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

Article abstract of DOI:10.1080/15421406.2012.710302

Novel Y-type polyurethane 3 containing 4-(4-nitrophenylazo)resorcinoxy groups as nonlinear optical (NLO) chromophores, which constitute part of the polymer backbone, was prepared and characterized. Polyurethane 3 is soluble in common organic solvents such

Full text of DOI:10.1080/15421406.2012.710302

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Synthesis and Nonlinear Optical
Properties of Novel Y-type Polyurethane
Containing Nitroazobenzene Group
Mi Sung Kim a , Mi Young Song a , Byeongman Jeon a & Ju-Yeon Lee a
a Institute of Basic Science, Department of Chemistry, Inje
University, 607 Obang-dong, Gimhae, 621-749, Korea
To cite this article: Mi Sung Kim , Mi Young Song , Byeongman Jeon & Ju-Yeon Lee (2012) Synthesis
and Nonlinear Optical Properties of Novel Y-type Polyurethane Containing Nitroazobenzene Group,
Molecular Crystals and Liquid Crystals, 568:1, 111-116, DOI: 10.1080/15421406.2012.710302
To link to this article: http://dx.doi.org/10.1080/15421406.2012.710302
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Mol. Cryst. Liq. Cryst., Vol. 568: pp. 111–116, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.710302
Synthesis and Nonlinear Optical Properties of Novel
Y-type Polyurethane Containing Nitroazobenzene
Group
MI SUNG KIM, MI YOUNG SONG, BYEONGMAN JEON,
AND JU-YEON LEE∗
Institute of Basic Science, Department of Chemistry, Inje University, 607
Obang-dong, Gimhae 621-749, Korea
Novel Y-type polyurethane 3 containing 4-(4-nitrophenylazo)resorcinoxy groups as non-
linear optical (NLO) chromophores, which constitute part of the polymer backbone, was
prepared and characterized. Polyurethane 3 is soluble in common organic solvents such
as acetone and N,N-dimethylformamide. It shows thermal stabilities up to 270◦C from
thermogravimetric analysis with glass transition temperature obtained from differential
scanning calorimetry of 102◦C. The second harmonic generation (SHG) coefficient (d33)
of poled polymer film at 1064 nm fundamental wavelength is 4.92 × 10−9 esu. Poly-
mer 3 exhibits a high thermal stability even at 8◦C higher than Tg, and no significant
SHG decay is observed below 110◦C, which is acceptable for nonlinear optical device
applications.
Keywords Differential scanning calorimetry (DSC); dipole alignment; NLO;
polyurethane; SHG coefficient; thermogravimetric analysis (TGA)
Introduction
During the past decades, second-order nonlinear optical (NLO) polymers have focused due
to their several advantages for photonic applications, such as high-speed optical switches
and electro-optic modulators [1–3]. In the developments of NLO polymers, stabilization of
electrically induced dipole alignment is one of important considerations. Two approaches
to minimize the randomization have been proposed, that is to use cross-linking methods
[4] and to utilize high Tg polymers such as polyimides [5]. Polyurethane matrix forms
extensive hydrogen bond between urethane linkages, with increased rigidity preventing the
relaxation of induced dipoles [6–7]. In general, main-chain NLO polymers exhibit good
thermal stability of dipole alignments, but they often have poor solubility in organic solvents
and difficulty in poling. Side-chain NLO polymers have good solubility, but they often suffer
from poor stability of dipole alignments at high temperatures. Recently we reported novel
NLO polyurethanes containing dioxynitrostilbene [8] or dioxybenzylidenemalononitrile [9]
with enhanced thermal stability of dipole alignments. In this work, we have prepared novel
polyurethane containing 4-(4-nitrophenylazo)resorcinoxy groups as NLO chromophores.
∗Address correspondence to Ju-Yeon Lee, Institute of Basic Science, Department of Chemistry,
Inje University, 607 Obang-dong, Gimhae 621-749, Korea. Tel.: +82-55-320-3221; Fax: +82-55-
321-9718. E-mail: chemljy@inje.ac.kr
[527]/111

