Solution, thermal and optical properties of novel poly(pyridinium salt)s derived from conjugated pyridine diamines

Article abstract of DOI:10.1002/pola.24228

Several novel poly(pyridinium salt)s with heterocyclic pyridine moieties in their backbones with tosylate and triflimide counterions were prepared by either ring-transmutation polymerization reaction of phenylated-bis(pyrylium tosylate) with isomeric pyri

Full text of DOI:10.1002/pola.24228

Solution, Thermal and Optical Properties of Novel Poly(pyridinium salt)s
Derived from Conjugated Pyridine Diamines*
ALEXI K. NEDELTCHEV, HAESOOK HAN, PRADIP K. BHOWMIK
Department of Chemistry, University of Nevada Las Vegas, 4505 Maryland Parkway, Las Vegas NV 89154-4003, USA
Received 21 May 2010; accepted 4 July 2010
DOI: 10.1002/pola.24228
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Several novel poly(pyridinium salt)s with heterocy-
clic pyridine moieties in their backbones with tosylate and trifli-
mide counterions were prepared by either ring-transmutation
polymerization reaction of phenylated-bis(pyrylium tosylate)
with isomeric pyridine diamines of 4-phenyl-2,6-bis(4-amino-
phenyl)pyridine in dimethyl sulfoxide (DMSO) for 48 h at 130–
140 ꢀC or by metathesis reaction of the respective tosylate
polymers with lithium triflimide in DMSO at about 60 ꢀC. Their
chemical structures were characterized by FTIR, 1H, 13C NMR
spectroscopy, and elemental analysis. Their number-average
molecular weights (Mn) were in the range of 8,000–51,000 and
their polydispersities in the range of 1.18–2.13 as determined
by gel permeation chromatography. They had excellent ther-
mal stabilities of 340–458 ꢀC and high glass transition tempera-
tures >200 ꢀC. As they showed good solubilities in common
organic solvents, their solution properties were also character-
ized for their lyotropic liquid-crystalline properties with polariz-
ing optical microscopy (POM) studies. Their photoluminescent
properties were examined by using a spectrofluorometer in
both solution and solid states. Their quantum yields were
rather low, which were in the range of 1.3–2.0%. Additionally,
hand-drawn fibers from the melts were examined to determine
their morphologies with a number of microscopic techniques
including POM, scanning electron microscopy, and transmis-
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sion electron microscopy.
2010 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 48: 4408–4418, 2010
KEYWORDS: gel permeation chromatography; LCPs; lumines-
cence; lyotropic; poly(pyridinium salt)s; TEM, UV–vis spectro-
scopy
INTRODUCTION Light-emitting, p-conjugated compounds,
oligomers, and polymers have received a lot of attention for
the preparation of semiconductors for applications in micro-
and optoelectronic devices, such as organic light emitting
diodes (OLEDs), organic field-effect transistors (OFETs), or-
ganic photovoltaic cells (OPVs), and organic lasers.1 Among
these, nitrogen-containing conjugated materials have shown
considerable promise as an electron transporting layers
(ETLs) in OLEDs. They have high electron affinities because
they have electron-deficient heterocycles and can effectively
transport electrons from the cathode throughout the ETL
materials until they successfully recombine with holes result-
ing in light emission.2–4 Moreover, it is demonstrated that
their light-emitting properties could be tuned through the
protonation of nitrogen heterocycles.2b,c,3f Different heterocy-
clic moieties, such as oxadiazole,3a azole,3b benzobisazole,3c
benzothiadiazole,3d pyridine,3e quinoxaline,3f quinoline,3g and
anthrazoline3h were incorporated in both molecular and
polymeric materials that were comprehensively reviewed by
Jenekhe et al.4 The 4-phenyl-2,6-bis(4-aminophenyl)pyridine,
prepared by Chichibabin reaction, and its derivatives were
used in the synthesis of various classes of polymers includ-
ing polyamides, polyimides, poly(amide-imide)s, and poly-
(ester-imides)s.5–7 They showed good thermal stabilities and
high thermal transitions; however, they were soluble only in
high boiling solvents, such as dimethyl sulfoxide (DMSO), N-
methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc),
and m-cresol, which makes their processability cumbersome.
There are only a few examples that exhibit light emitting
properties after protonation but not in their neutral forms.8
Two key methods that are generally applied to improve poly-
mers solubilities include attachment of flexible alkyl chains
as pendant groups as well as incorporation of kinks and
twists in the p-conjugated moieties. Another method of
achieving high solubility of a polymer is to introduce ionic
charges in the backbone of the polymer chain. Such are the
examples of viologen polymers and poly(pyridinium salt)s.9–11
The high solubility is usually achieved by solvation of ionic
charges in the polymer chain as well as of counterions result-
ing in substantial increase in their solubility in polar solvents.
Additionally, variation of different sizes of counterions could
result in a substantial change of their solubility in various or-
ganic solvents.11
Because the conception of poly(pyridinium salt)s, they have
undergone significant improvements with regard to synthesis
*This article is dedicated to the late Professor Robert W. Lenz (1926–2010).
Additional Supporting Information may be found in the online version of this article. Correspondence to: P. K. Bhowmik (E-mail: pradip.bhowmik@
unlv.edu)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 4408–4418 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
SCHEME 1 Synthesis of poly(pyridinium salt)s.
and physical properties. Initially, they had very low molecular
weights, limited solubility, and low thermal stability.9 Harris
et al. were able to modify the procedure of the ring-transmu-
tation polymerization reaction by using DMSO as a reaction
medium thereby significantly improving their molecular
weights.10a Their solubility and thermal stability were signifi-
cantly improved with the introduction of organic counterions
for the main-chain ionic polymers.10b,11a,b In recent years, we
have continuously explored various poly(pyridinium salt)s
containing both aromatic and aliphatic moieties for their vari-
ous interesting properties.11 They exhibited photolumines-
cence in various organic solvents as well as film states rang-
ing from UV to green light.11d Moreover, they are amorphous
having glass transition temperatures Tg > 150 ꢀC even for
polymers with oxyalkylene units in their main chains.11f,g
These ionenes are thermally robust with decomposition tem-
general structures and designations of these ionic polymers
used in this study are outlined in Scheme 1.
