Synthesis and properties of novel X-type nonlinear optical polyester containing nitrophenylazonitroresorcinoxy group with highly enhanced SHG thermal stability

Article abstract of DOI:10.1016/j.dyepig.2015.01.003

New X-type polyester (4) containing nitrophenylazonitroresorcinoxy groups as NLO chromophores, which are components of the polymer backbone, was prepared and characterized. Polyester 4 is soluble in common organic solvents such as acetone and N,N-dimethyl

Full text of DOI:10.1016/j.dyepig.2015.01.003

Dyes and Pigments 115 (2015) 197e203
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and properties of novel X-type nonlinear optical polyester
containing nitrophenylazonitroresorcinoxy group with highly
enhanced SHG thermal stability
,
*
Ji-Hyang Lee a, Ju-Yeon Lee b
a Institute of Basic Science, Department of Nano Science Engineering, Inje University, 607 Obang-dong, Gimhae 621-749, Republic of Korea
b Institute of Basic Science, Department of Chemistry and Nano Science Engineering, Inje University, 607 Obang-dong, Gimhae 621-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
New X-type polyester (4) containing nitrophenylazonitroresorcinoxy groups as NLO chromophores,
which are components of the polymer backbone, was prepared and characterized. Polyester 4 is soluble
in common organic solvents such as acetone and N,N-dimethylformamide. It shows a thermal stability up
to 280 ꢀC in thermogravimetric analysis with glass-transition temperature (Tg) obtained from differential
scanning calorimetry near 120 ꢀC. The second harmonic generation (SHG) coefficient (d33) of poled
polymer film at the 1064 nm fundamental wavelength is around 2.13 pm/V. The dipole alignment ex-
hibits a thermal stability even at 5 ꢀC higher than Tg, and there is no SHG decay below 125 ꢀC due to the
partial main-chain character of polymer structure, which is acceptable for NLO device applications.
© 2015 Elsevier Ltd. All rights reserved.
Received 7 November 2014
Accepted 5 January 2015
Available online 10 January 2015
Keywords:
Nonlinear optics (NLO)
Polyester
Differential scanning calorimetry (DSC)
Thermogravimetric analysis (TGA)
Atomic force microscopy (AFM)
Second harmonic generation (SHG)
1. Introduction
chromophores exhibit good thermal and temporal stability [1].
Polymers with H-type NLO chromophores exhibit a thermal sta-
Nonlinear optical (NLO) polymers received a great attention in
recent years because of their potential applications in the field of
electro-optic devices including ultrafast optical switches, high-
speed optical modulators, and high-density optical data storage
media [1e8]. NLO polymers have many advantages superior to
conventional inorganic ones such as light weight, low cost, ultrafast
response, wide response wave band, high optical damage
threshold, and good processability to form optical devices. A po-
tential NLO polymer must contain highly polarizable conjugated
electronic systems and has to be mechanically very strong and
thermally stable with a high glass transition temperature (Tg). NLO
polymers for electro-optic device applications, stabilization of
electrically induced dipole alignment is one of important consid-
erations; in this context, several approaches to minimize the
randomization have been proposed, namely the use of cross-linked
systems [9e13], the utilization of polymers with high glass transi-
tion temperature (Tg) such as polyimides [14e20], and H-shape
polymers [2]. Hosteguest polymer films doped with spindle-type
bility and large optical nonlinearity [2]. Phenyltetraene
chromophores-based inorganiceorganic hybrid polymer films
show a quite high thermal stability of optical nonlinearity [5].
Various polyesters with NLO chromophores in the main chain
[21,22] or side chain [23,24] have been prepared and their NLO
properties studied. Polyesters with amino-sulfone azobenzene
chromophores in the main chain generate strong and stable revers-
ible birefringence [21]. NLO polyesters containing azobenzene
mesogens in the main chain exhibit high thermal and temporal sta-
bilities [22]. Polyesters containing cyanophenylazoaniline moiety in
the side chain show good temporal stability of second-order nonlin-
earity [23]. Main-chain NLO polymers usually have good thermal
stability of dipole alignments, but they often do not dissolve in
organic solvents, and their intractability make them unusable to
fabricate stable films. Side-chain NLO polymers have the advantages
such as good solubility, homogeneity and high loading level of NLO
chromophores, but they often suffer from poor stability of dipole
alignments at high temperatures. Recently we prepared novel NLO
polyesters containing dioxynitrostilbene [25,26], dioxybenzylidene-
malononitrile [27e29], dioxybenzylidenecyanoacetate [30,31], tri-
cyanovinylthienylcatechol [32], tricyanovinylthienylresorcinol [33],
tricyanovinylthiazolylazoresorcinol [34], and dioxynitroazobenzene
* Corresponding author. Tel.: þ82 55 320 3221; fax: þ82 55 321 9718.
