Synthesis and nonlinear optical properties of novel Y-type polyurethane containing cyanovinylthiazole

Article abstract of DOI:10.1080/15421400903584382

A novel Y-type polyurethane 4 containing 5-methyl-4-{5-(1,2,2- tricyanovinyl)-2-thiazolylazo}resorcinoxy groups as nonlinear optical (NLO)-chromophores, which constituted parts of the polymer backbones, was prepared and characterized. Polyurethane 4 is soluble in common organic solvents such as acetone and N,N-dimethylformamide. It showed a thermal stability up to 250°C in thermogravimetric analysis thermogram and the glass-transition temperature (T_g) obtained from differential scanning calorimetry thermogram was around 118°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at 1560nm fundamental wavelength was around 8.43 × 10^-9esu. The dipole alignment exhibited a thermal stability even at 12°C higher than T_g, and there was no SHG decay below 130°C due to the partial main-chain character of the polymer structure, which is acceptable for nonlinear optical device applications.

Full text of DOI:10.1080/15421400903584382

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Synthesis and Nonlinear Optical
Properties of Novel Y-Type Polyurethane
Containing Cyanovinylthiazole
Hyo Jin No a , You Jin Cho a & Ju-Yeon Lee a
a Institute of Basic Science, Department of Chemistry, Inje
University, Gimhae, Korea
Version of record first published: 19 Apr 2010.
To cite this article: Hyo Jin No , You Jin Cho & Ju-Yeon Lee (2010): Synthesis and Nonlinear Optical
Properties of Novel Y-Type Polyurethane Containing Cyanovinylthiazole, Molecular Crystals and Liquid
Crystals, 520:1, 179/[455]-185/[461]
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Mol. Cryst. Liq. Cryst., Vol. 520: pp. 179=[455]–185=[461], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903584382
Synthesis and Nonlinear Optical
Properties of Novel Y-Type Polyurethane
Containing Cyanovinylthiazole
HYO JIN NO, YOU JIN CHO, AND JU-YEON LEE
Institute of Basic Science, Department of Chemistry, Inje University,
Gimhae, Korea
A novel Y-type polyurethane 4 containing 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2-
thiazolylazo}resorcinoxy groups as nonlinear optical (NLO)-chromophores,
which constituted parts of the polymer backbones, was prepared and characterized.
Polyurethane 4 is soluble in common organic solvents such as acetone and
N,N-dimethylformamide. It showed a thermal stability up to 250ꢀC in thermogravi-
metric analysis thermogram and the glass-transition temperature (Tg) obtained
from differential scanning calorimetry thermogram was around 118ꢀC. The second
harmonic generation (SHG) coefficient (d33) of poled polymer film at 1560 nm fun-
damental wavelength was around 8.43 ꢁ 10ꢂ9 esu. The dipole alignment exhibited a
thermal stability even at 12ꢀC higher than Tg, and there was no SHG decay below
130ꢀC due to the partial main-chain character of the polymer structure, which is
acceptable for nonlinear optical device applications.
Keywords Differential scanning calorimetry (DSC); NLO; polyurethane; relax-
ation of dipole alignment; SHG coefficient; thermogravimetric analysis (TGA)
Introduction
There has been an extensive effort to synthesize novel nonlinear optical (NLO)
polymers in recent years because of their potential applications in the field of
electro-optic devices, including ultrafast optical switches and high-density optical
data storage media [1–5]. The promise of NLO polymers lies in their light weight,
low cost, and good processability to form electro-optical devices. In the develop-
ments of NLO polymers, stabilization of electrically induced dipole alignment is
one of important considerations [6]. Two approaches have been proposed to mini-
mize the randomization, that is to use crosslinking method [7–9] and to utilize high
Tg polymers such as polyimides [10–12]. Polyurethane matrix forms extensive hydro-
gen bond between urethane linkage and increases rigidity preventing the relaxation
of induced dipoles. Polyurethanes with a NLO chromophore, whose dipole moment
is aligned transverse to the main chain backbone, showed enhanced thermal stability
Address correspondence to Ju-Yeon Lee, Institute of Basic Science, Department of
Chemistry, Inje University, 607 Obang-dong, Gimhae 621-749, Korea. Tel.: þ82-55-320-
3221; Fax: þ82-55-321-9718; E-mail: chemljy@inje.ac.kr
179=[455]