EXPERIMENTAL
Materials
30-nitroacetophenone, 40-nitroacetophenone, ammonium ace-
tate, and 10% Pd/C were purchased from Sigma-Aldrich. The
20-nitroacetophenone and hydrazine monohydrate were pur-
chased from TCI, America. Lithium triflimide and common
solvents and deuterated solvents were procured from Fisher
Scientific. All of the chemicals from various vendors were
used as received.
Monomer Synthesis
The 4,40-(1,4-phenylene)bis(2,6-diphenylpyrylium)ditosylate,
M, was synthesized according to the known procedure.11a
A
11a–d
ꢀ
peratures (Td) > 400 C
for fully aromatic polymers and
synthetic scheme for the preparation of isomeric pyridine
diamines is provided in the Supporting Information for con-
venience and their general synthetic procedures are
described below.
11e
ꢀ
Td > 200 C
for alkyl ones. They could be easily processed
into thin films and multilayer assemblies because they are
polycations having excellent solubility in common organic sol-
vents. Furthermore, they exhibit liquid-crystalline (LC) prop-
erties both thermotropic (in melt) and lyotropic (in solu-
tion).11 This combination of properties makes these polymers
potential candidates for various opto-electronic applications,
such as charge-transporting materials in OLEDs or materials
for LECs. In this article, we describe the synthesis of a new se-
ries of wholly aromatic poly(pyridinium salt)s, with tosylate
and triflimide counterions, having pyridine isomeric diamine
units in their backbones and characterize their physical prop-
erties by using a number of experimental techniques. The
Synthesis of 4-Phenyl-2,6-bis(4-nitrophenyl)pyridine
A total of 10.0 g (60.5 mmol) 40-nitroacetophenone, 3.21 g (30.6
mmol) benzaldehyde, and 30.0 g ammonium acetate were
heated to reflux in 75.0 mL of acetic acid for 2 h. As the reaction
progressed, the solution changed from light yellow to dark red
and eventually a precipitate formed. Upon cooling, the reaction
mixture was poured into 50% acetic acid solution. It was col-
lected and washed with hot ethanol. The compound was recrys-
tallized from N,N-dimethylformamide (DMF). An amount of
PROPERTIES OF NOVEL POLY(PYRIDINIUM SALT)S, NEDELTCHEV, HAN, AND BHOWMIK
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
3.30 g (8.30 mmol, 28% yield) of final product was collected hav-
ing the Tm ¼ 314 ꢀC at peak maximum. 1H NMR (400 MHz, d6-
DMSO) d ppm 8.54 (d, J ¼ 8.8 Hz, 4H), 8.40–8.24 (m, 6H), 8.05–
7.98 (m, 2H), 7.62–7.48 (m, 3H). 13C NMR (100 MHz, d6-DMSO)
d ppm 155.5, 151.2, 149.0, 145.2, 137.8, 129.5, 129.1, 128.8,
124.5, 124.3, 119.8, 119.5 Anal. Calcd. for C23H15N3O4 (397.40):
C, 69.52; H, 3.80; N, 10.57. Found: C, 69.28; H, 4.14; N, 10.57.
d ppm 8.01–7.91 (m, 2H), 7.88 (s, 2H), 7.69 (dd, J ¼ 7.82,
1.44 Hz, 2H), 7.61–7.47 (m, 3H), 7.21–7.11 (m, 2H), 6.83
(dd, J ¼ 8.12, 1.02 Hz, 2H), 6.75–6.66 (dt, J ¼ 7.4, 1.2 Hz,
2H), 6.21 (s, 4H). 13C NMR (100 MHz, d6-DMSO) d ppm
158.6, 150.1, 147.8, 138.8, 130.5, 130.4, 129.9, 129.8 128.0,
122.2, 118.2, 117.1, 116.9. Anal. Calcd. for
C23H19N3
(337.43): C, 81.87; H, 5.68; N, 12.45. Found: C, 82.06; H,
5.66; N, 12.48.
4-Phenyl-2,6-bis(3-nitrophenyl)pyridine
Polymer Synthesis
Synthesis of Polymers I-1-I-3
Yield ¼ 25%, recrystallized from DMF having the Tm
¼
242 ꢀC at peak maximum. 1H NMR (400 MHz, d6-DMSO)
d ppm 9.01 (t, J ¼ 1.90 Hz, 2H), 8.70 (t, J ¼ 14.11 Hz, 2H),
8.30 (s, 2H), 8.26 (m, 2H), 8.02 (dd, J ¼ 5.18, 3.10 Hz, 2H),
7.80 (t, J ¼ 7.98 Hz, 2H), 7.61–7.45 (m, 3H). 13C NMR (100
MHz, d6-DMSO) d ppm 155.3, 151.2, 149.5, 141.0, 137.8,
134.0, 133.7, 131.0, 130.4, 129.4, 128.1, 124.5, 122.2, 121.9,
118.9. Anal. Calcd. for C23H15N3O4 (397.40): C, 69.52; H,
3.80; N, 10.57. Found: C, 69.31; H, 3.99; N, 10.61.
The bis(pyrylium) salt was reacted with an appropriate dia-
mine by a ring-transmutation reaction to yield each of the
desired polymers. Equal mole ratio of each of monomers
was placed in three-necked, round-bottomed flask equipped
with a magnetic stirrer in DMSO. About 5 mL of toluene was
added to remove the water generated in the reaction by
forming a toluene/water azeotrope, which was facilitated by
using Dean-Stark trap and water condenser. The temperature
was monitored with a mercury thermometer and adjusted to
130–135 ꢀC for polymerization reaction. The reaction was
kept under nitrogen atmosphere for 48 h. Upon completion
of the polymerization reaction, the contents of the reaction
mixture were allowed to cool down to room temperature.
The DMSO was subsequently removed from the reaction mix-
ture in vacuo to form a viscous polymer solution. It was then
poured into water to precipitate the polymer, which was
then filtered to collect it. It was additionally washed a few
times with boiling water to remove DMSO completely. The
resulting polymer was dried in vacuo at 110 ꢀC for 72 h.
