Synthesis and nonlinear optical properties of novel Y-type polyester containing tricyanovinylthiazolylazoresorcinoxy group with enhanced thermal stability of dipole alignment

Article abstract of DOI:10.1002/pola.24603

Novel Y-type polyester 4 containing 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2- thiazolylazo}resorcinoxy groups as nonlinear optical (NLO) chromophores, which are parts of the polymer backbone, was prepared, and its NLO properties were investigated. Polyester 4 is soluble in common organic solvents such as N,N-dimethylformamide and dimethylsulfoxide. Polymer 4 shows a thermal stability up to 250 °C from thermogravimetric analysis with glass-transition temperature obtained from differential scanning calorimetry of approximately 94 °C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at 1560-nm fundamental wavelength is 8.12 × 10 ^-9 esu. The dipole alignment exhibits a thermal stability even at 6 °C higher than glass-transition temperature (T_g), and no significant SHG decay is observed below 100 °C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.

Full text of DOI:10.1002/pola.24603

Synthesis and Nonlinear Optical Properties of Novel Y-Type Polyester
Containing Tricyanovinylthiazolylazoresorcinoxy Group with Enhanced
Thermal Stability of Dipole Alignment
YOU JIN CHO, HYO JIN NO, MI SUNG KIM, JU-YEON LEE
Institute of Basic Science, Department of Chemistry, Inje University, 607 Obang-dong, Gimhae 621-749, Korea
Received 22 December 2010; accepted 26 January 2011
DOI: 10.1002/pola.24603
Published online 1 March 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Novel Y-type polyester 4 containing 5-methyl-4-{5-
(1,2,2-tricyanovinyl)-2-thiazolylazo}resorcinoxy groups as non-
linear optical (NLO) chromophores, which are parts of the poly-
mer backbone, was prepared, and its NLO properties were
investigated. Polyester 4 is soluble in common organic sol-
vents such as N,N-dimethylformamide and dimethylsulfoxide.
Polymer 4 shows a thermal stability up to 250 ꢀC from ther-
mogravimetric analysis with glass-transition temperature
obtained from differential scanning calorimetry of approxi-
mately 94 ꢀC. The second harmonic generation (SHG) coeffi-
wavelength is 8.12 ꢁ 10ꢂ9 esu. The dipole alignment exhibits a
thermal stability even at
6
ꢀC higher than glass-transition
temperature (Tg), and no significant SHG decay is observed
below 100 ꢀC due to the partial main-chain character of poly-
mer structure, which is acceptable for NLO device applications.
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2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
49: 1784–1790, 2011
KEYWORDS: atomic force microscopy; differential scanning cal-
orimetry; dipole alignment; NLO; polyester; second harmonic
generation; thermogravimetric analysis
cient (d33
) of poled polymer film at 1560-nm fundamental
INTRODUCTION Materials with nonlinear optical (NLO)
properties have been extensively studied because of their
potential applications in the field of electro-optic devices
including ultrafast optical switches, high-speed optical modu-
lators, and high-density optical data storage media.1–5 It is
well known that organic and inorganic materials with high
dipolar electronic systems exhibit large NLO effects. Organic
materials display higher optical nonlinearity and faster
response time than their inorganic counterparts; among the
organic materials, NLO polymers are receiving great atten-
tion of late, mainly because they offer many advantages such
as mechanical endurance, light weight, low cost, chemical re-
sistance, and good processability in terms of their use in
electro-optic devices. A potential 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). In the developments of
NLO polymers for electro-optic device applications, stabiliza-
tion of electrically induced dipole alignment is one of impor-
tant considerations; in this context, two approaches to mini-
mize the randomization have been proposed, namely the use
of cross-linked systems6–10 and the utilization of polymers
with high glass transition temperature (Tg) such as polyi-
mides.11–17 Various polyesters with NLO chromophores in
the main chain18,19 or side chain20,21 have been prepared
and their NLO properties studied. Polyesters with amino-sul-
fone azobenzene chromophores in the main chain generate
strong and stable reversible birefringence.18 NLO polyesters
containing azobenzene mesogens in the main chain exhibit
high thermal and temporal stabilities.19 Polyesters containing
cyanophenylazoaniline moiety in the side chain show good
temporal stability of second-order nonlinearity.20 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 fab-
ricate stable films. Side-chain NLO polymer systems 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 tempera-
tures. Recently, we prepared novel polyesters containing
dioxynitrostilbene,22,23 dioxybenzylidenemalononitrile,24–26
dioxybenzylidenecyanoacetate,27,28 and tricyanovinylthienyl
groups29,30 as NLO chromophores. The resulting polymers
exhibit enhanced thermal stability of second harmonic gener-
ation (SHG), which stems from the stabilization of dipole
alignment of the NLO chromophores.