180=[456]
H. J. No et al.
Figure 1. Side chain NLO polymers (a) and Y-type NLO polymers (b).
[13,14]. Polyurethane with a thiophene ring having a tricyanovinyl group in the poly-
mer side chain showed enhanced thermal stability [15,16]. Recently we reported
novel NLO polyurethanes containing dioxynitrostilbene [17] or dioxybenzylidene-
malononitrile [18] with enhanced thermal stability of dipole alignments. In this
work we have prepared a novel polyurethane containing 5-methyl-4-{5-(1,2,2-
tricyanovinyl)-2-thiazolylazo}resorcinoxy groups as NLO chromophores. We selec-
ted the latter because they are expected to have a large NLO activity. Furthermore, it
constitutes a novel Y-type NLO polyurthane (see Fig. 1b), in which the structure of
NLO chromophore, and this Y-type NLO polyurethane is not described in the litera-
ture. Thus we made a new type of NLO polyurethane, in which the pendant NLO
chromophores are components of the polymer backbones. This Y-type NLO poly-
mer 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 investigated its properties such as thermal
stability and second harmonic generation (SHG) activity (d33). We now report the
results of the initial phase of the work.
Experimental
Materials
Synthetic method of polymer 4 is summarized in Scheme 1. A representative syn-
thetic procedure was as follows. Tetracyanoethylene (1.28 g, 10 mmol) was added
slowly to a solution of polymer 3 (5.27 g, 8.5 mmol) dissolved in 15 mL of DMF with
stirring at room temperature under nitrogen. The resulting solution was heated in an
oil bath kept at 70ꢀC for 12 h under a nitrogen atmosphere. The resulting polymer-
ization solution was cooled to room temperature and poured into 400 mL of meth-
anol. 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 to give 6.49 g (90% yield) of polymer 4. Inherent
1
viscosity (ginh) ¼ 0.30 d L gꢂ1 (c ¼ 0.5 g dLꢂ1 in DMSO at 25ꢀC). H NMR (DMSO-
d6) d (ppm): 2.53 (s, 3H, ꢂCH3), 3.83–4.01 (m, 10H, 2 ꢂOCH3ꢂ, 2 ꢂOꢂCH2ꢂ,)
4.35–4.70 (q, 4H, 2 ꢂOꢂCH2ꢂOCOꢂ), 6.51 (s, 1H, aromatic), 6.68 (s, 1H, aro-
matic), 7.16–7.29 (m, 4H, aromatic), 7.64–7.73 (d, 1H, aromatic), 7.67–7.98 (t, 1H,

Synthesis and NLO Properties of Novel Y-Type Polyurethane 181=[457]
Scheme 1. Synthetic scheme and structure of polymer 4.
aromatic), 8.15–8.33 (t, 1H, aromatic), 8.65 (s, 1H, NꢂH), 8.97–9.07 (d, 1H, NꢂH).
IR (KBr disc) (cmꢂ1): 3392 (s, NꢂH), 2940 (m, CꢂH), 2200 (m, CN), 1687 (s, C O),
=
=
1594 (s, C C). 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.
Measurements
1
IR, H NMR, and UV-Vis spectra were taken on a Shimadzu FT IR-8201PC infra-
red spectrophotometer, Varian 300 MHz NMR spectrometer, and Shimadzu
UV-3100S spectrophotometer, respectively. Elemental analyses were performed
using a Perkin-Elmer 2400 CHN elemental analyzer. Tg values were measured on
a TA 2920 differential scanning calorimeter (DSC) in a nitrogen atmosphere. TA
Q50 thermogravimetric analyzer (TGA) with a heating rate of 10ꢀC=min up to
800ꢀC was used for the thermal degradation study of polymers under nitrogen.
The number average molecular weight (Mn) and weight average molecular weight
(Mw) of the polymer were estimated by gel permeation chromatography (GPC)
(columns styragel HR5E4E; solvent THF). The alignment of the NLO-chromophore
of the polymer was carried out by corona poling method. The refractive index of
the polymer sample was measured by the optical transmission technique [19].
Second harmonic generation (SHG) measurements were made using a Maker fringe
technique [20].
Results and Discussion
Synthesis and Characterization of Polymer 4
5-Methyl-4-(2-thiazolylazo)resorcinol was purchased from Aldrich and used as
received. Polyurethane 3 was prepared by the polyaddition reaction between a diol
2 and 3,30-dimethoxy-4,40-biphenylenediisocyanate in a dry DMF solvent. Polymer
3 was reacted with tetracyanoethylene in anhydrous DMF [15,16] to yield

