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

Article abstract of DOI:10.1080/15421400802330788

3,4-Di-(2'-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and condensed with terephthaloyl chloride to yield novel Y-type polyester (4) containing the NLO-chromophores dioxybenzylidenemalononitrile group, which constituted parts of the polymer ba

Full text of DOI:10.1080/15421400802330788

Mol. Cryst. Liq. Cryst., Vol. 492, pp. 312=[676]–321=[685], 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802330788
Synthesis and Properties of Novel Y-Type
Nonlinear Optical Polyester with Enhanced
Thermal Stability of Dipole Alignment
Han-Na Jang, Ga-Young Lee, and Ju-Yeon Lee
Institute of Functional Materials, Department of Chemistry,
Inje University, Obang-dong, Gimhae, Korea
0
3
,4-Di-(2 -hydroxyethoxy)benzylidenemalononitrile (3) was prepared and con-
densed with terephthaloyl chloride to yield novel Y-type polyester (4) containing
the NLO-chromophores dioxybenzylidenemalononitrile group, which constituted
parts of the polymer backbone. Polyester 4 is soluble in common organic solvents
such as acetone and dimethylsulfoxide. Polymer 4 showed thermal stability up
ꢀ
to 300 C in thermogravimetric analysis with glass-transition temperature (T
g
)
ꢀ
measured from differential scanning calorimetry around 91 C. The second
harmonic generation (SHG) coefficient (d33) of poled polymer films at the
ꢂ9
1
064 nm fundamental wavelength was around 4.39 ꢁ 10 esu. The dipole align-
g
ment exhibited high thermal stability up to T due to the partial main-chain
character of polymer structure, which is acceptable for NLO device applications.
Keywords: differential scanning calorimetry (DSC); NLO; polyester; relaxation of
dipole alignment; SHG coefficient; thermogravimetric analysis (TGA)
INTRODUCTION
Organic materials with nonlinear optical (NLO) effects have been
extensively studied over the past decade because of their potential
applications in the field of telecommunications, optical switching,
etc. Among the organic materials the NLO polymers are receiving
great attention, mainly because they offer many advantages such light
This work was supported by the Inje FIRST project of Inje University and partially by
the BK21 project of the Ministry of Education and the Korea Research Foundation Grant
funded by the Korean Government (MOEHRD, Basic Research Promotion Fund)
(KRF-2007-313-C00498).
Address correspondence to Ju-Yeon Lee Institute of Functional Materials, Depart-
ment of Chemistry, Inje University, 607 Obang-dong, Gimhae 621-749, Korea. E-mail:
chemljy@inje.ac.kr
3
12=[676]

Novel Y-Type NLO Polyester with Enhanced Thermal Stability
313=[677]
weight and good processability to form electro-optic devices [1–2]. In the
developments of NLO polymers for electro-optic device applications,
stabilization of electrically induced dipole alignment is important con-
siderations. Cross-linking method [3–5] and utilizing polymers with
high glass-transition temperature (T ) such as polyimides [6–8] have
g
been proposed to minimize the randomization. Polymers with the
NLO chromophores in the main chain [9] have good thermal stability
of dipole alignments, but they often do not dissolve in organic solvents,
and their intractability make them impossible to fabricate stable non-
centrosymmetric films. Side chain NLO polymers [10–11] have the
advantages such as good solubility, homogeneity and high level of
NLO chromophore, but they often suffer from poor stability of dipole
alignments at high temperatures. Recently we reported novel NLO
polyesters with enhanced thermal stability of dipole alignments
containing dioxynitrostilbenyl group [12]. In this work we prepared
novel polyester containing 3,4-dioxybenyzylidenemalononitrile group
as a NLO-chromophore. We selected the latter as a NLO-chromophore
because it has a large dipole moment and are rather easy to synthe-
size. Furthermore, 3,4-dioxybenyzylidenemalononitrile group consti-
tutes novel Y-type NLO polyester, in which the pendant NLO
chromophores are parts of the polymer backbones (Fig. 1(c)). These
FIGURE 1 (a) Main-chain NLO polymers, (b) side chain NLO polymers, and
(
c) Y-type NLO polymers.

