High refractive index materials: A structural property comparison of sulfide- and sulfoxide-containing polyamides

Article abstract of DOI:10.1002/pola.27764

High-refractive-index polyamides (PAs) are developed by incorporation of sulfide- or sulfoxide linkages and chlorine substituents. The PAs are synthesized through the polycondensation of two novel diamine monomers, 2,2′-sulfide-bis(4-chloro-1-(4-aminophenoxy) phenyl ether (3a) and 2,2′-sulfoxide-bis(4-chloro-1-(4-aminophenoxy) phenyl ether (3b), with various aromatic diacids (a-e). The ortho-sulfide or sulfoxide units, pendant chlorine groups, and flexible ether linkages in the diamine monomers endowed the obtained PAs with excellent solubilities in organic solvents. The resulting PAs showed high thermal stability, with 10% weight loss temperatures exceeding 415°C under nitrogen and 399°C in air atmosphere. The combination of chlorine substituents, sulfide or sulfoxide linkages, and ortho-catenated structures provided polymers with high transparency along with high refractive index values of up to 1.7401 at 632.8 nm and low birefringences (<0.0075). The structure-property relationships of the analogous PAs containing sulfide or sulfoxide linkages were also studied in detail by comparing the results.

Full text of DOI:10.1002/pola.27764

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High Refractive Index Materials: A Structural Property Comparison of
Sulfide- and Sulfoxide-Containing Polyamides
Ali Javadi,1 Ebrahim Abouzari-Lotf,2,3 Shahram Mehdipour-Ataei,4 Masoumeh Zakeri,2,3
Mohamed Mahmoud Nasef,2,3 Arshad Ahmad,2 Adnan Ripin2
1Department of Polymer Engineering, University of Akron, Akron, Ohio 44325
2Center of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia
3Malaysia–Japan International Institute of Technology, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia
4Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran
Correspondence to: E. Abouzari-Lotf (E-mail: ebrahim@utm.my)
Received 10 May 2015; accepted 29 June 2015; published online 28 July 2015
DOI: 10.1002/pola.27764
ABSTRACT: High-refractive-index polyamides (PAs) are devel-
oped by incorporation of sulfide- or sulfoxide linkages and
chlorine substituents. The PAs are synthesized through the pol-
ycondensation of two novel diamine monomers, 2,20-sulfide-
bis(4-chloro-1-(4-aminophenoxy) phenyl ether (3a) and 2,20-sulf-
oxide-bis(4-chloro-1-(4-aminophenoxy) phenyl ether (3b), with
various aromatic diacids (a–e). The ortho-sulfide or sulfoxide
units, pendant chlorine groups, and flexible ether linkages in
the diamine monomers endowed the obtained PAs with excel-
lent solubilities in organic solvents. The resulting PAs showed
high thermal stability, with 10% weight loss temperatures
exceeding 415 8C under nitrogen and 399 8C in air atmosphere.
The combination of chlorine substituents, sulfide or sulfoxide
linkages, and ortho-catenated structures provided polymers
with high transparency along with high refractive index values
of up to 1.7401 at 632.8 nm and low birefringences (<0.0075).
The structure–property relationships of the analogous PAs con-
taining sulfide or sulfoxide linkages were also studied in detail
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by comparing the results.
2015 Wiley Periodicals, Inc. J.
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KEYWORDS: halogenated; polyamide; refractive index; structure-
property relations; sulfide and sulfoxide
and versatile techniques for synthesis of processable high
performance polymers. In this regard, introduction of flexible
bonds, kinked structures, alicyclics, and bulky groups were
frequently considered.7–14 Another structural modification
approach to enhance the processability of rigid-rod polymers
is by modifying the substitution pattern of aromatic units in
the main chain.15–17 Hence, inclusion of n,n0-disubstituted
biphenylene (n 5 1–3)18 and 1,2-linked units derived from
ortho-catenated aromatic rings11,19 are mainly used. Since
2006, our group has started a systematic project to develop
processable polymers using various ortho-linked sulfide-, sulf-
oxide-, and sulfone-containing monomers.7,10,11,13,14,17,20–22 It
was found that the phenyl rings are forced by the ortho-
INTRODUCTION Demand for high refractive index (high-n)
polymeric materials has been rapidly increased because of
their wide range of applications in optical products, such as
optical eyewear, optical fibers, windowpanes, lenses, prisms,
components for charge-coupled devices, and as sensor mate-
rials in combination with optical waveguides.1–4 Their light-
weight, dying ability, and impact resistance, and good
processability are driving the industry toward the use of
polymeric materials rather than inorganic glasses.5 In
addition, aromatic polyamides (PAs) as a class of high-
performance polymers play important role in modern com-
mercial applications. Many investigations have been con-
ducted toward developing structurally modified PAs with
improved processability while retaining their excellent prop-
erties. Quite recently, progress in high-n polymers over the
past decade was described by Higashihara and Ueda.6
linked pattern into
a noncoplanar conformation which
improves processability through reducing intermolecular
forces between the polymer chains and lowering crystalliza-
tion tendency.
