Synthesis and properties of novel nonlinear optical polyester containing carbomethoxycyanovinylnitrocatecholic group

Article abstract of DOI:10.1080/15421406.2018.1450931

A new X-type polyester 5 containing 4-(2′-carbomethoxy-2′-cyano)vinyl-5-nitrocatecholic groups as nonlinear optical (NLO) chromophores, which are parts of the polymer backbone, was prepared and studied its properties. Polyester 5 is soluble in common organic solvents such as acetone and dimethylsulfoxide. Polymer 5 shows a thermal stability up to 280°C from thermogravimetric analysis with glass-transition temperature (T_g) obtained from differential scanning calorimetry of near 110°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer 5 film at the 1064?nm fundamental wavelength is 6.13 × 10^?9 esu. The dipole alignment of polymer 5 exhibits a thermal stability up to T_g, and no significant SHG decay is observed below 110°C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.

Full text of DOI:10.1080/15421406.2018.1450931

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and properties of novel
nonlinear optical polyester containing
carbomethoxycyanovinylnitrocatecholic group
Jung-Eun Kim & Ju-Yeon Lee
To cite this article: Jung-Eun Kim & Ju-Yeon Lee (2017) Synthesis and properties of novel
nonlinear optical polyester containing carbomethoxycyanovinylnitrocatecholic group, Molecular
Crystals and Liquid Crystals, 659:1, 77-83, DOI: 10.1080/15421406.2018.1450931
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Synthesis and properties of novel nonlinear optical polyester
containing carbomethoxycyanovinylnitrocatecholic group
Jung-Eun Kima and Ju-Yeon Leeb
aInstitute of Basic Science, Department of Chemistry, Inje University, Gimhae, Korea; bInstitute of Basic Science,
Department of Chemistry and Nano Science Engineering, Inje University, Gimhae, Korea
ABSTRACT
KEYWORDS
AnewX-typepolyester5containing4-(2ꢀ-carbomethoxy-2ꢀ-cyano)vinyl-
5-nitrocatecholic groups as nonlinear optical (NLO) chromophores,
which are parts of the polymer backbone, was prepared and studied its
properties. Polyester 5 is soluble in common organic solvents such as
acetone and dimethylsulfoxide. Polymer 5 shows a thermal stability up
to 280°C from thermogravimetric analysis with glass-transition temper-
ature (Tg) obtained from differential scanning calorimetry of near 110°C.
The second harmonic generation (SHG) coefficient (d33) of poled poly-
mer 5 film at the 1064 nm fundamental wavelength is 6.13 × 10−9 esu.
The dipole alignment of polymer 5 exhibits a thermal stability up to Tg,
and no significant SHG decay is observed below 110°C due to the partial
main-chain character of polymer structure, which is acceptable for NLO
device applications.
Differential scanning
calorimetry (DSC); dipole
alignment; NLO; polyester;
SHG coefficient;
thermogravimetric analysis
(TGA)
Introduction
Over the past few decades much effort have focused on nonlinear optical (NLO) materi-
als with potential applications in electro-optic devices for optical information processing
and optical data transmission [1–10]. Organic materials display higher optical nonlinear-
ity and faster response time than their inorganic counterparts; among the organic mate-
rials, NLO polymers are receiving great attention of late, mainly because they offer many
advantages such as mechanical endurance, light weight, low cost, chemical resistance, and
ease of processing in terms of their use in electro-optic devices. A potential NLO poly-
mer must contain highly polarizable conjugated electronic systems and has to be mechani-
cally very strong and thermally stable with a high glass transition temperature (Tg). In the
synthesis of NLO polymers for electro-optic device applications, stabilization of electrically
induced dipole alignment is one of important considerations. Several approaches to mini-
mize the randomization have been developed, namely the use cross-linking method [11–14],
utilization of high Tg polyimides [15–17], and host-guest polymers with spindle-type chro-
mophores [18]. 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 polymers have good solubility, but
they often suffer from poor stability of dipole alignments at high temperatures. Recently we
CONTACT Ju-Yeon Lee
chemljy@inje.ac.kr
Engineering, Inje University,  Obang-dong, Gimhae –, Korea.
Institute of Basic Science, Department of Chemistry and Nano Science
©  Taylor & Francis Group, LLC