4-Phenyl-2,6-bis(2-nitrophenyl)pyridine
Yield ¼ 12%, recrystallized from acetonitrile having the Tm
¼ 203 ꢀC at peak maximum. 1H NMR (400 MHz, CDCl3)
d ppm 7.93 (d, J ¼ 7.86 Hz, 2H), 7.78–7.64 (m, 8H), 7.54 (m,
5H). 13C NMR (100 MHz, CDCl3) d ppm 156.0, 150.7, 149.4,
137.8, 135.2, 132.9, 131.9, 129.8, 129.7, 129.5, 127.5, 124.6,
120.4. Anal. Calcd. for C23H15N3O4 (397.40): C, 69.52; H,
3.80; N, 10.57. Found: C, 69.52; H, 4.08; N, 10.78.
Synthesis of 4-Phenyl-2,6-bis(4-aminophenyl)pyridine
Ethanol suspension (50.0 mL) of the 3.10 g (78.0 mmol) 4-
phenyl-2,6-bis(4-nitrophenyl) pyridine was heated to 50 ꢀC
in presence of 0.10 g of Pd/C and 5.00 mL of hydrazine
monohydrate was added over 30 min as the reaction pro-
ceeded the solution cleared up. The reaction was kept at
reflux for another 12 h. After completion, the reaction mix-
ture was filtered over Celite twice to remove the Pd/C cata-
lyst. The compound was recrystallized from ethanol to yield
2.05 g (20.9 mmol, yield 78%) having the Tm ¼ 199 ꢀC at
peak maximum. 1H NMR (400 MHz, d6-DMSO) d ppm 8.01
(d, J ¼ 8.62 Hz, 4H), 7.95 (dd, J ¼ 5.23, 3.27 Hz, 2H), 7.80
(s, 2H), 7.59–7.45 (m, 3H), 6.68 (t, J ¼ 8.8 Hz, 4H), 5.43 (d, J
¼ 7.55 Hz, 4H). 13C NMR (100 MHz, d6-DMSO) d ppm 157.2,
150.6, 149.3, 139.2, 129.7, 129.5, 128.5, 128.4, 127.7, 127.2,
114.3, 113.4. Anal. Calcd. for C23H19N3 (337.43): C, 81.87; H,
5.68; N, 12.45. Found: C, 81.59; H, 5.75; N, 12.69.
Data for polymer I-1: Anal. Calcd. for
C77H57N3O6S2
(1184.45): C, 78.08; H, 4.85; N, 3.55; O, 8.11; S, 5.41. Found:
C, 73.73; H, 5.53; N, 3.40; S, 5.39; for polymer I-2: Anal.
Calcd. for C77H57N3O6S2 (1184.45): C, 78.08; H, 4.85; N, 3.55;
O, 8.11; S, 5.41. Found: C, 74.24; H, 5.56; N, 3.45; S, 5.77;
and for polymer I-3: Anal. Calcd. for
(1184.45): C, 78.08; H, 4.85; N, 3.55; O, 8.11; S, 5.41. Found:
C, 77.27; H, 6.11; N, 3.65; S, 5.95.
C77H57N3O6S2
Synthesis of Polymers II-1–II-3
Polymers II-1–II-3 were prepared by a metathesis reaction
from the respective tosylate polymers with excess lithium
triflimide salt in DMSO at 50 ꢀC for 48 h. At the end of the
metathesis reaction, the DMSO solvent was reduced in vacuo
to form a viscous solution, and it was then poured into
water to precipitate out the polymer. This procedure was
repeated once or twice until all the tosylate counterions
were exchanged to triflimide counterions, which was moni-
tored by the analysis of 1H NMR spectrum. After the final
precipitation, each polymer was washed extensively with
boiling water to remove any residual organic salts
and entrapped DMSO after which it was dried in vacuo at
110 ꢀC for 72 h. Data for polymer II-1: Anal. Calcd. for
4-Phenyl-2,6-bis(3-aminophenyl)pyridine
Yield ¼ 92%, recrystallized from ethanol having the Tm
¼
178 ꢀC at peak maximum. 1H NMR (400 MHz, d6-DMSO) d
ppm 7.98 (m, 4H), 7.62–7.54 (m, 2H), 7.54–7.49 (m, 3H),
7.43 (m, 2H), 7.23–7.16 (t, J ¼ 8.0 Hz, 2H), 6.69 (ddd, J ¼
7.92, 2.28, 0.87 Hz, 2H), 5.36–5.12 (s, 4H). 13C NMR (100
MHz, d6-DMSO) d ppm 157.9, 149.7, 149.6, 140.3, 138.7,
129.8, 127.8, 116.8, 115.5, 115.4, 113.1. Anal. Calcd. for
C
67H43N5O8S2F12 (1402.34): C, 57.39; H, 3.09; N, 4.99; O,
9.13; S, 9.14; F, 16.26. Found: C, 57.36; H, 3.33; N, 5.12; S,
9.29; Polymer II-2: Anal. Calcd. for 67H43N5O8S2F12
C23H19N3 (337.43): C, 81.87; H, 5.68; N, 12.45. Found: C,
81.49; H, 5.79; N, 12.48.
C
4-Phenyl-2,6-bis(2-aminophenyl)pyridine
(1402.34): C, 57.39; H, 3.09; N, 4.99; O, 9.13; S, 9.14; F,
16.26. Found: C, 57.39; H, 3.66; N, 5.00; S, 8.98; and Polymer
II-3: Anal. Calcd. for C67H43N5O8S2F12 (1402.34): C, 57.39; H,
Yield ¼ 89%, recrystallized from ethanol having the Tm
¼
172 ꢀC at peak maximum. 1H NMR (400 MHz, d6-DMSO)
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ARTICLE
3.09; N, 4.99; O, 9.13; S, 9.14; F, 16.26. Found: C, 59.30; H,
3.70; N, 5.00; S, 8.83.
plished using ViscoGel I-MBHMW-3078 columns purchased
from Viscotek. An aliquot of 100–200 mL of 2 mg/mL poly-
mer solution in DMSO containing 0.1 M LiBr was injected.
The dn/dc values were corrected by injecting different vol-
umes of polymers to assess their trends. All data analyses
were performed by using Viscotek TriSEC software. The X-
ray diffraction studies were performed on finely powdered
samples at room temperature with a PANalytical X’PERT Pro
X-ray diffraction spectrometer using Cu Ka radiation (k ¼
1.5418 Å) as an X-ray source operating at 45 kV and 40 mA.
