Synthesis and characterization of organo-soluble thioether-bridged polyphenylquinoxalines with ultra-high refractive indices and low birefringences

Article abstract of DOI:10.1016/j.polymer.2010.06.035

Two aromatic tetraketones, 4,4'-thiobis[(p-phenyleneoxy)benzil] (STK, 1) and 4,4'-thiobis[(p-phenylenesulfanyl)benzil] (3STK, 2) were synthesized by the nitro nucleophilic substituent reactions of 4-nitrobenzil and corresponding diol compounds. The two te

Full text of DOI:10.1016/j.polymer.2010.06.035

Polymer 51 (2010) 3851e3858
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of organo-soluble thioether-bridged
polyphenylquinoxalines with ultra-high refractive indices and low birefringences
*
*
Cheng Li, Zhuo Li, Jin-gang Liu , Xiao-juan Zhao, Hai-xia Yang, Shi-yong Yang
Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Two aromatic tetraketones, 4,40-thiobis[(p-phenyleneoxy)benzil] (STK, 1) and 4,40-thiobis[(p-phenyl-
enesulfanyl)benzil] (3STK, 2) were synthesized by the nitro nucleophilic substituent reactions of
4-nitrobenzil and corresponding diol compounds. The two tetraketones were polymerized with three
aromatic tetraamines, including 3,30-diaminobenzidine (a), 3,30,4,40-tetraaminodiphenylether (b) and
3,30,4,40-tetraaminodiphenylsulfone (c), respectively to afford six thioether-bridged polyphenylquinoxa-
lines (PPQs) e PPQ-1ae1c and PPQ-2ae2c. The obtained PPQs exhibited good solubility not only in
conventional m-cresol and chloroform, but in the aprotic solvent e N-methyl-2-pyrrolidinone (NMP).
PPQ-1c and 2c containing sulfone units were even soluble in tetrahydrofuran at room temperature with
a solid content of 15 wt%. Flexible and tough PPQ films cast from their NMP solution showed good thermal
stabilities, including glass transition temperatures in the range of 215e248 ꢀC and 5% weight loss
Article history:
Received 9 February 2010
Received in revised form
18 June 2010
Accepted 18 June 2010
Available online 1 July 2010
Keywords:
Polyphenylquinoxaline
Thioether
High refractive index
temperatures exceeding 500 ꢀC in nitrogen. The PPQ films at a thickness of w10
mm exhibited moderate
optical transparency at 450 nm. The best optical transmittance around 80% was achieved by PPQ-1c and 2c
containing electron-withdrawing sulfone moieties. The synergic effects of flexible thioether linkages and
highly conjugated quinoxaline rings in the present PPQs endowed them with ultra-high refractive indices
up to 1.7953 at 632.8 nm and birefringences close to zero.
Ó 2010 Elsevier Ltd. All rights reserved.
poly(ether sulfone), and poly(thioether ketone) [8e10] have been
well investigated as high-n optical materials. In addition, aromatic,
1. Introduction
particularly heteroaromatic components are also beneficial to
increase the polymers’ n values. For instance, Ueda lab developed
a series of sulfur-containing polyimides possessing n values as high
as 1.77 at 632.8 nm [11e16].
High refractive index (high-n) polymers have currently attracted
much attention in fabricating high performance optoelectronic
devices [1,2]. Polymer coatings with refractive index (n) higher than
1.70 are promising materials for a wide range of optoelectronic
applications due to their great potential in improving the perfor-
mance of the devices [3e6]. However, the n values of conventional
polymers are usually lower than 1.70 [7]. Based on LorentzeLorenz
equation [1], various methodologies have been now established to
increase the n values of polymers beyond 1.70, including intro-
duction of sulfur elements, halogens except fluorine, phosphorous
components, metallic elements as well as other substituents with
high molar refractions (RM) and low molar volumes (VM). Among
the substituents, sulfur-containing groups, such as linear thioether
(eSe), cyclic thioether (thiophene, thianthrene or thiadiazole) and
sulfone (eSO2e), seem to be the most optimal choices due to their
combined advantages to develop high-n optical polymers. Thus,
various sulfur-containing polymers, including poly(arylene sulfide),
Polyphenylquinoxaline (PPQ) is an important class of hetero-
aromatic polymer, first developed by Hergenrother in 1967 [17].
PPQ is characterized by its low moisture absorption, high thermal
and hydrolytic stability; thus initially developed to serve as high-
temperature repellent adhesives or composites to meet the severe
demands of extreme environments [18e20]. Lately, various specific
properties of PPQs have been gradually revealed, greatly expanding
their applications in high-tech fields. Up to now, PPQs with various
structural characteristics and functionalities have been developed.
