Synthesis and nonlinear optical properties of novel polyurethane containing cyanovinylnitroresorcinoxy group

Article abstract of DOI:10.1080/15421406.2014.935925

Novel X-type polyurethane 5 containing 4-(2′-carbomethoxy-2′-cyano)vinyl-6-nitroresorcinoxy groups as nonlinear optical (NLO) chromophores, which constitute part of the polymer backbone, was prepared and characterized. Polyurethane 5 is soluble in common organic solvents such as acetone and N,N-dimethylformamide. It shows thermal stabilities up to 260°C from thermogravimetric analysis with glass transition temperature obtained from differential scanning calorimetry near 108°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at 1064 nm fundamental wavelength is 9.83 × 10^-9 esu. Polymer 5 exhibits a high thermal stability even at 2°C higher than T_g, and no significant SHG decay is observed below 110°C, which is acceptable for nonlinear optical device applications.

Full text of DOI:10.1080/15421406.2014.935925

Mol. Cryst. Liq. Cryst., Vol. 599: pp. 72–78, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.935925
Synthesis and Nonlinear Optical Properties
of Novel Polyurethane Containing
Cyanovinylnitroresorcinoxy Group
JI-HYANG LEE,1 KANG YUN CHOE,1 AND JU-YEON LEE2,∗
1Institute of Basic Science, Department of Nano System Engineering, Inje
University, Gimhae, Korea
2Institute of Basic Science, Department of Chemistry and Nano System
Engineering, Inje University, Gimhae, Korea
Novel X-type polyurethane
5
containing 4-(2ꢀ-carbomethoxy-2ꢀ-cyano)vinyl-6-
nitroresorcinoxy groups as nonlinear optical (NLO) chromophores, which constitute
part of the polymer backbone, was prepared and characterized. Polyurethane 5 is solu-
ble in common organic solvents such as acetone and N,N-dimethylformamide. It shows
thermal stabilities up to 260◦C from thermogravimetric analysis with glass transition
temperature obtained from differential scanning calorimetry near 108◦C. The second
harmonic generation (SHG) coefficient (d33) of poled polymer film at 1064 nm funda-
mental wavelength is 9.83 × 10−9 esu. Polymer 5 exhibits a high thermal stability even
at 2◦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
Second-order nonlinear optical (NLO) polymers have been extensively studied during the
past decades because of their several advantages for photonic applications, such as high-
speed optical switches and electro-optic modulators [1–4]. In the developments of NLO
polymers, stabilization of electrically induced dipole alignment is one of important con-
siderations. Several approaches to minimize the randomization have been proposed such
as the use cross-linking methods [5–6], the utilization of high Tg polyimides [7–8], and
H-shape polymers [9]. Polyurethane matrix forms extensive hydrogen bond between ure-
thane linkages, with increased rigidity preventing the relaxation of induced dipoles [10]. 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 align-
ments at high temperatures. Recently we reported novel NLO polyurethanes containing
dioxybenzylidenecyanoacetate [11] with enhanced thermal stability of dipole alignments.
∗Address correspondence to Ju-Yeon Lee, Institute of Basic Science, Department of Chemistry
and Nano System Engineering, 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
[414]/72