The scanning electron microscopy (SEM) images of hand-
drawn fibers of polymers prepared from their melts were
taken with the JXA-8900 SuperProbe. The transmission elec-
tron microscopy (TEM) studies of polymers were performed
by using a TECNAI-F30-Super-twin TEM instrument operat-
ing at 300 kV field emission mode. For this study, a drop of
5 wt % of polymer I-2 in methanol was deposited on a cop-
per grid coated with carbon film that is suitable for electron
microscopy and the solvent was evaporated at room temper-
ature. In the case of polymer I-3, a drop of 2 wt % in metha-
nol was used in the deposition process.
Polymer Characterization
The 1H and 13C NMR spectra were recorded with a Varian
NMR 400 spectrometer equipped with three RF channels
operating at 400 and 100 MHz, respectively. Polymer solu-
tions were prepared by dissolving 35–40 mg of polymers per
mL of d6-DMSO with TMS as an internal standard. FTIR spec-
tra were recorded with a Shimadzu spectrometer. Polymer
samples were prepared by coating NaCl plates with polymers,
which were subsequently dried in vacuo at 70 ꢀC overnight.
The phase transition temperatures of monomers and poly-
mers were measured using a TA 2100 differential scanning
calorimetry (DSC) in nitrogen at heating and cooling rates of
ꢀ
10 C/min. The temperature axis of the DSC thermogram was
calibrated before using the reference standards of high purity
indium and tin. Polymers of 8–10 mg were used in these
measurements. Thermal stability of each of these polymers
was analyzed by thermogravimetric analysis (TGA) using TA
2100 instrument at a rate of 10 ꢀC/min in nitrogen using
sample not less than 10 mg. The LC properties of both lyo-
tropic and thermotropic were assessed using a polarized opti-
cal microscope (POM) Nikon Labophot 2 equipped with
crossed polarizers and hot stage. The UV–Vis absorption spec-
tra of polymer solutions in organic solvents were recorded
using Varian Cary 50 Bio UV–Visible spectrophotometer in
quartz cuvettes. Their photoluminescent properties in both
solution and film states were analyzed using a Perkin Elmer
LS-55 luminescence spectrophotometer. Quantum yields were
analyzed by adjusting the solution absorption using the UV–
Vis to about 0.05 at 350 nm wavelength. The outputs were
then measured using the luminescence spectrophotometer at
the same wavelength and compared it with known 9,10-
diphenylanthracene standard using eq 1:
RESULTS AND DISCUSSION
Chemical Structures
The chemical structures of the polymers in this study are
consistent with the spectra of FTIR, 1H and 13C NMR spec-
troscopy, and elemental analysis. The FTIR spectrum showed
the following representative characteristic peaks for polymer
I-1: 3057 (¼¼CAH aromatic stretching), 1619–1448 (C¼¼C
and C¼¼N aromatic ring stretching), 1200 (CANþ), 1120
(S¼¼O asymmetric stretching), 1033–1010 (S¼¼O symmetric
stretching, d), and 848–680 cmꢁ1 (¼¼CAH out-of-plane bend-
ing). Representative characteristic peaks for polymer II-1:
3065 (¼¼CAH aromatic stretching), 1619–1450 (C¼¼C and
C¼¼N aromatic ring stretching), 1348 (CAF stretching), 1194
(CANþ), 1133 (S¼¼O asymmetric stretching), 1058 (S¼¼O
symmetric stretching), and 844–698 cmꢁ1 (¼¼CAH out-of-
plane bending). Vinylogous signals in both 1H and 13C NMR
spectra of polymers I-1–I-3 were not observed suggesting
that the ring-transmutation polymerization reaction under-
went to completion. Disappearance of the characteristic pro-
ton and carbon peaks of the tosylate counterion in the spec-
tra of polymers II-1–II-2 suggested that the metathesis
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Astd
gunk
gstd
/
¼ /std
(1)
unk
Istd
Aunk
where f is the fluorescence quantum yield, I is the absorp-
tion of the excitation wavelength, A is the area under the
emission curve, and h is the refractive index of the solvents
used. Subscript std denotes the standard and subscript unk
denotes the unknown.12 To assess molecular weight of poly-
1
reaction also proceeds to completion. The H NMR spectra of
ꢀ
mer, gel permeation chromatography (GPC) was run at 50 C
polymers I-1 and II-1, which had para-linked pyridine dia-
mine moieties in the polymer chains, had very broad proton
signals similar to other high viscosity polymers. Further-
more, some of the carbon signals in their 13C NMR spectra
did not resolve well due to their high viscosity character.
The other polymers, which had meta- and ortho-linked pyri-
dine diamine moieties, showed significantly the well-resolved
features in their NMR spectra. All of the spectra for these
polymers are provided in Supporting Information for detail
analyses.
with a flow rate of 1 mL/min. The GPC instrument had con-
nected with Water 515 pump together with a Viscotek Model
301 Triple Detector Array. The array contained a laser re-
fractometer, a differential viscometer, and a light scattering
detector both right angle laser light scattering (RALS) as
well as low angle laser scattering (LALS) in a single instru-
ment with a fixed interdetector and temperature control that
can be regulated up to 80 ꢀC. In this system, the molecular
weight is available from light scattering and concentration
detectors, whereas the latter combines with a viscosity de-
tector to provide intrinsic viscosity [g]. The instrument was
calibrated with a pullulan standard of P-50 obtained from
Polymer Standard Services, USA. Separations were accom-
Molecular Weight Determination
The molecular weight data for the polymers in this study are
summarized in Table 1. Mark–Houwink constants a and K as
PROPERTIES OF NOVEL POLY(PYRIDINIUM SALT)S, NEDELTCHEV, HAN, AND BHOWMIK
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 GPC Data of Poly(pyridinium salt)s
Polymer
IV (dL/g)
Mn
Mw
60,339
Mw/Mn
dn/dc (mL/g)
aa
Ka
I-1
II-1
I-2
II-2
I-3
II-3
a
0.38
0.26
1.23
0.99
0.05
0.12
50,952
58,301
48,133
51,485
7,674
1.18
1.32
1.97
2.13
1.44
1.29
0.1400
0.1000
0.2400
0.2000
0.1800
0.2200
1.27
0.99
0.82
0.80
1.22
1.16
3.21 ꢂ 10ꢁ7
3.78 ꢂ 10ꢁ6
1.19 ꢂ 10ꢁ4
1.15 ꢂ 10ꢁ4
6.19 ꢂ 10ꢁ7
1.41 ꢂ 10ꢁ6
76,719
94,692
109,751
11,042
18,772
14,577
a and K were calculated by using Mark–Houwink equation: [g] ¼ KMa.
well as dn/dc values are also included. Their intrinsic viscos-
ities were distinctively related to their structures. Polymer
I-3, which contained tosylate counterion and an ortho-linked
pyridine diamine moiety in the main chain, had the lowest
viscosity value of 0.05 dL/g. Notably, polymer I-2-meta-
linked tosylate polymer had the highest viscosity value of
1.23 dL/g, which was significantly higher than the value for
the polymer with para-linked pyridine diamine moieties.