E-mail address: chemljy@inje.ac.kr (J.-Y. Lee).
http://dx.doi.org/10.1016/j.dyepig.2015.01.003
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

198
J.-H. Lee, J.-Y. Lee / Dyes and Pigments 115 (2015) 197e203
[35,36] as NLO chromophores. The resulting polymers exhibit
enhanced thermal stability of second harmonic generation (SHG),
which stems from the stabilization of dipole alignment of the NLO
chromophores.
2.2. Instrumentation
Infrared (IR) spectra were obtained with a Varian FT IR-1000 IR
spectrophotometer. 1H NMR spectra were obtained with a Varian
VNMRS 500 MHz NMR spectrometer. UVevisible absorption
spectra were obtained with a SECOMAM Model UVIKON XS 99-
90289 spectrophotometer. Elemental analyses were performed
using a PerkineElmer 2400 CHN elemental analyzer. Tg values were
measured using a TA 2920 differential scanning calorimeter (DSC)
in a nitrogen atmosphere. A TA Q50 thermogravimetric analyzer
(TGA) with a heating rate of 10 ꢀC minꢁ1 up to 800 ꢀC was used for
the thermal degradation of polymers under nitrogen. The number-
average molecular weight (Mn) and weight-average molecular
weight (Mw) of the polymers were estimated using gel permeation
chromatography (GPC; styragel HR5E4E columns; tetrahydrofuran
(THF) solvent). AFM images were recorded with a Park Science
Instrument Autoprobe CP, operated in a contact mode, which
measures topography. Viscosity values were obtained using a
Cannon-Fenske viscometer.
In this work reported here, we prepared new polyester con-
taining nitrophenylazonitroresorcinoxy group as NLO chromo-
phore. We selected the latter as NLO chromophore because it was
expected to have high optical nonlinearities and thermal stability.
Furthermore, this nitrophenylazonitroresorcinoxy group consti-
tutes a novel X-type NLO polyester (Fig. 1c), and this X-type NLO
polyester has not yet been reported in the literature. The X-type
NLO polymer is expected to exhibit higher NLO activity and thermal
stability due to a quadruple conjugation and planarity of the dipole
moments than Y-type NLO polymer of double conjugation [35].
Thus, we synthesized a new type of NLO polyester, in which the
pendant NLO chromophores are parts of the polymer backbone.
This X-type NLO polymer is expected to have the merits of both
main-chain (Fig. 1a) and side-chain (Fig. 1b) NLO polymers, namely
stable dipole alignment and good solubility. After confirming the
structure of the resulting polymer, we investigated its properties
such as solubility, Tg, thermal stability, surface morphology of
polymer films, second harmonic generation activity and relaxation
of dipole alignment.
2.3. Film preparation and SHG measurements
Polymer films were prepared from a 10 wt% polymer solution in
DMF deposited on an indiumetin oxide (ITO) covered glass. Prior to
film casting, the polymer solution was filtered through 0.45 mm
Teflon® membrane filter. The films were spin-cast at room tem-
perature in the range 1000e1200 rpm. The films were dried for 12 h
under vacuum at 60 ꢀC. The alignment of the NLO chromophore of
the polymers was carried out using a corona poling method. The
poling was performed in a wire-to plane geometry under in situ
conditions. The discharging wire to plane distance was 10 mm. As
the temperature was raised gradually to 5e10 ꢀC higher than Tg, a
corona voltage of 6.5 kV was applied and the temperature was
maintained for 30 min. The films were cooled to room temperature
in the presence of the electric field. Finally, the electric field was
removed. The refractive index of the sample was measured using
the optical transmission technique [37]. The transmittance of thin
film gives information on the thickness, refractive index and
extinction coefficient of the film. Thus, we can determine these
parameters by analyzing the transmittance. SHG measurement was
carried out one day after poling. A continuum PY61 mode-locked
2. Experimental
2.1. Materials
Reagent-grade chemicals were purchased from Aldrich or
AlfaAesar and purified by either distillation or recrystallization before
use. All glassware was thoroughly dried under vacuum with a hot air
gun before use. 4-(4-Nitrophenylazo)resorcinol, 2-chloroethyl vinyl
ether, and bismuth (III) trifluoromethanesulfonate were used as
supplied. Terephthaloyl chloride (TPC) was purified by sublimation
under vacuum. N,N-Dimethylformamide (DMF) was purified by dry-
ing with anhydrous calcium sulfate, followed by distillation under
reduced pressure.