78
J.-E. KIM AND J.-Y. LEE
Figure . Main chain NLO polymers (a) side chain NLO polymers (b) and X-type NLO polymers (c).
prepared novel NLO polyesters containing dioxybenzylidenecyanoacetate [19], nitrophenyla-
zoresorcinol [20], nitrophenylazocatechol [21], and dioxynitrobenzylidenemalononitrile [22]
as NLO chromophores, which showed enhanced thermal stability of dipole alignment. In this
work reported here, we prepared a new X-type polyester 5 containing 4-(2ꢀ-carbomethoxy-2ꢀ-
cyano)vinyl-5-nitrocatecholic groups as NLO chromophores. We selected the latter because
they are expected to have higher optical nonlinearities due to nitro group and double conju-
gation. Furthermore these 4-(2ꢀ-carbomethoxy-2ꢀ-cyano)vinyl-5-nitrocatecholic groups can
be incorporated into novel X-type NLO polyesters (see Fig. 1c). The structure of NLO chro-
mophores and these X-type NLO polyesters have not yet been described in the literature.
Thus, we synthesized a new type of NLO polyester 5, in which the pendant NLO chro-
mophores are parts of the polymer backbone. This X-type NLO polymer is expected to have
the advantages of both main-chain and side-chain NLO polymers, namely stable dipole align-
ment and good solubility. After confirming the structure of the resulting polymer 5, we inves-
tigated its properties such as SHG activity and relaxation of dipole alignment. We now report
the results of the initial phase of the work.
Experimental
Materials
Polymer 4 was prepared according to a procedure similar to that reported already [19]. Syn-
thetic method of polymer 5 was as follows. Polymer 4 (4.37 g, 0.10 mol) was dissolved in
25 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
(8 mL) and acetic acid (16 mL) were added dropwise to the solution and stirred for 20 hr
at 0°C. The reaction mixture was poured to 500 mL of ice water and the precipitated poly-
mer was separated with suction. The polymer was further purified by extraction in a Soxhlet

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
79
extractor with methanol and dried under vacuum, yielding 4.24 g (88% yield) of polymer
5. The degree of nitration estimated by 1H NMR spectrum was about 85%. ηinh = 0.30 dL
g−1 (c, 0.5 g−1 dL in DMSO at 25°C). 1H NMR (DMSO-d6) δ 3.90 (s, 3H, -COOCH3), 4.56–
4.68 (t, 8H, 2 -O-CH2-CH2-O-), 7.16–7.25 (s, 1H, aromatic), 7.59–7.64 (s, 1H, aromatic),
7.96–8.17 (m, 4H, aromatic), 8.83–8.86 (s, 1H, aromatic). IR (KBr) 3105 (w, = C-H), 2958
(m, C-H), 2224 (s, CN), 1725 (vs, C = O), 1596 (s, C = C), 1573, 1268 (s, N = O) cm-1.
Anal. Calcd for (C23H18N2O10)n: C, 57.26; H, 3.76; N, 5.81. Found: C, 57.33; H, 3.82;
N, 5.87.
Measurements
IR, 1H NMR, and UV-visible spectra were obtained with a Varian FT IR-1000 IR spectropho-
tometer, Varian VNMRS 500MHz 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 thermogravimetric analyzer
(TGA) with a heating rate of 10°C min−1 up to 800°C 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 chromatog-
raphy (GPC; styragel HR5E4E columns; tetrahydrofuran (THF) solvent). The alignment of
the NLO chromophore of the polymer was carried out by corona poling method. The refrac-
tive index of the polymer sample was measured by the optical transmission technique [23].
The transmittance of thin film gives information on the thickness, refractive index and extinc-
tion coefficient of the film. Thus, we can determine these parameters by analyzing the trans-
mittance. Second harmonic generation (SHG) measurements were made using a Maker fringe
technique [24].
Results and discussion
Synthesis and characterization of polymer
The synthetic route for polyester 5 is presented in Scheme 1. Diol 3 (Z and E) was condensed
with terephthaloyl chloride (TPC) in a dry DMF solvent to yield polyester 4. Polymer 4 was
reacted with nitric acid and acetic acid in anhydrous DMF to yield X-type polyester 5 con-
taining 4-(2ꢀ-carbomethoxy-2ꢀ-cyano)vinyl-5-nitrocatecholic groups as NLO chromophores.
The degree of nitration was about 85%. The chemical structure of the polymer was iden-
1
1
tified by H NMR, IR spectra, and elemental analysis. H NMR spectrum and elemental
analysis results of the polymer are fit the polymer structure. The IR spectrum of polymer 5
shows strong carbonyl peaks near 1725 cm−1 indicating the presence of ester bond. The spec-
trum also shows strong absorption peak near 2224 cm−1 indicating the presence of nitrile
group, and two strong absorption bands due to the nitro group in the NLO chromophore
appeared near 1573 cm−1 and 1268 cm−1. These results are consistent with the proposed
structure, indicating that the NLO chromophores remained intact during the polymeriza-
tion. The molecular weights were determined by GPC with polystyrene as the standard and
THF as the eluent. 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 15,400 g
mol−1 (Mw/Mn = 1.92). 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 in the