It is necessary to obtain a rich understanding of the struc-
ture–property relationships for rational design of high refrac-
tive index polymers. Among all designing strategies,
structural modification of monomers is the most powerful
High refractive index, low birefringence (Dn), high transpar-
ency, and good thermal, chemical, and mechanical properties
are desirable for optical applications of polymers which have
to be adjusted and controlled with respect to potential
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applications.23–28 Generally, the aromatic main chain poly-
mers are preferable for such applications owing to their
inherent high refractive indices.29 Based on the classical Lor-
entz–Lorenz equation,30 the refractive index could be
increased by introducing of substituents with high molar
refraction/molar volume ratios ([R]/V0). Consequently, sulfur-
containing substituents,31,32 heteroaromatic rings containing
AC@NACA or ANAN@CA bonds,33–35 and heavy halogen
atoms (Cl, Br, I)31 have been introduced into polymers to this
end.
from benzene. N-Methyl-2-pyrrolidone (NMP, Merck) and
pyridine (Py, Fluka) were purified by distillation under
reduced pressure over calcium hydride prior to use. Tri-
phenyl phosphite (TPP, Merck) was used without further
purification. LiCl (Merck) was dried for 24 h at 180 8C under
vacuum. The commercially available aromatic dicarboxylic
acids including terephthalic acid (a; Merck), isophthalic acid
(b; Merck), pyridine 2,5-dicarboxylic acid (c; Merck), pyri-
dine 3,5-dicarboxylic acid (d; Merck), and 2,20-dithiodiben-
zoic acid (e; Aldrich) were used as received.
In recent years, introduction of chlorine substituents has
attracted much attention to produce materials with tunable
electrical and optical properties which can be used as optical
waveguides, organic transistors, and photorefractive materi-
als. For instance, a series of colorless chlorinated polyimides
(PI)s derived from chlorinated benzidines having different
molecular structures were already developed by Ando and
coworkers.36 These materials showed high n values (average
refractive indices (nav) of 1.702–1.732 and low Dn of 0.028–
0.091 at 633 nm), high transparency, good thermal stability,
and low coefficient of thermal expansion. Recently, we also
reported a series of chlorinated PAs bearing flexible linkages
and bulky sulfonyl moieties.17 The synergic effects of these
functional groups provided PAs with nav of 1.7069–1.7245
and low Dn of 0.0061–0.0073 at 632.8 nm.
Characterization
An electrothermal engineering LTD 9200 apparatus was used
to measure the melting points. The inherent viscosities were
measured with an Ubbelohde suspended-level viscometer
using N,N-dimethylacetamide (DMAc) at 30 8C. The elemental
analyses were recorded by a Perkin Elmer 2004 (II) CHN
analyzer. Utilizing a Perkin Elmer FT spectrum RX1, the Fou-
1
rier transform infrared (FTIR) spectra were collected. The H
NMR (500 MHz) and 13C NMR (125 MHz) spectra were
obtained on a Bruker DRX 500 AVANCE apparatus. The
wide-angle X-ray diffraction (WAXD) patterns were obtained
on a PW 1800 diffractometer (Philips) by means of the
graphite monochromatized Cu Ka radiation (k 5 1.5401 Å)
with 2h increments of 0.08 8/s. The ultraviolet–visible (UV–
vis) optical transmittance spectra were recorded on a Per-
kin–Elmer Lambda 25 spectrophotometer. The thermo-
gravimetric analyses (TGA) were performed with a Du Pont
2000 thermal analyzer at a heating rate of 10 8C min21
under nitrogen and air atmospheres. The glass transition
temperatures (Tgs) were recorded on a 2010 DSC TA instru-
ment with a heating rate of 10 8C min21. The refractive indi-
ces of the PA films were measured with a prism coupler
(Metricon, model PC-2010) equipped with a He–Ne laser
As an important optical property of polymers, refractive
index is directly related to chemical structures and electrical
properties. The effect of sulfur-containing substituents on
increasing the refractive indices of PAs and PIs has been
already proved by different research groups.32,37–45 However,
the n values of sulfur-containing polymers depend on many
factors and acquiring a deep understanding of the structure–
property relationships of these materials is still crucial for
designing high-n polymers with improved properties and
applications.6,46 Following these considerations, herein, we
designed and synthesized two novel chlorinated diamine
monomers containing flexible sulfide (3a) and sulfoxide (3b)
linkages. Two series of new solution-processable PAs were
synthesized from 3a or 3b with various aromatic diacids.
These materials showed good thermal resistance and excel-
lent optical properties with high refractive indices (1.7190–
1.7401 at 632.8 nm), good optical transparency (>86% at
450 nm), and low birefringences (<0.0075). By comparing of
the prepared PAs, the effects of sulfur and sulfoxide groups
on the physicochemical and optical properties were investi-
gated in detail.
light source. The in-plane (nTE) and out-of-plane (nTM
)
refractive indices were measured by linearly polarized laser
light parallel and perpendicular polarizations to the film
plane, respectively. The average refractive index (nAV) was
calculated using the equation: nAV5½ð2n2TE1nT2MÞ=3ꢀ1=2. The
birefringence (Dn) was calculated as difference between nTE
and nTM. The dielectric constants (e) were optically estimated
from the average refractive indices based on the Max well
equation: e 5 1.10n2AV [32]. Spin-coating of the purified DMAc
solutions containing 5 wt % PAs on silicon (Si) wafer was
used to prepare PAs films. The prefiltration of polymer solu-
tions through a Teflon syringe filter was performed to elimi-
nate any particulates that could affect the quality of PA films.
