Synthesis and electro-optic properties of novel polyester containing dioxybenzylidenecyanoacetate as a NLO-chromophore

Article abstract of DOI:10.1080/15421400902950162

A novel polyester (4) containing dioxybenzylidenecyanoacetate group as a NLO-chromophore was prepared and characterized. Polyester 4 was soluble in common organic solvents such as acetone and DMSO. Polymer 4 showed thermal stability up to 280°C in TGA the

Full text of DOI:10.1080/15421400902950162

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Synthesis and Electro-
Optic Properties of Novel
Polyester Containing
Dioxybenzylidenecyanoacetate
as a NLO-Chromophore
Ga-Young Lee a , Dong-Seon Won a & Ju-Yeon Lee a
a Institute of Basic Science, Department of
Chemistry , Inje University , Gimhae, Korea
Published online: 25 Jun 2009.
To cite this article: Ga-Young Lee , Dong-Seon Won & Ju-Yeon Lee (2009)
Synthesis and Electro-Optic Properties of Novel Polyester Containing
Dioxybenzylidenecyanoacetate as a NLO-Chromophore, Molecular Crystals and Liquid
Crystals, 504:1, 189-195, DOI: 10.1080/15421400902950162
To link to this article: http://dx.doi.org/10.1080/15421400902950162
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Mol. Cryst. Liq. Cryst., Vol. 504, pp. 189–195, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902950162
Synthesis and Electro-Optic Properties of Novel
Polyester Containing Dioxybenzylidenecyanoacetate
as a NLO-Chromophore
Ga-Young Lee, Dong-Seon Won, and Ju-Yeon Lee
Institute of Basic Science, Department of Chemistry, Inje University,
Gimhae, Korea
A
novel polyester (4) containing dioxybenzylidenecyanoacetate group as a
NLO-chromophore was prepared and characterized. Polyester 4 was soluble in
common organic solvents such as acetone and DMSO. Polymer 4 showed thermal
stability up to 280ꢀC in TGA thermogram with Tg value measured from DSC
thermogram near 108ꢀC. The SHG coefficient (d33) of poled polymer film at the
1064 nm fundamental wavelength was around 3.54 ꢁ 10ꢂ9 esu. The dipole align-
ment exhibited a thermal stability up to near Tg and no significant SHG decay
was observed below 95ꢀC due to the partial main-chain character of polymer
structure, which was acceptable for NLO device applications.
Keywords: differential scanning calorimetry (DSC); NLO; polyester; relaxation of
dipole alignment; SHG coefficient; thermogravimetric analysis (TGA)
INTRODUCTION
Recently nonlinear optical (NLO) polymers are studied extensively
because of their potential applications in the field of electro-optic
devices [1–4]. In the developments of NLO polymers for electro-optic
device applications, stabilization of electrically induced dipole align-
ment is one of important considerations [5]. Cross-linking method
[6–7] and utilizing polymers with high glass-transition temperature
(Tg) such as polyimides [8–10] have been proposed to minimize the
randomization. In general, polymers with the NLO chromophores in
This work was supported by the 2008 Korea Research Foundation Grant (KRF-2007-
313-C00498).
Address correspondence to Ju-Yeon Lee, Institute of Functional Materials,
Department of Chemistry, Inje University, 607 Obang-dong, Gimhae 621-749, Korea.
E-mail: chemljy@inje.ac.kr
189

190
G.-Y. Lee et al.
FIGURE 1 (a) Side chain NLO polymers and (b) Mid-type NLO polymers.
the main chain 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 noncentrosymmetric films. On
the other hand, side chain NLO polymers 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 tem-
peratures. Recently we reported novel NLO polyesters with enhanced
thermal stability of dipole alignments containing dioxynitrostilbenyl
group [11] and dioxybenzylidenemalononitrile [12]. In this work we
prepared novel polyester containing 2,4-dioxybenyzylidenecyanoace-
tate group as a NLO-chromophore. We selected the latter as a NLO-
chromophore because it has a large dipole moment and is rather easy
to synthesize. Furthermore, 2,4-dioxybenyzylidenecyanoacetate group
constitutes a novel NLO polyester, in which the pendant NLO chromo-
phores are parts of the polymer backbones (Fig. 1b). This mid-type
NLO polymer is expected to have the advantages of both main-chain
and side-chain NLO polymers namely stabilization of dipole alignment
and good solubility. After confirming the structure of the resulting
polymer we investigated its properties such as thermal stability, sec-
ond harmonic generation (SHG) activity, and relaxation of dipole
alignment. We now report the results of the initial phase of the work.
EXPERIMENTAL
Materials
Synthetic method of diol 3 and polymer 4 are summarized in Scheme 1.
A
representative polycondensation procedure was as follows:
Terephthaloyl chloride (2.03 g, 0.01 mol) and diol 3 (2.74 g, 0.01 mol)