In this work reported here, we prepared a new Y-type poly-
ester containing 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2-thiazo-
lylazo}resorcinoxy groups as NLO chromophores. We
selected the latter, because they were expected to have large
NLO activities. Furthermore, these 5-methyl-4-{5-(1,2,2-tri-
cyanovinyl)-2-thiazolylazo}resorcinoxy
groups
can
be
Correspondence to: J.-Y. Lee (E-mail: chemljy@inje.ac.kr)
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ARTICLE
age molecular weight (Mn) and weight-average molecular
weight (Mw) of the polymers were estimated using gel per-
meation chromatography [GPC; styragel HR5E4E columns;
tetrahydrofuran (THF) solvent]. Atomic force microscopy
(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.
Film Preparation and SHG Measurements
The polymer film was prepared from a 10 wt % polymer so-
lution in DMF deposited on an indium-tin oxide-covered
glass. Before film casting, the polymer solution was filtered
R
through 0.45 lm TeflonV membrane filter. The films were
spin cast at room temperature in the range 1000–1200 rpm.
The films were dried for 12 h under vacuum at 60 ꢀC. The
alignment of the NLO chromophore of the polymers was car-
ried out by 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 tem-
perature was raised gradually to 5–10 ꢀ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 sam-
ple was measured by the optical transmission technique.31
The transmittance of thin film gives information on the thick-
ness, refractive index, and extinction coefficient of the film.
Thus, we can determine these parameters by analyzing the
transmittance. SHG measurement was carried out 1 day after
poling. The IR fundamental radiation of the 1.56-lm was
generated using an optical parametric oscillator (OPO)
pumped by a Q-switched Nd:YAG laser operating at a repeti-
tion rate of 10 Hz. The pulse width and beam diameter of
the pump laser were about 7 ns and 6 mm, respectively. The
OPO that we used has a well-established design. It consists
of a LiNbO3 crystal (12 ꢁ 15 ꢁ 50 mm3) cut at about 47ꢀ
for type-I angle phase matching inside a 70-mm long cavity
with plane reflectors. The 3.346-lm output of the idler wave
was eliminated by inserting a glass window in front of a
sample. The electric field vector of the incident beam was
either parallel (p-polarization) or perpendicular (s-polariza-
tion) to the plane of incidence. Only the p-polarized SH
beam was made to enter a photomultiplier tube (PMT) by
using a prism and a SH pass filter. An analyzer was used to
confirm the polarization direction of the SH signal. 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 thick-
ness of the plate is 3 mm. and d11 ¼ 0.3 pm Vꢂ1) was used
as a reference for determining the relative intensities of the
SH signals generated from the samples. The Maker Fringe
pattern was obtained by measuring the SHG signal at 0.5ꢀ
intervals using a rotation stage. SHG coefficients (d33) were
derived from the analysis of measured Maker fringes.32
FIGURE 1 Main Chain NLO polymers (a), side chain NLO poly-
mers (b), and Y-type NLO polymers (c).
incorporated into novel Y-type NLO polyesters [see Fig. 1(c)].
The structure of NLO chromophores and these Y-type NLO
polyesters have not yet been described in the literature.
Thus, we synthesized a new type of NLO polyester, in which
the pendant NLO chromophores are parts of the polymer
backbone. This Y-type NLO polymer is expected to have the
advantages of both main-chain and side-chain NLO polymers,
namely stable dipole alignment and good solubility. After
confirming the structure of the resulting polymer, we investi-
gated its properties such as solubility, Tg, thermal stability,
surface morphology of polymer films, SHG activity, and relax-
ation of dipole alignment.
EXPERIMENTAL
Materials
Reagent grade chemicals were purchased from Aldrich and
purified by either distillation or recrystallization before use.