Then, the films were dried under vacuum at 100 8C for 1.5 h.
The film thickness was controlled by regulating the spinning
rate and the viscosity of the PA solutions. For UV measure-
ments, the thickness of PA films was controlled to be
ꢁ10 lm.
EXPERIMENTAL
Materials
2,20-Sulfoxide-bis(4-chlorophenol) (1b) was synthesized
according to the modified procedure reported in our previ-
ous study17 and recrystallized twice from isopropyl alcohol.
The sulfide analogues, 2,20-sulfide-bis(4-chlorophenol) (1a),
was prepared by reduction of the sulfoxide linkage with zinc
dust/boiling acetic acid and followed by recrystallization
Synthesis of Dinitro Intermediates
1-Chloro-4-nitrobenzene (50 mmol, 7.87 g) was added to a
stirred mixture of bisphenol monomer 1a or 1b (25 mmol)
and potassium carbonate (52 mmol, 7.19 g) in dimethylsulf-
oxide (150 mL). The reaction mixture was heated at 100 8C
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for 15 h, cooled to room temperature, and poured in water
(500 mL) to gain a yellowish precipitate. This obtained solid
was further purified by recrystallization from ethanol.
2,20-Sulfide-bis(4-chloro-1-(4-nitrophenoxy) Phenyl Ether
(2a)
Yield: 12.66 g (96%). M.p.: 135–137 8C. 1H NMR (500 MHz,
CDCl3, d): 8.14 (d, 4H, J 5 9.2 Hz, He), 7.15 (m, 4H, Ha and
Hb), 6.97 (d, 2H, J 5 5.0 Hz, Hc), 6.83 (d, 4H, J 5 9.2 Hz, Hd);
13C NMR (125 MHz, CDCl3, d): 168.1 (C7), 156.1 (C6), 147.7
(C10), 135.4 (C2), 131.9 (C3), 130.9 (C9), 127.0 (C4), 119.2
(C1), 118.4 (C5), 114.5 (C8); IR (KBr): m 5 3026 (w, ArAH
stretching), 1606, 1587 (s, asymmetrical nitro stretching),
1526, 1475, 1353 (s, symmetrical nitro stretching), 1262,
1243, 1201, 1162, 1144, 1088, 1058, 897, 865, 824,
783,737, 693, 656 cm21. Anal. Calcd. for C24H14Cl2N2O6S
(%): C 54.45, H 2.67, N 5.29; found, C 54.40, H 2.65, N 5.27.
SCHEME 1 Synthesis of the diamine monomers.
2,20-Sulfoxide-bis(4-chloro-1-(4-nitrophenoxy) Phenyl
Ether (2b)
2,20-Sulfoxide-bis(4-chloro-1-(4-aminophenoxy) Phenyl
Ether (3b)
Yield: 25.5 g (94%). M.p.: 208–210 8C. 1H NMR (500 MHz,
CDCl3, d): 8.26 (d, 4H, J 5 10.2 Hz, He), 8.01 (d, 2H, J 5 1.6 Hz,
Ha), 7.54 (dd, 2H, J 5 8.8, 1.6 Hz, Hb), 7.36 (d, 2H, J 5 8.8 Hz,
Hc), 7.20 (d, 4H, J 5 10.2 Hz, Hd); 13C NMR (125 MHz, CDCl3,
d): 160.8 (C7), 150.4 (C6), 141.2 (C10), 129.6 (C4), 129.2 (C2),
127.5 (C3), 124.6 (C9), 123.2 (C5), 121.9 (C1), 116.9 (C8); IR
(KBr): m 5 3025 (w, ArAH stretching), 1642, 1604, 1565 (s,
asymmetrical nitro stretching), 1532, 1490, 1362 (s, sym-
metrical nitro stretching), 1345, 1265, 1205, 1086, 917, 804,
670 cm21. Anal. Calcd. for C24H14Cl2N2O7S (%): C 52.86, H
2.94, N 5.14; found, C 52.84, H 2.96, N 5.13.
Yield: 3.75 g (77%). M.p.: 129–131 8C. 1H NMR (500 MHz,
CDCl3, d): 7.96 (d, 2H, J 5 1.5 Hz, Ha), 7.54 (dd, 2H, J 5 9.0,
1.5 Hz, Hb), 7.30 (d, 2H, J 5 9.0 Hz, Hc), 6.75 (d, 4H,
J 5 10.2 Hz, Hd), 6.71 (d, 4H, J 5 10.2 Hz, He), 4.75–5.23
(broad, NH); 13C NMR (125 MHz, CDCl3, d): 153.4 (C6), 143.7
(C7), 136.5 (C10), 129.4 (C4), 129.2 (C2), 127.5 (C3), 123.26
(C9), 121.9 (C1), 121.6 (C5), 115.8 (C8); IR (KBr): m 5 3455
(s, NAH stretching), 3348 (m, NAH stretching), 3036 (w,
ArAH stretching), 1623 (m, NAH bending), 1500, 1486,
1447, 1336, 1273, 1230, 1200, 1125, 1038, 838 cm21. Anal.