182=[458]
H. J. No et al.
polyurethane 4 containing 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2-thiazolylazo}resor-
cinoxy groups as NLO-chromophores. The synthetic route for polymer 4 is pre-
sented in Scheme 1. The resulting polymer was purified by Soxhlet extraction for
2 days with methanol solvent. The chemical structure of the polymer was identified
1
by H NMR, IR spectra, and elemental analysis. Elemental analysis results fit the
1
polymer structure. H NMR spectrum of the polymer showed a signal broadening
due to polymerization, but the chemical shifts are consistent with the proposed poly-
mer structure. The signal at 8.65–9.07 ppm assigned to the amine proton indicates
the formation of urethane linkage. The IR spectrum of polymer 4 showed strong
absorption peak near 2200 cmꢂ1 indicating the presence of nitrile group. The IR
spectra of the same polymer sample also show 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 thiophene ring. The molecular weights were determined by GPC with polystyrene
as the standard and THF as the eluent. The number average molecular weight (Mn)
of the polymer 4, determined by GPC, was 18400 (Mw=Mn ¼ 1.96). The polymer 4
was soluble in common solvents such as acetone, DMF, and DMSO, but was not
soluble in methanol and diethyl ether. The inherent viscosity values were in the range
0.30–0.31 dL=g. Polymer 4 showed strong absorption near 422 nm by the
NLO-chromophore 5-methyl-4-{5-(1,2,2-tricyanovinyl)-2-thiazolylazo}resorcinoxy
group. The striking feature of this polymerization system is that it gives unpre-
cedented Y-type NLO polymers, in which the pendant NLO chromophores are part
of the polymer backbones. These Y-type NLO polymers are expected to have the
advantages of both main-chain and side-chain NLO polymers. In general,
main-chain NLO polymers 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 polymer systems have the advan-
tages such as good solubility, homogeneity and high loading level of NLO chromo-
phores relative to the main-chain systems, but they often suffer from poor stability of
dipole alignments at high temperatures. Thus, we obtained a new type of NLO poly-
urethane 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 by TGA and DSC to determine
the thermal degradation pattern and Tg. Polymer 4 showed a thermal stability up to
250ꢀC according to its TGA thermogram. The Tg value of the polymer 4 measured by
DSC was around 118ꢀC. This Tg value is lower than those of the polyurethanes con-
taining dioxybenzylidenemalononitrile, which are in the range 145–159ꢀC [17] or
dioxynitrostilbene, which are in the range 142–143ꢀC [18]. The TGA and DSC studies
showed that the decomposition temperature of the polyurethane 4 was higher than
the corresponding Tg value. 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 by the SHG method. To induce non-
centrosymmetric polar order, the spin-coated polymer film was corona-poled. As the

