
to 700 ◦ C or higher is related to flame retardancy of a polymer.40 In addition, the absence of significant weight
loss up to 500 ◦ C shows that the PTSIs have high fire resistance and thermal stability. Increasing char yield at
800 ◦ C could limit the production of combustible gases and thermal conductivity of the polymers, consequently
limiting the polymer’s flammability.30,31 LOI values of PTSIs were found in the range of 50–56. These values
are larger than those reported for phosphorous-containing and other flame retardant polyimides.32,33 Moreover,
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the LOI values of PTSIs are higher than those without thiourea moiety in the polymer backbone as reported in
our previous studies.40 There appears to be a correlation between LOI and polymer structure. The introduction
of sulfur-containing thiourea and sulfone groups is found to significantly increase the flame retardancy of the
polyimides without reducing their thermal stability. Hence, the thiourea functionality in the PTSIs backbones
renders these polymers flame retardant.
Table 5. Thermal analysis data of PTSIs in a nitrogen flow.
Polymer
Specific heat capacity
Tg (◦C) T10 (◦C)b Tmax (◦C)c LOI Yc at
◦
a
◦
at 200 C (J g−1K−1
)
800 C (%)d
SPT-BPDA 1.7888
SPT-PMDA 1.7299
SPT-ODPA 1.6472
268
262
258
253
526
510
487
478
605
592
577
571
56
54
50
53
54
51
49
48
SPT-6FDA
1.5787
aMeasurement by DSC at a heating rate of 10 C/min.
bTemperature for 10% weight loss.
◦
cMaximum decomposition temperature obtained from differential curves.
dChar yield (weight of the polymer remaining).
100
1
0
SPT-BPDA
SPT-PMDA
SPT-ODPA
SPT-6FDA
80
60
40
SPT-BPDA
SPT-PMDA
SPT-ODPA
SPT-6FDA
-1
100
200
300
200
400
600
800
Temperature (°C)
(b)
Temperature (°C)
(a)
Figure 6. TGA curves of PTSIs obtained at heating rate of 10 ◦ C min−1 in an inert atmosphere of nitrogen (a). DSC
thermograms of PTSIs obtained at a heating rate of 10 ◦ C min−1 under nitrogen flow (b).
4. Conclusion
A series of novel PTSIs containing flexible C=S and -SO2 - moieties were synthesized from SPT and aromatic
dianhydrides using 2-step chemical imidization. In this way, SPT was used as starting material for the synthesis
of new highly thermally stable and high molecular weight PTSIs.
In general, the synthesized polyimides showed excellent solubility and medium to high inherent viscosity
as well as molecular weight. These polyimides exhibited an amorphous nature and a fine balance of properties
(thermal stability and solubility). PTSIs were observed to have excellent electrical properties with dielectric
constants of 2.89–3.21 and outstanding mechanical properties with tensile strengths of 119–127 MPa and tensile
modulus values of 2.31–2.67 GPa. Thus, the introduction of thiourea and sulfone groups into the main chain
of PTSIs was successful and SPT diamine monomer can be considered as a candidate for the fabrication of
processable as well as heat resistant materials. These pronounced favorable properties of PTSIs make them
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attractive for the electronics and aerospace industries. The reasons for such stability in these polymers might
originate primarily from their chemical structure, which is composed of units usually known to be highly flame
retardant, such as sulfur containing thiourea and sulfone moieties.
Acknowledgments
One of the authors (GW) is much indebted to the Higher Education Commission of Pakistan (HEC) for financial
support in this work under the Indigenous 5000 PhD Fellowship Scheme Batch-IV, and the Department of
Chemistry, Quaid-i-Azam University, Islamabad, Pakistan, for providing the necessary research facilities.
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