Neutral polyamides7a,c and polyesters5,7b based on 4-aryl-
2,6-bis(4-aminophenyl)pyridine and 4-aryl-2,6-bis(4-chloro-
carbonylphenyl)pyridine moieties, respectively, showed in-
herent viscosities of 0.32–0.497a,c and 0.68–0.87,5,7b respec-
tively. Number-average molecular weight (Mn) was the
lowest of 8,000 for polymer I-3 and that was the highest of
58,000 for polymer II-1. Their weight-average molecular
weights (Mw) ranged from 11,000 to 110,000. The polydis-
persity indices (Mw/Mn) of these polymers ranged from 1.18
to 2.13 (Table 1). Representative GPC plots of polymers I-1
and II-1 are provided in Supporting Information in Figure
S19. Overall, they had a wide range of molecular weights,
intrinsic viscosities and polydispersities. Nonetheless, their
molecular weights are sufficiently high enough to assess
their solutions, thermal and optical properties without con-
cerns with regard to the effect of molecular weights on these
properties.
esters based on 4-aryl-2,6-bis(4-phenyl)-pyridines, which are
generally soluble only in high boiling solvents.5–7 We were
motivated to study their lyotropic LC properties, because
there are many examples of poly(pyridinium salt)s exhibiting
lyotropic LC phases, such as mosaic texture or Maltese
crosses in various polar organic solvents.11 As all of the poly-
mers described here are based on different isomeric pyridine
monomers, it is of significant interest to establish how struc-
tural differences would impact their solution properties (Ta-
ble 2). This is particularly interesting, because the well-
known polymer Kevlar forms lyotropic phase in strongly
acid media including concentrated sulfuric acid, whereas
Nomex, which is meta-linked aramide, does not self-assemble
into a lyotropic phase.13 Polymer I-1, which had a tosylate
counterion and para-linked pyridine moiety, did not show
solubility in methanol or acetonitrile but had a very good
solubility in DMSO. Increase of the polymer concentration in
DMSO eventually passed the critical concentration (*C) at 31
wt % and fully-grown lyotropic LC texture was observed as
shown in Figure 1(a). On the other hand, polymer II-1 con-
taining triflimide as counterion showed improved solubility
in acetonitrile and DMSO but not in methanol. A full grown
lyotropic LC phase was developed in these solvents at 20
and 50 wt %, respectively. Figure 1(b) shows the lyotropic
LC texture of polymer II-2 in DMSO. Polymer I-2, which was
the polymer with meta-linked pyridine diamine moiety and
tosylate counterion, showed better solubility but some of the
lyotropic LC had diminished. It was soluble in methanol and
showed lyotropic LC phase above 30 wt % in this solvent.
Biphasic solutions were observed in acetonitrile and DMSO.
Polymer II-2 containing triflimide counterion had very high
solubilities in acetonitrile and DMSO but only isotropic solu-
tions were observed, whereas in methanol a biphasic solu-
tion formed at low concentration of 5 wt %. The polymers I-
3 and II-3, which had an ortho-linked pyridine diamine moi-
ety, had very high solubilities in methanol, acetonitrile, and
Solution Properties
Solubility of highly conjugated polymers is always limited
due to their rigid structures. Therefore, their processability
into thin films and fibers is indeed a challenging task. Nota-
ble example is Kevlar, which is a para-linked aramide, that is
only soluble in concentrated sulfuric acid.13 Even though, the
poly(pyridinium salt)s in this study are highly conjugated
they still had good solubility in common organic solvents,
such as tetrahydrofuran (THF), methanol, acetonitrile, DMSO,
among others, compared with neutral polyamides and poly-
TABLE 2 Solution Properties of Poly(pyridinium salt)s
Polymer
I-1
II-1
I-2
II-2
I-3
II-3
CH3OH (e ¼ 32.6)
CH3CN (e ¼ 37.5)
–
–
–
30% Lyotropic 5% Biphasica
50% Biphasica
10% Biphasica
20% Lyotropic 10% Biphasica Up to 20% Isotropic 59% Biphasica
Up to 50% Isotropic
DMSO (e ¼ 48.9) 31% Lyotropic 50% Lyotropic 41% Biphasica Up to 30% Isotropic Up to 50% Isotropic Up to 50% Isotropic
a
The coexistence of anisotropic (bright) and isotropic (dark) phase.
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ARTICLE
results of other poly(pyridinium salt)s.11 This characteristic
may be attributed to the following two reasons. First, sodium
tosylate (155 ꢀC) had much lower decomposition tempera-
ture than lithium triflimide (363 ꢀC). This trend is mani-
fested in the degradation temperatures of these ionic poly-
mers. These results are also supported by the study of the
decomposition products of poly(pyridinium salt)s with tetra-
fluoroborate counterion, where the counterion decomposes
before that of the main-chain polymer.10b Second, triflimide
counterion has a significantly weaker nucleophilic character
compared with tosylate and, therefore, it starts to act as a
nucleophile to cause the decomposition of main-chain of the
polymer at high temperature. A weight loss of 2–3% was
observed in all tosylate polymers, which is due to a solvent
loss (Fig. S20). The measured thermal properties of these
polymers including decomposition and glass transition tem-
peratures are summarized in Table 3.