Nd:YAG laser (
l
¼ 1064 nm) with pulse width of 40 ps and repe-
tition rate of 10 Hz was used as the fundamental light source and Y-
cut quartz was used as reference. A poled polymer film was
mounted on the rotator coupled to a step motor. The output signals
from the photodiode and PMT were detected as a function of the
incident angle. A 3-mm-thick Y-cut quartz crystal (a piece of quartz
plate whose plane is perpendicular to the crystalline y-axis and the
thickness of the plate is 3 mm and d11 ¼ 0.3 pm/V) was used as a
reference for determining the relative intensities of the SH signals
generated from the samples. The Maker Fringe pattern was ob-
tained by measuring the SHG signal at 0.5ꢀ intervals using a rota-
tion stage. SHG coefficients (d33) were derived from the analysis of
measured Maker-fringes [38].
2.4. Synthesis
2.4.1. Preparation of 2,4-di-(20-vinyloxyethoxy)-40-nitroazobenzene
(1)
4-(4-Nitrophenylazo)resorcinol (25.9 g, 0.10 mol), anhydrous
potassium carbonate (82.9 g, 0.60 mol) and 2-chloroethyl vinyl
ether (32.0 g, 0.30 mol) were dissolved in 400 mL of dry DMF under
nitrogen. The mixture was refluxed in an oil bath kept at 90 ꢀC for
24 h under nitrogen. The resulting solution was cooled to room
temperature, diluted with 300 mL of water, and extracted with
Fig. 1. (a) Main-chain NLO polymers, (b) side-chain NLO polymers, and (c) X-type NLO
polymers.

J.-H. Lee, J.-Y. Lee / Dyes and Pigments 115 (2015) 197e203
199
OH
HO
O
HO
OH
O
O
O
Cl
K2CO3, DMF
O
H2O, DMF
HCl
O
O
N
N
N
N
2
N
N
1
NO2
NO2
NO2
O
C
O
C
OH
HO
O
O
n
O
O
O2N
O
Cl C
O
C Cl
O
O
HNO3, Bi(OTf)3
ClCH2CH2Cl
N
N
N
O2N
3
Pyridine
4
N
NO2
NO2
Scheme 1. Synthetic scheme and structure of polymer 4.
300 mL of diethyl ether three times. The organic layer was washed
with saturated aqueous sodium chloride solution and dried with
anhydrous magnesium sulfate. Rotary evaporation of diethyl ether
gave crude product, which was purified by column chromatog-
raphy (ethyl acetate/hexane ¼ 1/2 by volume). Thus obtained
product was washed with 10% aqueous ethanol and dried in a
vacuum oven yielded 30.3 g (76% yield) of pure product 1. 1H NMR
aromatic). IR (KBr disc) (cmꢁ1): 3325 (m, OeH), 2948 (m, CeH),
1600 (vs, N]N), 1519, 1337 (vs, N]O). Anal. Calcd for C16H17N3O6:
C, 55.33; H, 4.93; N, 12.10. Found: C, 55.25; H, 4.98; N, 12.15.