5-Methyl-4-(2-thiazolylazo)resorcinol and 2-chloroethyl vinyl
ether were used as received. Tetracyanoethylene (TCNE) and
terephthaloyl chloride (TPC) were purified by sublimation
under vacuum. N,N-Dimethylformamide (DMF) was purified
by drying with anhydrous calcium sulfate, followed by distil-
lation under reduced pressure.
Instrumentation
Infrared (IR) spectra were obtained with a Shimadzu FT IR-
1
8201PC IR spectrophotometer. H NMR and 13C NMR spectra
were obtained with a Varian 300 MHz NMR spectrometer.
UV–visible absorption spectra were obtained with a Shi-
madzu UV-3100S spectrophotometer. Elemental analyses
were performed using a Perkin-Elmer 2400 CHN elemental
analyzer. Tg values were measured using a TA 2920 differen-
tial scanning calorimeter (DSC) in a nitrogen atmosphere. A
TA Q50 thermogravimetric analyzer (TGA) with a heating
rate of 10 ꢀC minꢂ1 up to 800 ꢀC was used for the thermal
degradation of polymers under nitrogen. The number-aver-
NLO POLYESTER CONTAINING TRICYANOVINYLTHIAZOLE, CHO ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Preparation of 5-Methyl-4-(2-thiazolylazo)resorcinoxy
Ethyl Vinyl Ether (1)
matic), 7.80 (d, 1H, aromatic), 7.97 (d, 1H, aromatic), 8.07 (s,
4H, aromatic). IR (KBr) 3090 (w, ¼¼CAH), 2957 (w, CAH),
1718 (vs, C¼¼O), 1626 (m, C¼¼C) cmꢂ1. Anal. Calcd for
(C22H27N3O6S)n: C, 57.25; H, 5.90; N, 9.10; S, 6.95. Found: C,
57.33; H, 5.98; N, 9.17; S, 7.02.
5-Methyl-4-(2-thiazolylazo)resorcinol (23.5 g, 0.10 mol),
anhydrous potassium carbonate (82.9 g, 0.60 mol), and 2-
chloroethyl vinyl ether (26.6 g, 0.25 mol) were dissolved in
400 mL of dry DMF under nitrogen. The mixture was
Synthesis of Polymer 4
ꢀ
refluxed in an oil bath kept at 80 C for 15 h under nitrogen.
A representative reaction procedure was as follows. TCNE
(1.28 g, 10 mmol) was added slowly to a solution of polymer
3 (5.27 g, 8.5 mmol) dissolved in 30 mL of DMF with stir-
ring at room temperature under nitrogen. The resulting solu-
The resulting solution was cooled to room temperature,
diluted with 300 mL of water, and extracted with 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 chromatography (hexane/ethyl acetate ¼ 2/1 by
volume) yielded 33.0 g (yield 88%) of pure product 1.
1H NMR (acetone-d6) d 2.56 (d, 3H, ACH3), 3.92–4.41 (m,
12H, 2 CH2¼¼, 2 AOACH2ACH2AOA), 6.43–6.74 (m, 4H, 2
¼¼CHAOA, aromatic), 7.62–7.65 (q, 1H, aromatic), 7.93–7.98
(m, 1H, aromatic). IR (KBr) 3112, (m, ¼¼CAH), 2922 (m,
CAH), 1622 (vs, C ¼¼ C) cmꢂ1. Anal. Calcd for C18H21N3O4S:
C, 57.58; H, 5.64; N, 11.19; S, 8.54. Found: C, 57.65; H, 5.68;
N, 11.14; S, 8.58.
ꢀ
tion was heated in an oil bath kept at 70 C for 16 h under
a nitrogen atmosphere. The resulting polymerization solution
was cooled to room temperature and poured into 400 mL of
methanol. The precipitated polymer was collected and
reprecipitated from DMSO into methanol. The polymer was
further purified by extraction in a Soxhlet extractor with
methanol and dried under vacuum, yielding 4.94 g (88%
yield) of polymer 4.