Calcd. for C24H18Cl2N2O3S (%): C, 59.39; H, 3.74; N, 5.77;
found, C, 59.37; H, 3.80; N, 5.68.
Synthesis of Diamine Monomers
General Polymerization Procedure
A 200-mL round-bottom flask was charged with a mixture of
dinitro intermediate 2a or 2b (10 mmol), 100 mL dry metha-
nol, Zn (25 mmol, 1.63 g), and ammonium chloride (32
mmol, 1.71 g). The reaction mixture was heated to reflux
temperature and maintained at this temperature for 14 h.
Then, the reaction mixture was cooled to ambient tempera-
ture, filtered, and poured into 500 mL water. The white pre-
cipitate was filtered, dried, and recrystallized from ethanol.
All polymerizations were carried out in a nitrogen atmos-
phere and a representative example is as follows: 3a (5
mmol, 2.347 g,), 1.1 mL of TPP, 1.2 mL of pyridine, and 7 mL
of freshly distilled NMP were charged in a 20-mL flask
equipped with a magnetic stirrer, a nitrogen inlet, and an oil
bath. After dissolving 3a, terephthalic acid (5 mmol, 0.830 g)
and 0.9 g of LiCl was added and heated at 110 8C for 4 h
under nitrogen atmosphere to yield a viscous solution. Usu-
ally after 2 h the reaction solution became too viscous and
additional 2 mL of NMP was added. At the end of the reac-
tion, the polymer solution was poured slowly into 200 mL of
methanol under rapid stirring. The stringy polymer was fil-
tered, washed thoroughly with hot water (2 3 100 mL) and
methanol (2 3 100 mL), and dried under vacuum at 100 8C
overnight. Further purification was carried out by dissolving
the polymer in DMAc or dimethylsulfoxide (DMSO), filtering
the polymer solution, and then precipitating it into methanol.
The inherent viscosity of this polymer in DMAc was 0.84 dL
g21, measured at a concentration of 0.5 g dL21 at 30 8C.
Yield: 93%. 1H NMR (500 MHz, DMSO-d6, d): 10.03–10.14
(broad, NH), 8.04 (4H, Hg), 7.52 (d, 2H, J 5 1.4 Hz, Ha), 7.35
(dd, 2H, J 5 8.1, 1.4 Hz, Hb), 6.86 (m, 6H, Hc and Hd), 6.72 (d,
2,20-Sulfide-bis(4-chloro-1-(4-aminophenoxy) Phenyl Ether
(3a)
Yield: 3.61 g (75%). M.p.: 124–126 8C. 1H NMR (500 MHz,
CDCl3, d): 7.43 (d, 2H, J 5 1.4 Hz, Ha), 7.26 (dd, 2H, J 5 8.2,
1.4 Hz, Hb), 6.81 (m, 6H, Hc and Hd), 6.67 (d, 4H, J 5 8.8 Hz,
He), 4.43–5.61 (broad, NH); 13C NMR (125 MHz, CDCl3, d):
148.4 (C6), 142.2 (C7), 134.8 (C10), 132.7 (C2), 128.2 (C3),
127.4 (C4), 125.9 (C1), 119.0 (C5), 116.9 (C9), 113.9 (C8); IR
(KBr): m 5 3450 (m, NAH stretching), 3342 (w, ArAH
stretching), 3029 (w, ArAH stretching), 1628 (m, NAH bend-
ing), 1500, 1483, 1450, 1343, 1275, 1232, 1214, 1129, 1043,
835 cm21. Anal. Calcd. for C24H18Cl2N2O2S (%): C, 61.41; H,
3.87; N, 5.97; found, C, 61.39; H, 3.89; N, 5.93.
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to the amino protons confirmed that the nitro groups have
been completely converted into amino groups. The 13C NMR
spectra of both dinitro and diamine compounds exhibited
similar patterns. Because of the resonance effect arising
from the electron-donating nature of the amino groups,
upfield shifts of the aromatic carbons (especially for those
para (C7) and ortho (C9) to the amino group) were observed
in the 13C NMR spectrum of diamine monomers.
Polymer Synthesis
The direct polycondensation was successfully adopted here
to prepare aromatic PAs from diamine monomers with vari-
ous aromatic dicarboxylic acids a–e, as shown in Scheme 2.
The inherent viscosities and synthesis conditions of the PAs
are summarized in Table 1.
All the polycondensations proceeded readily in a homogene-
ous NMP solution. When the viscous polymer solutions were
trickled into the stirring methanol, stringy and tough PAs
precipitates formed. These polymers were obtained in high
yields (90–93%), and the inherent viscosities were in the
range of 0.59–0.90 dL g21
.
The structural features of these PAs were verified by elemen-
tal analysis and FTIR and 1H NMR spectroscopies. Figure 3
1
shows a typical H NMR spectrum of PA-4a, in which all pro-
tons are in agreement with the proposed structure.