Systemic work includes the low-cost PPQs synthesized from self-
polymerizable quinoxaline monomers developed by Harris group
[21e25]; sulfonated PPQs for proton exchange membrane fuel cells
[26,27]; and nanoporous PPQ foams as potential low-k interlayer
dielectric materials for integrated circuit [28,29]. Recently, the
applications of PPQs in optical fields have been increasingly
investigated. Various electron-transporting PPQs [30e32] and
nonlinear optical PPQs [33,34] were reported.
* Corresponding authors. Tel.: þ86 10 62564819; fax: þ86 10 62569562.
E-mail addresses: liujg@iccas.ac.cn (J.-g. Liu), shiyang@iccas.ac.cn (S.-y. Yang).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.06.035

3852
C. Li et al. / Polymer 51 (2010) 3851e3858
Although PPQs have been widely evaluated as optical materials,
solid content), which was then mechanically stirred in nitrogen for
24 h. The solubility was determined visually as three grades:
completely soluble (þþ), partially soluble (þ), and insoluble (ꢃ).
The complete solubility is defined as a homogenous and clean
solution is obtained, in which no phase separation, precipitation or
gel formation is detected.
Refractive index of the PPQ film formed on a 3-inch silicon wafer
was measured at room temperature with a prism coupler (Metri-
con, model PC-2010) equipped with a HeeNe laser light source
to our knowledge, researches concerning refractive indices of the
polymers have rarely been addressed until now. Actually, from the
viewpoint of structure characteristics, there exists a high content of
aromatic components with high molar refractions in PPQs, which
might be propitious to increasing their n values. This fact attracted
us to investigate the structureerefractive index relationships in
PPQs. Our preliminary research has confirmed that ether-contain-
ing PPQs exhibited intrinsic high refractive indices and low bire-
fringence [35]. Thus, as a part of our continuous endeavor to
develop high-n polymers, the objective of the present work is to
further increase the n values of PPQs by introduction of sulfur units.
Meanwhile, the optical transparency of the PPQs in ultra-
violetevisible light region was also taken into consideration. The
synergic effects of thioether and quinoxaline ring on the solubility,
thermal stability, especially refractive index and birefringence of
the PPQs were investigated in detail.
(wavelength: 632.8 nm). The in-plane (nTE) and out-of-plane (nTM
)
refractive index were determined using linearly polarized laser
light parallel (transverse electric, TE) and perpendicular (transverse
magnetic, TM) polarizations to the film plane, respectively. In-plane
(nTE)/out-of-plane (nTM) birefringence (Dn) was calculated as
a difference between nTE and nTM. The average refractive index (nav
)
was calculated according to equation (1):
nav ¼ ð2nTE þ nTMÞ=3
(1)
2. Experimental
2.3. Monomer synthesis
2.1. Materials
2.3.1. 4,40-Thiobis[(p-phenyleneoxy)benzil] (STK, 1)
In a 500-mL three-necked flask equipped with a mechanical
stirrer, a nitrogen inlet, and a condenser, a mixture of 4-nitrobenzil
(53.60 g, 0.21 mol), 4,40-thiodiphenol (21.8 g, 0.1 mol), and anhy-
drous DMSO (240 mL) was heated to 60 ꢀC. Then, anhydrous
potassium carbonate (69.11 g, 0.5 mol) was added. The reaction was
maintained at 60 ꢀC for 20 h. Upon confirmation of the completion
of the reaction by thin-layer chromatography, the solution was
cooled to room temperature and then poured into a mixed solvent
containing hydrochloric acid (1 mol/L, 2400 mL) and chloroform
(600 mL). The organic phase was collected and washed thoroughly
with deionized water. Then, the chloroform solution was dried with
MgSO4. After distilling off the solvent, a pale-yellow solid was
obtained. The crude product was purified by a two-step recrystal-
lizations, first from acetic acid and then from a mixture of
benzeneeethanol (4:3, v/v). The purified tetraketone STK was
obtained as pale-yellow crystals (39.55 g, yield: 62.3%).