2.4.3. Preparation of 2,4-di-(20-hydroxyethoxy)-5-nitro-40-
nitroazobenzene (3)
Compound 2 (2.0 g, 5.76 mmol) was dissolved in 40 mL of dry
1,2-dichloroethane under nitrogen. To the resulted solution was
added bismuth (III) trifluoromethanesulfonate (0.30 g, 0.46 mmol)
and 60 wt% nitric acid (60%, 0.91 g, 8.64 mmol). The mixture was
refluxed in an oil bath kept at 75 ꢀC for 8 h under nitrogen. The
resulting solution was cooled to room temperature, neutralized
with 5 g of anhydrous sodium bicarbonate, diluted with 100 mL of
water with stirring, extracted with 50 mL of diethyl ether three
times, and separated. Rotary evaporation of solvent gave product,
which was dissolved in 50 mL of dichloromethane, washed with
water (100 mL) and saturated aqueous sodium bicarbonate (50 mL),
dried with anhydrous sodium carbonate, filtered and concentrated
by rotary evaporator. The combined product was purified by col-
umn chromatography (ethyl acetate/methanol ¼ 1/2 by volume)
(DMSO-d6, d): 4.02e4.49 (m, 12H, 2 CH2], 2 eOeCH2eCH2eOe),
6.56e6.63 (m, 2H, 2]CHeOe), 6.70e6.74 (m, 1H, aromatic),
6.89e6.91 (m, 1H, aromatic), 7.71e7.74 (d, 1H, aromatic), 7.95e7.98
(d, 2H, aromatic), 8.40e8.43 (d, 2H, aromatic). IR (KBr disc) (cmꢁ1):
3082 (w, ¼CeH), 2934 (m, CeH), 1600 (vs, N]N), 1583 (s, C]C),
1515, 1332 (vs, N]O), 1180 (m, N]N). Anal. Calcd for C20H21N3O6:
C, 60.15; H, 5.30; N, 10.52. Found: C, 60.24; H, 5.26; N, 10.45.
2.4.2. Preparation of 2,4-di-(20-hydroxyethoxy)-40-nitroazobenzene
(2)
Aqueous hydrochloric acid (1.5 mol Lꢁ1, 30 mL) was slowly
added to a solution of compound 1 (3.99 g,10 mmol) in 50 mL of dry
DMF with stirring under nitrogen at room temperature. The
mixture was stirred at 50 ꢀC for 6 h under nitrogen. The resulting
solution was cooled to room temperature and poured into 100 mL
of ice water, stirred, separated by suction, and washed with 50 mL
of water. The obtained product was dried in a vacuum oven to give
gave 1.69 g (75% yield) of pure product 3. 1H NMR (DMSO-d6,
d):
3.78e3.82 (t, 2H, eOeCH2e), 3.84e3.88 (t, 2H, eOeCH2e),
4.37e4.40 (t, 2H, PheOeCH2e), 4.43e4.46 (t, 2H, PheOeCH2e),
5.02e5.04 (t, 1H, eOH), 5.05e5.07 (t, 1H, eOH), 7.17 (s, 1H, aro-
matic), 8.05e8.08 (d, 2H, aromatic), 8.28 (s, 1H, aromatic),
8.42e8.45 (d, 2H, aromatic). IR (KBr disc) (cmꢁ1): 3320 (m, OeH),
2943 (m, CeH), 1610 (vs, N]N), 1519, 1344 (vs, N]O). Anal. Calcd
for C16H16N4O8: C, 48.98; H, 4.11; N, 14.28. Found: C, 48.92; H, 4.06;
N, 14.22.
2.78 g (80% yield) of pure 2. 1H NMR (DMSO-d6,
d): 3.74e3.78 (t, 2H,
eOeCH2e), 3.79e3.83 (t, 2H, eOeCH2e), 4.12e4.16 (t, 2H,
PheOeCH2e), 4.24e4.28 (t, 2H, PheOeCH2e), 4.92e4.96 (t, 2H, 2
eOH), 6.65e6.69 (d, 1H, aromatic), 6.84 (s, 1H, aromatic), 7.70e7.73
(d, 1H, aromatic), 7.96e7.99 (d, 2H, aromatic), 8.39e8.42 (d, 2H,
2.4.4. Preparation of polymer 4
Table 1
A representative polycondensation reaction procedure was as
follows. Terephthaloyl chloride (2.03 g, 10 mmol) and diol 3 (3.92 g,
10 mmol) were dissolved in 25 mL of anhydrous pyridine under
nitrogen. The resulting solution was refluxed in an oil bath kept at
90 ꢀC under a nitrogen atmosphere. After heating 20 h with stirring
the resulting polymerization solution was poured into 400 mL of
methanol. The precipitated polymer was collected and re-
precipitated from DMSO into methanol. The polymer was further
purified by extraction in a Soxhlet extractor with methanol and
dried under vacuum, yielding 4.70 g (90% yield) of polymer 4.