3
14=[678]
H.-N. Jang et al.
mid-type NLO polymers are expected to have the advantages of both
main-chain and side-chain NLO polymers: stabilization of dipole align-
ment and good solubility. After confirming the structure of the result-
ing polymer we investigated its properties such as solubility, Tg,
thermal stability, second harmonic generation (SHG) activity and
relaxation of dipole alignment. We now report the results of the initial
phase of the work.
EXPERIMENTAL
Materials
0
3
,4-(2 -Vinyloxyethoxy)benzaldehyde
(1). 3,4-Dihydroxybenz-
aldehyde (13.8 g, 0.10 mol), anhydrous potassium carbonate (82.9 g,
.60 mol), and 2-chloroethyl vinyl ether (16.0 g, 0.25 mol) were dis-
0
solved 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. The result-
ing 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 evap-
oration of diethyl ether gave crude product, which was recrystallized
from 1-butanol yielded 24.4 g (88% yield) of pure product 1.
ꢀ
1
Mp ¼ 56–57 C. H NMR (CDCl ) d 4.04–4.36 (m, 12H, 2 CH =,
3
2
2
1
ꢂOꢂCH ꢂCH ꢂOꢂ), 6.50–6.62 (m, 2H, 2=CHꢂOꢂ), 7.01–7.06 (d,
2
2
H, aromatic), 7.45–7.42 (d, 2H, aromatic), 9.86 (s, 1H, ꢂCHO). IR
(
1
KBr) 3099, 3080 (w, =CꢂH), 2952, 2872 (m, CꢂH), 1672 (vs, C=O),
ꢂ1
612 (vs, C=C), 1575 (s, C=C) cm
.
0
3
,4-Di-(2 -vinyloxyethoxy)benzylidenemalononitrile (2). Piperidine
0
(
0.13 g, 1.5 mmol) was added to a solution of 3,4-di-(2 -vinyl-
oxyethoxy)benzaldehyde
1
(8.35 g, 30 mmol) and malononitrile
ꢀ
(
2.18 g, 33 mmol) in 170 mL of 1-butanol with stirring at 0 C under
ꢀ
nitrogen. After stirring for 4 h at 0 C, the reaction mixture was cooled
ꢀ
to ꢂ10 C for crystallization. The product was filtered and washed
successively with cold 1-butanol (80 mL), water (30 mL), and cold
1
-butanol (20 mL). The obtained pale yellow product was recrystallized
ꢀ
1
from 1-butanol to give 8.42 g (86% yield) of 2. Mp ¼ 66–68 C. H NMR
(
CDCl ) d 4.02–4.39 (m, 12H, 2 CH =, 2ꢂOꢂCH ꢂCH ꢂOꢂ), 6.47–
3
2
2
2
6
1
3
1
8
.58 (m, 2H, 2 =CHꢂOꢂ), 6.96–7.03 (d, 1H, aromatic), 7.37–7.45 (d,
H, aromatic), 7.58–7.69 (d, 2H, aromatic, ꢂPhꢂCH=). IR (KBr)
036 (w, =CꢂH), 2941, 2890 (m, CꢂH), 2222 (s, CN), 1636, 1618,
ꢂ1
583 (s, C=C) cm . Anal. Calcd for C H N O :C, 66.25; H, 5.56; N,
1
8
18
2
4
.58. Found: C, 66.36; H, 5.66; N, 8.48.