Synthesis and Electro-Optic Properties of Novel Polyester
191
were dissolved in 30 mL of anhydrous pyridine under nitrogen. The
resulting solution was refluxed in an oil bath kept at 80ꢀC under a
nitrogen atmosphere. After heating 20 h with stirring the resulting
polymerization solution was poured into 400 mL of methanol. The pre-
cipitated 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, yielding 3.49 g (91%
yield) of polymer 4: ginh ¼ 0.30 dL=g (c, 0.5 g=dL in DMSO at 25ꢀC). 1H
NMR (DMSO-d6) d3.65–3.73 (s, 3H, -COOCH3), 4.55 (s, 4H, 2 -Ph-O-
CH2-), 4.68 (s, 4H, 2 COO-CH2-), 6.75–6.95 (m, 2H, aromatic), 8.05
(s, 4H, aromatic), 8.18 (d, 1H, aromatic), 8.54 (s, 1H, aromatic). IR
(KBr) 2955 (m, C-H), 2220 (s, CN), 1722 (vs, C ¼ O), 1586 (s, C ¼ C)
cmꢂ1. Anal. Calcd for (C23H19NO8)n: C, 63.16; H, 4.37; N, 3.20. Found:
C, 63.24; H, 4.43; N, 3.17.
Measurements
IR, 1H 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.
Tg value was 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 of polymer 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 [13]. Second harmonic generation
(SHG) measurements were made using a Maker fringe technique [14].
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymer 4
Monomer 3 was condensed with terephthaloyl chloride in a dry DMF
solvent to yield polyester 4 containing 2,4-dioxybenzylidenecyanoace-
tate group as a NLO-chromophore. The synthetic route for polymer 4
is presented in Scheme 1. The chemical structure of the resulting poly-
mer was confirmed by 1H NMR, IR spectra, and elemental analysis. 1H
NMR spectrum of the polymer showed a signal broadening due to poly-
merization, but the chemical shifts are consistent with the proposed
polymer structure. The IR spectrum of the polymer showed a strong

192
G.-Y. Lee et al.
SCHEME 1 Synthetic scheme and structure of polymer 4.
carbonyl peak near 1722 cmꢂ1 indicating the presence of ester bond.
The same polymer sample also shows strong absorption peaks near
2220 cmꢂ1 and 1586 cmꢂ1 due to the nitrile and olefin group, respec-
tively. These results are consistent with the proposed structure, indi-
cating that the NLO-chromophore remained intact during the
polymerization. The number average molecular weight (Mn) of the
polymer 4 determined by GPC was 18200 (Mw=Mn ¼ 1.91). The struc-
tural feature of this polymer is that it has pendant NLO chromophores,
which are parts of the polymer main chains. Thus the resulting poly-
mer (4) is a mid type of side chain- and main chain NLO polymer,
and is expected to have both of their merits. 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 was near
0.30 dL=g. Polymer 4 showed strong absorption near 364 nm by the
NLO-chromophore 2,4-dioxybenzylidenecyanoacetate group. Having
well defined polyester (4) and we investigate 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 280ꢀC according to its TGA thermo-
gram. The Tg value of the polymer 4 measured by DSC was around
108ꢀC. This Tg value is lower than those of polyurethane derivatives
[15]. The TGA and DSC studies showed that the decomposition
temperature of the polyester 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.

Synthesis and Electro-Optic Properties of Novel Polyester
193
Nonlinear Optical Properties of Polymer
The NLO properties of polymer were studied by the SHG method. To
induce noncentrosymmetric polar order, the spin-coated polymer films
were corona-poled. As the temperature was raised gradually to 115ꢀC,
6.5 kV of corona voltage was applied and this temperature was main-
tained for 30 min. The poling was confirmed by UV-Vis spectrum.
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 0.13. The refractive index
of the sample was measured by the optical transmission technique
[13]. SHG measurements were performed at a fundamental wave-
length of 1064 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 depen-
dence of SHG signal in a poled polymer 4. The SHG values were com-
pared 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 to the samples and recorded. SHG coefficients (d33) were
derived from the analysis of measured Maker-fringes with Pascal
fitting program according to the literature procedure [14]. The values
of d33 and d31 for polymer
4
were (3.54 ꢃ 0.09) ꢁ 10ꢂ9 and
(1.26 ꢃ 0.06) ꢁ 10ꢂ9 esu, respectively. These values are somewhat
larger than those of T-type polyurethane derivatives [15]. Since the
second harmonic wavelength was at 532 nm, which is not in the
absorptive region of the resulting polyester, there was not resonant
contribution to this d33 value.
TABLE 1 Nonlinear Optical Properties of Polymer 4
a
kmax
Film
thicknessd (mm)
b
c
Polymer (nm)
D33 (esu)
U
d31b(esu)
n
4
364 (3.54 ꢃ 0.09) ꢁ 10ꢂ9 0.13 (1.26 ꢃ 0.06) ꢁ 10ꢂ9
0.52
1.60
aPolymer film.
bSHG coefficients (d33) were derived from the analysis of measured Maker-fringes [14].
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 [13].

194
G.-Y. Lee et al.
FIGURE 2 Angular dependence of SHG signal in a poled film of polymer 4.
To evaluate the high-temperature stability of the polymer, 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
FIGURE 3 Normalized SHG signal of polymer 4 as a function of temperature
at a heating rate of 3.5ꢀC=min.

Synthesis and Electro-Optic Properties of Novel Polyester
195
of the poled polymer film as a function of temperature, in situ SHG
measurements were performed at a heating rate of 3.5ꢀC=min from
25ꢀC to 140ꢀC. The polymer film exhibited a thermal stability up to
near Tg and no significant SHG decay was observed below 95ꢀC, as
shown in Figure 3. In general, side chain NLO polymers lose thermal
stability of dipole alignment below Tg. Stabilization of dipole alignment
is a characteristic of main chain NLO polymers. The high thermal sta-
bility of second harmonic generation of polymer 4 is due to the stabili-
zation of dipole alignment of NLO chromophore, which stems from the
partial main chain character of the polymer structure. Thus, we
obtained a new NLO polyester having both of the merits of main chain-
and side-chain NLO polymers namely stable dipole alignment and good
solubility.
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