Synthesis and nonlinear optical properties of novel Y-type polyurethane containing tricyanovinylthiazole with enhanced thermal stability of second harmonic generation

Article abstract of DOI:10.1002/pola.23875

A novel Y-type poly[iminocarbonyloxyethyl-5-methyl4-{2-thlazolylazo-4-(1,2, 2-tricyanovinyl)}resorcinoxyethyloxycarbonylimino-(3,3′-dimethoxy-4, 4′-biphenylene)] 4 containing 5methyl-4-(5-(1,2,2-tricyanovinyl)-2- thiazolylazo}resorcinoxy groups as nonline

Full text of DOI:10.1002/pola.23875

Synthesis and Nonlinear Optical Properties of Novel Y-Type Polyurethane
Containing Tricyanovinylthiazole with Enhanced Thermal Stability of
Second Harmonic Generation
HYO JIN NO, HAN-NA JANG, YOU JIN CHO, JU-YEON LEE
Department of Chemistry, Institute of Basic Science, Inje University, 607 Obang-dong, Gimhae 621-749, Korea
Received 5 November 2009; accepted 8 December 2009
DOI: 10.1002/pola.23875
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A novel Y-type poly[iminocarbonyloxyethyl-5-methyl-
1560 nm fundamental wavelength was around 8.43 ꢁ 10ꢂ9 esu.
The dipole alignment exhibited a thermal stability even at 12 ꢀC
higher than Tg, and there was no SHG decay below 130 ꢀC due
4-{2-thiazolylazo-4-(1,2,2-tricyanovinyl)}resorcinoxyethyloxycarbo-
nylimino-(3,30-dimethoxy-4,40-biphenylene)]
4
containing 5-
methyl-4-{5-(1,2,2-tricyanovinyl)-2-thiazolylazo}resorcinoxy groups
as nonlinear optical (NLO) chromophores, which constitute part
of the polymer backbone, was prepared and characterized. Poly-
urethane 4 is soluble in common organic solvents such as ace-
tone and N,N-dimethylformamide. It showed a thermal stability
up to 250 ꢀC in thermogravimetric analysis thermogram and the
glass-transition temperature (Tg) obtained from differential scan-
ning calorimetry thermogram was around 118 ꢀC. The second
harmonic generation coefficient (d33) of poled polymer films at
to the partial main-chain character of the polymer structure,
C
V
which is acceptable for NLO device applications.
2010 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1166–1172,
2010
KEYWORDS: atomic force microscopy (AFM); differential scan-
ning calorimetry (DSC); NLO; polyurethane; second harmonic
generation (SHG); thermal stability; thermogravimetric analysis
(TGA)
INTRODUCTION Nonlinear optical (NLO) materials have been
extensively studied recently because of their potential appli-
cations in the field of electro-optic devices, including ultrafast
optical switches, high-speed optical modulators, and high-
density optical data storage media.1–5 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. One of the current
tasks is to design novel NLO polymers having optimized prop-
erties. In the developments of NLO polymers for electro-optic
device applications, stabilization of electrically induced dipole
alignment is one of important considerations; in this context,
two approaches to minimize the randomization have been
proposed namely the use of crosslinked systems6–10 and the
use of polymers with high glass transition temperature (Tg)
such as polyimides.11–17 The polyurethane matrix forms
extensive hydrogen bonding between urethane linkage and
increases rigidity preventing the relaxation of induced
dipoles. Polyurethanes functionalized with hemicyanine18 and
thiophene ring19 in side chain showed an enhanced thermal
stability of aligned dipoles. Polyurethanes with dipole
moments aligning transverse to the main chain showed large
second-order nonlinearity with good thermal stability.20–21
Physically crosslinked systems via hydrogen bonds have the
advantages such as homogenity and good processability
relative to chemically crosslinked systems, which suffer from
significant optical loss and poor solubility. Recently, we pre-
pared novel polyurethanes,22–28 polyimides,29–30 and polyes-
ters31–32 containing dioxynitrostilbene, dioxybenzylidenemalo-
nonitrile, and 1-(2,4-dioxyphenyl)-2-{5-(1,2,2-tricyanovinyl)-
2-thienyl}ethene as NLO chromophores. The resulting poly-
mers exhibited enhanced thermal stability of second harmonic
generation (SHG), which stemmed from the stabilization of
dipole alignment of the NLO chromophores.