The formation of amide linkages are supported by the reso-
nance peak appearing in the most downfield region. As
FIGURE 1 1H NMR (top) and 13C NMR (down) spectra of the
diamine monomer (3a) in CDCl3. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
4H, J 5 9.0 Hz, He); IR (film): m 5 3290 cm21 (NAH stretch),
1420cm21 (NAH bending), 1670 cm21 (C@O), phenyl alkyl
ether 1050 and 1255cm21. Anal. Calcd. for C32H20Cl2N2O4S
(%): C, 46.11; H, 3.36; N, 4.67, found: C, 64.03; H, 3.42; N, 4.60.
RESULTS AND DISCUSSION
Monomer Synthesis
In this study, two new sulfur-containing diamines were pre-
pared in a three-step procedure as shown in Scheme 1. The
first step was the preparation of bisphenols 1a and 1b and
the second step was an aromatic nucleophilic chloro-
displacement reaction of 4-nitrochlorobenzene with the
potassium phenolate of 1a or 1b formed in situ by the treat-
ment with K2CO3 in DMSO. The target diamine monomers 3a
and 3b were obtained in high yields by the Zn/NH4Cl reduc-
tion of the intermediate dinitro compounds (2a and 2b). The
structures of the dinitro compounds and the diamine mono-
mers were confirmed by 1H NMR, 13C NMR, FTIR, and ele-
mental analysis. Figures 1 and 2 illustrate the 1H NMR and
13C NMR spectra of the synthesized diamine compounds 3a
and 3b, respectively.
FIGURE 2 1H NMR (top) and 13C NMR (down) spectra of the
diamine monomer (3b) in CDCl3. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
The upfield shift of the He protons as well as the appearance
of new signal centered around 5 ppm (not shown) peculiar
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SCHEME 2 Synthesis of sulfide- or sulfoxide-containing chlorinated PAs.
shown in Table 2, the values of elemental analysis agreed
with the calculated values for the proposed structures of
PAs. The values of carbon were slightly lower than the calcu-
lated values because of the hygroscopic nature of the amide
groups. These aromatic PAs exhibited low moisture absorp-
tion values (1.88–2.31%) which ensure that these PAs have
stable dielectric performance. Although the presence of
sulfur-containing substituents generally leads to lower mois-
ture absorption in polymers,47 because of potential for inter-
molecular hydrogen bonding between sulfoxide linkages and
water molecules, the moisture uptakes of the PAs 5a–5e
were slightly higher than those of the PAs 4a–4e. The FTIR
spectra of these PAs showed the characteristic absorptions
of PAs at 3250–3400 cm21 (NAH stretching), 1420–
1425 cm21 (NAH bending), and 1655–1670 cm21 (C@O
stretching). These results further confirmed that the PAs
have the expected chemical structures.
Figure 4. The diffractograms patterns exhibited broad halo
patterns without any noticeable sharp peaks demonstrating
that these sulfur-containing PAs were amorphous in nature.
This amorphicity was related to the presence of the ortho-
catenated sulfide or sulfoxide linkages between two aromatic
rings, flexible ether linkages, and pendent chlorine groups.
The introduction of pendent groups and less symmetric
ortho-catenated linkages can easily reduce the backbone
symmetry and regularity, by means of lowering chain pack-
ing efficiency and weakening intermolecular hydrogen bond-
ing which results in decreased crystallinity.
The lowered chain packing efficiency in these PAs influences
on their processability by increasing the solubility in com-
mon organic solvents. The solubility of these polymers was
tested through dissolving 0.05 g of PAs in 1 mL of solvent,
including NMP, DMSO, DMAc, dimethylformamice (DMF), tet-
rahydrofuran (THF), Py, and acetone for 24 h at room tem-
perature or upon heating in tightly closed vials. The results
showed that almost all the PAs were dissolved not only in
highly polar solvents, but also in moderate polar solvents
such as THF, pyridine, and acetone. As indicated in Table 3,
Solubility and Morphology of the Polymers
The morphology of all PAs was studied using WAXD in a
spectral window ranging from 2h 5 4 8–80 8. The representa-
tive diffractograms of the PA-4a and PA-5a is shown in
TABLE 1 Synthetic Conditions and Inherent Viscosities of PAs
Amount of Reagent Useda
Polymer Code
NMP (mL)
Py (mL)
LiCl (g)
ginhb(dL/g)
PA-4a
PA-4b
PA-4c
PA-4d
PA-4e
PA-5a
PA-5b
PA-5c
PA-5d
PA-5e
7 1 2c
1.2
1.2
1.2
1.2
1.5
1.2
1.2
1.2
1.2
1.5
0.9
0.8
0.9
0.8
0.8
0.9
0.8
0.9
0.8
0.8
0.84
0.70
0.83
0.66
0.59
0.90
0.75
0.88
0.72
0.65
7
7 1 2c
7
6
7 1 2c
7
7 1 2c
7
6
a
c
Amount of diamine and diacid monomer 5 5.0 mmol; TPP 5 1.1 mL;
reaction temperature 5 110 8C; reaction time 5 4 h.