Novel Y-Type NLO Polyester with Enhanced Thermal Stability
315=[679]
0
3
,4-Di-(2 -hydroxyethoxy)benzylidenemalononitrile (3). Aqueous
hydrochloric acid (1.5 M, 150 mL) was slowly added to a solution
0
of 3,4-di-(2 -vinyloxyethoxy)benzylidenemalononitrile (2) (6.89 g,
ꢀ
0
.021 mol) in 50 mL of DMF with stirring under nitrogen at 0 C. The
ꢀ
mixture was stirred at 0 C for 20 h under nitrogen. The resulting sol-
ution was filtered and washed with 50 mL of water. The obtained pale
yellow product was recrystallized from ethyl acetate to give 4.72 g
ꢀ
1
(
82% yield) of 3. Mp: 108–110 C. H NMR (acetone-d ) d 2.78–2.87
6
(
d, 2H, ꢂOH), 3.87–3.98 (m, 4H, 2 ꢂCH -OH), 4.09–4.28 (m, 4H, 2
2
ꢂOꢂCH ꢂ), 7.21–7.27 (d, 1H, aromatic), 7.63–7.65 (d, 1H, aromatic),
2
7
.76 (s, 1H, aromatic), 8.14 (s, 1H, ꢂPhꢂCH=). IR (KBr) 3337 (s,
OꢂH), 3045 (w, =CꢂH), 2930 (m, CꢂH), 2222 (s, CN), 1607, 1583 (s,
ꢂ1
C=C) cm . Anal. Calcd for C H N O : C, 61.31; H, 5.14; N, 10.21.
1
4
14
2
4
Found: C, 61.40; H, 5.25; N, 10.28.
Synthesis of polyester 4. A representative polycondensation
procedure was as follows: Terephthaloyl chloride (2.03 g, 0.01 mol)
and diol 3 (2.74 g, 0.01 mol) were disolved in 30 mL of anhydrous pyri-
dine under nitrogen. The resulting solution was refluxed in an oil bath
ꢀ
kept at 80 C under a nitrogen atmosphere. After heating 10 h with
stirring the resulting polymerization solution was poured into
4
00 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 diethyl ether and
dried under vacuum, yielding 3.56 g (88% yield) of polymer 4.
ꢀ
1
ginh ¼ 0.26 dL=g (c, 0.5 g=dL in DMSO at 25 C). H NMR (DMSO-d )
6
d 4.17–4.68 (m, 8H, 2ꢂOꢂCH ꢂCH ꢂOꢂ), 7.12–7.38 (m, 1H, aro-
2
2
matic), 7.25–7.97 (m, 6H, aromatic), 8.15–8.37 (m, 1H, aromatic). IR
ꢂ1
(
KBr) 2959 (m, CꢂH), 2227 (s, CN), 1722 (vs, C=O), 1584 (s, C=C) cm
.
Anal. Calcd for (C H N O ) :C, 65.34; H, 3.99; N, 6.93. Found: C,
2
2
16
2
6 n
6
5.42; H, 4.08; N, 6.98.
Measurements
1
IR, H NMR, and UV-Vis spectra were taken on a Shimadzu FT
IR-8201PC infrared spectrophotometer, Varian 300 MHz NMR spec-
trometer, and Shimadzu UV-3100S spectrophotometer, respectively.
Elemental analyses were performed using a Perkin-Elmer 2400
CHN elemental analyzer. T values were measured on a TA 2920 dif-
g
ferential scanning calorimeter (DSC) in a nitrogen atmosphere.
DuPont 951 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

3
16=[680]
H.-N. Jang et al.
(
M ) and weight average molecular weight (M ) of the polymers were
n w
estimated by gel permeation chromatography (GPC) (columns styragel
HR5E4E; solvent THF). The alignment of the NLO-chromophore of
the polymers was carried out by corona poling method. The refractive
index of the polymer sample was measured by the optical transmission
technique [13]. Second harmonic generation (SHG) measurements
were made using a Maker fringe technique [14].
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymer 4
0
3
,4-Di-(2 -vinyloxyethoxy)benzaldehyde (1) was prepared by the
reaction of 2-chloroethyl vinyl ether with 3,4-dihydroxybenaldehyde.
0
3
,4-Di-(2 -vinyloxyethoxy)benzylidenemalononitrile (2) was prepared
by the condensation reaction of 1 with malononitrile, and was hydro-
lyzed to yield acetaldehyde and diol 3. The synthetic route for com-
pound 3 is presented in Scheme 1. Diol 3 was condensed with
terephthaloyl chloride in a dry DMF solvent to yield Y-type polyester
4
containing the NLO-chromophore 3,4-dioxybenzylidenemalononi-
trile group (Scheme 1). The polymerization yield was 90%. The chemi-
1
cal structures of the resulting polymer were confirmed by H NMR, IR
spectra, and elemental analysis. Elemental analysis results fit the
1
polymer structure. H NMR spectra of the polymer showed a signal
broadening due to polymerization, but the chemical shifts are consist-
ent with the proposed polymer structure. The IR spectrum of the same
SCHEME 1 Synthetic scheme and structure of polymer 4.