4-Nitrobenzil was synthesized in our laboratory according to the
literature [36]. 4,40-Thiobisbenzenethiol, 4,40-thiodiphenol and
3,30-diaminobenzidine (a) were purchased from Aldrich Chemical
Co. and used as received. 3,30,4,40- Tetraaminodiphenylether (b)
was synthesized according to the reported procedure [37]. 3,30,4,40-
Tetraaminodiphenylsulfone (c) was kindly supplied by Konishi
Chemical Ind. Co., Japan and recrystallized from acetonitrile before
use. N-methyl-2-pyrrolidinone (NMP), dimethylsulfoxide (DMSO),
m-cresol, N,N-dimethylacetamide (DMAc), cyclopentanone (CPA),
tetrahydrofuran (THF) and other solvents were purified by distil-
lation prior to use. The other commercially available reagents were
used without further purification.
2.2. Measurements
Inherent viscosity was measured using an Ubbelohde viscometer
with a 0.5 g/dL NMP solution at 25 ꢀC. Absolute viscosity was
measured using a Brookfield DV-IIþ Pro viscometer at 25 ꢀC. Gel
permeation chromatography (GPC) measurements were performed
using a Waters 1515 HPLC pump equipped with a Waters 2414
refractive index detector. Two Waters Styragel columns (HR 5E) kept
at 35 ꢀC ꢁ 0.1 ꢀC were used with HPLC grade tetrahydrofuran (THF) as
the mobile phase at a flow rate of 1.0 mL/min. Fourier transform
infrared (FT IR) spectra were obtained with a Tensor 27 Fourier
transform spectrometer. Ultravioletevisible (UVevis) spectra were
recorded on a Hitachi U-3210 spectrophotometer at room tempera-
ture. The cutoff wavelength was defined as the point where the
transmittance drops below 1% in the spectrum. Prior to test, PPQ
sampleswere driedat100 ꢀC for 1 h to remove the absorbed moisture.
Nuclearmagneticresonances (1HNMRand13CNMR)wereperformed
ona AV 400 spectrometeroperating at 400 MHz inDMSO-d6 orCDCl3.
Differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) were recorded on a TA-Q series thermal analysis
system at a heating rate of 10 ꢀC/min and 20 ꢀC/min in nitrogen or air,
respectively. The tensile properties were performed on an Instron
3365 Tensile Apparatus with 80 ꢂ 10 ꢂ 0.05 mm3 specimens in
accordance with GB 1447-83 at a drawing rate of 2.0 mm/min. Seven
Melting point: 127.5 ꢀC (DSC peak temperature). FT IR (KBr,
cmꢃ1): 1672, 1600, 1582, 1483, 1248, 1163 and 881. 1H NMR (CDCl3):
7.04e7.08 (m, 8H), 7.39e7.42 (d, 4H), 7.51e7.55 (t, 4H), 7.66e7.70 (t,
2H), and 7.97e7.99 (m, 8H). 13C NMR (CDCl3): 117.3, 120.6, 127.4,
128.5, 129.4, 131.4, 131.9, 132.5, 132.6, 134.4, 153.9, 162.5, 192.4, and
193.9. Mass [m/e (relative intensity)]: 529 (Mþ-105,100). Elemental
analysis: calculated for C40H26O6S: C, 75.70%; H, 4.13%. Found: C,
75.46%; H, 4.09%.
2.3.2. 4,40-Thiobis[(p-phenylenesulfanyl)benzil] (3STK, 2)
The monomer was similarly synthesized by the procedure as
STK except that 4,40-thiobisbenzenethiol was used instead of 4,40-
thiodiphenol.
Melting point: 158.9 ꢀC (DSC peak temperature). FT IR (KBr,
cmꢃ1): 1666, 1585, 1473, 1215, 1176 and 878. 1H NMR (DMSO-d6):
7.35e7.37 (d, 4H), 7.45e7.47 (d, 4H), 7.55e7.57 (d, 4H), 7.60e7.64 (t,
4H), 7.77e7.81 (t, 2H), 7.84e7.86 (d, 4H), and 7.89e7.91 (d, 4H). 13C
NMR (CDCl3): 127.6, 129.0, 129.9, 130.4, 130.7, 132.0, 132.9, 134.7,
134.9, 136.6, 147.1, 193.3, and 194.3. Mass [m/e (relative intensity)]:
561 (Mþ105, 100). Elemental analysis: calculated for C40H26O4S3: C,
72.05%; H, 3.93%. Found: C, 71.76%; H, 3.88%.
samples of each PPQ film were tested. Dielectric constants (3) were
measured on a QBG-3B high-frequency Q meter in accordance with
GB 1409-88 at 1 MHz at room temperature. Samples were dried at
120 ꢀC for 1 h to eliminate absorbed moisture prior to testing.