Inherent viscosity (hinh) ¼ 0.31 dL gꢁ1 (c, 0.5 g dLꢁ1 in DMSO at
Thermal properties of polymer 4.
Degradation temp (ꢀC)b
Residue at
800 ꢀC (%)b
a
Polymer
Tg
(ꢀC)
5 wt%-loss
20 wt%-loss
40 wt%-loss
4
120
116
298
241
325
284
421
388
36.1
34.2
PEc
a
Determined from DSC curves measured on a TA 2920 differential scanning
calorimeter with a heating rate of 10 ꢀC/min under nitrogen atmosphere.
b
Determined from TGA curves measured on
a TA Q50 thermogravimetric
analyzer with a heating rate of 10 ꢀC/min under nitrogen atmosphere.
c
Y-type polyester containing nitrophenylazoresorcinol [35].

200
J.-H. Lee, J.-Y. Lee / Dyes and Pigments 115 (2015) 197e203
Fig. 2. TGA thermogram of polymer 4 at a heating rate of 10 ꢀC/min under nitrogen.
Fig. 4. UVeVis absorption spectra of a film of polymer 4 before and after poling.
25 ꢀC). 1H NMR (DMSO-d6,
d): 4.52e4.80 (m, 8H,
2
analysis. Elemental analysis results fit the polymer structure. 1H
NMR spectrum of the polymer showed a signal broadening due to
polymerization, but the chemical shifts are consistent with the
proposed polymer structure. The IR spectrum of polymer 4 shows
strong carbonyl peak near 1723 cmꢁ1 indicating the presence of
ester bond. The spectrum also shows strong absorption peaks near
absorptions at 1520 and 1341 cmꢁ1 due to nitro group and ab-
sorptions at 1420 and 1182 cmꢁ1 due to azo group indicating the
presence of nitroazobenzene unit. These results are consistent with
the proposed polymer structure, indicating that the NLO chromo-
phore remained intact during the polymerization. The molecular
weights were determined by GPC using polystyrene as the standard
and THF as an eluent. The number average molecular weight (Mn) of
the polymer 4, determined by GPC, was 16,300 g molꢁ1 (Mw/
Mn ¼ 1.94). The polymer 4 is soluble in common solvents such as
acetone, DMF, and DMSO, but is not soluble in methanol and diethyl
eOeCH2eCH2eOe), 6.76e7.12 (d, 1H, aromatic), 7.78e8.45 (m, 9H,
aromatic). IR (KBr disc) (cmꢁ1): 3084 (w, ¼CeH), 2935 (m, CeH),
1723 (vs, C]O), 1520 (s, N]O), 1341 (vs, N]O), 1420 (m, N]N),
1182 (vs, N]N). Anal. Calcd for (C24H18N4O10)n: C, 55.18; H, 3.47; N,
10.73. Found: C, 55.28; H, 3.56; N, 10.68.
3. Results and discussion
3.1. Synthesis and characterization of polymer
Compound 1 was prepared by the reaction of 2-chloroethyl vinyl
ether with 4-(4-nitrophenylazo)resorcinol. Compound 2 was pre-
pared by acid-catalyzed hydrolysis of 1 in DMF. Compound 2 was
reacted with nitric acid and bismuth (III) trifluoromethanesulfonate
in anhydrous 1,2-dichloroethane according to a literature proce-
dure [39] to yield nitrated compound 3. Monomer 3 was condensed
with terephthaloyl chloride in a dry DMF solvent to yield X-type
polyester 4 containing nitrophenylazonitroresorcinoxy groups as
NLO chromophores. The synthetic route for polymer 4 is presented
in Scheme 1. The resulting polymer was purified by Soxhlet
extraction for two days with methanol as a solvent. The polymer-
ization yield was 90%. The chemical structure of the resulting
polymer was confirmed by 1H NMR, IR spectra, and elemental
ether. The inherent viscosity is in the range 0.30e0.31 dL gꢁ1
.
Polymer 4 shows strong absorption near 399 nm due to the NLO
chromophore nitrophenylazonitroresorcinoxy group. The striking
feature of this polymerization system is that it gives unprecedented
X-type NLO polymer, in which the pendant NLO chromophores are
parts of the polymer backbone. This X-type NLO polymer is ex-
pected to have the advantages of both main-chain and side-chain
NLO polymers. Thus, we obtained a new type of NLO polyester
with side-chain and main-chain characteristics. Having obtained
the well-defined X-type polyester 4, we investigated its properties.