112/[528]
M. S. Kim et al.
We selected the latter as NLO chromophores because they are expected to have high
optical nonlinearities. Furthermore, these 4-(4-nitrophenylazo)resorcinoxy groups can be
incorporated into novel Y-type NLO polyurthane, and this NLO polyurethane has not yet
been described in the literature. This Y-type NLO polymer is expected to have the merits
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. We now report the results of the initial phase of the work.
Experimental
Materials
A representative polyaddition reaction procedure was as follows. 3,3ꢀ-Dimethoxy-4,4ꢀ-
biphenylenediisocyanate (2.96 g, 0.01 mol) was added slowly to a solution of 3.47 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 collected and re-precipitated from
dimethylsulfoxide (DMSO) into methanol. The polymer was further purified by extraction
in a Soxhlet extractor with methanol and dried under vacuum to give 5.79 g (90% yield)
of dark orange colored polymer 3. Inherent viscosity (ηinh): 0.32 dL g−1 (c, 0.5 g dL−1
1
in DMSO at 25◦C). H NMR (DMSO-d6) δ 3.42 (s 6H, -OCH3), 3.86–4.07 (m, 4H, 2
-O-CH2-), 4.43–4.67 (m, 4H, 2 Ph-O-CH2-), 6.76–6.83 (s, 1H, aromatic), 7.18–7.36 (m,
5H, aromatic), 7.73–7.81 (d, 2H, aromatic), 7.93–8.05 (d, 2H, aromatic), 8.25–8.35 (t, 3H,
aromatic), 8.63–8.74 (d, 2H, N-H). IR (KBr) 3401 (m, N-H), 2943 (m, C-H), 1729 (s, C
O), 1591 (s, N N), 1514, 1339 (vs, N O) cm−1. Anal. Calcd for (C32H29N5O10)n: C,
59.72; H, 4.54; N, 10.88. Found: C, 59.82; H, 4.62; N, 10.82.
Measurements
1
IR, H NMR, and UV-visible spectra were obtained with a Varian FT IR-1000 IR spec-
trophotometer, Varian 300MHz NMR spectrometer, and SECOMAM Model UVIKON XS
99-90289 spectrophotometer, respectively. Tg values were measured on a TA 2920 differ-
ential scanning calorimeter (DSC) in a nitrogen atmosphere. TA Q50 thermogravimetric
analyzer (TGA) with a heating rate of 10◦C/min up to 800◦C was used for the thermal
degradation study of polymers under nitrogen. The number average molecular weight 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 [10]. Second harmonic generation (SHG) measurements were made
using a Maker fringe technique [11].
Results and Discussion
Synthesis and Characterization of Polymer 3
The synthetic route for polymer 3 is presented in Scheme 1. The chemical structure of
1
1
the polymer was identified using H NMR, IR spectra, and elemental analysis. H NMR

Synthesis of Novel Y-type NLO Polyurethane
[529]/113
OH
HO
HO
OH
O
O
O
O
CH2=CHOCH2CH2Cl
K2CO3, DMF
H2O, DMF
HCl
O
O
N
N
N
N
2
N
N
1
NO2
NO2
NO2
CH3O
O
C
O
C
CH3O
OCN
O
N
H
N
H
O
n
NCO
OCH3
O
O
OCH3
N
N
DMF
3
NO2
Scheme 1. Synthetic scheme and structure of polymer 3.
spectrum of the polymer 3 has a signal broadening due to polymerization, but the chemical
shifts are consistent with the proposed polymer structure. The signal at 8.63–9.08 ppm
1
of N NMR spectrum of polymer 3 assigned to the amine proton indicates the formation
of urethane linkage. The IR spectrum of polymer 3 shows strong carbonyl peaks near
1729 cm−1 indicating the presence of urethane bond. These results are consistent with
the proposed structure. The molecular weight (Mn) of the polymer 3, determined using
GPC, is 17600 g mol−1 (Mw/Mn = 1.88). The polymer 3 is soluble in common solvents
such as acetone and DMF, but is not soluble in methanol and diethyl ether. Polymer 3
shows strong absorption near 399 nm due to the NLO chromophore. The striking feature of
this polymerization system is that it gives unprecedented Y-type NLO polymers, in which
the pendant NLO chromophores are part of the polymer backbone. These Y-type NLO
polymers are expected to have the advantages of both main-chain and side-chain NLO
polymers. Having obtained the well defined Y-type polyurethane 3, 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 TGA thermogram of
the polymer 3 is shown in Fig. 2. Polymer 3 has a thermal stability up to 270◦C according
to its TGA thermogram. The Tg value of the polymer 3 measured using DSC is near 102◦C.
This Tg value is lower than those of the polyurethanes containing dioxynitrostilbene, which
are in the range 143-150◦C [12-13] or dioxybenzylidenemalononitrile, which are in the
range 135-159◦C [14–15].
Nonlinear Optical Properties of Polymer
The NLO properties of polymers were studied using the SHG method. To induce noncen-
trosymmetric polar order, the spin-coated polymer films were corona-poled. The film was
transparent and dark orange colored. As the temperature was raised gradually to 5–10◦C