In this work reported here, we have prepared a novel poly-
urethane containing 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2-
thiazolylazo}resorcinoxy groups as NLO-chromophores. We
selected the latter because they have large dipole moment.
Furthermore, 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2-thiazolyla-
zo}resorcinoxy groups can be incorporated into novel Y-type
NLO polyurthane [see Fig. 1(c)]. The structure of NLO chro-
mophores and this Y-type NLO polyurethane has not yet
been described in the literature. Thus, we formulated a new
type of NLO polyurethane in which the pendant NLO chro-
mophores are components of the polymer backbone. This
mid-type NLO polymer is expected to have both the merits
of main-chain [Fig. 1(a)] and side-chain [Fig. 1(b)] NLO poly-
mers, namely stable dipole alignment and good solubility. Af-
ter confirming the structure of the resulting polymer, we
Correspondence to: J.-Y. Lee (E-mail: chemljy@inje.ac.kr)
C
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1166–1172 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
operated in a contact mode, which measures topography.
Melting points were measured with a Buchi 530 melting
point apparatus. Viscosity values were obtained using a Can-
non-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 (ITO) cov-
ered glass. Before film casting, the polymer solution was fil-
R
V
tered through 0.45-lm Teflon 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
carried out by corona poling method. The poling was per-
formed 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 from 5 to 10 ꢀC higher
than Tg, a corona voltage of 6.5 kV was applied and the tem-
perature 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 by the optical transmission
technique.33 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. 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
repetition 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 con-
sists of a LiNbO3 crystal (12 mm ꢁ 15 mm ꢁ 50 mm) 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-polarization) to the plane of incidence. Only the p-polar-
ized 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 sig-
nal. A poled polymer film was mounted on the rotator
coupled to a step motor. The output signals from the photo-
diode 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
FIGURE 1 Main Chain NLO polymers (a), side chain NLO poly-
mers (b), and Y-type NLO polymers (c).
investigated its properties: Tg, thermal stability, surface mor-
phology of polymer films, and SHG activity.
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) was
purified by sublimation under vacuum. 3,30-Dimethoxy-4,40-
biphenylenediisocyanate (DMBPI) was recrystallized from
ethyl acetate. N,N-Dimethylformamide (DMF) was purified by
drying with anhydrous calcium sulfate, followed by distilla-
tion under reduced pressure.
Instrumentation
Infrared (IR) spectra were taken on a Shimadzu FT IR-
8201PC infrared spectrophotometer. 1H-NMR spectra were
obtained on a Varian 300MHz NMR spectrometer. UV–visible
absorption spectra were measured on a Shimadzu UV-3100S
spectrophotometer. Elemental analyses were performed
using a Perkin-Elmer 2400 CHN elemental analyzer. The
glass transition temperatures (Tg) were measured using a TA
2920 differential scanning calorimeter (DSC) in a nitrogen
atmosphere. TA Q50 thermogravimetric analyzer (TGA) with
V
ꢂ1) 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 0.5 intervals using a rotation stage. SHG coefficients
(d33) were derived from the analysis of measured Maker-
fringes.34
ꢂ1
ꢀ
ꢀ
a heating rate of 10 C min up to 800 C was used for the
thermal degradation of polymers under nitrogen. The num-
ber-average molecular weight (Mn) and weight-average mo-
lecular weight (Mw) of the polymers were estimated using
gel permeation chromatography (GPC; styragel HR5E4E col-
umns; tetrahydrofuran (THF) solvent). AFM images were
recorded with a Park Science Instrument Autoprobe CP,
Preparation of 5-Methyl-4-(2-thiazolylazo)
resorcinoxyethyl Vinyl Ether (1)
5-Methyl-4-(2-thiazolylazo)resorcinol (23.5 g, 0.10 mol),
anhydrous potassium carbonate (82.9 g, 0.60 mol), and 2-
NOVEL Y-TYPE POLYURETHANE CONTAINING TRICYANOVINYLTHIAZOLE, NO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
chloroethyl vinyl ether (26.6 g, 0.25 mol) were dissolved in
400 mL of dry DMF under nitrogen. The mixture was
refluxed in an oil bath kept at 80 C for 15 h under nitrogen.