Synthesis and NLO Properties of Novel Y-Type Polyurethane 183=[459]
Table 1. Nonlinear optical properties of polymer 4
Uc
d31 (esu) Film thicknessd (mm)
n
a
b
b
Polymer kmax (nm) d33 (esu)
4
422
8.43 ꢁ 10ꢂ9 0.19 2.62 ꢁ 10ꢂ10
0.52
1.55
aPolymer film after poling.
bSHG coefficients (d33) were derived from the analysis of measured Maker-fringes [20].
cOrder parameter U ¼ 1ꢂA1=A0, where A0 and A1 are the absorbances of the polymer film
before and after corona poling, respectively.
dFilm thickness was determined by the optical transmission technique [19].
temperature was raised gradually to 125ꢀC, 6.5 kV of corona voltage was applied and
this temperature was maintained for 30 min. The UV-Vis absorption spectra of the
polymer before and after poling were recorded. After electric poling, the dipole
moments of the NLO-chromophores were aligned and UV-Vis spectrum of polymer
4 exhibited a slight blue shift and a decrease in absorption due to birefringence.
From the absorbance change, the order parameter of the poled film could be esti-
mated, which is related to the poling efficiency. The estimated order parameter value
U of polymer 4 was equal to 0.19. SHG measurements were performed at a funda-
mental wavelength of 1560 nm using a mode locked Nd-YAG laser. NLO properties
of polymer 4 are summarized in Table 1. In order to determine the microscopic
second-order susceptibility of the polymer, the angular SHG dependence was
recorded. Figure 2 shows the angular dependence of SHG signal in a poled polymer
4. The SHG values were compared with those obtained from a Y-cut quartz plate.
SHG coefficients (d33, d31) were derived from the analysis of measured Maker-fringes
with Pascal fitting program according to the literature procedure [21]. The measured
values of d33 and d31 for polymer 4 were 8.43 ꢁ 10ꢂ9 and 2.62 ꢁ 10ꢂ9 esu, respect-
ively. Since the second harmonic wavelength was at 780 nm, which is not in the
absorptive region of the resulting polymer, there was not resonant contribution to
this d33 value.
Figure 2. Angular dependence of SHG signal for a poled film of polymer 4.

184=[460]
H. J. No et al.
Figure 3. Normalized SHG signal of polymer 4 as a function of temperature at a heating rate
of 4.5ꢀC=min.
To evaluate the high-temperature stability of the polymers, we studied the
temporal stability of the SHG signal. In Figure 3, we present the dynamic thermal
stability study of the NLO activity of the film 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 4.5ꢀC=min from
25ꢀC to 220ꢀC. The polymer film exhibited a greater thermal stability even at
12ꢀC higher than Tg, and there was no SHG decay below 130ꢀC, as shown in
Figure 3. In general, side-chain NLO polymers lose thermal stability of dipole align-
ment below Tg. Stabilization of dipole alignment is a characteristic of main-chain
NLO polymer [21,22]. The enhanced thermal stability of SHG of polymer 4 is due
to the stabilization of dipole alignment of NLO chromophore, which stems from
the partial main-chain character of the polymer structure and partly by hydrogen
bonds between the neighboring urethane linkages. Thus, we obtained a new type
of NLO polyurethane having both of the merits of main chain- and side-chain
NLO polymers namely stabilization of dipole alignment and good solubility.
Conclusions
We synthesized novel Y-type polyurethane 4 with pendant NLO chromophores,
which are parts of the polymer main chains. This mid-type NLO polyurethane is sol-
uble in common organic solvents. The resulting polymer 4 showed a thermal stability
up to 250ꢀC from TGA thermograms with Tg value near 118ꢀC. The SHG coefficient
(d33) of corona-poled polymer film was 8.43 ꢁ 10ꢂ9 esu. The striking feature of this
polymer is that it exhibits SHG stability even at 12ꢀC higher than Tg, and no
SHG decay was observed below 130ꢀC. This high thermal stability of optical non-
linearity stemmed from the stabilization of dipole alignment of the NLO chromo-
phore, which constituted a part of the polymer backbone, and partly by hydrogen
bonds between the neighboring urethane linkages.

Synthesis and NLO Properties of Novel Y-Type Polyurethane 185=[461]
Acknowledgements
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. 2009-0070572).
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