80
J.-E. KIM AND J.-Y. LEE
Scheme . Synthetic scheme and structure of polyester 5.
range 0.30–0.32 dL g−1. Polymer 5 shows strong absorption near 362 nm due to the NLO chro-
mophore 4-(2ꢀ-carbomethoxy-2ꢀ-cyano)vinyl-5-nitrocatecholic group. The striking feature of
this polymerization system is that it gives unprecedented X-type NLO polymers, in which the
pendant NLO chromophores are parts of the polymer backbone. These X-type NLO polymers
are expected to have the advantages of both main-chain and side-chain NLO polymers. Thus,
we obtained a new type of NLO polyester with side-chain and main-chain characteristics.
Thermal properties of polymer
The thermal behavior of the polymer was investigated using TGA and DSC to determine
the thermal degradation pattern and Tg. The TGA thermogram of the polymer 5 is shown in
Fig. 2. Polymer 5 shows a thermal stability up to 280°C from its TGA thermogram. Tg value of
polymer 5 measured using DSC is near 110°C. This Tg value is higher than that of the Y-type
polyester 4, which is near 105°C [19].
Nonlinear optical properties of polymer
The NLO properties of polymer 5 were studied using the SHG method. To induce noncen-
trosymmetric polar order, the spin-coated polymer films were corona-poled. As the tempera-
ture 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 removed. The poling was confirmed
from UV-visible spectra. Figure 3 shows the UV-visible absorption spectra of the polymer 5
before and after poling. After electric poling, the dipole moments of the NLO chromophores
were aligned and UV-visible absorption of polymer 5 exhibits a decrease in absorption due

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
81
Figure . TGA thermogram of polymer 5 obtained at a heating rate of °C min− under nitrogen.
to birefringence. From the absorbance change, the order parameter of the poled films can be
estimated, which is related to the poling efficiency. The estimated order parameter value ꢀ
is equal to 0.19 for polymer 5 (ꢀ = 1-A1/A0, where A0 = 1.2436 and A1 = 1.0126 are the
absorbances of the polymer film before and after poling, respectively). SHG measurements
were performed at a fundamental wavelength of 1064 nm using a mode locked Nd-YAG laser.
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 (d11 for quartz was 0.79 × 10−9 esu). To calculate the d31 and d33 value,
both s-polarized and p-polarized IR laser were directed at the samples and recorded. SHG
coefficients (d33) were derived from the analysis of measured Maker-fringes with the Pascal
fitting program according to the literature procedure [24]. The NLO properties of polymer 5
are summarized in Table 1. The values of d31 and d33 for polymer 5 are 2.18 × 10−9 esu and
6.13 × 10−9 esu, respectively. This d33 value is higher than that of Y-type polyester 4, which is
5.78 × 10−9 esu [19].
To evaluate the high-temperature stability of the polymer, we studied the temporal stability
of the SHG signal. Figure 4 shows the dynamic thermal stability study of the NLO activity of a
Figure . UV-visible absorption spectra of a film of polymer 5 before and after poling.

82
J.-E. KIM AND J.-Y. LEE
Table . Nonlinear optical properties of polyesters 4–5.
a
b
b
λmax
d
ꢀc
d
Film thicknessd
Polymer
(nm)
(esu)
(esu)
(μm)
n
4e
5


. × −
. × −
.
.
. × −
. × −
.
.
.
.
aPolymer film after corona poling.
bSHG coefficients (d) were derived from the analysis of measured Maker-fringes [].
cOrder parameter ꢀ = -A/A, where A and A are the absorbances of the polymer film before and after corona poling, respec-
tively.
dFilm thickness was determined by the optical transmission technique [].
eY-type polyester containing ,-dioxybenzylidenecyanoacetate [].
Figure . Normalized SHG signal of polymer 5 as a function of temperature at a heating rate of °C min−.
film of polymer 5. To investigate the real time NLO decay of the SHG signal of the poled poly-
mer film as a function of temperature, in situ SHG measurements were performed at a heating
rate of 4°C min−1 from 30 to 200°C. The polymer film exhibits a thermal stability up to Tg
and no SHG decay is observed below 110°C. This SHG thermal stability is higher than that of
the Y-type polyester 4 (100°C) [19], which is probably due to a nitro group and a double con-
jugation. 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 chromophore, which stems from the partial main-
chain character of the polymer structure. Thus, we obtained a new NLO polyester having both
the merits of main-chain and side-chain NLO polymers, namely stable dipole alignment and
good solubility.
Conclusions
We prepared a new X-type polyester 5 with pendant NLO chromophores which constitute
part of the polymer main chain. The chemical structure of the polymer was confirmed by 1H
NMR, IR spectra, and elemental analysis. The X-type NLO polyester 5 is soluble in common
organic solvents and shows a thermal stability up to 280°C from its TGA thermogram with Tg
value measured using DSC near 110°C. The NLO properties of polymer 5 were studied using

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
83
the SHG method. The SHG coefficient (d33) of corona-poled polymer film is 6.13 × 10−9 esu.
Polyester 5 exhibits SHG stability up to Tg and no significant SHG decay is observed below
110°C. This high thermal stability of optical nonlinearity is due to the stabilization of dipole
alignment of NLO chromophore, which stems from the partial main-chain character of the
polymer structure. We are now in the process of extending the polymerization system to the
synthesis of other type of NLO polymers and the results will be reported elsewhere.
Funding
This research was supported by Basic Science Research Program through the National Research Foun-
dation of Korea (NRF) funded by the Ministry of Education (No. 2015R1D1A1A09058972).
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