Solubility was determined as follows: 1.5 g of the PPQ resin was
mixed with 8.5 g of the tested solvent at room temperature (15 wt%
2.4. Polymer synthesis and film preparation
Six PPQs, including PPQ-1ae1c based on STK and PPQ-2ae2c
based on 3STK were synthesized via a two-step procedure with
m-cresol as the solvent (Scheme 2). As a typical embodiment,

C. Li et al. / Polymer 51 (2010) 3851e3858
3853
PPQ-2c was synthesized according to the procedure as follows. To
a stirred solution of 3,30-diaminobenzidine (a) (4.2854 g, 0.02 mol)
in m-cresol (50 g), 3STK (13.3364 g, 0.02 mol) was gradually added.
An additional volume of m-cresol (20 g) was added to wash the
residual monomers, and at the same time to adjust the solid
content of the reaction system to be 20 wt%. The mixture was
stirred at room temperature for 24 h under a nitrogen flow to form
a viscous brown solution. Then, the reaction mixture was heated to
120 ꢀC and maintained for another 4 h. The obtained viscous pale-
brown solution was cooled to room temperature and slowly
precipitated in an excess of methanol (500 mL). The yellow, fibrous
PPQ-2c resin was collected by filtration. The resin was further
heated in methanol with refluxing for 24 h, then, dried at 80 ꢀC in
vacuum overnight. Yield: 15.21 g (94%).
A PPQ-2c resin (1.5 g) was dissolved in NMP (8.5 g) at room
temperature to afford a 15 wt% solution. The solution was filtered
through a 0.45 mm Teflon syringe filter to remove any undissolved
impurities. Then, the solution was spin-coated on a clean silicon
wafer or quartz substrate. The thickness of the PPQ film was
controlled by regulating the spinning rate. PPQ-2c films with
Fig. 1. FT IR spectra of tetraketones and PPQs.
thicknesses from 10 to 100 mm were obtained by thermally baking
to the relatively high electron-withdrawing characteristics of thi-
oether compared to oxygeneether unit [39]. This variety was also
clearly reflected in the 13C NMR spectra of the monomers. As
depicted in Fig. 3, for both compounds, 14 signals are clearly
revealed due to the structure symmetry; among these, 7 carbons
induce signals in the DEPT-135 measurements due to the existence
of the attached protons. This result is consistent with the proposed
structures. The variety of chemical shift sequences of C11 and C12 in
Fig. 3a and b reflects the effects of oxygeneether and thioether
units on the structures of the monomers.
the solution in flowing nitrogen according to the following heating
procedure: 80 ꢀC/2 h, 150 ꢀC/1 h, 200 ꢀC/1 h, and 250 ꢀC/1 h.
The other PPQ resin and films were prepared according to
a similar procedure as mentioned above.
3. Results and discussion
3.1. Monomer synthesis
The synthesis route of the novel tetraketones incorporating
thioether linkages in the main chain is shown in Scheme 1. The
target compounds were easily synthesized by the nitro nucleophilic
substituent reaction of 4-nitrobenzil with 4,40-thiodiphenol for STK
and 4,40-thiobisbenzenethiol for 3STK with good yields. The nitro
group in 4-nitrobenzil was activated by the para-substituted elec-
tron-withdrawing C6H5COCO-group [38], thus its nucleophilic
In addition, elemental analysis results also supported the
successful preparation of the target tetraketone compounds.
3.2. Polymer synthesis
Six PPQs were prepared via a two-step polycondensation
procedure shown in Scheme 2. It has been well established that the
solution polymerization reaction of tetraketones and tetraamines
was strongly dependent on the monomer reactivity and the
substituent reaction proceeded smoothly under
a moderate
condition. Fig. 1 shows the FT IR spectra of the tetraketones, in
which the characteristic bands of carbonyl groups in benzil moiety
were both clearly observed at 1672 cmꢃ1 for STK and 3STK.
The 1H and 13C NMR spectra of the monomers are shown in
Figs. 2 and 3, respectively, together with the assignments of the
observed resonances. As shown in Fig. 2, the protons ortho to the
electron-withdrawing carbonyl (Hc and Hd) appeared at the lowest
field in both of the spectra. In Fig. 2a, the protons ortho to the ether
bond (He and Hf) resonated at the highest field in the spectrum.
However, when the ether bond was replaced by thioether bond, the
resonances apparently changed. It can be clearly seen in Fig. 2b that
the chemical shifts of He and Hf were higher than that of Hg in 3STK,
which was contrary to that in STK (Fig. 2a). This is mainly attributed
Scheme 1. Synthesis of sulfur-containing tetraketones.