3.2. Thermal properties of polymer
The thermal behavior of the polymer was investigated by TGA
and DSC to determine the thermal degradation pattern and glass
transition temperature. The results are summarized in Table 1. The
TGA and DSC thermograms of the polymer 4 are shown in Figs. 2
and 3, respectively. Polymer 4 has a thermal stability up to 280 ꢀC
according to its TGA thermogram. This thermal stability is much
higher than that of the Y-type polyester containing nitro-
phenylazoresorcinol, which is near 240 ꢀC [35]. The Tg value of the
polymer 4 measured using DSC is near 120 ꢀC. This Tg value is
higher than that of the Y-type polyester [35] containing nitro-
phenylazoresorcinol, which is probably due to a nitro group and
planarity of the dipole moments. These are also relatively high
values compared to those of common polyesters, which can prob-
ably be attributed to the rigid ring unit in the polymer pendant
group. The TGA and DSC studies show that the decomposition
Fig. 3. DSC thermogram of polymer 4 at a heating rate of 10 ꢀC/min under nitrogen.

J.-H. Lee, J.-Y. Lee / Dyes and Pigments 115 (2015) 197e203
201
Fig. 5. AFM images of a spin-coated film of polymer 4: (a) before corona-poling; (b) after corona-poling.
temperature of the polymer 4 is higher than the corresponding Tg
value. This indicates that high-temperature poling for a short term
is feasible without damaging the NLO chromophore.
Table 2
Nonlinear optical properties of polymer 4.
Fc
Film thicknessd
m)
d31
n
a
b
b
Polymer
lmax
d33
3.3. Nonlinear optical properties of polymer
(nm)
(pm/V)
(
m
(pm/V)
4
397
2.13
0.32
0.28
0.50
0.48
0.72
n1 ¼ 1.712
n2 ¼ 1.725
n1 ¼ 1.724
n2 ¼ 1.736
The NLO properties of polymers were studied using the SHG
method. To induce noncentrosymmetric polar order, the spin-
coated polymer films were corona-poled. As the temperature was
raised to 5e10 ꢀC higher than Tg, a corona voltage of 6.5 kV was
applied and this temperature was maintained for 30 min. The films
were cooled to room temperature in the presence of the electric
field. Finally, the electric field was removed. The poling was
confirmed from UVevisible spectra. The UVevisible absorption
spectra of the polymer 4 before and after poling are presented in
PEe
398
1.94
0.66
a
Polymer film after corona poling.
b
SHG coefficients (d33) were derived from the analysis of measured Maker-
fringes [38].
c
Order parameter
F
¼ 1 ꢁ A1/A0, where A0 and A1 are the absorbances of the
polymer film before and after corona poling, respectively.
d
Film thickness was determined using the optical transmission technique [37].
Y-type polyester containing nitrophenylazoresorcinol [35].
e

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J.-H. Lee, J.-Y. Lee / Dyes and Pigments 115 (2015) 197e203
The refractive index of the sample was measured using the
optical transmission technique [37]. The transmittance of thin film
gives information on the thickness, refractive index and extinction
coefficient. So we could determine these parameters by analyzing
the transmittance. SHG measurements were performed at a
fundamental wavelength of 1064 nm using a mode locked Nd-YAG
laser. Nonlinear optical properties of polymer 4 are summarized in
Table 2. In order to determine the microscopic second-order sus-
ceptibility of the polymer, the angular SHG dependence was
recorded. Fig. 6 shows the angular dependence of SHG signal for a
poled polymer 4. The SHG values were compared with those ob-
tained from a Y-cut quartz plate. To calculate the d31 and d33 values,
both s-polarized and p-polarized IR laser were directed at the
samples. SHG coefficients (d33) were derived from the analysis of
measured Maker-fringes with the Pascal fitting program according
to the literature procedure [38]. The values of d33 and d31 for
polymer 4 are 2.13 pm/V and 0.72 pm/V, respectively. This d33 value
is higher than that of the Y-type polyester containing nitro-
phenylazoresorcinol, which is near 1.94 pm/V [35]. Since the sec-
ond harmonic wavelength is at 532 nm, which is not in the
absorptive region of the resulting polyester, there is no resonant
contribution to this d33 value. In the isotropic model, the ratio of
d33/d31 is predicted to be about 3. Our d33/d31 value of 2.95 is in good
agreement with the predicted value.