Thermal Transitions
Amorphous materials are a particularly interesting class of
materials, because they are easy to process into thin films
and fibers, they are transparent, they have no grain bounda-
ries, they have homogenous properties and they have high
quantum efficiencies.14 For opto-electrical devices and other
applications that operate at high temperature, high glass tran-
sition temperatures (Tgs) are required.4 In their reviewed
articles, Shirota et al. had identified a number of structural
characteristics necessary to achieve a stable glassy state,
while keeping high thermal stabilities.14 These structural
designs have been applied successfully in both molecular14
and polymeric15 materials. The synthesized ionic polymers
had high Tgs as compiled in Table 3, similar to neutral poly-
mers based on 4-aryl-2,6-bis(4-aminophenyl)-pyridine.5–7
Polymer I-1 had only one endotherm, which is attributed to
solvent loss as supported by the TGA analysis (vide supra)
and no other thermal transitions before its decomposition
temperature, implying that this polymer had exceptionally
high theoretical Tg. Polymer II-1, which was the triflimide
adduct of polymer I-1, had a distinct Tg observed in all heat-
FIGURE 1 Photomicrographs of (a) polymer I-1 at 31 wt % in
DMSO and (b) polymer II-1 at 50 wt % in DMSO under crossed
polarizers exhibiting lyotropic LC phases, respectively (magnifi-
cation 400ꢂ). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
DMSO but only biphasic solutions were observed at very
high concentrations in several cases. Overall, two important
trends stem from these results. First, the solubility was
increased by introducing meta- and ortho-linked pyridine
diamines but their lyotropic liquid crystal character
decreased as is the case with Kevlar and Nomex. Second, the
change in counterion from tosylate to triflimide of polymer
increased the solubility in acetonitrile and DMSO but
decreased in methanol.
ꢀ
ing and cooling cycles at 324 C. The polymers I-2 and II-2,
with meta-linked pyridineꢀmoiety, as expected, had distinctly
lower Tgs at 285 and 251 C, respectively. Finally, polymers I-
3 and II-3 (Fig. S21) had the lowest Tgs 223 and 199 ꢀC,
respectively. Overall, two notable trends were clearly
observed. Change of the counterion from tosylate to triflimide
significantly lowered the glass transition temperature by
Thermal Properties
Thermal Stability
ꢀ
ꢀ
34 C (polymer I-2 vs. II-2) and 24 C (polymer I-3 vs. II-3).
This decrease in Tg was presumably related to the fact that
triflimide is a bulkier counterion compared with tosylate.
High thermal stability is generally desirable for polymers
that are used at high operating temperatures to extend their
life time. To enhance it, highly p-conjugated aromatic and
heterocyclic structures are introduced into polymer chains,
but often they lead to poor solubility.4–8 In this study, poly
(pyridinium salt)s with heterocyclic pyridine diamine moi-
eties were found to have excellent thermal stabilities in the
range of 340–458 ꢀC, while maintaining their solubility in
common organic solvents (vide supra). Polymers with trifli-
mide counterions had 87–102 ꢀC higher thermal stability
than the polymers with tosylate counterions (Fig. S20),
which were in excellent agreements with the previous
TABLE 3 Thermal Properties of Poly(pyridinium salt)s
Polymer
I-1
II-1
I-2
II-2
I-3
II-3
a
Tg (ꢀC)
–
324
458
285
355
251
457
223
340
199
427
b
Td (ꢀC)
365
a
Glass transition was recorded from the second heating cycle of DSC
thermogram.
b
Thermal decomposition was recorded at 5 wt % loss in nitrogen.
PROPERTIES OF NOVEL POLY(PYRIDINIUM SALT)S, NEDELTCHEV, HAN, AND BHOWMIK
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 4 Optical Properties of Poly(pyridinium salt)s
Polymer
I-1
II-1
I-2
II-2
I-3
II-3
UV abs (nm)
–
–
–
–
–
–
–
–
260,340
3.23
–
335
3.27
–
335
3.27
–
255,345
3.20
–
350
3.22
483
471
–
Band gap (eV)
PL kem THF (nm)
PL kem acetone (nm)
PL kem CH3OH (nm)
PL kem CH3CN (nm)
PL kem film (nm)
533
–
–
525
–
–
489
520
521
1.3
477
484
460
2.0
528
490
1.3
525
474
1.8
481
455
1.9
/
(%)a
F
a
Quantum yield was calculated against diphenyl anthracene as standard (/F ¼ 0.9).
Similar trend was also observed in many ionic liquids where
triflimide counterion lowered their melting points signifi-
cantly.16 Second trend was the decrease in Tg of these ionic
polymers from para- to meta- to ortho-linked pyridine dia-
mine moieties. This phenomenon is quite common in many
classes of polymers, where kinks and twists are introduced in
the main chain of polymers,13 most notable examples of such
properties are exhibited in the cases of para-linked Kevlar (Tg
¼ 330 ꢀC) and meta-linked Nomex (Tg ¼ 285 ꢀC) polymers.13c
bonding p* orbital that accounts for the hypsochromic shift.