Fig. 2. 1H NMR spectra of tetraketones. (a) STK (CDCl3); (b) 3STK (DMSO-d6).

3854
C. Li et al. / Polymer 51 (2010) 3851e3858
temperature (>100 ꢀC) had to be adopted [41]. In the present work,
the reactivity of amino groups in 3,30,4,40-tetraaminodiphe-
nylsulfone was decreased by the electron-withdrawing sulfone
groups. Thus, a two-step polymerization procedure, including an
initial reaction at room temperature for 24 h, and a following
reaction at 120 ꢀC for another 4 h was utilized.
As tabulated in Table 1, PPQs with inherent viscosities ranging
from 0.73 to 0.85 dL/g were obtained, indicating the feasibility of
the polymerization conditions and high purity of the monomers.
The average molecular weights of PPQ-1c and 2c were determined
by GPC with THF as an eluent and polystyrene standards as the
reference. The GPC plots of PPQ-1c and PPQ-2c are shown in Fig. 4,
in which the number average molecular weights (Mn), weight
average molecular weights (Mw) and the polydispersity indices
(PDIs) of the polymers are inserted. According to the data, the PPQs
exhibited moderate Mn values around 11,000 and Mw around
15,000. This corresponds to the average degree of polymerization of
13 and PDI around 1.3. The somewhat low molecular weight values
might be ascribed to the low reactivity of amino groups in 3,30,4,40-
tetraaminodiphenylsulfone. Self-standing PPQ films, including thin
films (thickness: w10
mm) for IR and UV measurements and thick
ones (thickness: 30e50
mm) for thermal and mechanical evalua-
tions were prepared by spin-coating the PPQ-NMP solutions on
silicon wafer. After curing, flexible and tough PPQ films were
obtained.
The structures of the PPQ films were confirmed by FT IR
measurements. Fig. 1 shows the typical FT IR spectra of the poly-
mers. The characteristic absorptions of carbonyl in STK and 3STK
located at 1672 cmꢃ1 disappear in the spectra of PPQ-1c and PPQ-
2c, indicating the complete conversion from the monomers to
polymers. Meanwhile, the characteristic bands due to sulfone (SO2)
groups at 1320 and 1160 cmꢃ1 are observed in both of the polymers.
In addition, the typical absorptions of thioether (AreSeAr) linkages
appear at 1257 cmꢃ1 in the spectra.
In Section 3.1, we discussed the effects of oxygeneether and
thioether groups on the absorptions of protons in STK and 3STK.
Similar effects were also observed in PPQs, as illustrated in Fig. 5.
Both of the 1H NMR spectra of PPQ-1c and PPQ-2c are clearly
divided into two parts. The lower fields were occupied by the
absorptions of protons in the tetraamine moiety (Ha, Hb, and Hc)
due to the electron-withdrawing quinoxaline ring and sulfone unit.
In the higher fields, the effects of structure variety on the absorp-
tions of the protons (He, Hf, and Hg) were obviously revealed.
Fig. 3. 13C NMR spectra of tetraketones (DMSO-d6). (a) STK; (b) 3STK (DEPT-135 was
embedded).
solubility of the obtained PPQs in the reaction medium. For
instance, Wrasidlo synthesized a series of high molecular weight
soluble PPQs containing flexible groups at ambient temperature
[40]. However, in some other reports, higher polymerization
Scheme 2. Synthesis of PPQ-1ae1c and PPQ-2ae2c.

C. Li et al. / Polymer 51 (2010) 3851e3858
3855
Table 1
Inherent viscosities and solubility of PPQs.
Solventb
NMP DMAc m-cresol CPA THF CHCl3 Ethanol
a
Sample
[h]
inh
(dL/g)
PPQ-1a 0.85
PPQ-1b 0.82
þþ
þþ
þþ
þþ
þþ
þþ
þ
þþ
þþ
þþ
þþ
þþ
þþ
ꢃ
ꢃ
þ
ꢃ
ꢃ
þ
ꢃ
þþ
þþ
þþ
þþ
þþ
þþ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
þ
þ
PPQ-1c
0.77
þþ
þ
þþ
ꢃ
PPQ-2a 0.83
PPQ-2b 0.79
þ
þ
PPQ-2c
0.73
þþ
þþ
a
Inherent viscosities measured with a PPQ resin at a concentration of 0.5 g/dL in
NMP at 25 ꢀC.
b
þþ: Wholly soluble at room temperature; þ: partially soluble; ꢃ: insoluble;
CPA: cyclopentanone.