1H-NMR (DMSO-d6) d 2.52 (s, 3H, ACH3), 3.84–4.01 (m, 10H,
AOCH3A, AOACH2A,) 4.35–4.43 (d, 4H, 2
2
2
ꢀ
AOACH2AOCOA), 6.51 (s, 1H, aromatic), 6.67 (s, 1H, aro-
matic), 7.16–7.28 (m, 4H, aromatic), 7.65–7.68 (d, 1H, aro-
matic), 7.76–7.79 (d, 1H, aromatic), 7.95–7.98 (d, 1H, aro-
matic), 8.16–8.21 (d, 1H, aromatic), 8.65 (s, 1H, NAH), 9.01
(s, 1H, NAH). IR (KBr) 3421 (s, NAH), 2957 (m, CAH), 1687
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 recrystallized
from 1-butanol yielded 33.0 g (yield 88%) of pure product
1.
(s, C¼¼O), 1617 (s, C¼¼C) cmꢂ1
.
Anal. Calcd. for
(C30H29N5O8S)n: C, 58.15; H, 4.72; N, 11.30; S, 5.18. Found:
C, 58.26; H, 4.79; N, 11.38; S, 5.26.
Synthesis of Poly[iminocarbonyloxyethyl-5-methyl-4-
{2-thiazolylazo-4-(1,2,2-tricyanovinyl)}resorcinoxyethyloxy-
carbonylimino-(3,30-dimethoxy-4,40-biphenylene)] (4)
A representative reaction procedure was as follows. Tetra-
cyanoethylene (1.28 g, 10 mmol) was added slowly to a so-
lution of polymer 3 (5.27 g, 8.5 mmol) dissolved in 15 mL of
DMF with stirring at room temperature under nitrogen. The
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, C¼¼C) 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.
ꢀ
resulting solution was heated in an oil bath kept at 70 C for
12 h under a nitrogen atmosphere. The resulting polymeriza-
tion solution was cooled to room temperature and poured
into 400 mL of methanol. The precipitated polymer was col-
lected and reprecipitated from DMSO into methanol. The
polymer was further purified by extraction in a Soxhlet ex-
tractor with methanol and dried under vacuum to give 6.49
g (90% yield) of polymer 4.
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
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 product obtained was separated by suction and
washed with 30% aqueous ethanol. Thus obtained product
was recrystallized from ethanol to give 2.78 g (yield 86%) of
2.
Inherent viscosity (ginh) ¼ 0.30 dL gꢂ1 (c ¼ 0.5 g dLꢂ1 in
DMSO at 25 ꢀC). 1H-NMR (DMSO-d6) d 2.53 (s, 3H, ACH3),
3.83–4.01 (m, 10H, 2 AOCH3A, 2 AOACH2A,) 4.35–4.70 (q,
4H, 2 AOACH2AOCOA), 6.51 (s, 1H, aromatic), 6.68 (s, 1H,
aromatic), 7.16–7.29 (m, 4H, aromatic), 7.64–7.73 (d, 1H, ar-
omatic), 7.67–7.98 (t, 1H, aromatic), 8.15–8.33 (t, 1H, aro-
matic), 8.65 (s, 1H, NAH), 8.97–9.07 (d, 1H, NAH). IR (KBr)
3392 (s, NAH), 2940 (m, CAH), 2200 (m, CN), 1687 (s,
C¼¼O), 1594 (s, C¼¼C) cmꢂ1. Anal. Calcd. for (C35H28N8O8S)n:
C, 58.33; H, 3.91; N, 15.55; S, 4.45. Found: C, 58.42; H, 3.98;
N, 15.46; S, 4.38.