Figure 2(b) shows the light emission of the triflimide-contain-
ing polymers II-1–II-3. Polymers II-1 and II-2 emitted light in a
similar range of kem at 528 and 525 nm when excited with 330
and 340 nm wavelength of light, respectively. Polymer II-3, sim-
ilar to polymer I-3, was hypsochromically shifted exhibiting
kem at 484 nm when excited with 335 nm light, compared with
polymers II-1 and II-2. These results suggested that extensive
kinks in the main chain had an effect on light emission. This
phenomenon was consistent in different solvents and as well as
with different counterions. In some cases, we observed a signif-
icant difference of light emission by simply changing the sol-
vent. Such as the case with polymer I-2 that exhibited a batho-
chromic shift of 31 nm on changing from methanol to
acetonitrile. These were the evidence for a solvatochromic
effect. When considering the effect of counterions from tosylate
to triflimide no appreciable change in light emission was
detected in solution state. Polymers based on 4-aryl-2,6-bis(4-
aminophenyl)pyridine are shown to have light emission in their
protonated form using HCl exhibiting kem at 500 and 560 nm,
which were in most cases bathochromically shifted compared
with those of poly(pyridinium salt)s in this study.8 Overall, the
emission peaks of this series of poly(pyridinium salt)s had
broad FWHM values over 100 nm. These optical properties
were in excellent agreements with the FWHM values of other
poly(pyridinium salt)s.11 The large FWHM values are attrib-
uted to the fact that the light emission stemmed from more
than one chromophoric species. Quantum yield is an important
factor considering light emitting polymers (LEPs).1 The poly-
mers in this study had moderate quantum efficiencies ranging
from 1.3 to 2.0%. The optical band gaps (Eg) were determined
from the onset values of UV–Vis absorption spectra. Their val-
ues ranged from 3.20–3.27 eV, which were comparable with
previously reported poly(pyridinium salt)s11 but generally
lower than other p-conjugated LEPs.1b,c The poly(pyridinium
salt)s in this study were made into thin films cast from acetoni-
trile, to study their light emitting properties in solid state. The
photoluminescence spectra of the tosylate polymers I-2 and I-3
are displayed in Figure 2(c). They exhibited kem 521 and 460
nm when excited at 390 and 340 nm light, respectively. There
was a hypsochromic shift of 61 nm, which suggested that poly-
mer I-3 had less ordered structures in solid state compared
with polymer I-2, which also exhibited a hypsochromic shift
Optical Properties
Light emission in organic materials both molecular and poly-
meric ones have been of a significant interest in recent years
for the development of various opto-electronic devices.1
Introducing heterocyclic pyridine moieties not only improves
thermal stabilities of materials but also provides tunability
through protonation of the nitrogen heterocycle.2b,c,3f Many
neutral polymers with 4-aryl-2,6-bis(phenyl)pyridine moieties
were developed, which show interesting thermal properties,5–8
but only a few of them were photoactive in their protonated
and in their neutral states.8 All of the polymers described
herein, we introduced isomeric forms of 4-phenyl-2,6-bis(phe-
nyl)pyridine in their backbones, because poly(pyridinium
salt)s were already known for their interesting photolumines-
cent properties11 and introduction of additional pyridine heter-
ocyclic unit would help understand their light-emitting proper-
ties in both solution and solid states. Because of their good
solubility the light-emitting properties of these polymers were
studied in various solvents having a wide range of polarities
(e ¼ 7.5ꢁ37.5). Their measured optical properties including
UV–Vis, photoluminescence spectra and quantum yields are
summarized in Table 4. Figure 2(a) displays photolumines-
cence spectra of tosylate polymers I-2 and I-3 in methanol.
Note here that polymer I-1 had limited solubility in methanol
that precluded the measurement of photoluminescence proper-
ties in this solvent. There was a slight bathochromic shift in the
light emission of polymer I-2 compared with polymer I-3
exhibiting kem at 489 and 477 nm when excited with 375 and
380 nm light, respectively. This shift in light emission was prob-
ably because polymer I-2 had meta-linked pyridine moiety,
whereas polymer I-3 had ortho-linked pyridine moiety. The p–
p stacking in polymer I-3 was precluded in contrast with poly-
mer I-2, which resulted in less decrease in energy of the anti-
4414
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ARTICLE
FIGURE 2 Emission spectra of (a) polymers I-2 and I-3 in methanol, (b) polymers II-1–II-3 in acetonitrile, (c) polymers I-2 and I-3 in
thin films cast from methanol, and (d) polymers II-1–II-3 in thin films cast from acetonitrile at various excitation wavelengths.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
when compared with its solution spectrum. Figure 2(d) showed
the spectra for the triflimide polymers where similar hypso-
chromic shifts were observed when going from para- to meta-
to ortho-linked pyridine diamine moieties. They showed a kem
at 490, 474, and 455 nm when excited at 340 nm wavelength of
light. Exchanging the counterion had an effect on the light emis-
sion in film state. A difference of 47 nm between the light emis-
sion of polymers I-2 and II-2 was observed. The reason for this
shift was presumably because triflimide prevented extensive
p–p stacking in the solid state. It is noteworthy that a significant
change did not occur in the light emission of polymers I-3 and
II-3 probably because these polymers had significantly disor-
dered structures because they had ortho-linked pyridine dia-
mine moieties in their main chains. In general, these polymers
resulted in a significant hypsochromic shift when changing
from solution to solid state. These results suggested that they
were less ordered in film states compared with their solution
spectra, which are in good agreement with the previously
reported results of this class of poly(pyridinium salt)s.11 The
blue shifts in our ionic polymers, as opposed to red shifts of p-
conjugated polymers in ordered structures, may be related to
the fact that the extensive pꢁp stacking phenomena of aromatic
moieties do not occur in ionic polymers leading to less ordered
structures presumably because of repulsive interactions of
charges resulting in blue shifts.
Morphology
As the polymers in this study are envisioned as materials for
fabrication of different opto-electronic devices, we examined
their microstructures with X-ray scattering and morphology
with various microscopic techniques. Figure S22 shows the
representative X-ray powder diffraction (XRD) plots of poly-
mers I-1–I-3 and II-3 recorded at room temperature. They
show broad diffraction peaks with relatively low intensities
at both small and wide angles, which are the characteristic
features of glassy phases of these ionic polymers. These
results are in consistent with the DSC thermograms. These
peaks can be assigned to the intermolecular short-range
interactions that are parallel and perpendicular to the long
axes of the polymer chains. The broad wide-angle diffraction
peaks at around 2y ¼ 20ꢀ that corresponded to the d-spac-
ings in the range of 4.43–4.96 Å are the results of p–p stack-
ing of the polymer chains. The relative sharp, wide-angle dif-
fraction peaks at around 2y ¼ 10ꢀ that corresponded to the
d-spacings in the range of 8.51–9.21 Å are related to some
long-range order between the polymer chains. Thin films of
these polymers were examined after annealing at tempera-
tures above their glass transition temperatures. They had
quite uniform textures. Figure 3(a–c) show photomicro-
graphs of polymers II-2, I-3, and II-3, respectively. Polymers
I-3 and II-3 exhibited some weak birefringent textures that
PROPERTIES OF NOVEL POLY(PYRIDINIUM SALT)S, NEDELTCHEV, HAN, AND BHOWMIK
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 Photomicrographs of (a) polymer II-2, (b) polymer I-3, and (c) polymer II-3 exhibiting their glassy phases; and those of
(d) polymer II-2, (e) polymer I-3, and (f) polymer II-3 exhibiting hand-drawn fiber morphologies.
may be an indication of thermotropic LC phase as observed
in previously reported poly(pyridinium salt)s.11h From the
melt of these poly(pyridinium salt)s, we were able to draw
thin fibers with thickness ranging from about 50 to 300 lm.