Inherent viscosity (ginh) ¼ 0.30 dL gꢂ1 (c, 0.5 g dLꢂ1 in
DMSO at 25 ꢀC). 1H NMR (DMSO-d6) d 2.52 (s, 3H, ACH3),
3.93–4.08 (m, 4H, 2 ACH2AOA), 4.33–4.76 (m, 4H, 2 Ph-
OACH2A), 7.15–7.43 (s, 4H, aromatic), 7.62–7.85 (m, 1H, ar-
omatic), 8.16–8.28 (m, 1H, aromatic), 8.93–9.12 (d, 1H, aro-
matic). 13C NMR (DMSO-d6) d 22.80, 60.43, 60.46, 64.90,
100.31, 110.01, 111.12, 116.93, 119.01, 128.74, 131.50,
133.85, 134.50, 148.92, 149.12, 153.09, 154.34, 173.44. IR
(KBr) 3085 (w, ¼¼CAH), 2956 (m, CAH), 2220 (s, CN), 1719
Preparation of 5-Methyl-4-(2-
thiazolylazo)resorcinoxyethanol (2)
Aqueous hydrochloric acid (1.5 mol Lꢂ1, 12 mL) was slowly
added to a solution of compound 1 (3.75 g, 10 mmol) in
ꢀ
30 mL of dry DMF with stirring under nitrogen at 0 C. The
(vs, C¼¼O), 1590 (s, C¼¼C) cmꢂ1
.
Anal. Calcd for
mixture was stirred at 0 ꢀC for 5 h under nitrogen. The
resulting solution was poured into 100 mL of ice water and
stirred. The crude product obtained was separated by suc-
tion and washed with 30% aqueous ethanol to give 2.78 g
(yield 86%) of 2.
(C27H26N6O6S)n: C, 57.64; H, 4.66; N, 14.94; S, 5.70. Found:
C, 57.73; H, 4.72; N, 14.86; S, 5.78.
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymer
1H NMR (acetone-d6) d 2.57 (s, 3H, ACH3), 3.88–3.94 (m,
2H, ACH2AOA), 4.21–4.27 (m, 2H, Ph-OACH2A), 6.42 (d,
1H, aromatic), 6.64 (d, 1H, aromatic), 7.65 (d, 1H, aromatic),
7.95 (d, 1H, aromatic). IR (KBr) 3312 (vs, OAH), 3084 (w,
¼¼CAH), 2943 (m, CAH), 1631 (vs, C¼¼C) cmꢂ1. Anal. Calcd
for C14H17N3O4S: C, 52.00; H, 5.30; N, 12.99; S, 9.92. Found:
C, 52.08; H, 5.36; N, 12.92; S, 9.96.
Compound 1 was prepared by the reaction of 2-chloroethyl
vinyl ether with 5-methyl-4-(2-thiazolylazo)resorcinol. Diol 2
was prepared by acid-catalyzed hydrolysis of 2 in DMF. Poly-
ester 3 was synthesized through the reaction of monomer 2
containing NLO chromophore with stoichiometric amount of
TPC in pyridine (see Scheme 1). The polymerization yield
was 90–92%. Polymer 3 was reacted with TCNE in anhy-
drous DMF33 to yield polyester 4 containing 5-methyl-4-{5-
(1,2,2-tricyanovinyl)-2-thiazolylazo}resorcinoxy groups as
NLO chromophores. The synthetic route for polymer 4 is
presented in Scheme 1. The resulting polymer was purified
by Soxhlet extraction for 2 days with methanol as a solvent.
The chemical structure of the resulting polymer was con-
firmed using 1H NMR, IR spectra, and elemental analysis.
Synthesis of Polyester 3
A representative polycondensation reaction procedure was
as follows. TPC (2.03 g, 10 mmol) and diol 2 (3.23 g, 10
mmol) were dissolved in 25 mL of anhydrous pyridine under
nitrogen. Tꢀhe resulting solution was heated in an oil bath
kept at 80 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 reprecipitated from dimethylsulfoxide
(DMSO) into methanol. The polymer was further purified by
extraction in a Soxhlet extractor with methanol and dried
under vacuum, yielding 4.15 g (90% yield) of polymer 3.
1
Elemental analysis results fit the polymer structure. H NMR
spectrum of the polymer showed a signal broadening due to
polymerization, but the chemical shifts are consistent with
the proposed polymer structure. The 13C NMR spectrum of
the polymer 4 shows a signal at 117–119 ppm assigned to
the cyano carbon, which indicates the presence of nitrile
group. The IR spectrum of the polymer 4 shows a strong
carbonyl peaks near 1719 cmꢂ1 indicating the presence of
ester bond. The spectrum also shows strong absorption peak
near 2220 cmꢂ1 indicating the presence of nitrile group.