1H-NMR (acetone-d6) d 2.57 (s, 3H, ACH3), 3.88–3.94 (m,
2H, ACH2AOA), 4.21–4.27 (m, 2H, PhAOACH2A), 6.42 (d,
1H, aromatic), 6.64 (d, 1H, aromatic), 7.65 (d, 1H, aromatic),
7.95 (d, 1H, aromatic). IR (KBr) 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.
RESULTS AND DISCUSSION
Synthesis of Poly{iminocarbonyloxyethyl-5-methyl-4-
(2-thiazolylazo)resorcinoxyethyloxycarbonylimino-
(3,30-dimethoxy-4,40-biphenylene)} (3)
Synthesis and Characterization of Polymer
5-Methyl-4-(2-thiazolylazo)resorcinoxyethyl vinyl ether (1)
was prepared by the reaction of 2-chloroethyl vinyl ether
with 5-methyl-4-(2-thiazolylazo)resorcinol. Diol 2 was pre-
pared by acid-catalyzed hydrolysis of 2 in DMF. Polyurethane
3 was prepared by the polyaddition reaction between a diol
2 and 3,30-dimethoxy-4,40-biphenylenediisocyanate in a dry
DMF solvent. The polymerization yield was 88–90%. Poly-
mer 3 was reacted with tetracyanoethylene in anhydrous
DMF19 to yield polyurethane 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 polymer was identified using
1H-NMR, IR spectra, and elemental analysis. Elemental analy-
sis results fit the polymer structure. 1H-NMR spectrum of
the polymer showed a signal broadening due to polymeriza-
tion, but the chemical shifts are consistent with the proposed
A representative polyaddition reaction procedure was as fol-
lows: DMBPI (2.96 g, 0.01 mol) was added slowly to a solu-
tion of 3.23 g of diol 2 (0.01 mol) in 25 mL of anhydrous
DMF. The resulting solution was degassed by a freeze–thaw
process under vacuum and placed in an oil bath kept at 80
ꢀC. After heating 12 h with stirring, the polymerization tube
was opened and the viscous polymer solution was poured
into 400 mL of cold water. The precipitated polymer was col-
lected and reprecipitated from DMSO into methanol. The
polymer was further purified by extraction in a Soxhlet ex-
tractor with methanol and dried under vacuum to give 5.46
g (88% yield) of polymer 3.
Inherent viscosity (ginh) ¼ 0.31 dL gꢂ1 (c ¼ 0.5 g dLꢂ1 in
ꢀ
DMSO at 25 C). The number average molecular weight (Mn)
of the polymer 3, determined by GPC, was 18,200 (Mw/Mn ¼
1.93).
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SCHEME 1 Synthetic route to and structure of polymer 4.
polymer structure. The signal at 8.65–9.07 ppm assigned to
the amine proton indicates the formation of urethane link-
age. The IR spectrum of polymer 4 showed strong absorp-
tion peak near 2200 cmꢂ1 indicating the presence of nitrile
group. The spectrum also shows a strong carbonyl peak near
1687 cmꢂ1 indicating the presence of urethane bond. These
results are consistent with the proposed structure, indicating
that the tricyanovinyl groups are introduced well to the thio-
phene 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 18,400 g
molꢂ1 (Mw/Mn ¼ 1.96). The polymer 4 is soluble in common
solvents such as acetone, DMF, and DMSO, but is not soluble
in methanol and diethyl ether. The inherent viscosity is
0.30–0.31 dL gꢂ1. Polymer 4 showed strong absorption near
422 nm due to the 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2-thia-
zolylazo}resorcinoxy group NLO chromophore. The striking
feature of this polymerization system is that it gives unprec-
edented Y-type NLO polymers in which the pendant NLO
chromophores are part of the polymer backbone. These mid-
type NLO polymers are expected to have the advantages of
both main-chain and side-chain NLO polymers. Thus, we
obtained a new type of NLO polyurethane with side-chain
and main-chain characteristics. Having well defined Y-type
polyurethane 4, we investigated its properties.