3.3. Solubility
The solubility of PPQs is summarized in Table 1. As we know, the
commercial available PPQ e IP 200Ò PPQ was only soluble in phenol
and its derivatives. Although it is also soluble in chloroform and
1,1,2,2-tetrachloroethane, insoluble gels often form during storage
[20]. The potential hazards of phenol solvents to environments and
operators limited the wide applications of IP 200Ò PPQ. Enhance-
ment of solubility of PPQs in common low-toxic solvents, such as
NMP has been successfully achieved by introduction of flexible
ether substituents or bulky alkyl groups in our previous work [35].
Similarly, in the present work, all the PPQs were easily soluble in
NMP, m-cresol, and chloroform at a concentration of 15 wt%.
Among the PPQs, 1c and 2c showed the best solubility due to the
presence of bulky sulfone units. They were also wholly soluble in
DMAc and THF and partially soluble in CPA.
The better solubility of 1c and 2c could be further ascertained by
Fig. 5. 1H NMR spectra of PPQs (CDCl3). (a) PPQ-1c; (b) PPQ-2c.
the absolute viscosities (
h
) of the polymers. Fig. 6 illustrates the
correlations between the
h
values and solid contents of PPQ-2ae2c.
NMP was used as the solvent. Although all the PPQs were soluble in
NMP, the values were quite different. For instance, at the same
solid content of 20 wt%, PPQ-2c had a value of 18,917 mPa s, which
was much lower than that of PPQ-2a (51,023 mPa s). The relatively
lower values of PPQ-2c indicate its higher solvability in the
solvent. The good solubility is mainly attributed to the synergic
effects of flexible thioether linkages, lateral phenyl groups, bulky
sulfone moieties and quinoxaline rings. The good solubility of the
present PPQs in low-toxic solvents facilitates their applications in
high-tech fields.
3.4. Thermal and mechanical properties
h
h
The thermal properties of PPQs were investigated by thermog-
ravimetric analysis (TGA) and differential scanning calorimetry
(DSC) measurements and the results are presented in Table 2. TGA
curves (Fig. 7) reveal several structureethermal stability correla-
tions of the PPQ films. First, all the polymers exhibit good thermal
stability up to 500 ꢀC both in nitrogen and in air, which is mainly
ascribed to the highly-conjugated quinoxaline rings. Secondly,
h
Fig. 4. GPC plots and molecular weights of PPQ-1c and PPQ-2c.
Fig. 6. Correlations of absolute viscosities (h) and solid contents of PPQ-2ae2c.

3856
C. Li et al. / Polymer 51 (2010) 3851e3858
Table 2
sulfide) (PPS) film under different atmospheres using a high-reso-
Thermal and mechanical properties of PPQ films.
lution thermogravimetry analyzer. They observed that the thermal
decomposition temperatures of PPS increased in the following
order: in helium < in nitrogen < in argon < in air. The char yield at
700 ꢀC increased in the order: in air < in helium < in nitrogen < in
argon. The author ascribed this phenomenon to the different
thermal conductivities and densities of the gases. In the present
investigation, with the increase of the temperatures, the PPQ
samples exhibited more rapid decomposition behavior in air,
leaving little residues at 750 ꢀC. This phenomenon is consistent
with the literature.
The glass transition temperature (Tg) values of the PPQs are in
the range of 215e248 ꢀC, as shown in Fig. 8 and Table 2. These
values are much lower than that of the standard PPQ (IP 200Ò PPQ;
Tg ¼ 317 ꢀC) [20] due to the existence of flexible thioether linkages.
For the same tetraketone, the Tg values of the PPQs decreased in the
following order: PPQ-a > PPQ-c > PPQ-b. This trend, on one hand,
depends on the rigidity of the linkages in the tetraamine moiety; on
the other hand, the molecular volumes of the bridges. The bulky
sulfone (SO2) segments are less prone to move at elevated
temperatures in comparison with ether segments, resulting in
higher Tg values.
a
a
a
a
c
c
c
Sample
Tg
T5% (ꢀC)
T10% (ꢀC)
Rw750
(%)
Ts
Eb
TM
(ꢀC)
(MPa)
(%)
(GPa)
PPQ-1a
PPQ-1b
PPQ-1c
PPQ-2a
PPQ-2b
PPQ-2c
248
221
244
235
215
230
559 (560)b
551 (537)
502 (527)
537 (547)
545 (546)
500 (520)
570 (584)
562 (573)
526 (552)
552 (586)
554 (560)
523 (548)
68 (2.2)
62 (0.7)
56 (3.7)
67 (2.7)
61 (1.1)
57 (3.7)
81
76
72
86
73
71
6.0
6.3
5.4
6.2
5.8
5.2
2.1
2.1
2.2
2.3
2.1
2.3
a
Tg: glass transition temperature; T5%, T10%: temperatures at 5% and 10% weight
loss, respectively; Rw750: residual weight ratio at 750 ꢀC.
b
The thermal data measured in air.