Synthesis and NLO Properties Novel Polyurethane
[415]/73
(a)
(b)
X X X
(c)
Figure 1. Main Chain NLO polymers (a), side chain NLO polymers (b), and X-type NLO polymers
(c).
In this work, we have prepared novel polyurethane containing 4-(2ꢀ-carbomethoxy-2ꢀ-
cyano)vinyl-6-nitroresorcinoxy groups as NLO chromophores. We selected the latter as
NLO chromophores because they are expected to have higher optical nonlinearities than
those without nitro group due to a quadruple conjugation and planarity of the dipole mo-
ments. Furthermore, these 4-(2ꢀ-carbomethoxy-2ꢀ-cyano)vinyl-6-nitroresorcinoxy groups
can be incorporated into novel X-type NLO polyurethane (see Fig. 1c), and this NLO
polyurethane has not yet been described in the literature. This X-type NLO polymer, sim-
ilar to the Y-type [11], 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 electro-optic properties. We now
report the results of the initial phase of the work.
Experimental
Materials
Compounds 1–3 and polymer 4 was prepared according to a procedure similar to that
reported already [11]. A representative synthetic procedure for polymer 5 was as follows.
Polymer 4 (4.75 g, 0.01 mol) was dissolved in 22 mL of DMF at room temperature
and stirred for 30 min. The reaction mixture was cooled with ice bath, added 4 mL of
sulfuric acid, and stirred for 30 min. A mixture of nitric acid (9 mL) and sulfuric acid
(9 mL) were added drop-wise to the solution and stirred for 24 hr at 0◦C. The reaction
mixture was poured to 500 mL of ice water and the precipitated polymer was separated
with suction. The polymer was further purified by extraction in a Soxhlet extractor with

74/[416]
J.-H. Lee et al.
O
O
O
O
HO
OH
CH2=CHOCH2CH2Cl
K2CO3, DMF
H2O, DMF
HCl
CH2(CN)CO2CH3
piperidine, DMF
O
O
O
O
CN
CO2CH3
CHO
CHO
2
1
O
C
O
OH
HO
N
H
C O
O
N
n
H
O
O
O
O
NCO
OCN
CN
CN
CO2CH3
DMF
CO2CH3
3
4
O
O
C
N
O
O
C
N
H
n
H
O
O
HNO3, DMF
H2SO4
CN
CO2CH3
O2N
5
Scheme 1. Synthetic scheme and structure of polymer 5.
methanol and dried under vacuum, giving 4.68 g (90% yield) of polymer 5. Inherent
1
viscosity (ηinh) = 0.28 dL/g (c = 0.5 g/dL in DMSO at 25◦C). H NMR (DMSO-d6)
δ 1.13–1.39 (m, 8H, (CH2)4)-, 2.86–2.98 (m, 4H, 2 N CH2-), 3.76–3.83 (m, 3H,
COOCH3), 4.243–4.38 (m, 8H, 2 O CH2 CH2 O ), 6.73–6.79 (m, 2H, 2 NH ),
7.21–7.33 (d, 1H, aromatic), 8.18–8.25 (m, 1H, aromatic), 8.53–8.56 (m, 1H, Ph CH ).
IR (KBr) 3335 (s, N H), 2933, 2855 (m, C H), 2220 (w, CN), 1710 (s, C O), 1656, 1582
(s, C C) cm−1. Anal. Calcd for (C23H28N4O10)n: C, 53.07; H, 5.42; N, 10.76. Found: C,
53.16; H, 5.48; N, 10.71.
Measurements
1
IR, H NMR, and UV-visible spectra were obtained with a Varian FT IR-1000 IR spec-
trophotometer, Varian VNMRS 500 MHz NMR spectrometer, and SECOMAM Model
UVIKON XS 99-90289 spectrophotometer, respectively. Tg values were measured on a
TA 2920 differential scanning calorimeter (DSC) in a nitrogen atmosphere. TA Q50 ther-
mogravimetric 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 molec-
ular 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 [12]. Second harmonic generation (SHG)
measurements were made using a Maker fringe technique [13].
Results and Discussion
Synthesis and Characterization of Polymer 5
The synthetic route for polymer 5 is presented in Scheme 1. Diol 3 was polyadded with
1,6-hexamethylenediisocyanate in a dry DMF solvent to yield polyurethane 4. Polymer 4
was reacted with nitric acid and sulfuric acid in anhydrous DMF to yield novel polyurethane