Measured at a concentration of 0.5 g/dL in DMAc at 30 8C.
“7 1 2” means that an initial amount of 7 mL NMP was used and an
additional 2 mL of NMP was added when the reaction solution became
too viscous to be stirred.
b
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FIGURE 3 1H NMR spectrum of PA-4a in DMSO-d6. [Color figure can be viewed in the online issue, which is available at wileyonli-
nelibrary.com.]
PAs based on flexible diacids, such as PA-4b, PA-4d, PA-4e,
PA-5b, PA-5d, and PA-5e, revealed a higher solubility because
of decreased intermolecular interactions and increased chain
packing distance. The higher entropy state of the PAs chains
resulting from meta linkages was also responsible for
improved solubility of the meta-related polymers. PAs 4e
and 5e were soluble even in solvents such as THF, pyridine,
and acetone at room temperature or upon heating. This
TABLE 2 Elemental Analysis and Moisture Uptake of PAs
Elemental Analysis (%)
Polymer Code
PA-4A
Formula (Formula Weight)
C
H
N
Moisture Uptakea (%)
1.88
(C32H20Cl2N2O4S)n
(598.05)n
Cb 64.11
Fc 64.03
Cb 64.11
Fc 64.00
Cb 62.01
Fc 61.90
Cb 62.01
Fc 61.88
Cb 61.70
Fc 61.55
Cb 62.45
Fc 62.34
Cb 62.45
Fc 62.32
Cb 60.40
Fc 60.27
Cb 60.40
Fc 60.25
Cb 60.39
Fc 60.22
3.36
3.42
3.36
3.45
3.19
3.28
3.19
3.30
3.27
3.41
3.28
3.36
3.28
3.38
3.11
3.23
3.11
3.27
3.20
3.36
4.67
4.60
4.67
4.58
7.00
6.90
7.00
6.87
3.79
3.63
4.55
4.45
4.55
4.42
6.82
6.68
6.82
6.65
3.71
3.53
PA-4b
PA-4c
PA-4d
PA-4e
PA-5a
PA-5b
PA-5c
PA-5d
PA-5e
(C32H20Cl2N2O4S)n
(598.05)n
1.90
1.95
2.03
2.22
1.92
1.95
2.01
2.10
2.31
(C31H19Cl2N3O4S)n
(600.47)n
(C31H19Cl2N3O4S)n
(600.47)n
(C38H24Cl2N2O4S3)n
(739.71)n
C32H20Cl2N2O5S
(615.48)n
(C32H20Cl2N2O5S)n
(615.48)n
(C31H19Cl2N3O5S)n
(616.47)n
(C31H19Cl2N3O5S)n
(616.47)n
(C38H24Cl2N2O5S3)n
(755.71)n
a
b
Moisture uptake (%) 5 (W 2 W0/W0)3100%; W: weight of powder poly-
mer sample after standing at room temperature, W0: weight of powder
polymer sample after being dried in vacuum at 100 8C for 12 h.
Calculated elemental analysis.
Found elemental analysis.
c
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led to a decrease in Tg. The PAs of diacids b and d, which
contained meta-catenated phenylene rings, showed lower Tg
than those of the analogous PAs of diacids a and c, which
contained para-substituted phenylene rings, respectively. In
addition, PAs 4e and 5e showed the lowest Tg value of all
the PAs, which could be attributed to the flexibility and low
rotation barrier of their diacids moieties. In comparison with
the sulfide-containing PAs 4a–4e, the analogous sulfoxide-
containing PAs 5a–5e showed higher Tg values which could
be attributed to the less flexibility of sulfoxide versus sulfide
linkages in the repeating units.
FIGURE 4 WAXD diffraction patterns of PA-4a (a) and PA-5a
The thermal and thermo-oxidative stabilities of the polymers
were characterized in both nitrogen and air atmospheres.
These PAs showed high thermal stability and had no notable
weight loss below 415 8C in nitrogen atmosphere. The tem-
perature for 10% weight loss (T10) as one of the main crite-
ria to determine the thermal resistance of polymers were
determined from the original thermograms. As shown in
Table 4, T10 values of these PAs were in the range of 415–
450 8C in nitrogen and in the range of 399–431 8C in air,
respectively. The amount of residue of the chlorinated PAs at
700 8C in nitrogen atmosphere was higher than 30%. The
comparative thermogravimetric curves of sulfide and analo-
gous sulfoxide containing PAs in nitrogen atmosphere are
shown in Figure 6. Due to the introduction of thermostable
sulfoxide groups in the main chains, PAs 5a–5e had higher
T10 values when compared with sulfide-containing analogous
PAs 4a–4e. In general, these PAs possessed good thermal sta-
bility which could withstand elevated processing tempera-
tures. Consequently, the incorporation of flexible sulfide or
sulfoxide linkages and pendant chlorine substituents did not
deteriorate their thermal stability.
(b).
unique solubility was attributed to the ortho-catenated struc-
ture and highly flexible ASASA linkage in 2,20-dithiodiben-
zoic acid, which further increased disorder in the PA chains,
thereby reducing chain interactions. In comparison with the
analogous sulfide-containing PAs 4a–4e, the sulfoxide-
containing PAs 5a–5e showed better solubility. This could be
attributed to the presence of the polarized SAO bonds which
disturbed the planarity of aromatic rings to reduce close
packing and crystallinity.