Ts: tensile strength; Eb: elongation at break; TM: tensile modulus.
c
PPQs derived from sulfone-bridged tetraamine (PPQ-1c and 2c)
exhibit lower thermal stability compared to their ether-linked
(PPQ-1b and 2b) and biphenyl (PPQ-1a and 2a) analogs. The
unstable nature of sulfone groups at elevated temperatures has
been observed in other polymers [15]. Thirdly, most of the PPQs
show higher initial thermal decomposition temperatures in air
than those in nitrogen. For instance, the 5% weight loss of PPQ-2c
occurred at 500 ꢀC in nitrogen, which was 20 ꢀC lower than that in
air (520 ꢀC). This phenomenon has also been reported on the other
sulfur-containing polymers in the literature [39,42]. In Ref. [42], the
authors investigated the thermal decomposition of poly(phenylene
The tensile properties of the PPQ films are listed in Table 2. The
isotropic films exhibited acceptable properties with the tensile
strength higher than 70 MPa, tensile modulus higher than 2.1 GPa,
and the elongation in the range of 5.2e6.3%.
3.5. Optical properties
As mentioned in Introduction, standard PPQs were initially
designed as high-temperature resistant adhesives or composites
for applications in extreme environments. Thus, highly conjugated
and fused structures were preferred. These structures afford
conventional PPQs good thermal and environmental stability,
however, greatly sacrifice their optical transparency in the visible
light region. It has been well-known that introduction of flexible
spacers, such as ether (eOe), thioether (eSe), and methylene
(eCH2e) in heteroaromatic polymers would be beneficial to sepa-
rating the chromophoric groups and disturbing the intramolecular
conjugation interactions, thus, be helpful for the penetration of
visible light. In addition, some external factors including the purity
of the PPQ resin, residual solvent in the resin, and film-forming
history might also affect the colors of the PPQ films.
Fig. 7. TGA curves of PPQ films (a) in nitrogen; (b) PPQ-2ae2c in nitrogen and air
(heating rate: 20 ꢀC/min).
Fig. 8. DSC curves of PPQ films (heating rate: 10 ꢀC/min, nitrogen).

C. Li et al. / Polymer 51 (2010) 3851e3858
3857
could mainly be attributed to the contribution of sulfur compo-
nents in the tetraketone moiety.
The nav values of the PPQs decrease in the order of
2a > 1a > 2c > 2b > 1c > 1b. This sequence well reflects the general
trends for the specific structural compositions of the PPQs. First, the
PPQs with higher sulfur contents generally exhibited higher nav
values. For instance, the nav values of 3STK-PPQs (1.7590e1.7953)
are correspondingly higher than those of their STK-based analogs
(1.7277e1.7665).
Secondly, in each series, the highest nav value was achieved by
the polymer derived from 3,30-diaminobenzidine (a) despite its
relatively lower sulfur content in comparison with the polymer
from 3,30,4,40-tetraaminodiphenylsulfone (c). For instance, PPQ-2a
has a nav value of 1.7953, which is 0.0347 higher than that of PPQ-
2c, although the former’s sulfur content (11.89%) is lower than that
of the latter (14.69%). This fact indicates that the solid state struc-
tures of PPQs including chain rigidity and degrees of molecular
packing should play an important role in affecting the refractive
index of the polymers [43]. Actually, it has been well established
that the refractive index of one polymer is the result of competition
among several factors, including molecular polarizability, chain
flexibility, molecular geometry and orientation in the polymer
backbone [44]. For thioether substituents, the dominant factor
affecting the n values is their high molar refractions. However, for
sulfone group, its contribution to n values of the polymers includes
not only its high molar refraction but its high molar volume. These
two interinhibitive factors usually bring a more complex influence
on the n values of the polymers. For instance, for PPQ-2c, the looser
molecular packing originating from the bulky sulfone groups in the
tetraamine moiety decreases its refractive index despite the exis-
tences of high molar refraction thioether groups in the tetraketone
moiety. On the contrary, for PPQ-2a, the higher molecular packing
density of the molecular chains induced by the rigid biphenyl unit
in the tetraamine moiety and the high molar refractions caused by
the thioether linkages in the tetraketone moiety synergistically
endow the polymer with the highest nav value among the PPQs.