Thermal Properties of Polymer
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 7 is
shown in Figure 2. Polymer 4 has a thermal stability up to
ꢀ
250 C according to its TGA thermogram. Thꢀe Tg value of the
polymer 4 measured using DSC is near 118 C. This Tg value
is lower than those of the polyurethanes containing dioxyni-
trostilbene, which are in the range 142–143 ꢀC,22 dioxyben-
23–24
ꢀ
zylidenemalononitrile, which are in the range 145–159 C,
or 1-(2,4-dioxyphenyl)-2-{5-(1,2,2-tricyanovinyl)-2-thienyl}ethene,
which is near 163 ꢀC.25 The TGA and DSC studies show that
the decomposition temperature of the polymer 4 is higher
than the corresponding Tg value. This indicates that high-
TABLE 1 Thermal Properties of Polymer 4
Degradation Temp (ꢀC)b
Residue at 800 ꢀC (%)b
37.0
a
Polymer
Tg (ꢀC)
5 wt %-Loss
256
20 wt %-Loss
40 wt %-Loss
455
4
118
308
a
b
Determined from DSC curves measured with a TAꢂ12920 differential
scanning calorimeter with a heating rate of 10 ꢀC min under nitrogen
atmosphere.
Determined from TGA curves measured with a TA Q50 thermogravi-
metric analyzer with
atmosphere.
a
heating rate of 10 ꢀC minꢂ1 under nitrogen
NOVEL Y-TYPE POLYURETHANE CONTAINING TRICYANOVINYLTHIAZOLE, NO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 TGA thermogram of polymer 4 obtained at a heating
rate of 10 ꢀC minꢂ1 under nitrogen.
temperature poling for a short term is feasible without dam-
aging the NLO chromophore.
NLO Properties of Polymer
The NLO properties of polymer were studied using the SHG
method. To induce noncentrosymmetric polar order, the
spin-coated polymer films were corona-poled. As the temper-
ature was raised gradually from 5 to 10 ꢀC higher than Tg,
6.5 kV of corona voltage was applied and this temperature
was maintained for 30 min. The poling was confirmed by
UV–visible spectra. Figure 3 shows the UV–visible absorption
spectra of the polymer 4 before and after poling. After elec-
tric poling, the dipole moments of the NLO chromophores
were aligned, and UV–visible absorption of polymer 4 exhib-
its a decrease in absorption due to birefringence. From the
absorbance change, the order parameter of the poled film
could be estimated, which is related to the poling efficiency.
The estimated order parameter value U was equal to 0.19
for polymer 4 (U ¼ 1 ꢂ A1/A0, where A0 ¼ 1.1726 and A1
¼ 0.9521 are the absorbances of the polymer film before
and after poling, respectively).
FIGURE 4 AFM images of spin-coated film of polymer 4: (a)
before corona-poling and (b) after corona-poling.
For the purpose of investigating surface morphology of poly-
mer films, domain structures of NLO-chromophores for the
thin-film samples were obtained using atomic force
FIGURE 3 UV–visible absorption spectra of a film of polymer 4
FIGURE 5 Angular dependence of SHG signal for a poled film
of polymer 4.
before and after poling.
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TABLE 2 Nonlinear Optical Properties of Polymer 4
Uc
Film Thicknessd (lm)
d31 (esu)
n
a
b
b
Polymer
kmax (nm)
d33 (esu)
4
422
8.43 ꢁ 10ꢂ9
0.19
0.52
2.62 ꢁ 10ꢂ9
n1 ¼ 1.521, n2 ¼ 1.553
a
c
Polymer film after corona poling.
Order parameter U ¼ 1 ꢂ A1/A0, where A0 and A1 are the absorbances
b
SHG coefficients (d33) were derived from the analysis of measured
Maker-fringes.34
of the polymer film before and after corona poling, respectively.