Inherent viscosity (ginh) ¼ 0.30 dL gꢂ1 (c, 0.5 g dLꢂ1 in
DMSO at 25 ꢀC). 1H NMR (DMSO-d6) d 2.51 (s, 3H, ACH3),
4.45–4.54 (m, 4H, 2 AOACH2A,) 4.62–4.68 (m, 4H, 2
AOACH2AOCOA), 6.55 (d, 1H, aromatic), 6.70 (d, 1H, aro-
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ARTICLE
SCHEME 1 Synthetic scheme and structure of polymer 4.
ꢀ
These results are consistent with the proposed structures,
indicating that the tricyanovinyl groups are attached well to
the thiazole rings. The molecular weights were determined
using GPC with polystyrene as the standard and THF as the
eluent. Mn of the polymer 4, determined using GPC, is
17,200 g molꢂ1 (Mw/Mn ¼ 1.94). The inherent viscosity is
0.30–0.32 dL gꢂ1. Polymer 4 shows strong absorption near
394 nm due to the 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2-thia-
zolylazo}resorcinoxy group NLO chromophore. The structural
feature of this polymer is that it has pendant NLO chromo-
phores, which are parts of the main chain polymer. Thus, the
resulting polymer 4 is mid type of side-chain and main-chain
NLO polymer, and is expected to have both of their merits.
The polymer 4 is soluble in common solvents such as ace-
tone, DMF, and DMSO, but is not soluble in methanol and
diethyl ether. Having obtained the well-defined polyester 4,
we investigated its properties.
measured using DSC is approximately 94 C. This Tg value is
similar with those of the polyesters containing dioxynitrostil-
bene, which are in the range 90–95 ꢀC,34 but is higher than
those with dioxybenzylidenemalononitrile, which are in the
25,26
ꢀ
range 83–94 C.
The TGA and DSC studies show that the
decomposition temperature of the polyester 4 is higher than
the corresponding Tg. This indicates that high-temperature
poling for a short term is feasible without damaging the NLO
chromophore.
Nonlinear Optical Properties of Polymer
The NLO properties of polymer were studied using the SHG
method. To induce noncentrosymmetric polar order, the
spin-coated polymer filmsꢀwere corona poled. As the temper-
ature was raised to 5–10 C higher than Tg, a corona voltage
of 6.5 kV was applied, and this temperature was maintained
for 30 min. The poling was confirmed from UV–visible spec-
tra. After electric poling, the dipole moments of the NLO
chromophores were aligned and UV–visible absorption of
polymer 4 exhibits a decrease in absorption due to align-
ment of the chromophores perpendicular to the substrate
plain. From the absorbance change, the order parameter of
the poled films can be estimated, which is related to the
poling efficiency. The estimated order parameter value U is
Thermal Properties of Polymers
The thermal behavior of the polymer was investigated using
TGA and DSC to determine the thermal degradation pattern
and glass transition temperature. The results are summar-
ized in Table 1. The TGA thermogram of the polymer 4 is
shown in Figure 2. Polymer 4 shows a thermal stability up
to 250 ꢀC from its TGA thermogram. Tg value of polymer 4
equal to 0.15 for polymer 4 (U ¼ 1 ꢂ A1/A0, where A0
¼
TABLE 1 Thermal Properties of Polymer 4
Degradation Temperature (ꢀC)b
Residueb at
800 ꢀC (%)
a
Tg
5 wt %
Loss
10 wt %
Loss
20 wt %
Loss
40 wt %
Loss
Polymer
(ꢀC)
4
94
252
271
286
317
31.0
a
Determined from DSC curves measured with a TA 2920 differential scanning calorimeter with a heating
rate of 10 ꢀC minꢂ1 under nitrogen atmosphere.
b
Determined from TGA curves measured with a TA Q50 thermogravimetric analyzer with a heating rate of
10 ꢀC minꢂ1 under nitrogen atmosphere.
NLO POLYESTER CONTAINING TRICYANOVINYLTHIAZOLE, CHO ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
thin-film samples were obtained with AFM. Figure 3 shows
AFM scans of a spin-coated film of polymer 4 before and af-
ter poling. AFM images show that the surface of the film
sample is extremely flat and smooth before poling [Fig.
3(a)]. 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 chromo-
phores were aligned in the poling direction, as shown in Fig-
ure 3(b).