Synthesis and NLO Properties Novel Polyurethane
[417]/75
5
0
-5
-10
-15
-20
0
50
100
150
200
250
300
Temperature
Figure 2. DSC thermogram of polymer 5 obtained at a heating rate of 10◦C/min under nitrogen.
5 containing 4-(2ꢀ-carbomethoxy-2ꢀ-cyano)vinyl-6-nitroresorcinoxy groups as NLO chro-
mophores. The chemical structure of the polymer was identified using 1H NMR, IR spectra,
and elemental analysis. The chemical shifts of the polymer 5 in 1H NMR spectrum are con-
sistent with the proposed polymer structure. The signal at 6.73–6.79 ppm assigned to the
amine proton indicates the formation of urethane linkage. The IR spectrum of polymer
5 shows strong absorption peak near 2220 cm−1 indicating the presence of nitrile group.
The spectrum also shows a strong carbonyl peak near 1710 cm−1 indicating the presence
of urethane bond. These results are consistent with the proposed structure. The molecular
weights were determined using GPC with polystyrene as the standard and THF as the
eluent. Mn of the polymer 5, determined using GPC, is 14,500 g mol−1 (Mw/Mn = 1.98).
The polymer 5 is soluble in common solvents such as acetone, DMF, and DMSO, but is
not soluble in methanol and diethyl ether. The inherent viscosity is 0.28 dL/g. The striking
feature of this polymerization system is that it gives unprecedented X-type NLO polymers,
in which the pendant NLO chromophores are part of the polymer backbone. This X-type
NLO polymer is expected to have the advantages of both main-chain and side-chain NLO
polymers. Thus, we obtained new NLO polyurethane with side-chain and main-chain char-
acteristics. Having obtained the well defined X-type polyurethane 5, 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 (Tg). The TGA thermogram of

76/[418]
J.-H. Lee et al.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
20
40
60
80 100 120 140 160 180
Temperature (oC)
Figure 3. Normalized SHG signal of polymer 5 as a function of temperature at a heating rate of
4◦C/min.
the polymer 5 is shown in Fig. 2. Polymer 5 has a thermal stability up to 260◦C according
to its TGA thermogram. The Tg value of the polymer 5 measured using DSC is near 108◦C.
Nonlinear Optical Properties of Polymer
The NLO properties of polymers were studied using the SHG method. To induce non-
centrosymmetric polar order, the spin-coated polymer films were corona-poled. As the
temperature was raised gradually 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 films were cooled to
room temperature in the presence of the electric field. Finally, the electric field was re-
moved. SHG measurements were performed at a fundamental wavelength of 1064 nm with
a mode locked Nd-YAG laser and optical parametric oscillator. 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 5 are summarized in Table 1. SHG coef-
ficients (d33) were derived from the analysis of measured Maker-fringes with the Pascal
Table 1. Nonlinear optical properties of polymer 5
Film thicknessc
b
b
d33
d31
Polymer
(esu)
(esu)
(μm)
n
d33/d31
5
9.83 × 10−9
3.46 × 10−9
0.62
1.618
2.84
aSHG coefficients (d33) were derived from the analysis of measured Maker-fringes [13].
bFilm thickness was determined by the optical transmission technique [12].

Synthesis and NLO Properties Novel Polyurethane
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fitting program according to the literature procedure [13]. The values of d31 and d33 for
polymer 5 are 3.46 × 10−9 and 9.83 × 10−9 esu, respectively. This d33 value is higher than
that of the Y-type polyurethane containing dioxybenzylidenecyanoacetate, which is near
8.43 × 10−9 esu [11].
To evaluate the high-temperature stability of the polymer, we studied the temporal
stability of the SHG signal. Fig. 3 shows the dynamic thermal stability study of the NLO
activity of a film of polymer 5. 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 from 30 to 200◦C. The polymer film exhibits a high
thermal stability even at 2◦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
5 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.
Conclusions
We synthesized novel X-type polyurethane 5 with pendant NLO chromophores, which are
part of the polymer main chain. This X-type NLO polyurethane is soluble in common
organic solvents. The resulting polymer 5 shows a thermal stability up to 260◦C from TGA
thermogram with Tg value near 108◦C. The SHG coefficient (d33) of corona-poled polymer
film is 9.83 × 10−9 esu. Polymer 5 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. We are now in the process
of measuring other properties of the polymer and the full accounts of the work will be
reported shortly.
Funding
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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