Figure 3(d–f) shows photomicrographs of fibers from poly-
mers II-2, I-3, and II-3, respectively. In Figure 3(d), the fiber
from polymer II-2 shows quite rough surface with some
voids and furrows. On the other hand, fibers from polymers
I-3 and II-3 [Fig. 3(e,f)] have smoother surfaces even though
some imperfections were also evident. To further elucidate
the morphologies of these fibers, we examined them with
SEM [Fig. 4(a–d)]. The fiber prepared from polymer II-1
[Fig. 4(a)] is flat with a distinctive texture on its surface,
while inside it seemed quite uniform. In contrast, the fiber
produced from polymer I-2 [Fig. 4(b)] has a smooth surface,
while inside it there are numerous hollow cavities. In Figure
4(c), fiber from polymer II-2 also has some roughness to the
surface. The internal structure of this fiber are revealing
where the surface is thinned out. It appears to have some
hollowness similarly to the fiber made from polymer I-2. As
these two polymers have the same backbone, this interesting
morphology, exhibited by both them, is presumably due the
molecular structure of their backbones. The fiber prepared
from polymer I-3 is flat in nature similar to that of the fiber
from polymer I-1 but it also has smooth surface [Fig. 4(d)]
and seemed filled and uniform in the cross-section (not
shown). Next, we examined these materials with TEM stud-
ies. The samples were prepared from dilute solutions (1–5
wt %) of these polymers in either methanol or acetonitrile.
Each of the triflimide polymers (II-1–II-2) exhibited an
aggregated structure of unidentifiable nature as observed in
Figure S23. On the other hand, the tosylate polymers (I-1–I-
3) had quite distinct, different morphology as displayed in
Figure 4(e,f). Polymer I-2 formed a small spheroid clusters
[Fig. 4(e)], whereas polymer I-3 formed an interesting inter-
connected networks of spheroid objects [Fig. 4(f)]. The for-
mation of spheroid objects of these ionic polymers are pre-
sumably related to the aggregation phenomenon, which may
also related to the formation of lyotropic phase of these poly-
mers. The aggregation phenomena of ionic polymers, irre-
spective of the nature of polyions either rigid-rod or flexible,
in water or in organic solvents are quite remarkable in
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ARTICLE
FIGURE 4 SEM images of (a) polymer II-1, (b) polymer I-2, (c) polymer II-2, and (d) polymer I-3 exhibiting their fiber morphologies;
and TEM images of (e) polymer I-2 (magnification 3,900ꢂ) from 5 wt % CH3OH solution and (f) polymer I-3 ( magnification 5,000ꢂ)
from 2 wt % CH3OH solution exhibiting aggregated particles.
general, because the macroions approach one another leading
to the formation of aggregates instead of repulsion between
like charges. The mechanism of this aggregation process
remains unknown to date, but aggregation processes do occur
in both flexible and rod-like polyelectrolytes as detected by
various experimental techniques including optical microscopy,
dynamic and static light scattering, X-ray scattering, scanning
force microscopy and TEM, among others.17 However, here
we presented some interesting morphology of this class of
poly(pyridinium salt)s in different conditions, which could
shed light how these polymers will perform when they are
used for the fabrication of functional devices.
essential for their fabrication into thin films by spin coating
techniques or inkjet printing. Additionally, in several of these
polymers, a fully grown lyotropic phase was developed in
methanol, acetonitrile and DMSO and, therefore, much like
Kevlar, these polymers could be processed into high-per-
formance materials.13
They exhibited light-emitting properties in solvents of vari-
ous polarities as well as in solid states. In solution they
exhibited photoluminescence in the green region of visible
light and their FWHM values were quite broad. Changing
from solution to solid state, they generally exhibited hypso-
chromic shifts of about 20–30 nm, which suggested less or-
dered structures in film states compared to solution states.
Exchange of the counterion from tosylate to triflimide did
not affect the light emission in solutions but did affect this
property in solid states of these polymers. They also showed
reasonable quantum yields comparable to other p-conjugated
LEPs.1 They showed interesting morphologies in films and
fibers as examined with POM, SEM, and TEM studies.
CONCLUSIONS
Several poly(pyridinium salt)s with tosylate and triflimide
counterions, having pyridine diamine moieties, were pre-
pared by ring-transmutation polymerization or metathesis
reaction. Their chemical structures were verified by spectro-
scopic techniques including FTIR, NMR spectroscopy, and
elemental analysis. Their intrinsic viscosities were as low as
0.05 dL/g and as high as 1.23 dL/g and their weight-average
molecular weights (Mw) were in a range of 11,000 to
110,000 as determined by GPC. All of these polymers had
high thermal stabilities >340 ꢀC for the tosylate polymers
and >427 ꢀC for the triflimide ones. As determined by DSC
and XRD, these polymers are amorphous with high Tgs. They
were highly soluble in common organic solvents, which is
As they contain positively charged nitrogen ions that are
associated with counterions in their backbones, they could
also be used for light-emitting electrochemical cells.18 More-
over, their thermal and optical properties could be fine tuned
by exchange of the counterion to other counterions as
required without the need of synthesizing new polymers.
Finally, these ionic polymers could be used as polycations
with appropriate polyanions to produce multilayered
PROPERTIES OF NOVEL POLY(PYRIDINIUM SALT)S, NEDELTCHEV, HAN, AND BHOWMIK
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
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nanoassemblies by either dipping or spin-assisted method to
create functional materials.19
P.K.B. acknowledges the University of Nevada Las Vegas (UNLV)
for New Investigation Award (NIA), Planning Initiative Award
(PIA), and Applied Research Initiative (ARI) grants, and the
donors of the Petroleum Research Fund, administered by the
American Chemical Society, and an award (CCSA# CC5589)
from Research Corporation for the support of this research.
A.K.N. acknowledges the Graduate College (UNLV) for providing
him a Nevada Stars Graduate Assistantship for the period
of 2006-2008. The authors also sincerely acknowledge to
Dr. Longzhou Ma for his expertise in the analyses of TEM studies.
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