Thermal Properties of the Polyamides
Thermal decomposition temperatures and glass transition
temperatures (Tg) of high-n polymers are considered as
important factors in optical device design and fabrication.48
In this study, the thermal properties of the PAs were eval-
uated by means of TGA and differential scanning calorimetry
(DSC) and the obtained results are summarized in Table 4.
The DSC analysis shows that for each sample there is no evi-
dence of crystallization and melting processes, supporting
the amorphous nature of these polymers. As shown in
Figure 5, the Tg values of aromatic PAs were in the range of
181–239 8C. This range of Tg values followed the increasing
order of chain flexibility and steric hindrance of the PAs
backbones. The incorporation of less symmetric structures
Optical Properties of the Polyamides
Table 5 summarizes the optical properties of the PA films.
Figure 7 shows the UV–vis transmission spectra of PA films
of 4e and 5e. The cutoff wavelengths (kcutoff), determined
from wavelengths of the intersection points of lines tangent
TABLE 3 Solubilitya of PAs
Solvent
Polymer Code
NMP
DMSO
DMAc
DMF
Py
THF
Acetone
PA-4a
PA-4b
PA-4c
PA-4d
PA-4e
PA-5a
PA-5b
PA-5c
PA-5d
PA-5e
1
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
1
6
–
–
11
11
11
11
1
11
11
11
11
11
11
11
11
11
1
S
6
–
S
6
11
11
1
1
6
1
–
11
6
11
11
11
11
1
1
1
6
1
11
1
6
11
11
1
11
a
(11) Soluble at room temperature, (1) soluble after heating, (6) par-
tially soluble, (S) swelling, (2) insoluble
Solubility: measured at a polymer concentration of 0.05 g/mL.
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TABLE 4 Thermal Characterizations of PAs
TGA
Decomposition Temperatureb
Polymer Code
DSC Tga (8C)
In Air
In Nitrogen
Char Yieldc (%)
PA-4a
PA-4b
PA-4c
PA-4d
PA-4e
PA-5a
PA-5b
PA-5c
PA-5d
PA-5e
220
208
217
205
181
239
228
235
226
197
421
413
418
412
399
431
424
427
424
411
442
435
436
430
415
450
447
448
445
426
39
37
36
36
30
41
39
40
39
33
a
b
Tg measured by DSC at scanning rate of 10 8C min–1 under flowing
Temperature of 10% weight loss determined in nitrogen and air
nitrogen.
atmospheres.
c
Residual weight (%) at 700 8C under nitrogen.
to UV–vis curves,37,40 were in the range of 351–382 nm.
Reducing the coloration of PAs is necessary for developing
high-n polymers with high optical transparency. For the
resulting PAs, the optical transmittance values were higher
than 86% at 450 nm. This colourlessness is due to the amor-
phous nature of these PAs. In fact, the bulky pendant chlo-
rine groups as well as flexible ether and sulfide or sulfoxide
linkages prevented interchain packing. PAs 4e and 5e having
flexible disulfide linkages between the ortho-substituted
phenylene rings exhibited the highest optical transparency
among these PAs. As shown in Table 5, a comparison of the
optical transmittance values of the sulfide-containing PAs with
the sulfoxide-containing PAs indicated that the introduction of
sulfoxide linkages in the main chain improves the optical
transparency. This increment in optical transparency is
ascribed to the bulkiness of sulfoxide groups, which are more
likely to disrupt packed intermolecular interactions than the
sulfide groups.
The average refractive index values (nav) of these PAs ranged
between 1.7190 and 1.7401. PAs 4e and 5e showed the high-
est nav values among the prepared PAs because of the higher
sulfur contents (Sc) owing to the presence of ASASA link-
ages. Because the value of molar refraction/molecular vol-
ume ratio ([R]/V0) for pyridine is higher than that of
benzene, pyridine-containing PAs (4c, 4d, 5c, and 5d) exhib-
ited higher refractive indices despite slightly lower Sc values
than those of phenylene-containing PAs (4a, 4b, 5a, and 5b).
These results confirmed that sulfur content is not the only
parameter which affects refractive index of polymers. The
FIGURE 6 Comparative TGA curves of sulfide- and sulfoxide-
containing PAs obtained with a heating rate of 10 8C min21 in
nitrogen atmosphere.
FIGURE 5 DSC curves of PA-4a and PA-5a in nitrogen
atmosphere.
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TABLE 5 Optical Properties of PA Films
Refractive Indices and Birefringences
Polymer
Dielectric
Constantsg
Code
Sc (wt %)a
T450 (%)b
nTE
nTM
nAV
Dnf
c
d
e
PA-4a
PA-4b
PA-4c
PA-4d
PA-4e
PA-5a
PA-5b
PA-5c
PA-5d
PA-5e
5.35
5.35
5.34
5.34
13.00
5.21
5.21
5.20
5.20
12.73
86
90
87
90
92
90
93
91
93
96
1.7223
1.7213
1.7239
1.7229
1.7392
1.7255
1.7249
1.7269
1.7262
1.7422
1.7151
1.7145
1.7168
1.7162
1.7333
1.7180
1.7177
1.7195
1.7192
1.7359
1.7199
1.7190
1.7215
1.7207
1.7372
1.7230
1.7225
1.7244
1.7239
1.7401
0.0072
0.0068
0.0071
0.0067
0.0059
0.0075
0.0072
0.0074
0.0070
0.0063
3.25
3.25
3.26
3.26
3.32
3.27
3.26
3.27
3.27
3.33
1=2
a
e
f
Sulfur content.