114/[530]
M. S. Kim et al.
Figure 1. TGA thermogram of polymer 3 obtained at a heating rate of 10◦C min−1 under nitrogen.
higher than Tg, a corona voltage of 6.5 kV was applied and this temperature was main-
tained for 30 min. The poling was confirmed from UV-visible spectra. Figure 2 shows the
UV-visible absorption spectra of the polymer film deposited on a slide glass before and
after poling. After electric poling, the dipole moments of the NLO chromophores were
aligned and UV-visible absorption of polymer 3 exhibits a slight blue shift and a decrease
in absorption due to birefringence. SHG measurements were performed at a fundamental
wavelength of 1064 nm with a mode locked Nd-YAG laser and OPO. In order to determine
the microscopic second-order susceptibility of the polymer, the angular SHG dependence
was recorded. 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 3 are summarized in Table 1.
SHG coefficients (d33) were derived from the analysis of measured Maker-fringes with
1.2
Before Poling
After Poling
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
Wavelength (nm)
Figure 2. UV-visible absorption spectra of a film of polymer 3 before and after poling.

Synthesis of Novel Y-type NLO Polyurethane
[531]/115
Table 1. Nonlinear Optical Properties of Polymer 3
λamax
db33
db31
film thicknessc
Polymer
(nm)
(esu)
(esu)
(µm)
n
3
399
4.92×10−9
1.65×10−9
0.52
1.716
aPolymer film after corona poling.
bSHG coefficients (d33) were derived from the analysis of measured Maker-fringes [11].
cFilm thickness was determined by the optical transmission technique [10].
the Pascal fitting program according to the literature procedure [11]. The values of d31
and d33 for polymer 3 are 1.65 × 10−9 and 4.92 × 10−9 esu, respectively. These SHG
values are smaller than those of the polyurethanes containing 3,4-dioxynitrostilbene [12]
or 2,4-dioxybenzylidenemalononitrile [14], but are larger than those of the polyurethanes
containing 2,3-dioxynitrostilbene [13].
To evaluate the high-temperature stability of the polymer, we studied the temporal
stability of the SHG signal. Figure 3 shows the dynamic thermal stability study of the NLO
activity of a film of polymer 3. To investigate the real time NLO decay of the SHG signal
of the poled polymer film as a function of temperature, in situ SHG measurement was
performed at a heating rate of 4◦C min−1 from 30 to 200◦C. The polymer film exhibits a
high thermal stability even at 8◦C higher than Tg, and no significant SHG decay is observed
below 110◦C. 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 second harmonic generation of polymer
3 is due to the stabilization of dipole alignment of NLO chromophores, which stems from
the partial main-chain character of the polymer structure. Thus, we obtained a new type of
NLO polyurethane with stable dipole alignment and good solubility.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
Temperature (oC)
Figure 3. Normalized SHG signal of polymer 3 as a function of temperature at a heating rate of
4◦C/min.

116/[532]
M. S. Kim et al.
Conclusions
We synthesized novel Y-type polyurethane 3 with pendant NLO chromophores, which are
part of the polymer main chain. This Y-type NLO polyurethane is soluble in common
organic solvents. The resulting polymer 3 shows a thermal stability up to 270◦C from TGA
thermogram with Tg value near 102◦C. The SHG coefficient (d33) of corona-poled polymer
film is 4.92 × 10−9 esu. Polymer 3 exhibits SHG stability up to 110◦C. This high thermal
stability of optical nonlinearity stems from the stabilization of dipole alignment of the NLO
chromophores, which constitute part of the polymer backbone.
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. 2011-0026534).
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