Film thickness was determined using the optical transmission technique.33
d
microscopy (AFM). Figure 4 shows AFM scans of the spin-
coated film of polymer 4 before and after poling. AFM
images show that the surface of the film sample is extremely
flat and clean before poling [Fig. 4(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-chromophores are aligned in the poling
direction Figure 4(b).
respectively. As the second harmonic wavelength is at 780
nm, which is not in the absorptive region of the resulting
polyurethane, there was not resonant contribution to this
d33 value. In the isotropic model, the ratio of d33/d31 is pre-
dicted to be about 3. Our d33/d31 value of 3.22 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 6
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 func-
tion of temperature, in situ SHG measurements were per-
The refractive index of the sample was measured using the
optical transmission technique.33 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.
ꢂ1
ꢀ
ꢀ
ꢀ
formed at a heating rate of 4.5 C min from 30 to 220 C.
The polymer film exhibited a thermal stability even at 12 C
SHG measurements were performed at a fundamental wave-
length of 1560 nm using a mode locked Nd-YAG laser and
OPO. To determine the microscopic second-order susceptibil-
ity of the polymers, the angular SHG dependence was
recorded. Figure 5 shows the angular 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. The NLO
properties of polymer 4 are summarized in Table 2. SHG
coefficients (d33) were derived from the analysis of mea-
sured Maker-fringes with the Pascal fitting program accord-
ing to the literature procedure.34 The values of d31 and d33
for polymer 4 are 2.62 ꢁ 10ꢂ9 esu and 8.43 ꢁ 10ꢂ9 esu,
higher than Tg, and no significant SHG decay was observed
below 130 ꢀC. Figure 7 shows 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 stabilization of dipole
alignment of NLO chromophore, which stems from the par-
tial main-chain character of the polymer structure and partly
by hydrogen bonds between the neighboring urethane link-
ages. Thus, we obtained a new type of NLO polyurethane
having the advantages of both main-chain and side-chain
NLO polymers: stabilization of dipole alignment and good
solubility.
FIGURE 6 Normalized SHG signal of polymer 4 as a function of
FIGURE 7 Normalized SHG signal of polymer 4 as a function of
baking time at 80 ꢀC in air.
temperature at a heating rate of 4.5 ꢀC minꢂ1
.
NOVEL Y-TYPE POLYURETHANE CONTAINING TRICYANOVINYLTHIAZOLE, NO ET AL.
1171

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
CONCLUSIONS
12 Chen, T.; Jen, A. K. Y.; Cai, Y. Macromolecules 1996, 29,
535–539.
We synthesized novel Y-type polyurethane 4 with pendant
NLO chromophores, which are part of the polymer main
chain. This mid-type NLO polyurethane is soluble in common
organic solvents. The resulting polymer 4 shows a thermal
stability up to 250 ꢀC from TGA thermograms with Tg value
near 118 ꢀC. The SHG coefficient (d33) of corona-poled poly-
mer film was 8.43 ꢁ 10ꢂ9 esu. The striking feature of this
polymer is that it exhibits a SHG thermal stability even at 12
ꢀC higher than Tg, and no SHG decay was observed below 130
ꢀC. This enhanced thermal stability of optical nonlinearity
stems from the stabilization of dipole alignment of the NLO-
chromophores, which constitute part of the polymer backbone
and partly by hydrogen bonds between the neighboring ure-
thane linkages. 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.
13 Yu. D.; Gharavi, A.; Yu, L. Macromolecules 1996, 29,
6139–6142.
14 Saadeh, H.; Gharavi, A.; Yu, D.; Yu, L. Macromolecules
1997, 30, 5403–5407.
15 Tsutsumi, N.; Morishima, M.; Sakai, W. Macromolecules
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16 Liu, J.-G.; Nakamura, Y.; Shibasaki, Y.; Ando, S.; Ueda, M. J
Polym Sci Part A: Polym Chem 2007, 45, 5606–5617.
17 Hoang, M. A.; Kim, M. H.; Cho, M. J.; Kim, K. H.; Kim, K. N.;
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18 Moon, K. J.; Shim, H. K.; Lee, K. S.; Zieba, J.; Prasad, P. N.
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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
Technology (No. 2009-0070572).
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