Novel Y-Type NLO Polyester with Enhanced Thermal Stability
317=[681]
ꢂ1
polymer sample showed a strong carbonyl peak near 1722 cm
indicating the presence of ester bond. The same polymer sample also
ꢂ1
showed a strong nitrile peak near 2227 cm . These results are
consistent with the proposed structure, indicating that the NLO-
chromophore remained intact during the polymerization. The mole-
cular weights were determined by GPC using polystyrene as the
standard and THF as eluent. The number average molecular weight
(
M ) of the polymers was determined to be 16800 (M =M ¼ 1.90) for
n
w
n
polymer 4. The structural feature of this polymer is that it has pen-
dant NLO chromophores, which are parts of the polymer main chains.
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 was soluble in common solvents such as acetone,
DMF, and DMSO, but was not soluble in methanol and diethyl ether.
Polymer 4 isolated from methanol were yellow colored amorphous
materials. The inherent viscosities were around 0.26 dL=g. Polymer
4
showed strong absorption near 364 nm by the NLO-chromophore
dioxybenzylidenemalononitrile group. Having well defined polyester
4) and we investigate its properties.
(
Thermal Properties of Polymer
The thermal behavior of the polymers were investigated by thermo-
gravimetric analysis (TGA) and differential scanning calorimeter
(
DSC) to determine the thermal degradation pattern and glass tran-
sition temperature (T ). The results are summarized in Table 1. Poly-
g
ꢀ
mer 4 showed a thermal stability up to 300 C from TGA thermogram.
T value of the polymer 4 measured by DSC was around 91 C. The
ꢀ
g
TGA and DSC studies showed that the decomposition temperature
of the polyester 4 was higher than the corresponding T . This indicates
g
TABLE 1 Thermal Properties of Polymer 4
ꢀ
b
Degradation temp, C
b
Residue at
aꢀ
ꢀ
Polymer
T
C
5 wt%-loss 10 wt%-loss 20 wt%-loss 40 wt%-loss 800 C, wt%
g
4
91
318
335
376
448
31.6
a
Determined from DSC curves measured on a TA 2920 differential scanning
ꢀ
calorimeter with a heating rate of 10 C=min under nitrogen atmosphere.
b
Determined from TGA curves measured on a DuPont 951 theromogravimetric
ꢀ
analyzer with a heating rate of 10 C=min under nitrogen atmosphere.

3
18=[682]
H.-N. Jang et al.
that high-temperature poling for a short term is feasible without
damaging the NLO chromophore.
Nonlinear Optical Properties of Polymer
The NLO property of polymer was studied by the SHG method. To
induce noncentrosymmetric polar order, the spin-coated polymer films
ꢀ
were corona-poled. As the temperature was raised to 105 C, 6.5 kV of
corona voltage was applied and this temperature was maintained for
3
0 min. The poling was confirmed by UV-Vis spectra. Figure 2 shows
the UV-Vis absorption spectra of the polymer 4 before and after poling.
After electric poling, the dipole moment of the NLO-chromophore was
aligned and the UV-Vis spectrum of polymer 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 was 0.30 for polymer 4 (U ¼ 1 ꢂ A =A , where
1
0
A ¼ 0.982 and A ¼ 0.684 are the absorbances of the polymer film
0
1
before and after poling, respectively). The refractive index of the
sample was measured by the optical transmission technique [13]. The
NLO properties were studied by the SHG method. SHG measurements
were performed at a fundamental wavelength of 1064 nm using a mode
locked Nd-YAG laser. Linear and nonlinear optical properties of poly-
mer 4 are summarized in Table 2. In order to determine the macro-
scopic second-order susceptibility of the polymer, the angular SHG
dependence was recorded. Figure 3 shows the angular dependence of
FIGURE 2 UV-Vis absorption spectra of a film of polymer 4 before and after
poling.