Actually, from another point of view, PPQ-2c has the best optical
transparency among the 3STK-PPQs, revealing the lowest degree of
molecular packing in the polymer.
Fig. 9. UVeVis spectra of PPQ films (thickness: w10 mm).
Fig. 9 shows the typical UVeVis spectra of PPQ films. The cutoff
wavelengths ( ) and transmittances at 450 nm (T450) are summa-
rized in Table 3. The PPQ films (thickness: w10 m) were cast in an
l
m
oven filled with flowing nitrogen at curing temperatures up to
250 ꢀC. No apparent weight loss was detected before 300 ꢀC in TGA
measurements, indicating a low content of residual NMP. The
obtained films exhibited colors ranging from pale-yellow (1c and
2c) to pale-brown (1a and 2a) depending on their structures. In
each series, PPQ-1c and 2c showed the best optical transmittances
due to the existence of electron-withdrawing sulfone moiety. For
instance, the T450 value of PPQ-1c is 86%, which is 12% and 24%
higher than that of PPQ-1b and PPQ-1a, respectively. In addition,
the l values of PPQ-1c (406 nm) and 2c (412 nm) are the shortest in
the series, indicating their superior optical transparency in a wider
spectra range.
The PPQ samples for refractive index measurements were
prepared by spin-coating on a quartz substrate. The thickness of the
films ranged from 4.6 to 7.9 mm. The refractive indices measured at
632.8 nm wavelength are summarized in Table 3. The nav values
ranging from 1.7277 (PPQ-1b) to 1.7953 (PPQ-2a) make the present
PPQs rank first in the families of high-n polymers in the literature.
In our previous work [35], a maximum n value of 1.7739 was ach-
ieved by a PPQ derived from 4,40-bis(4-benzilyloxy)biphenyl and
3,30-diaminobenzidine (PPQ-IIa). In the present work, the value was
increased by 0.0214 (1.7953 for PPQ-2a). Considering the same
tetraamine moiety in PPQ-IIa and PPQ-2a, the improvement of n
Thirdly, the PPQ films exhibited good optical isotropic charac-
teristics as evidenced by the small birefringence values. The facts
that the nTE values are slightly higher than nTM ones reflect the
slight chain orientation parallel to the film plane. Such small bire-
fringences can mainly be contributed to the small polarizability
anisotropy of quinoxaline rings and the low degree of molecular
orientation due to the flexible thioether linkages in the molecular
chains [45].
Table 3
Optical properties of the PPQ films.
b (nm)
T450c (%)
dd
(m
m)
Refractive indices at 632.8 nm
3f
a
Sample
SC (%)
l
D
ne
e
e
e
nTE
nTM
nav
PPQ-1a
PPQ-1b
PPQ-1c
PPQ-2a
PPQ-2b
PPQ-2c
4.13
4.04
7.62
11.89
11.66
14.69
426
424
406
429
428
412
62
74
86
56
64
78
6.4
4.6
6.6
7.9
7.3
6.2
1.7667
1.7278
1.7296
1.7954
1.7590
1.7607
1.7662
1.7276
1.7293
1.7950
1.7589
1.7605
1.7665
1.7277
1.7295
1.7953
1.7590
1.7606
0.0005
0.0002
0.0003
0.0004
0.0001
0.0002
3.12 (3.24)g
2.98 (3.15)
2.99 (3.08)
3.22 (3.28)
3.09 (3.19)
3.10 (3.17)
a
Sulfur content.
b
c
d
e
f
Cutoff wavelength.
Transmittance at 450 nm.
Film thickness for refractive index measurements.
See Section 2.2.
Estimated dielectric constant from Maxwell’s equation as
Experimental dielectric constants.
3
¼ n2av
.
g

3858
C. Li et al. / Polymer 51 (2010) 3851e3858
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a higher nav value usually exhibits a higher
PPQ-2a has a maximum value of 3.28, which is 0.20 higher than
the minimum value of PPQ-1c.
The values of the PPQs are a bit lower than those of the wholly
aromatic polyimides (
can be processed with their fully cyclized form in organic solvents.
Thus, the good combined properties make the present PPQs good
candidates not only for optoelectronic fabrication, but for advanced
microelectronic assembly.
3
values revealed the similar trend. PPQ film with
3
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synthesized. The introduction of thioether groups indeed increased
the n values of the PPQ matrix. Refractive index up to 1.7953 is one
of the highest values reported in the literature so far. Introduction
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The authors greatly appreciate the financial supports from
National Nature Science Foundation of China (NSFC) (No.
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