To evaluate the high-temperature stability of the polymer, we
studied the temporal stability of the SHG signal. Fig. 7 shows the
dynamic thermal stability study of the NLO activity of a film of
polymer 4. To investigate the real time NLO decay of the SHG signal
of the poled polymer film as a function of temperature, in situ SHG
measurements were performed at a heating rate of 4 ꢀC/min from
30 to 200 ꢀC. The dipole alignment exhibits a thermal stability even
at 5 ꢀC higher than Tg, and there is no SHG decay below 125 ꢀC due
to the partial main-chain character of the polymer structure. This
SHG thermal stability is higher than that of the Y-type polyester
containing nitrophenylazoresorcinol, which is near 120 ꢀC [35]. In
Fig. 8, we present the temporal stability of the polymer film in
which there was no negligible decay of the SHG signal over hun-
dreds of hours. We measured normalized SHG signal of polymer 4
as a function of baking time at 80 ꢀC, which is enough for electro-
optic device applications. In general, side chain NLO polymers
lose the thermal stability of dipole alignment below Tg. Stabilization
of dipole alignment is a characteristic of main chain NLO polymers.
The high thermal stability of second harmonic generation of
Fig. 6. Angular dependence of SHG signal in a poled film of polymer 4.
Fig. 4. Polyester 4 shows strong absorption near 399 nm before
electric poling. After electric poling, the dipole moments of the NLO
chromophores were aligned and the UVevisible spectrum of
polymer 4 exhibits a slight blue shift showing absorption near
397 nm, and a decrease in absorption due to birefringence, namely
by the Pockels effect, where an electric field induces molecules to
line up asymmetrically, inducing anisotropy [40]. From the absor-
bance change, the order parameter of the poled film could be
estimated, which is related to the poling efficiency. The estimated
order parameter value
F
is equal to 0.32 for polymer 4 (
F
¼ 1 ꢁ A1/
A0, where A0 ¼ 1.0184 and A1 ¼ 0.6922 are the absorbances of the
polymer film before and after poling, respectively).
For the purpose of investigating the surface morphology of
polymer film, domain structures of NLO chromophores for the thin-
film samples were obtained using atomic force microscopy (AFM).
Fig. 5 shows AFM scans of a spin-coated film of polymer 4 before
and after poling. AFM images show that the surface of the film
sample is quite flat and smooth before poling (Fig. 5a). However,
this good-quality film was dramatically changed after poling,
resulting in numerous hills and valleys in the surface structure,
which means that the NLO chromophores were aligned in the
poling direction (Fig. 5b).
Fig. 7. Normalized SHG signal of polymer 4 as a function of temperature at a heating
rate of 4 ꢀC/min.
Fig. 8. Normalized SHG signal of polymer 4 as a function of baking time at 80 ꢀC in air.

J.-H. Lee, J.-Y. Lee / Dyes and Pigments 115 (2015) 197e203
203
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polymer 4 at high temperature is due to the stabilization of dipole
alignment of NLO chromophore, which stems from the partial main
chain character of the polymer structure. Thus, we obtained a new
type of NLO polyester having the merits of both main-chain and
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4. Conclusion
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New X-type polyester 4 containing pendant NLO chromophore,
which constitutes a part of the polymer main chain, was synthe-
sized and characterized. This X-type NLO polyester 4 is soluble in
common organic solvents and displayed a thermal stability up to
280 ꢀC with a Tg of 120 ꢀC. The SHG coefficient (d33) of corona-poled
polymer film is 2.13 pm/V. Polymer 4 exhibits SHG stability even at
5
ꢀC higher than glass transition temperature (Tg), and no SHG
decay is observed below 125 ꢀC. This high thermal stability of op-
tical nonlinearity stems from the stabilization of dipole alignment
of the NLO chromophores, which are parts of the polymer back-
bone. We are now in the process of extending the polymerization
system to the synthesis of other type of NLO polymers and the
results will be reported elsewhere.
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harmonic generation. Polym Int 2006;55:248e56.
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Acknowledgments
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (No. 2010-
0020951).
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Y-type polyesters with highly enhanced thermal stability of second harmonic
generation. Eur Polym J 2008;44:1814e21.
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