Novel Y-Type NLO Polyester with Enhanced Thermal Stability
319=[683]
TABLE 2 Linear and Nonlinear Optical Properties of Polymer 4
a
d
k
max
Film thickness
b
c
b
Polymer (nm)
d
33 (esu)
U
d
31 (esu)
(mm)
n
4
364 (4.39 ꢃ 0.13) ꢁ 10ꢂ9 0.30 (1.58 ꢃ 0.08) ꢁ 10ꢂ9
0.52
1.60
a
b
Polymer film.
SHG coefficients (d33
) were derived from the analysis of measured Maker-
fringes [14].
c
Order parameter U ¼ 1 ꢂ A
1 0 0 1
=A , Where A and A are the absorbances of the polymer
film before and after corona poling, respectively.
d
Film thickness was determined by the optical transmission technique [13].
SHG signal in a poled polymer 4. The SHG values were compared
with those obtained from a Y-cut quartz plate. SHG coefficient (d )
33
was derived from the analysis of measured Maker-fringes with the
Pascal fitting program according to the literature procedure [14].
ꢂ9
The values of d31 and d for polymer 4 were (1.58 ꢃ 0.08) ꢁ 10
33
ꢂ9
and (4.39 ꢃ 0.13) ꢁ 10
esu, respectively. Because the second
harmonic wavelength was at 532 nm, which is not in the absorptive
region of the resulting polyurethane, there was not resonant contri-
bution to this d33 value. To evaluate the high-temperature stability
of the polymer, we studied the temporal stability of the SHG signal.
FIGURE 3 Angular dependence of SHG signal in a poled film of polymer 4.

3
20=[684]
H.-N. Jang et al.
FIGURE 4 Normalized SHG signal of polymer 4 as a function of temperature
ꢀ
at a heating rate of 3 C=min.
In Figure 4, we present the dynamic thermal stability study of the
NLO activity of the film 4. To investigate the real time decay of the
SHG signal of the poled polymer film as a function of temperature,
in situ SHG measurements were performed under a temperature raise
ꢀ
ꢀ
at a heating rate of 3 C=min from 30 to 150 C. The polymer film exhib-
ited a high thermal stability up to T and no significant SHG decay
g
ꢀ
was observed below 95 C for polymer 4. In general, side chain NLO
polymers lose thermal stability of dipole alignment below T . Stabili-
g
zation of dipole alignment is a characteristic of main chain NLO poly-
mers. The high thermal stability of second harmonic generation of
polymer 4 is due to the stabilization of dipole alignment of NLO chro-
mophore, which stems from the partial main chain character of the
polymer structure. Thus, we obtained a new type of NLO polyester
having both of the merits of main chain- and side-chain NLO poly-
mers: stabilization of dipole alignment and good solubility.
REFERENCES
[
[
1] Cho, M. J., Lee, S. K., Jin, J.-I., & Choi, D. H. (2006). Macromol. Res., 14, 603.
2] Samoc, A., Samoc, M., Luther-Davies, B., Kolev, V. Z., Bagien, R. K., Luo, X., &
Zah, C. (2006). Mol. Cryst. Liq. Cryst., 446, 123.
[
[
3] Lee, K. S., Moon, K. J., & Shim, H. K. (1996). Korea Polym. J., 2, 191.
4] Han, K. S., Park, S. K., Shim, S. Y., Jahng, W. S., & Kim, N. J. (1998). Bull. Korean
Chem. Soc., 19, 1165.
[5] Seo, D. K., Lim, H. S., Lee, J. Y., & Kim, W. G. (2006). Mol. Cryst. Liq. Cryst.,
4
45, 323.

Novel Y-Type NLO Polyester with Enhanced Thermal Stability
321=[685]
[
[
[
6] Yu, D., Gharavi, A., & Yu, L. (1995). J. Am. Chem. Soc., 117, 11680.
7] Tsutsumi, N., Morishima, M., & Sakai, W. (1998). Macromolecules, 31, 7764.
8] Rau, I., Armatys, P., Kajzar, F., Chollet, P. A., Centore, R., & Carella, A. (2006).
Mol. Cryst. Liq. Cryst., 446, 161.
[9] Lee, J.-H. & Lee, K.-S. (2000). Bull. Korean Chem. Soc., 21, 847.
[10] Winkelhahn, H.-J., Schrader, S., Neher, D., & Wegner, G. (1995). Macromolecules,
2
8, 2882.
[
[
[
[
11] Lee, S.-H., Kim, Y.-K., & Won, Y.-H. (1999). Macromolecules, 32, 342.
12] Lee, J.-Y., Jung, W.-T., & Lee, W.-J. (2006). Polym. Int., 55, 248.
13] Cisneros, J. I. (1998). Appl. Opt., 37, 5262.
14] Maker, P. D., Terhune, R. W., Nisenoff, M., & Savage, C. M. (1962). Phy. Rev. Lett.,
8, 21.