The refractive index of the sample was measured using the
optical transmission technique.31 The transmittance of thin
film gives information on the thickness, refractive index, and
extinction coefficient. Thus, we could determine these pa-
rameters by analyzing the transmittance. SHG measurements
were performed at a fundamental wavelength of 1560 nm
using a mode-locked Nd-YAG laser. To determine the micro-
scopic second-order susceptibility of the polymer, the angu-
lar SHG dependence was recorded. Figure 4 shows the angu-
lar dependence of SHG signal for a poled sample of polymer
4. The SHG values were compared with those obtained 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 and recorded. SHG coefficients (d33) were derived
from the analysis of measured Maker fringes with the Pascal
fitting program according to the literature procedure.32 The
NLO properties of polymer 4 are summarized in Table 2.
The values of d31 and d33 for polymer 4 are 2.56 ꢁ 10ꢂ9 esu
and 8.12 ꢁ 10ꢂ9 esu, respectively. As the second harmonic
wavelength is at 780 nm, which is not in the absorptive
region of the resulting polyester, there is no resonant contri-
bution to this d33 value. In the isotropic model, the ratio of
FIGURE 2 TGA thermogram of polymer 4 obtained at a heating
rate of 10 ꢀC minꢂ1 under nitrogen.
1.5515 and A1 ¼ 1.3226 are the absorbances of the polymer
film before and after poling, respectively).
For the purpose of investigating surface morphology of poly-
mer film, domain structures of NLO chromophores for the
d
33/d31 is predicted to be about 3. Our d33/d31 value of 3.17
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. Figure 5
shows the dynamic thermal stability study of the NLO activ-
ity of a film of polymer 4. To investigate the real time NLO
decay of the SHG signal of the poled polymer film as a
FIGURE 3 AFM images of spin-coated film of polymer 4: (a)
FIGURE 4 Angular dependence of SHG signal for a poled film
of polymer 4.
before corona-poling and (b) after corona-poling.
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ARTICLE
TABLE 2 Nonlinear Optical Properties of Polymer 4
Film Thicknessd
a
b
b
kmax
d33
d31
Polymer
(nm)
(esu)
Uc
(esu)
(lm)
n
4
394
8.12 ꢁ 10ꢂ9
0.15
2.56 ꢁ 10ꢂ9
0.51
1.532
a
Polymer film after corona poling.
SHG coefficients (d33) were derived from the analysis of measured Maker fringes.32
Order parameter U ¼ 1 ꢂ A1/A0, where A0 and A1 are the absorbances of the polymer film before and af-
b
c
ter corona poling, respectively.
d
Film thickness was determined using the optical transmission technique.31
function of temperature, in situ SHG mꢂea1surements were
ꢀ
performed at a heating rate of 3.5 C min from 30 to 200
ꢀ
ꢀC. The polymer film exhibits a thermal stability even at 6 C
ꢀ
higher than Tg, and no SHG decay is observed below 100 C
for polymer 4. In Figure 6, we present the temporal stability
of the polymer film in which there was no negligible decay
of the SHG signal over hundreds of hours. 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 enhanced
thermal stability of SHG of polymer 4 is due to the stabiliza-
tion of dipole alignment of NLO chromophore, which stems
from the partial main-chain character of the polymer struc-
ture. Thus, we obtained a new type of NLO polyester having
both the merits of main-chain and side-chain NLO polymers,
namely stable dipole alignment and good solubility.
FIGURE 6 Normalized SHG signal of polymer 4 as a function of
baking time at 60 ꢀC in air.
CONCLUSION
New Y-type polyester 4 with pendant NLO chromophores as
parts of the main chain polymer was synthesized. This Y-
type NLO polyester 4 is soluble in common organic solvents
mer backbone. 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.
ꢀ
and displays a thermal stability up to 250 C with Tg values
near 94 ꢀC. The SHG coefficient (d33) of corona-poled poly-
mer film is 8.17 ꢁ 10ꢂ9 esu. Polymer 4 exhibits SHG stabil-
This research was supported by Basic Science Research Pro-
gram through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and Tech-
nology (No. 2010-0020951).
ꢀ
ity even at 6 C higher ꢀthan Tg, and no significant SHG decay
is observed below 100 C. This high-thermal stability of opti-
cal nonlinearity stems from the stabilization of dipole align-
ment of the NLO chromophores, which are parts of the poly-
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