Average refractive index: nAV5½ð2nT2E1nT2MÞ=3ꢀ
.
b
Transmittance at 450 nm (10 mm thick).
Birefringence: Dn 5 nTE – nTM.
c
g
n
TE: in-plane refractive index at 632.8 nm at ambient temperatures.
Optically estimated dielectric constant: e 5 1.10nA2V
.
d
nTM
:
out-of-plane refractive index at 632.8 nm at ambient
temperatures.
lower nav values of PAs 4b, 4d, 5b, and 5d than those of PAs
4a, 4c, 5a, and 5c showed that the meta-catenated diacids
could reduce molecular packing and led to decreased refrac-
tive index values.
All PA films showed higher values of nTE than nTM, indicat-
ing that they have positive birefringences (Dn), meaning
that the molecular chains of these PAs aligned in the film
plane preferentially. The Dn values of the PA films ranged
from 0.0059 to 0.0075 (Table 5). The low birefringence val-
ues could have been because of the flexible sulfide or sulf-
oxide linkages and pendant chlorine groups, which
decreased in-plane orientation of the PA main chains. PAs
4e and 5e exhibited the smallest Dn, which was attributable
to their highly flexible ASASA linkages. In comparison with
the sulfoxide-containing PAs, the sulfide-containing ana-
logues exhibited lower birefringence values owing to the
more flexible structure of sulfide linkage than that of sulfox-
ide linkage.
Several factors including geometry, chain flexibility, molecular
polarizability per volume, and orientation of bonds in polymer
backbones can affect n values of polymers. Hence, refractive
indices of polymers are
a consequence of competition
between these factors. Even though the substitution of sulfide
with sulfoxide linkages in the polymer backbones naturally
decreases the sulfur contents, comparison of the sulfoxide-
containing PAs with the sulfide-containing ones demonstrated
that their refractive indices were higher by ꢁ0.003. Therefore,
the incorporation of sulfoxide substituents can compensate
for decrease in the Sc values of the resulting PAs.
According to the Maxwell equation (eq 1), the refractive
index of a material is directly related to dielectric constant
(e) and relative magnetic permittivity (l):
1:10n2AV5El
(1)
where l is ꢁ1 and has an insignificant effect on the
refractive index values of most polymers, and e is a func-
tion of frequency of the electromagnetic radiation. In this
study, the dielectric constants of these sulfur-containing
PAs were roughly estimated from the refractive index val-
ues by the modified Maxwell equation, where e is the
dielectric constant at approximately 1 MHz. The estimated
dielectric constant values of these PAs were in the range
of 3.25–3.33.
CONCLUSIONS
In summary, we successfully developed two series of highly
refractive polymers via introducing ortho-linked sulfide or
sulfoxide linkages into the molecular structures of
FIGURE 7 Representative UV–vis spectra of sulfide- and
sulfoxide-containing PA films (thickness ꢁ 10 mm).
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13 A. Shockravi, A. Javadi, E. Abouzari-Lotf, Polym. J. 2011, 43,
816–825.
chlorinated PAs. The Tg values of these PAs were in the
range of 181–220 8C for sulfide-containing PAs and 197–
239 8C for sulfoxide-containing PAs. The T10 values were in
the range of 415–442 8C for sulfide-containing PAs and 426–
450 8C for sulfoxide-containing PAs in nitrogen atmosphere.
The high refractive indices ranging from 1.7190 to 1.7372
for sulfide-containing PAs and 1.7225 to 1.7401 for
sulfoxide-containing PAs as well as the low birefringences
between 0.0059 and 0.0072 for sulfide-containing PAs and
0.0063 and 0.0075 for sulfoxide-containing PAs were
obtained for these polymers. In comparison of sulfide versus
sulfoxide linkages, according to the X-ray diffractograms and
solubility data, polarized S@O bond is more efficient in
reducing close packing and crystallinity. Although introduc-
ing sulfoxide naturally decreases the sulfur contents (which
is an important factor for high-n polymers), the comparisons
demonstrated that the refractive indices of sulfoxide-
containing PAs were higher than sulfide-containing ones by
ꢁ0.003. Therefore, the incorporation of sulfoxide substitu-
ents could compensate for decrease in the Sc values of the
resulting PAs. Consequently, sulfoxide-containing PAs with
higher thermal stability and better optical properties could
be good candidates for optical applications such as the com-
ponents for advanced optical device fabrications.
14 E. Abouzari-Lotf, A. Shockravi, A. Rafieimanesh, M. Saremi,
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under Research University Fund Scheme 9 (vote no. #01K23)
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